Inheritance of sex & sex linkage (Edexcel GCSE Biology): Revision Notes
Inheritance of sex & sex linkage
Human body cells have 23 pairs of chromosomes.
- 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
This means colour blindness is much rarer in women than men
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