Inheritance of sex & sex linkage

• The 23rd pair carries sex determining genes, and the two chromosomes can look different to each other (Y chromosomes are much smaller than X chromosomes)
• 22 control characteristics, and the chromosomes in each pair look very similar

How gender is determined at fertilisation

• Remember gametes contain 1⁄2 the genetic information of the parent
• Half the fathers gametes contain the Y chromosome, the other the X chromosome
• All the mothers gametes contain X chromosomes Males**:** have an X and Y chromosome (the Y causes male characteristics)

Female: have two X chromosomes (XX) (the XX combination allows female characteristics to develop

Punnett squares can be used to show sex inheritance. There is a 50% chance of the child being male, and a 50% chance of the child being female.

Sex linkage

This refers to the inheritance of particular disorders on the X or Y chromosome only

• Most common in men as they only have one X chromosome
• If they inherit a genetic disorder allele on the X it will be expressed even if recessive (as will only have 1 allele)
• For females the second X chromosome acts as a 'protective' function

Colour blindness

Caused by faulty allele carried on X chromosome

Its sex linked so both the chromosome and the allele are written in the genetic diagram, eg. Xn where X represents the X chromosome and n the faulty allele for colour vision. The Y chromosomes don't have an allele for colour vision so is just represented by Y

Women- need two copies of the recessive allele to be colour blind Men- only need one copy

A women with only one copy of the recessive allele is a carrier of colour blindness, this means that she isn't colour blind herself but she can pass the allele on to her offspring

Here's a genetic cross between a carrier female & unaffected (non-colour blind) male:

Haemophilia

Disease where blood doesn't clot properly Another sex linked disorder caused by faulty allele carried on the x chromosome so its inherited the same way as colour blindness A genetic diagram for the inheritance of haemophilia will look the same as above