Topic 7: Genetics, populations, evolution and ecosystems

Define genotype and phenotype

Genotype:

• genetic constitution of an organism (the alleles it has for a gene)

Phenotype:

• expression of the genetic constitution

• and its interaction with the environment

Define homozygous and heterozygous

Homozygous:

• pair of homologous chromosomes carrying 2 identical alleles for a particular gene

Heterozygous:

• pair of homologous chromosomes carrying 2 different alleles for a particular gene

What are recessive and dominant alleles?

Recessive allele:

• only expressed when there’s no dominant allele present

• alleles effect is only apparent in an organisms phenotype when in the presence of another recessive allele

Dominant allele:

• always expressed in an organisms phenotype

Define codominance

• both alleles of a gene are equally dominant

• so both are expressed in the phenotype

Coding example:

• gene^allele

• IᴬIᴮ

What are monohybrid and dihybrid?

Monohybrid:

• genetic inheritance cross of a characteristic determined by 1 gene

Dihybrid:

• genetic inheritance cross for a characteristic determined by 2 genes

• crossing 2 heterozygous parents produces 9:3:3:1 ratio

Define locus

• the fixed position of a gene on a chromosome

• alleles of a gene are found at the same position on each chromosome pair

What is sex linkage?

• a gene whose locus is on the X chromosome

• fathers can only pass X linked alleles to daughters

• mothers can pass them to both son and daughter

Coding example:

• chromosome^allele

• XᴴXʰ or XᴴY

What is autosomal linkage?

• 2 different genes located on the same chromosome (autosome not sex)

• so the alleles for each gene linked on the same chromosome will be inherited together, impacting the predicted gametes

• these genes stay together during independent segregation and split up when crossing over

Coding example:

• Aa Bb

What is epistasis?

• when a gene blocks/modifies/masks the expression of a different gene

• many genes can contribute to one feature, interacting to create the phenotype e.g. animal fur colour

Coding example:

• EE Bb

When would you not get a 9:3:3:1 ratio when crossing 2 heterozygous parents in a dihybrid cross?

If there’s…

• autosomal linkage

• crossing over in meiosis

• epistasis

What does chi squared measure?

• it’s a statistic used to investigate differences between frequencies

• determines whether there’s a significant difference between the expected and observed frequencies

How to calculate chi squared?

• (O-E)²/E }=x²

• (add together to get x²

• degrees of freedom = categories - 1

• critical value (p) is generally 0.05

• if chi squared is less than cv, results are likely due to chance

• so null hypothesis can be accepted and there’s no significant difference

What is the Hardy Weinberg principle?