Sex and Autosomal linkage

Question 1

What is sex linkage

Answer 1

Genes that are carried on either the X or Y chromosome

Question 2

Why are recessive alleles more likely to be expressed in males?

Answer 2

• The X chromosome is much longer than the Y and therefore for most of the length of the X chromosome there is no homologous Y area
• This means that the recessive alleles carried on the non - homologous area of the X chromosome will appear more frequently in males as there is no dominant allele to prevent the recessive allele from being expressed

Question 3

Give an example of a sex linked disease?

Answer 3

Haemophilia which is a disease that prevents the clotting of blood and is caused by a detective X chromosome.

As males can only inherit the X chromosome from their mother (use note diagram) they always inherit, if any, the diseased allele from their mother. Males can also never pass haemophilia to their sons as the Y chromsome does not carry this allele

Question 4

What is autosomal linkage?

Answer 4

Any genes that occur on the same autosomal chromosome
Autosome = the remaining 22 chromsomes which are not sex linked
If there is no crossing over, all linked genes remain together and pass this on to gametes

Question 5

Why is there limited options for autosomal linkage?

Answer 5

If there is no crossing over, each chromsome keeps the alleles that they carry therefore there are only two versions (AB or ab)
Compared to when alleles are carried on different chromosomes, the possibilities are AB, Ab,ab,aB
This simplifies genetic crosses