Linkages

Define Autosomal Linkage.

• When two or more genes are located on the same autosome (non-sex chromosome) and tend to be inherited together.

How does autosomal linkage affect the phenotypic ratio of a dihybrid cross?

• It produces a higher proportion of parental phenotypes and significantly fewer recombinant phenotypes than expected (deviating from 9:3:3:1).

What process in Meiosis I can separate linked genes?

• Crossing over (in Prophase I).

Define Sex Linkage.

• When a gene is found on one of the sex chromosomes (usually the X chromosome).

Why are men more likely to suffer from X-linked recessive disorders (like haemophilia) than women?

• Men are XY. They only have one X chromosome, so they only need one recessive allele to express the phenotype. Women are XX, so they need two recessive alleles.

Define Epistasis.

• An interaction between genes at different loci, where one gene masks or affects the expression of another gene.

In epistasis, what is the difference between the epistatic and hypostatic gene?

• Epistatic: The gene that suppresses/masks the other.

• Hypostatic: The gene that is suppressed/masked.

What phenotypic ratio suggests Recessive Epistasis?

• 9:3:4.

What phenotypic ratio suggests Dominant Epistasis?

• 12:3:1.

If two genes are linked, what determines the frequency of recombinant offspring?

• The distance between the loci on the chromosome. The further apart they are, the more likely crossing over will occur between them.

Why can a father never pass an X-linked disorder to his son?

• Because a father always passes his Y chromosome to his son. The X chromosome always comes from the mother.

In a fruit fly cross involving body colour and wing length, the offspring appeared in the following numbers:

• Grey body, Normal wings: 415

• Black body, Vestigial wings: 405

• Grey body, Vestigial wings: 48

• Black body, Normal wings: 52 Explain why these results indicate autosomal linkage. (3 marks)

• The offspring are mostly parental phenotypes (Grey/Normal and Black/Vestigial) (1).

• There are very few recombinants (1).

• If genes were unlinked, we would expect a 1:1:1:1 ratio (or 9:3:3:1 depending on parents), but here the ratio is skewed, suggesting the genes are on the same chromosome (1).