Achondroplasia is an autosomal genetic disorder caused by the dominant allele (A). This genetic disorder leads to a form of dwarfism. Offspring who inherit both dominant alleles, die during the baby/infant stage. The pedigree diagram below shows the inheritance of achondroplasia.

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QUESTION 2.5

2.5.1 State what is meant by a dominant allele.

A dominant allele is an allele that expresses its characteristic phenotype even when only one copy is present in the genotype (i.e., in a heterozygous individual), masking the effect of any recessive allele present.

2.5.2 How many generations are represented in this diagram?

3 generations

2.5.3 State the:

• (a) Phenotype of individual 2: Achondroplasia

• (b) Genotype of individual 1: Since achondroplasia is caused by a dominant allele (A) and individuals with two dominant alleles (AA) die, individual 1, who has achondroplasia, must be heterozygous. Aa

• (c) Number of individuals that are heterozygous: Individuals 1, 2, and 4 have achondroplasia. Since the homozygous dominant genotype (AA) is lethal, these individuals must be heterozygous (Aa). All other individuals (3, 5, 6, 7, and X) do not have achondroplasia, meaning they are homozygous recessive (aa). 3

2.5.4 Individuals 6 and 7 had one daughter X. Explain her phenotype.

Individuals 6 and 7 both do not have achondroplasia, which means their genotype must be homozygous recessive ($aa$). When two individuals with genotype $aa$ reproduce, they can only pass on the recessive allele ($a$) to their offspring. Therefore, their daughter X will inherit an $a$ allele from each parent, resulting in a genotype of $aa$. The phenotype associated with the $aa$ genotype is without achondroplasia (normal height).