Genetics, Variation, Natural Selection and Speciation

What is a gene

A sequence of DNA bases that codes for a polypeptide

What does genotype mean?

• The genetic constitution of an organism

• Describes all the alleles an organism contains

What does phenotype mean?

The observable characteristics expressed of an organism caused by the genotype and the enviroment

What is an allele?

An alternative form of the same gene

What is a dominant allele?

An allele that only needs one copy in the genotype to be expressed in the phenotype

What is a recessive allele?

An allele that requires to copies in the genotype to be expressed in the phenotype

What is locus?

The position of a gene on a chromosome

What does homozygous dominant/recessive mean?

A gene containing two dominant/recessive alleles

What does heterozygous mean?

A genotype containing one copy of a dominant allele and one copy of a recessive allele

What does codominance mean?

Both alleles are expressed in the phenotype when the genotype is heterozygous

What are homologous chromosomes?

A pair of chromosomes that have the same genes on the same loci but different alleles

What is monohybrid inheritance?

Inheritance of a characteristic controlled by a single gene

What does pure bred mean?

An organism is either homozygous dominant or homozygous recessive

When a genetics question asks the probability of producing a male or female with X condition, what should you always do? (Assuming the disorder is not sex linked)

Divide by two

How do you write codominance alleles?

A large letter with a superscript for each allele in capital letters (C^RC^R)

What is a common codominance cross ratio for two heterozygous parents?

1:2:1

What are multiple alleles?

• When are are several alleles of the same gene

• These therefore show a hierarchy of dominance

What blood types are there and what is the hierarchy of dominance?

• AB, A, B and O

• I^A and I^B are codominant and lead to the production of antigen A and B respectively

• I^O is recessive to both of them and does not lead to the production of any antigen

Why is blood type O a universal donor?

The red blood cells of people with type O blood do not have any antigens on them and therefore do not trigger and immune response that would destroy the foreign cell

Why is blood type AB a universal acceptor?

Can accept A and B and well as O as O has no antigens on it’s surface to trigger an immune response

What are lethal alleles?

Alleles that often prevents the development of an organism at early stages leading to their death

What is dihybrid inheritance?

• The inheritance of two characteristics controlled by different genes on different chromosomes

• The inheritance of one allele has no influence on the inheritance of another

What is the expected ratio of a heterozygous dihybrid cross?

9:3:3:1

What are polygenes?

• Two or more different genes combining to affect one characteristic

• For example one gene may control fur colour- black is the dominant allele (B) and white is the recessive (b)

• Another gene controls the dilution of the pigment, dense being D and dilute being d

How is a chi-squared carried out?

• List all the observed and expected frequencies

  • The expected frequencies depends on the type of cross that is happening. For example a heterozygous dihybrid cross would be 9:3:3:! ratio

• Make another column for observed minus expected

• Another column for (O-E)²

• Another column for (O-E)²/E

How do we know if our calculated value for the chi-squared test is significant and how do we interpret this?

• You need to find the degrees of freedom which is the number of categories -1

• Then you need to find the critical value at that degree of freedom at P=0.05

• If the calced (thats just short for calculated I’m just using slang guys) value is greater than the critical, there is a significant difference between our results with a less than 5% probability this difference is due to chance

Why may observed ratios not matched expected?

• Small sample sizes

• Random fertilisation/fusion of gametes

• Lethal alleles

• Gametes not produced in equal numbers

What is epistasis?

When the expression of one gene/genotype masks another

What is dominant epistasis?

When the allele causing epistasis is dominant

What is recessive epistasis?

When the allele causing epistasis is recessive

What is the expected phenotypic ratio for dominant epistasis?

12:4:3

What is the expected phenotypic ratio for recessive epistasis?

9:4:3

What is sex linkage?

Alleles located on X or Y chromosomes

Why do X and Y chromosomes not form a typical bivalent during meiosis?

They are not homologous chromosomes

What is the size of the X chromosomes compared to the Y and what does this mean?

• The X chromosome is much larger than the Y

• The X chromosome contains thousands of genes for things like rhodopsin, blood clotting proteins and muscle proteins

• The Y is small and therefore has few genes

How are sex chromosomes inherited?

• Females inherit one X chromosome from their father and the other X from their mother

• Males will always inherit the Y from their father and the X from their mother

Why are males more susceptible to sex linked disorders caused by recessive alleles than females?

• Recessive alleles for these disorders tend to be on the X chromosome since it’s larger

• Female would have to inherit two recessive alleles- one from their father and one from their mother

• Males only have to inherit one X chromosome with the recessive allele from their mother

If two parents who carry a characteristic caused by a dominant allele give birth to a child with a condition caused by a recessive allele, what evidence is there that a condition is caused by a recessive allele? (Explain in terms of normal alleles and sex linked separately)

Normal alleles

• The child inherited a condition from their parents even though they are both normal

• The parents must be heterozygous and therefore pass on two recessive alleles to the child

Sex Linked

• An unaffected mother and an affected father give birth to an affected offspring

• For females, they must have inherited two recessive alleles from a heterozygous mother and a homozygous recessive father

• For males, they would only inherit the recessive allele on the x chromosome from their mother meaning their mother must be heterozygous

What evidence can we find on a genetic diagram that a condition is not caused by a gene on the X chromosome?

• A male offspring will always have inherited the recessive allele on the X chromosome from it’s mother which would cause the condition

• If an unaffected father and an affected mother produce an unaffected male offspring, it’s shows the gene is not sex-linked

What is an autosome?

Any chromosome that isn’t a sex chromosome

What is autosomal linkage?

Genes on the same autosome are said to be linked as they stay together during independent segregation and therefore will be inherited together

Why are two genes on an autosome closer together said to be more linked?

The closer the two genes are together, the less likely they are to be split up by crossing over

One chromosome has the alleles A and B, another has the alleles a and b. What gamete would be produced in large quantities and which would rarely be produced and why?

• AB and ab would be produced in large quantities as they are likely inherited together during independent segregation

• In rare cases, crossing over of non-sister chromatids may cause alleles to be swapped between chromosomes causing aB or Ab gametes to be formed in small quantities

What is a species?

A group of similar organisms that can reproduce to give fertile offspring

What does population mean?

All the organisms of the same species living in a particular area at a particular time

What is a gene pool?

All the alleles of all the genes of all the individuals within a population

What is the Hardy-Weinberg principle?

A principle that predicts the frequency of the alleles of one gene in a particular population

What assumptions does the Hardy-Weinberg Principle make?

• The populations are large- in large populations mating is more random and chance events are less likely to cause large changes in allele frequencies

• No immigration or emigration

• There is random mating between members of the population

• All the genotypes have the same reproductive success- natural selection should not be happening as this changes allele frequencies

• There is no gene mutation

What equation can be used to find different allele frequencies?

p+q = 1

p= frequency of the dominant allele

q= frequency of the recessive allele

What equation relating to the Hardy-Weinberg principle can be used to find frequency of homozygous and heterozygous alleles?

p²+2pq+q²=1

p²= frequency of homozygous dominant genotype

2pq= frequency of heterozygous genotype

q²= frequency of homozygous recessive genotype

In the Hardy-Weinberg equation, what part shows all the frequencies of the dominant phenotype?

p²+2pq

• This includes the homozygous dominant individuals and heterozygous individuals who, because of their dominant allele, will express the dominant phenotype

What is variation?

The differences that exist between individuals meaning that they have a wide range of phenotypes

What is interspecific and intraspecific variation?

• Inter- between species

• Intra- within the same species

What is variation caused by?

• Genetic factors- mutations, meiosis (crossing over, independent segregation, random fertilisation)

• Environmental factors

• Can be a mixture of the two

How can individuals of the same species have variation if they all have the same genes?

They have different alleles

What is the difference between continuous and discontinuous variation?

Continuous

• There is a complete range of measurements from one extreme to the other

• An example of this would be height measured in cm

• It is the combined effect of many genes (known as polygenic inheritance) and is often significantly affected by environmental factors

Discontinuous

• These are categoric variables like hair colour- there is no range of measurement of the values

• This is controlled by the alleles of a single or small number of genes

• The environment has little to no effect on this type variation

How does natural selection work?

• There is variation within the population due to random mutations that produce different alleles

• Individuals with beneficial alleles have a selective advantage against selection pressures compared to individuals that do not

  • If this is for bacteria- they have a resistance allele against an antibiotic

• The organisms with beneficial alleles are more likely to survive and reproduce (differential survival)

• The beneficial alleles is passed onto the next generation

• The frequency of the allele within the population increases, causing the mean phenotype to change (directional selection)

What is stabilising selection and when does it occur?

• Where individuals with alleles for characteristics towards the mean phenotype are more likely to survive and reproduce

• It occurs when the environment is not changing

• It reduces the range of possible phenotypes- the extremes are eliminated

What is directional selection and when does it occur?

• Where individuals with a single extreme phenotype are more likely to survive and reproduce

• This can be in response into an environmental change

• This causes a change in the mean phenotype of a population

What is disruptive selection?

• Where individuals with alleles for two extreme phenotypes on opposite sides of the spectrum are more likely to survive and reproduce

• It occurs when the environment favours more than one phenotypes and selects against the mean

What is speciation?

The development of a new species from an existing one

What is required for speciation to occur?

• Populations of a species must become reproductively isolated from one another- this cause be because of geographical isolation or populations becoming reproductively isolated because of incompatible sex organs, different mating seasons, different flowering times, etc.

• Changes in allele frequency causes alters the gene pool so greatly that the organisms can no longer interbreed to produce fertile offspring

What is allopatric speciation?

• A physical barrier divides a population- geographical isolation

• There is no gene flow between the two populations

What is sympatric speciation?

• A population may become reproductively isolated without a physical separation

• There is no gene flow between the two populations

Structure of an allopatric speciation question

• Populations of a species become geographically isolated from each other due to a barrier- they therefore become geographically isolated

• There is no gene flow between the two populations

• There is a different allele frequency within the two populations and mutations can further change these allele frequencies

• They will have different selection pressures as they live in different environments- some individuals will have beneficial alleles meaning they have a selective advantage against the selection pressures meaning they are more likely to survive and reproduce compared to individuals without the beneficial allele

• They beneficial alleles in each population become increasingly common as alleles are passed onto the next generation

• Eventually the allele frequencies/gene pools are so different over a long period of time that the two species can no longer interbreed to produce fertile offspring- they are different species

Structure of a sympatric speciation question

• Populations of a species become reproductively isolated- could be due to a mutation causing the sex organs of one individual to be incompatible with another, different mating seasons, different flowering seasons, etc.

• There is no gene flow between the two populations

• There is a different allele frequency within the two populations and mutations can further change these allele frequencies

• They may have different selection pressures as they live in different environments- some individuals will have beneficial alleles meaning they have a selective advantage against the selection pressures meaning they are more likely to survive and reproduce compared to individuals without the beneficial allele

  • The different selection pressures may be due to the difference in the time of mating or flowering seasons

• They beneficial alleles in each population become increasingly common as alleles are passed onto the next generation

• Eventually the allele frequencies/gene pools are so different over a long period of time that the two species can no longer interbreed to produce fertile offspring- they are different species

What is genetic drift?

• Chance dictates which alleles are passed on unlike natural selection where the environment dictates what alleles are being passed on

• Individuals within a population show variation by their genotype

• By chance, the allele for one genotype is passed on to more offspring than the others

• The number of individuals with the alleles increases

• If by chance the same allele is passed on more often over and over again, it can lead to evolution as the allele becomes more common in the population

How can genetic drift lead to speciation?

• Genetic drift can lead to difference in allele frequency between two isolated populations

• If there is enough of a difference in allele frequency built up over time, it could lead to reproductive isolation and speciation

When does genetic drift have a greater affect on allele frequency

In smaller populations where chance has a greater influence

What is genetic diversity?

The range of all the different alleles in a population

What is a genetic bottleneck?

A single chance vents that leads to a large reduction in the population size, causing genetic diversity to decrease (the range of alleles decrease)

What is the founder effect?

When a small number of the original population move to somewhere new leading to a new, smaller population (genetic diversity decreases)