Question

What does recessive genetic disease mean?

Answer 1

Recessive diseases are single gene disorders that occur only when an individual carries two malfunctioning copies (mutant alleles) of the relevant gene. Such individuals are described as homozygous, and arise most frequently as the offspring of heterozygous parents - parents who each possess one normal allele and one mutant allele (see Figure 1).

In most cases, the parents are perfectly healthy because the mutant allele has no adverse effect when a normal allele is also present. The parents are said to be carriers of the disease. Examples of recessive diseases include cystic fibrosis, spinal muscular atrophy and phenylketonuria.

Because carriers do not display disease symptoms, recessive diseases are difficult to trace through family pedigrees. Such diseases tend to affect groups of brothers and sisters, which makes it appear as if the disease is spreading horizontally (see Figure 2).

This inheritance pattern is completely different to the vertical spreading characteristic ofdominant diseases .

The risk of recessive disease increases where there is consanguinity (relationships between members of the same family, such as first cousin marriages) because there is a much greater chance that related parents will be carriers of the same disease allele.

The above applies to genes that are present on the autosomes - the chromosomes that are present in both sexes. Recessive diseases caused by genes on the X chromosome show a different type of inheritance pattern because males only have one copy of this chromosome, while females have two