Summary Unit 11C applied science DISTINCTION

P5-
For a recessive, a dominant and a sex-linked disorder, use Punnett squares to show how different
genotypes result in different genotypic and phenotypic ratios.
dominate disorder= Huntington disease
Recessive disorder= cystic fibrosis
Sex linked= haemophilia
Huntington disease
Key
H- Huntington gene
h- normal gene
Hh- Huntington disease
hh- normal non effected gene




There is a 1:2 or 50% chance of the offspring inheriting Huntington disease if only one of the parents
has Huntington, and since it cannot be a carrier you either have the disease or do not have it, this is
because Huntington is a dominate disorder.

, There is a 3:4 or 75% chance of the offspring inheriting Huntington’s disease if both of the parents
have the disease. However, there is also a 1:4 or 25% chance that the offspring does not have
Huntington as indicated by the hh on the punnet square.




There is a 0:4 or 0% chance of the offspring inheriting Huntington of neither parents have the diesis,
we can see this in the punnet square as they all indicate the normal non effected gene (hh).
Cystic fibrosis
Key
c- cystic fibrosis gene (carrier)
C- non affected gene
CC- Normal gene
Cc- carrier of CF
cc- has inherited CF