3.3 Inheritance

The pedigree chart below shows the inheritance of Tay-Sachs disease which is an autosomal recessive trait, characterised by neurological problems caused by the death of nerve cells. 

In the pedigree chart, a filled in circle or square means the individual is affected and shows the genetic condition.

What is the probability that individuals 3 and 4 will have another child with Tay-Sachs disease?

Which statements best explain why the ABO blood system in humans is an example of co-dominance?

1. Allele I^A and the allele I^B are both expressed in the heterozygote/AB type blood.
2. Neither allele I^A or the allele I^B can mask the expression of the other allele.
3. There are three alleles of the gene that controls the blood group.
4. There are more than two blood groups.

Two parents with normal factor VIII have a child with haemophilia.

Which is the correct statement?

Which statements best explain why a significant increase in the incidence of cancer was reported following the nuclear bombing of Hiroshima and Nagasaki, and the nuclear accident of Chernobyl?

I. Ionising radiation can induce breaks in the DNA structure.

II. Control of the cell cycle can be affected by mutagens.  

III. Mutations were passed down to offspring leading to increased cancer rates. 

IV. Mutations arise when the body repairs a break in the DNA structure.