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Genetic Variation
Genetic variation describes naturally occurring genetic differences among individuals of the same species. All organisms are slightly or greatly different. This variation permits flexibility and survival of a population in the face of changing environmental circumstances and can also produce variation in the gene pools. This variation is important, especially in New Zealand as the habitat is constantly changing living (biotic) and non living (abiotic factors) change the populations gene pools and pressures. Genetic Variation is a biological advantage as having individuals in a species that are all slightly "different" leads to the ability of some of these individuals being able to adapt and change to the changes that may occur with their environment - this leads to survival of these individuals. This standard is about what brings on this variation in populations and how this leads to different frequencies of traits and eventually natural selection. What leads to the variation in a species? The Genetic difference/variability between individuals is what makes us all different from each other. Brothers and sisters may look similar to each other, but there are always significant differences (unless twins) = Genetic Variation HOW is every individual genetically different ?? ..........By the production of Gametes & the mixing up of existing alleles into new combinations by Meiosis and by Mutations occuring. |
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DNA - do we need a Recap: Check this out !
Click on the button to check out the link re: more info on DNA |
Chromosomes, Genes, Alleles and Mutations...how are they linked ?? Watch the clip below.
What is the LINK and differences between Chromosome, chromatids, loci and alleles
Do we need to re-cap DNA Replication ?? A quick reminder that is worth watching....see below...
Mutations:
Just to Recap for you......
Mutation is a permanent / random change in the BASE PAIR SEQUENCE in DNA.
Mutations must occur in gamete-producing cells in order to enter the gene pool of the population = Gametic Mutations
It can also be defined as a permanent change in the nucleotide sequence in a gene or a chromosome/DNA.
A mutation is a permanent (unrepaired) change in an organisms DNA.
They introduce new alleles into a population. Most mutations are harmful.
Beneficial ones tend to occur more often in organisms with short generation times
Mutations are caused by mutagens.
Somatic mutations occur in any cells of the body other than in the gametes so are Alterations in DNA that occur after conception. Somatic mutations can occur in any of the cells of the body except the germ cells (sperm and egg) and therefore are not passed on to the offspring
somatic mutations are not passed on from one generation to the next
somatic mutations only affect the individual organism in which the cells have mutated
Gametic mutations only occur in gametes, eg, sperm / eggs (accept pollen).
A heritable change in the DNA that occurred in a gamete (germ cell) – a cell destined to become an egg or sperm.
Gametic mutations are (heritable) transferred to the next (& possibly subsequent) generations
Gametic mutations are not limited to the individual in which the original mutations has occurred, the new alleles created by gametic mutation are available to the gene pool and may become established in that gene pool if they create a "selective advantage" to the individual and increase reproductive success.
Many may be silent – not observed – and may only be selected for or against at a later date.
Neutral mutations make no change at all.
Mutations must happen in gamete producing cells to enter the gene pool of a population. This is important!!
The following Mutations clip is worth a nosey ladies.
Just to Recap for you......
Mutation is a permanent / random change in the BASE PAIR SEQUENCE in DNA.
Mutations must occur in gamete-producing cells in order to enter the gene pool of the population = Gametic Mutations
It can also be defined as a permanent change in the nucleotide sequence in a gene or a chromosome/DNA.
A mutation is a permanent (unrepaired) change in an organisms DNA.
They introduce new alleles into a population. Most mutations are harmful.
Beneficial ones tend to occur more often in organisms with short generation times
Mutations are caused by mutagens.
Somatic mutations occur in any cells of the body other than in the gametes so are Alterations in DNA that occur after conception. Somatic mutations can occur in any of the cells of the body except the germ cells (sperm and egg) and therefore are not passed on to the offspring
somatic mutations are not passed on from one generation to the next
somatic mutations only affect the individual organism in which the cells have mutated
Gametic mutations only occur in gametes, eg, sperm / eggs (accept pollen).
A heritable change in the DNA that occurred in a gamete (germ cell) – a cell destined to become an egg or sperm.
Gametic mutations are (heritable) transferred to the next (& possibly subsequent) generations
Gametic mutations are not limited to the individual in which the original mutations has occurred, the new alleles created by gametic mutation are available to the gene pool and may become established in that gene pool if they create a "selective advantage" to the individual and increase reproductive success.
Many may be silent – not observed – and may only be selected for or against at a later date.
Neutral mutations make no change at all.
Mutations must happen in gamete producing cells to enter the gene pool of a population. This is important!!
The following Mutations clip is worth a nosey ladies.
Causes of Mutation
Mutations can be:
MutagensMutagens are environmental factors that induce a change in the base sequence of an individual's DNA. Common mutagens include
Mutations can be:
- Inherited
- Spontaneous (errors made during replication)
- Induced by a mutagen
MutagensMutagens are environmental factors that induce a change in the base sequence of an individual's DNA. Common mutagens include
- Radiation (UV rays / X-rays etc)
- Chemicals (including environmental poisons, diet, etc)
- Viruses & some other microorganisms
Chromosome Mutations:
Chromosome mutations or ‘block’ mutations occur as a result of errors in crossing over during meiosis. Certain mutagens may also induce Chromosomal mutations.
Chromosome mutations affect large segments of DNA containing many genes.
Chromosome Mutations can cause changes in "chromosome number"(aneuploidy) in a cell OR changes in "chromosome Structure".
There are four different types of chromosomal mutations: Deletions, Translocations, Duplications and Inversions (pictured below).
Note that any chromosome mutation resulting in a significant loss of genetic material (Deletion) is most likely to be lethal. While many chromosome mutations do not result in a loss of genetic material, the position of a gene on a chromosome can affect its expression. Moving genes from one location to another can affect their expression especially during early developmental stages. In humans significant changes to the position of many genes can prevent proper foetal development (lethal). As with gene mutations, chromosome mutations can be neutral, deleterious, lethal or even beneficial. However, because chromosome mutations affect much larger regions of DNA potentially carrying hundreds or even thousands of genes, they are much more likely to be deleterious or lethal.
EXAMPLE 1:
Translocations Can Cause Down Syndrome:
Down syndrome is associated with some impairment of cognitive ability and physical growth as well as characteristic body / facial features. Down syndrome is most commonly the result of Aneuploidy.
However, some cases (2-3%) are caused by a translocation during meiosis that transfers most of chromosome 21 onto chromosome 14.
The resulting recombinant chromosome 14 effectively now carries a copy of chromosome 21. If a gamete receives this new recombinant chromosome 14 as well as the normal chromosome 21 the resulting zygote will effectively inherit an additional copy of chromosome 21.
Chromosome mutations or ‘block’ mutations occur as a result of errors in crossing over during meiosis. Certain mutagens may also induce Chromosomal mutations.
Chromosome mutations affect large segments of DNA containing many genes.
Chromosome Mutations can cause changes in "chromosome number"(aneuploidy) in a cell OR changes in "chromosome Structure".
There are four different types of chromosomal mutations: Deletions, Translocations, Duplications and Inversions (pictured below).
Note that any chromosome mutation resulting in a significant loss of genetic material (Deletion) is most likely to be lethal. While many chromosome mutations do not result in a loss of genetic material, the position of a gene on a chromosome can affect its expression. Moving genes from one location to another can affect their expression especially during early developmental stages. In humans significant changes to the position of many genes can prevent proper foetal development (lethal). As with gene mutations, chromosome mutations can be neutral, deleterious, lethal or even beneficial. However, because chromosome mutations affect much larger regions of DNA potentially carrying hundreds or even thousands of genes, they are much more likely to be deleterious or lethal.
EXAMPLE 1:
Translocations Can Cause Down Syndrome:
Down syndrome is associated with some impairment of cognitive ability and physical growth as well as characteristic body / facial features. Down syndrome is most commonly the result of Aneuploidy.
However, some cases (2-3%) are caused by a translocation during meiosis that transfers most of chromosome 21 onto chromosome 14.
The resulting recombinant chromosome 14 effectively now carries a copy of chromosome 21. If a gamete receives this new recombinant chromosome 14 as well as the normal chromosome 21 the resulting zygote will effectively inherit an additional copy of chromosome 21.
EXAMPLE 2: Chronic Myeloid Leukaemia
Chromic myeloid leukaemia is a type of leukaemia (cancer of the blood) caused by a translocation between chromosomes 9 and 22.
It can clearly be seen here that crossing over has occurred between chromosomes 9 and 22. As a result the two chromosomes have exchanged chromosome tips altering the position and thus expression of many different genes.
Extra for Experts: Chronic myeloid leukaemia is not actually the result of altered gene expression. Rather the point at which the two chromosomes cross over happens to be within coding regions (genes) on both chromosomes. The result is a gene which is an abnormal fusion of the two existing genes. The resulting protein contains a region or domain capable of stimulating cell division, but no longer requires to be activated by other cell signals. Instead it is constantly switched on, continuously stimulating cell division resulting in a loss of the ability to regulate cellular growth
Chromic myeloid leukaemia is a type of leukaemia (cancer of the blood) caused by a translocation between chromosomes 9 and 22.
It can clearly be seen here that crossing over has occurred between chromosomes 9 and 22. As a result the two chromosomes have exchanged chromosome tips altering the position and thus expression of many different genes.
Extra for Experts: Chronic myeloid leukaemia is not actually the result of altered gene expression. Rather the point at which the two chromosomes cross over happens to be within coding regions (genes) on both chromosomes. The result is a gene which is an abnormal fusion of the two existing genes. The resulting protein contains a region or domain capable of stimulating cell division, but no longer requires to be activated by other cell signals. Instead it is constantly switched on, continuously stimulating cell division resulting in a loss of the ability to regulate cellular growth
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Gregor Mendel’s law of segregation states that the two alleles for each trait segregate, or separate, during the formation of gametes, and that during the formation of new zygotes, the alleles will combine at random with other alleles. The law of segregation ensures that a parent, with two copies of each gene, can pass on either allele. Both alleles will have the same chance of ending up in a zygote.
HOW is every individual genetically different ?? ..........By the production of Gametes & the mixing up of existing alleles into new combinations = Meiosis
Asexual v Sexual Reproduction......
click here for some notes on this if needed
Sexual Reproduction:
The process of sexual reproduction involves the fusion of sex cells called Gametes - 2 parents each contributing one gamete. Gametes(sex cells) are produced by the cell process called Meiosis.
Meiosis ...
The process of cell division that occurs in Germ Cells = ovaries and testes ( germ cells).
It produces the sex cells (gametes) - egg and sperm.
It comes from the Greek word to "make smaller" ie: so results in 4 daughter cells each with half the normal number of chromosomes ie: 4 haploid cells .
Meiosis is the reason you look similar, but not identical to your parents and siblings.
There are 3 areas of focus for us at Level 2 in Meiosis -that leads to Genetic Variation:
- independent assortment,
-crossing over and
- segregation !!
These processes are the reasons for the genetically different daughter cells that have NEW COMBINATIONS of alleles
Meiosis is a vital process because it reduces the original number of chromosomes to half and allows genetic variability by genetic recombination and independent assortment.
Recombination is where there is an exchange of genetic material between adjacent chromatids of homologous chromosomes= crossing over. This ‘random’ exchange of DNA results in novel combinations of alleles on the chromosomes, creating almost infinite potential for variation.
Independent Assortment:
During meiosis I (first division), Metaphase 1- chromosomes pairs line up up side by side at the "cell equator". This "lining up" is a totally random "independent" event ( no set pattern)
Each of the resulting daughter cells will receive one chromosome from each pair. It is totally random as to which combo of alleles ends up in particular gamete (one of the 4 new ones produced in this process)
The random arrangement of homologous chromosomes during meiosis I results in gametes with a unique combinations of alleles. This is known as independent assortment.
If we have 23 pairs of chromosomes and each time we randomly take one chromosome from each pair, then we can make more than 8 million different combinations.
Eg, in humans there are 23 pairs of chromosomes, so 223 kinds of gamete could be produced (>8 million)
For example,See diagram below:
Here we have started with a cell that only has two pairs of chromosomes (smaller and larger). When this cell undergoes its first division, each daughter cell will receive one of the larger chromosomes and one of the smaller chromosomes. However, which one of the large chromosomes the daughter cell receives is completely independent of which smaller chromosome it receives.
We call this Independent Assortment.
You can think of this like a deck of cards, where each card represents a chromosome. Homologous chromosomes might be represented by cards with same number and colour (so there is always a pair). The cards are shuffled and then dealt between two players ensuring that each player always get's one card from each pair (e.g. that both players get a red 10). Independent assortment essentially acts like shuffling the deck. Each time a hand is dealt (gametes are formed), a new combination of chromosomes results.
Crossing Over:
During meiosis I (Metaphase) the replicated homologous pairs of chromosomes come together and often sections of the chromatids "exchange" pieces of their DNA in a process called recombination (crossing over). The point at which the chromosomes cross over is known as the chiasma. Theses non-sister chromatids exchange chuncks so the resultant chromosome is neither maternal or paternal BUT made up of a new combo of genes of BOTH parental chromosomes.This random crossing over and exchanging of chunks of DNA/sections of chromatid results in novel combinations of alleles - so new allele combos end up in the gametes which are " different" to the parental allele combinations.
Copy pics from diagrams below..........
Law of segregation:
Organisms that reproduce sexually inherit at least two versions of every gene, called alleles.
During Meiosis, when each of the chromosome pairs separate (segregate) - the two alleles separate and so each gamete only recieves one allele of each pair and so only one of each allele is passed onto the offspring. This separation is often referred to as the Law of Segregation.
The segregation of alleles is essential for sexual reproduction, allowing offspring to be produced with a NEW combination of alleles from each parent.
Gregor Mendel’s law of segregation states that the two alleles for each trait segregate, or separate, during the formation of gametes. The law of segregation ensures that a parent, with two copies of each gene, can pass on either allele.
Both alleles will have the same chance of ending up in a zygote.
See below for Example of Independent Assortment:
CROSSING OVER IMAGES.... (SEE NOTES ABOVE)
NEED MORE to read on this process...click on this link to see some animations on Meiosis OR watch the videos below.....
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PREDICTING INHERITANCE:
- Monohybrid inheritance is the inheritance of ONE characteristic/trait controlled by ONE gene at a particular locus on a homologous chromosome. The gene will have alternative forms called alleles. The alleles will be dominant or recessive. An organisms genotype is determined by the combination of alleles inherited from each parent.
- Complete dominance is when the info one allele, the dominant allele is always expressed in the phenotype and its presence will MASK the presence of the recessive alllele.
- The study of these patterns of inheritance is often referred to as Mendelian Genetics -named after an Austrian monk who first realised that these patterns could only be explained if sexually reproducing organisms had two "factors" (which later came to be called alleles) and that they could be dominant and recessive.
Test Cross:
If an organism displays a dominant trait how do you know if they are homozygous dominant (pure breed)for that phenotype or heterozygous for that desired trait ? DO A TEST CROSS OF COURSE !
A test cross is crossing the individual in question with a homologous recessive individual. You can be fairly certain of the phenotype of the parent (BB or Bb), HOWEVER due to the random nature of fertilisation offspring may not reflect predicted outcomes in the first generation of offspring. Multiple crosses will need to be done to ensure your individual is a pure bred.
Incomplete dominance:
Is a form of intermediate inheritance in which one allele for a specific trait is NOT completely dominant over the other allele. (There are two different versions of a gene and neither is dominant over the other. )
When both alleles are present in the "heterozygous genotype" they BOTH contribute to produce a phenotyype that is an intermediate or blend of the genetic info !! So the Heterozygote Individual is producing BOTH PROTEINS that contribute to the final phenotype
This results in a third phenotype in which the expressed physical trait is a combination /blend of the dominant and recessive phenotypes in the heterozygous genotype.
When two F1's ie: Pink are crossed the Phenotypic Ratio for Incomplete Dominance is 1:2:1
Ie: 1 red : 2 Pink : 1 white
A common example of this is found in snapdragons (a type of flower). There are two versions of a gene for flower colour, one codes for Red flowers, the other White flowers. When a plant inherits one copy of each (is heterozygous) the colours mix and the resulting flowers are pink (see below, note the resulting phenotype ratio). This ‘mixing’ of characteristics is known as incomplete dominance. Because neither allele is dominant over the other we can’t simply use upper and lower case letters. Refer to diagram above.
CO-DOMINANCE:
Inheritance occurs when BOTH alleles are involved and expressed independently in the phenotype of offspring.
BOTH alleles are equally and independently expressed - so the genes for producing the protein for each allele is fully expressed.
Hint: can recognise co-dominance when offspring produce a "third" phenotype that contains characteristics of BOTH the parental phenotypes. 2 different uppercase letters for alleles is typically used in co-dominance.
In some flower varieties we find it is possible for two different versions of a gene (alleles) that code for different characteristics to both be expressed independently. You can think of both alleles as being dominant. Here the characteristics do not mix; rather heterozygotes will display both characteristics. Using flowers as an example, heterozygote's display both red and white patches. When two FI (from a co-dominance are crossed) again you get a 1:2:1 ratio for Phenotype. Where you have 3 different phenotypes.
Inheritance occurs when BOTH alleles are involved and expressed independently in the phenotype of offspring.
BOTH alleles are equally and independently expressed - so the genes for producing the protein for each allele is fully expressed.
Hint: can recognise co-dominance when offspring produce a "third" phenotype that contains characteristics of BOTH the parental phenotypes. 2 different uppercase letters for alleles is typically used in co-dominance.
In some flower varieties we find it is possible for two different versions of a gene (alleles) that code for different characteristics to both be expressed independently. You can think of both alleles as being dominant. Here the characteristics do not mix; rather heterozygotes will display both characteristics. Using flowers as an example, heterozygote's display both red and white patches. When two FI (from a co-dominance are crossed) again you get a 1:2:1 ratio for Phenotype. Where you have 3 different phenotypes.
LETHAL ALLELES:
Are alleles that produce a phenotypic effect that causes the death of the individual.
Usually result due to a mutation which results in an allele that produces a non-functional version of an essential protein.
If an individual inherits a lethal combination of mutated alleles it will die, before or shortly after birth.
The Mutated Lethal Allele may be completely dominant, incompletely dominant and or recessive
MULTIPLE ALLELES:
So far we have always looked at genes for which there are only two possible versions (alleles). But what happens if there are more than two possible versions of a gene? This is referred to as Multiple alleles.
Occurs with genes where there are 3 or more alleles for the same/ single gene -(characteristics/Phenotype) ie: there are more than 2 alternatives available at one locus for one gene. (although individual will only have 2 of the alleles in its genotype)
These arise due to Mutations in a single gene which results in that gene having more than 2 different alleles (versions)
A good example of Multiple Alleles in action is the Human Blood groups
The blood group of humans is coded for by the ABO gene - it has 3 different versions of the allele rather than 2
Blood cells express a particular carbohydrate groups on their surface. There are three different versions of the gene that code for this surface carbohydrate. These three alleles cannot be represented by a single letter and are thus denoted A, B and O.
I A = A Blood group
I B = B
i = O
(see table below) I stands for Immunoglobin
The alleles A and B code for different carbohydrate groups, while the allele O codes for no carbohydrate group at all!
Because the O allele does not code for a surface carbohydrate, it is considered recessive to the other two alleles.
If the alleles A and B are both present, they are both expressed simultaneously and are thus considered co-dominant to one another and that individual has the blood type AB
As a person can only carry two versions of a gene (alleles), the following genotypes and phenotypes are possible:
So far we have always looked at genes for which there are only two possible versions (alleles). But what happens if there are more than two possible versions of a gene? This is referred to as Multiple alleles.
Occurs with genes where there are 3 or more alleles for the same/ single gene -(characteristics/Phenotype) ie: there are more than 2 alternatives available at one locus for one gene. (although individual will only have 2 of the alleles in its genotype)
These arise due to Mutations in a single gene which results in that gene having more than 2 different alleles (versions)
A good example of Multiple Alleles in action is the Human Blood groups
The blood group of humans is coded for by the ABO gene - it has 3 different versions of the allele rather than 2
Blood cells express a particular carbohydrate groups on their surface. There are three different versions of the gene that code for this surface carbohydrate. These three alleles cannot be represented by a single letter and are thus denoted A, B and O.
I A = A Blood group
I B = B
i = O
(see table below) I stands for Immunoglobin
The alleles A and B code for different carbohydrate groups, while the allele O codes for no carbohydrate group at all!
Because the O allele does not code for a surface carbohydrate, it is considered recessive to the other two alleles.
If the alleles A and B are both present, they are both expressed simultaneously and are thus considered co-dominant to one another and that individual has the blood type AB
As a person can only carry two versions of a gene (alleles), the following genotypes and phenotypes are possible:
Dihybrid Inheritance:
Dihybrid crosses are genetic crosses involving the inheritance of two genes and, therefore, two traits. Dihybrid crosses show how the production of these gametes bring about the inheritance of traits. Four types of gamete are produced for each parent. This is the result of independent assortment of the homologous pairs of chromosomes and their separation during the process of meiosis.
What does a Dihybrid Cross look like ???
Dihybrid crosses are genetic crosses involving the inheritance of two genes and, therefore, two traits. Dihybrid crosses show how the production of these gametes bring about the inheritance of traits. Four types of gamete are produced for each parent. This is the result of independent assortment of the homologous pairs of chromosomes and their separation during the process of meiosis.
What does a Dihybrid Cross look like ???
DIHYBRID and a TEST CROSS:
Remember just like in a monohybrid cross multiple breeding's need to be done to determine if an individual is "pure Breeding" for a desired Phenotype
Remember just like in a monohybrid cross multiple breeding's need to be done to determine if an individual is "pure Breeding" for a desired Phenotype
LINKED GENES:
Chromosomes carry many genes.
Genes (alleles) on the same chromosome are called " linked genes " and often they move together during meiosis because they are physically "tethered" together and ( therefore cannot separate randomly during Meiosis..)
Therefore they do NOT assort independently (like genes on different chromosomes)as they are PHYSICALLY LLINKED together = reduces variation AND will affect Ratios in term of new recombinants. There will be less recombinants. (so you do not get the expected 9:3:3:1) ie: low number of recombinants in a ratio .
(this is determined by their relative distance apart they are on the chromosome) see pic
If the linked genes are very close to each other on the chromosome it is likely that they will be exchanged together.....so the crossing over has no affect as they will "go over" or " be exchanged" together.......does this increase variation ??....NO !
If linked genes are far apart on the same chromosome then crossing over may independently affect them and therefore influence the phentoypic ratio...YES..this produces recombinants !!
ICheck out the video below:
Chromosomes carry many genes.
Genes (alleles) on the same chromosome are called " linked genes " and often they move together during meiosis because they are physically "tethered" together and ( therefore cannot separate randomly during Meiosis..)
Therefore they do NOT assort independently (like genes on different chromosomes)as they are PHYSICALLY LLINKED together = reduces variation AND will affect Ratios in term of new recombinants. There will be less recombinants. (so you do not get the expected 9:3:3:1) ie: low number of recombinants in a ratio .
(this is determined by their relative distance apart they are on the chromosome) see pic
If the linked genes are very close to each other on the chromosome it is likely that they will be exchanged together.....so the crossing over has no affect as they will "go over" or " be exchanged" together.......does this increase variation ??....NO !
If linked genes are far apart on the same chromosome then crossing over may independently affect them and therefore influence the phentoypic ratio...YES..this produces recombinants !!
ICheck out the video below:
Early genetic experiments showed that there were some traits that did not follow the normal expected patterns of inheritance in Dihybrid crosses.
The expected ratios of 9:3:3:1 (for a heterozygous cross) or 1:1:1:1 (for a back or test cross) were not seen at all. In fact, in some cases only two different phenotypes were produced! If we look at the diagram (left), we can see that this is because the genes are linked. As a result only two different types of gamete can be produced. Independent assortment/crossing over does NOT affect linked genes as it only results in a random combination of chromosomes and does not alter the combination of alleles within a chromosome itself. |
Recombination crossing over) between homologous chromosomes can result in a new combination of alleles within a chromosome. The chromosomes / offspring that carry these new combinations of alleles are referred to as recombinants.
The diagram (left) shows how these recombinants are produced. It's worth noting that the further apart two genes are, from each-other, the greater the chance of crossing over occurring somewhere between the two. Likewise, if two genes are very close to each other, the chance of crossing over occurring in-between them is small. This is why in some cases the recombinant are rarely seen -the genes are simply so close together that recombination (crossing over) doesn't often occur between them.
The dihybrid cross shown in the You Tube clip example assumed the genes were on different pairs of chromosomes. Now, we want to look at an example where the genes involved are on the same chromosome. One such example is the flower color and pollen shape experiment done by Bateson and Punnett. In the plants that they studied, the genes for pollen shape and flower color are located on the same chromosome (pair) as each other, thus are inherited together.
Linked Genes If the parents are PPLL × ppll, the first parent will only make gametes with PL and the second with pl, which doesn’t seem too different so far. From these parents, the F1 generation would all be PpLl. However, when calculating what the F2 generation will be, since the genes are located on the same pair of chromosomes, then theoretically, the only possible gametes are PL and pl (not Pl or pL). So
PL pl
PL PPLL PpLl
pl PpLl ppll
The phenotype ratio for this cross is 3:1, not 9:3:3:1 as would be expected for a “normal” dihybrid cross. Because these genes are on the same chromosome pair, they are called linked genes. Interestingly, Bateson and Punnett’s results showed just a few, unexpected ppL- and P-ll offspring, more than would be predicted by linked genes, but far less than would be predicted by unlinked genes in a “regular” dihybrid cross. This is due to the fact that occasionally, during synapsis in meiosis I, while the homologous chromosomes are paired up, sister chromatids from the homologous chromosomes exchange equal segments. This is called crossing over. In the flower example, a few of the plants could exhibit crossing over during meiosis I, producing a few pL and Pl gametes, which would account for the small number of ppL- and P-ll offspring. T. H. Morgan and his grad students, who studied fruitflies, found that the farther apart two genes are on a chromosome, the more likely there is to be crossing over between those two genes. They found that for any given two genes on the same chromosome as each other, the amount of crossing over that occurs is a fairly constant quantity that can be measured. From their crossing over data, Morgan et al. were able to arrange fruit fly genes in the order in which they occur on the fruit fly chromosomes. Interestingly, if two genes are very far apart on the same chromosome pair, there is so much crossing over that the results obtained look like a regular dihybrid cross between unlinked genes.
Biological ideas and processes relating to factors affecting allele frequencies in a gene pool:
Factors that affect Allele Frequency.
If the allele frequency is no longer changing in a population a genetic equilibrium has been reached. However, several factors can easily disrupt this genetic equilibrium and later the allele frequency.
The factors or agents for this include:
Click on the button below for a great animation to start this topic...
Factors that affect Allele Frequency.
If the allele frequency is no longer changing in a population a genetic equilibrium has been reached. However, several factors can easily disrupt this genetic equilibrium and later the allele frequency.
The factors or agents for this include:
- Natural Selection,
- Artificial Selection
- Sexual Selection
- Gene Flow (Immigration & Emigration)
- Genetic Drift
- Founder Effect
- Population Bottlenecks
- mutations - see earlier notes
Click on the button below for a great animation to start this topic...
Allele frequency...what do we mean by this ??
This is the proportion of each allele (different versions of a gene) within a population.
It can be calculated by converting the number of each allele to a percentage.
This is the proportion of each allele (different versions of a gene) within a population.
It can be calculated by converting the number of each allele to a percentage.
Gene Pool:
The total of all the genes /alleles present in a population.
The total of all the genes /alleles present in a population.
Natural Selection:
Darwin's grand idea of evolution by "natural selection" was proposed more than 150 years ago - it is relatively simple but often misunderstood. However it is the best explanation of "adaptive evolution"
To find out how it works, imagine a population of beetles:
There is variation in traits.
1. For example, some beetles are green and some are brown
2.There is differential reproduction.
Since the environment can't support unlimited population growth, not all individuals get to reproduce to their full potential. In this example, green beetles tend to get eaten by birds and survive to reproduce less often than brown beetles do.
3.There is heredity.
The surviving brown beetles have brown baby beetles because this trait has a genetic basis.
4.End result:
The more advantageous trait, brown coloration, which allows the beetle to have more offspring, becomes more common in the population. If this process continues, eventually, all individuals in the population will be brown.
Darwin's grand idea of evolution by "natural selection" was proposed more than 150 years ago - it is relatively simple but often misunderstood. However it is the best explanation of "adaptive evolution"
To find out how it works, imagine a population of beetles:
There is variation in traits.
1. For example, some beetles are green and some are brown
2.There is differential reproduction.
Since the environment can't support unlimited population growth, not all individuals get to reproduce to their full potential. In this example, green beetles tend to get eaten by birds and survive to reproduce less often than brown beetles do.
3.There is heredity.
The surviving brown beetles have brown baby beetles because this trait has a genetic basis.
4.End result:
The more advantageous trait, brown coloration, which allows the beetle to have more offspring, becomes more common in the population. If this process continues, eventually, all individuals in the population will be brown.
NATURAL SELECTION is defined as: the gradual process by which heritable traits become either more or less common in a population as a result of their impact on reproductive success ie: presence of more offspring.
Click here for some introductory notes if wanted....
Click here for some introductory notes if wanted....
This is also worth having a look at - explains the concept more simply !!
Darwins Finches And the Galapagos islands:
TYPES of Natural Selection:
Populations are rarely stable and natural selection pressures are always acting upon them...this means that frequency of phenotypic traits are dynamic.
Natural selection may favour the most common phenotype (stabilizing), one extreme of the phenotype trait (directional) or favours the alleles that code for the two extremes ( Disruptive).
Stabilising Selection:
Stabilising selection is probably the most common type of natural selection; it favours the most common phenotype as the best adapted
Stabilising selection reduces variation by selecting against alleles that produce more extreme phenotypes at either end of the phenotypic range. The resulting bell shaped curve is narrower.
A good example is birth weight. Babies that are too light are often under-developed and therefore have a reduced chance of survival. Babies that are too heavy are usually larger and therefore they may be an increased risk of complications during birth.
Directional Selection:
This type of natural selection is most common during periods of environmental change.
Directional selection favours alleles that produce phenotypes at one extreme of a phenotypic range. Selection reduces variation at one extreme of the range while favouring variants at the other end. The resulting bell shaped curve shifts in the direction of the selection.
For instance a population of snails may exhibit some variation in shell colour. Directional selection might act against the lightest coloured individuals, reducing the frequency of alleles that code for lighter colours.
Disruptive Selection:
Disruptive selection favours alleles that code for phenotypes at both extremes of a phenotypic range. The bell shaped curve acquires two peaks. Disruptive selection may occur when environmental conditions are varied or when the population covers a large area.
For instance, a population of snails might live in a region that has areas with white rocks and areas with black rocks. Disruptive selection would act against the individuals with intermediate colours (grey or beige individuals), reducing the frequency of alleles that code for these colours. Disruptive selection can result in two distinct groups and if they become adapted to a different way of life (niche) they could eventually evolve into separate species (speciations).
Stabilising selection is probably the most common type of natural selection; it favours the most common phenotype as the best adapted
Stabilising selection reduces variation by selecting against alleles that produce more extreme phenotypes at either end of the phenotypic range. The resulting bell shaped curve is narrower.
A good example is birth weight. Babies that are too light are often under-developed and therefore have a reduced chance of survival. Babies that are too heavy are usually larger and therefore they may be an increased risk of complications during birth.
Directional Selection:
This type of natural selection is most common during periods of environmental change.
Directional selection favours alleles that produce phenotypes at one extreme of a phenotypic range. Selection reduces variation at one extreme of the range while favouring variants at the other end. The resulting bell shaped curve shifts in the direction of the selection.
For instance a population of snails may exhibit some variation in shell colour. Directional selection might act against the lightest coloured individuals, reducing the frequency of alleles that code for lighter colours.
Disruptive Selection:
Disruptive selection favours alleles that code for phenotypes at both extremes of a phenotypic range. The bell shaped curve acquires two peaks. Disruptive selection may occur when environmental conditions are varied or when the population covers a large area.
For instance, a population of snails might live in a region that has areas with white rocks and areas with black rocks. Disruptive selection would act against the individuals with intermediate colours (grey or beige individuals), reducing the frequency of alleles that code for these colours. Disruptive selection can result in two distinct groups and if they become adapted to a different way of life (niche) they could eventually evolve into separate species (speciations).
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Artificial selection AND Sexual selection... Click here to know more on these two types....
GENE FLOW:
This is another process that can affect the allele frequency within a population
Is the movement of Gens (alleles) into and out of a population....
Click here to get more theory behind this
This is another process that can affect the allele frequency within a population
Is the movement of Gens (alleles) into and out of a population....
Click here to get more theory behind this
Gene Flow - Migration
Is the movement of individuals from one population to another - so Gene flow can change the Frequency and/or range of alleles in population gene pools.
Immigration = individuals migrate into a population - may add NEW alleles so may increase the genetic diversity within the population
Emigration = individuals migrating out of a population - may REMOVE alleles so decrease the genetic diversity
If populations are larger in numbers - Migration may have little or no effect on the allele frequencies
If populations are small - Migration may have a significant/major effect on allele frequencies
Is the movement of individuals from one population to another - so Gene flow can change the Frequency and/or range of alleles in population gene pools.
Immigration = individuals migrate into a population - may add NEW alleles so may increase the genetic diversity within the population
Emigration = individuals migrating out of a population - may REMOVE alleles so decrease the genetic diversity
If populations are larger in numbers - Migration may have little or no effect on the allele frequencies
If populations are small - Migration may have a significant/major effect on allele frequencies
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GENETIC DRIFT:
The change in allele frequencies due to "chance".....not "selection pressure"
It is particularly important if the population is small because in large populations the allele frequencies remain relatively stable.
Genetic Drift is more likely to occur after a "founders effect or a Bottleneck effect"
Click here for some theory behind the Founders Effect....
Click here for some theory behind the Bottleneck Effect
The change in allele frequencies due to "chance".....not "selection pressure"
It is particularly important if the population is small because in large populations the allele frequencies remain relatively stable.
Genetic Drift is more likely to occur after a "founders effect or a Bottleneck effect"
- The Founders Effect - a population founding from a few "founders" (major role in evolution of NZ native species)
- The Bottleneck Effect - a sharp decline in population size due to natural disaster or Human intervention
Click here for some theory behind the Founders Effect....
Click here for some theory behind the Bottleneck Effect
Founders Effect - check out the You Tube Clips below.....
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Bottleneck effect.....
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Past Exam Paper