Autosomal Linkage

What is an autosome?

A non-sex chromosome

Linkage

- Linked genes are genes found on the same chromosome

- Because they are physically close together, they tend to be inherited together

- This is because they don't get separated by independent assortment during metaphase I of meiosis

Autosomal linked genes

When genes are on the same autosome, they are called autosomal linked genes

These genes are usually passed on together, rather than being mixed up randomly

Are linked genes inherited as one unit?

Yes

What does autosomal linkage then mean?

- There is no random assortment of alleles at different loci

- The phenotypic ratios observed in dihybrid crosses are different from those expected from independent assortment

- Parental allele combinations are preserved across generations

Crossing over

During meiosis, crossing over can exchange genetic material between homologous chromosomes at regions called chiasmata. Offspring that have different combinations of alleles from their parents due to crossing over are called recombinant offspring.

Effects of Crossing over on Autosomal linkage

- Crossing over potentially separates linked genes. However, when genes are linked, fewer recombinant offspring tend to be produced

- This indicates less genetic variation being introduced from crossing over when genes are linked

- The probability of linked genes being separated during crossing over is inversely proportional to how close they are on a chromosome, i.e. the closer the genes, the more likely they will be inherited together

Why won't the F2 dihybrid cross (double heterozygous parents) 9:3:3:1 be seen?

The chances of some allele combinations occurring in gametes is less likely due o linkage.

Fruit flies

Body colour and wing length are determined by genes that are linked on the same autosome

B - Brown body

b - Black body

V - Long wings

v - Short wings

If a brown, long winged fly (BbVv) is crossed with a black short winged fly (bbvv), then the autosomal linkage results in mainly parental phenotypes among offspring (BbVv and bbvv).

This is because the linked alleles for body colour and wing length are usually inherited together. This produces very few recombinants with new genetic combinations.