Genetics, populations, evolution and ecosystems

Define genotype

The genetic constitution of an organism

What is a gene?

A small section of DNA which codes for a polypeptide

What is an allele?

An alternative form a gene

Define phenotype

The expression of an organisms genetic constitution combined with its interaction with the environment

Define homozygous

A pair of homologous chromosomes carrying the same alleles for a single gene - either both dominant or both recessive

Define heterozygous

A pair of homologous chromosomes carrying 2 different alleles for a single gene - one dominant and one recessive

Define recessive allele

An allele only expressed if no dominant allele is present

Define dominant allele

An allele that will always be expressed in the phenotype

What is meant by codominant alleles?

Both alleles are equally dominant and contribute to the phenotype

How many alleles can there be for a single gene?

Multiple

What is monohybrid inheritance?

When one phenotypic characteristic is controlled by a single gene

What is dihybrid inheritance?

The study of how 2 phenotypic characteristics are determined by 2 different genes present on 2 different chromosomes at the same time, simultaneously

What is meant by sex-linkage?

When an allele is located on one of the sex chromosomes, meaning its expression depends on the sex of the individual

Why are males more likely to express a recessive sex-linked allele?

• Most sex-linked alleles are located on the X chromosome, as the X chromosome has more genes

• Men only have 1 X chromosome so only have 1 allele for any X-linked gene

• Because there is no second allele to mask it, it will always be expressed

• Since females get 2 alleles (as they have XX chromosomes), this is less likely

What is meant by autosomal linkage?

When 2 genes are on the same (non-sex) chromosome so they are inherited together

What is meant by epistasis?

When one gene modifies or masks the expression of a different gene at a different locus

What is the chi-squared test used for?

To determine whether there is a significant difference between the frequency you expect and the frequency you observe

What is the chi-squared formula?

x² = (SUM OF (O-E)²/E)

How does the chi-squared test work?

• The formula results in a number, which is then compared to a critical value for the corresponding degrees of freedom

• If the number is greater than or equal to the critical value, we conclude there is a significant difference between the observed and expected data and the results did not occur due to chance. We reject the null hypothesis

• If the number is less than the critical value, there is no significant difference between the observed and expected data and the results are due to chance. We accept the null hypothesis

How do you work out degrees of freedom?

Number of categories - 1

What 6 things must be shown on a genetic cross?

1. Parental phenotype

2. Parental genotype

3. Possible gametes

4. Offspring genotype

5. Offspring phenotype

6. Proportion of each phenotype

When homozygous dominant is crossed with homozygous recessive, the offspring will all be…?

Heterozygous

In a dihybrid cross, if 2 parents who are heterozygous for both genes cross, what will the phenotype ratio for offspring always be? What are the 2 exceptions to this ratio?

9:3:3:1

Unless there is autosomal linkage or crossing over in meiosis

What does crossing over in meiosis result in?

A new combination of alleles in the gametes

Why do alleles assort independently in a dihybrid cross?

Because the genes are on different chromosomes, which separate randomly during meiosis

What gametes do linked genes usually produce?

Mostly parental combinations, not new one

Why do linked genes not follow the 9:3:3:1 ratio, like in dihybrid inheritance?

Because they do not separate independently - they move together as part of the same chromosome during meiosis

What’s the term for offspring with new combinations of alleles?

Recombinants

When are recombinant gametes rare?

When linked genes are close together on the chromosome, as crossing over is less likely

How would it be possible for recombinant phenotypes to arise if genes are linked?

If crossing over occurs during meiosis, where homologous chromosomes exchange alleles, recombinant gametes are formed

What are pedigree diagrams?

Family trees of the inheritance of one or more genes

On a pedigree diagram, what pattern would you look for to disprove sex-linkage?

If the condition is caused by an X-linked dominant allele, any affected father would always pass on the condition to any daughter they have (as daughter has XX chromosomes and one X is inherited from father)

What is the Hardy-Weinberg principle?

A mathematical model which is used to predict the allele frequencies within a population, as well as if they are changing over time

Define gene pool

All of the alleles of all the genes within a population at one time

Define population

All of the organisms of a particular species in one area at one time

Define allele frequency

The proportion of an allele within the gene pool, expressed as a decimal or percentage

Give the 5 assumptions made by the Hardy-Weinberg principle in order to estimate allele frequency

• No mutations to create new alleles

• No migration to introduce or remove alleles from the population

• No selection so alleles are all equally passed on

• Random mating

• Large population

Give the Hardy-Weinberg equation for calculating allele frequency

p + q = 1

p: frequency of dominant alleles

q: frequency of recessive allele

Give the Hardy-Weinberg equation for calculating genotype frequency

p² + 2pq + q² = 1

p²: frequency of homozygous dominant genotype

q²: frequency of homozygous recessive genotype

2pq: frequency of heterozygous genotype