Genetics
There are 46 chromosomes in the human genome, that is 23 pairs of homologous
chromosomes.
This means that for any gene we will have two different versions of it, one on each
homologous chromosome.
There are normally many different versions of each gene so it is possible that each of the
two homologous chromosomes has a different version of the gene, if this is the case the
persons is said to be heterozygous for this gene (e.g. Ff).
Someone who has two alleles which are the same is homozygous for that gene (e.g. FF).
Put a definition for the following words and then write a sentence using the word in the
space below
ALLELE:
GENOTYPE:
PHENOTYPE:
Predicting inheritance
If we know the genotype of the mother and the father then we can predict the possible
genotype of the offspring by using a punnett square.
For cystic fibrosis the normal gene (F) is dominant over the cystic fibrosis gene (f) which
is recessive.
In this first example both the parents are heterozygous (Ff) for the cystic fibrosis gene
By drawing a punnett square we can predict the likelihood of a child acquiring CF.
Egg
Sperm
F
F
f
f
Predict the likelihood of the child having cystic fibrosis?
What is the likelihood of the child being a carrier of cystic
fibrosis?
Try these:
1) Sickle cell anaemia is caused by a recessive allele of the gene that codes for
Haemoglobin.
The normal, dominant allele is H, and the faulty recessive allele is h.
What would be the result of a man with a genotype Hh having a child with a woman with
sickle cell anaemia?
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2) Use a punnett square diagram to show why there is an equal chance of a parent having a
girl or a boy.
Eggs from female
X
Sperm
from
X
male
X
Y