AQA GCSE Pedigree diagrams(Biology)

Pedigree diagrams

A pedigree diagram is a chart showing how a trait is inherited through generations of a family.

Pedigree diagrams are normally based on either a dominant or recessive trait.

Pedigree diagram (Dominant trait)

If characteristic or trait is caused by a dominant allele, then the individual will only need to inherit one copy of the dominant allele to express the characteristic.

Polydactyly is a disorder caused by dominant allele.

D = polydactyly, d = normal

Inidividual 2 is dd because they do not suffer from the condition. However, we can see that individual 1 could be DD or Dd.

Below are examples of a punnett square that will allow us to identify the genotype of individual 1.

As the offspring of 1 and 2 are a mixture of some having polydactyly and some not, it means that individual 1 is Dd and not DD.

Pedigree diagram (Recessive trait)

If characteristic or trait is caused by a recessive allele, then the individual will need to inherit two copies of the recessive allele to express the characteristic.

Cystic fibrosis is a disorder caused by a recessive allele.

F= normal, f = cystic fibrosis

The individuals that do not have cystic fibrosis (1,2,4,5,6) could be FF or Ff.

However, individual 3 with cystic fibrosis can only be ff.

This means that individual 3 would had received one recessive allele from each of its parents (individuals 1 and 2).

However, as neither 1 or 2 suffer from the disease, it means that both of them must also carry a dominant allele in addition to their recessive allele

Practice Questions

1.Describe what a pedigree diagram shows us.

2.Huntingdon’s disorder is caused by a dominant allele H. People with two copies of the recessive alleles (hh) are unaffected. Below is a pedigree diagram for huntingdon’s disease.

Both individuals 1 and 2 have Huntingdon’s disease. Explain how it is possible for them to have children that will not suffer from the disorder.

3. Tay Sachs disease is caused by a recessive allele t. People with T are unaffected by the disease. Below is a pedigree diagram for Tay Sachs disease.