sex linkage- lqc 8b

Question 1

sex linked genes

Answer 1

An allele or gene responsible for a trait is located on a sex chromosome (either the X or Y chromosome – not both!)
Expression of the trait is related to ‘gender’ e.g. males will only have one allele but females will have two alleles for the trait

Question 2

when a gene is found on the X chromosome

Answer 2

females have two copies of the gene and males have one copy of the gene

Question 3

autosomal chromosomes

Answer 3

Chromosomes that are not sex chromosomes. Humans have
22 pairs of autosomal chromosomes and 44 in total.

Question 4

examples of sex linked conditions

Answer 4

colour blindness, haemophilia, eye colour in Drosophila

Question 5

explain why more men than women are colour blind (or have any recessive X linked condition)

Answer 5

• Colour blindness is caused by a recessive allele on the X chromosome
• Males (XY) inherit only one X chromosome, so have only one copy of the gene
• So if the recessive mutated allele which causes colour blindness (Xn) is found on the male’s X chromosome it will always be expressed, as the male has the genotype Xn Y
• Females (XX) inherit two X chromosomes
• So if they inherit an X chromosome with a dominant allele for colour vision (XN) and an X chromosome with the recessive allele for colour blindness (Xn) , they will have the genotype XN Xn and the colour blindness allele will not be expressed
• For a female to be colour blind they would need to inherit two recessive alleles, one from each parent , giving them the genotype Xn Xn which is rare