Phenotypic Ratios and Linkage

Linkage

Genes located on the same chromosome

Sex linkage

Sex linked genes are genes carried on the sex chromosomes, which determine certain characteristics e.g. colour blindness, haemophilia

Autosomal Linkage

Genes present on the same non-sex chromosome

• Linked genes are inherited as 1 unit

• There is no independent assortment

How are linked genes inherited?

As a single unit

What is meant by recombinant offspring

Offspring w. diff combination of alleles to their parents

• The closer the genes on a chromosome, the less likely they will be separated during crossing over

• Fewer recombinant offspring will be produced

What is recombination frequency

Measure of the amount of crossing over that has happened in meiosis

What does a recombination frequency of 50% and above indicate?

There is no linkage, genes are on separate chromosomes

What does a recombination frequency of less than 50% indicate?

There is gene linkage

• Independent assortment has been hindered

• As degree of crossing over reduces, recombination frequency gets smaller.

  • Degree of crossing over is determined by how close the genes are on a chromosome.

  • The closer they are the less likely they will be separated during crossing over and vice versa.

What does null hypothesis for chi-squared test mean?

There’s no significant difference between what we expect & what we observe

• Any differences we see are due to chance.

Chi squared example

Epistasis

Interaction of genes at diff loci to form the phenotype

• Occurs when two genes on different chromosomes affect the same feature

  • One gene affects the expression of another gene

• Occurs frequently in plants and animals showing continuous variation

What is a form of epistasis

Gene regulation

• regulatory genes control activity of structural genes, e.g. the lac operon.

• Gene interaction also occurs in biochemical pathways involving only structural genes.

What is meant by a hypostatic gene

A gene that is affected by another gene

What is meant by an epistatic gene

Gene that affects the expression of another gene

Dominant epistasis

Occurs when dominant allele results in a gene having an effect on the other gene

Expected ratios that arise from epistasis

• 9:3:4 → recessive epistasis

• 12:3:1 → dominant epistasis

• 9:7 → duplicate recessive epistasis

• 13:3 → dominant allele of one gene suppresses the expression of another gene.