Question

Early-onset myopia in humans is inherited as a simple dominant trait. Determine the genotypes of the parents and offspring for the following families. Mention alternate genotypes wherever applicable. (a) One normal (without early-onset myopia) parent and one abnormal (with early-onset myopia) parent produce six children, out of which only one is normal. (b) An abnormal male and a normal female produce five normal children.

Short answer

In Scenario (a), one parent has normal vision (mm) and the other has early-onset myopia (Mm). Their children's genotypes are as follows: 1 normal child (mm) and 5 abnormal children (Mm). In Scenario (b), the male parent has early-onset myopia (Mm) and the female has normal vision (mm). Their children's genotypes are as follows: 5 normal children (mm).

Step-by-step solution

Scenario (a) - Normal Parent and Abnormal Parent, One Normal Child and Five Abnormal Children

In this scenario, we have one parent with normal vision and one parent with early-onset myopia. Since early-onset myopia is a dominant trait, the abnormal parent must have at least one dominant allele (M). Now, there are two possible genotypes for the normal parent: they can be homozygous recessive (mm) or heterozygous (Mm). However, since the couple produced at least one normal child, the abnormal parent must be heterozygous (Mm). Genotypes: - Normal parent: mm - Abnormal parent: Mm Moving on to their offspring, since one child has normal vision, they must have inherited two recessive alleles (mm). Considering that five children are abnormal, they have inherited at least one dominant allele (Mm or MM). However, based on the genotype of the parents, it's not possible for the offspring to have a homozygous dominant genotype (MM), so their genotype must be heterozygous (Mm). Offspring genotypes: - 1 normal child: mm - 5 abnormal children: Mm

Scenario (b) - Abnormal Male and Normal Female, Five Normal Children

In this scenario, the male has early-onset myopia and the female has normal vision. Since early-onset myopia is a dominant trait, the abnormal male parent must have at least one dominant allele (M). However, since the couple produced all normal children, the abnormal male must be heterozygous (Mm) because, if he was homozygous dominant (MM), he would pass on the dominant allele (M) to all his offspring, which is not the case in this scenario. Genotypes: - Normal female parent: mm - Abnormal male parent: Mm As for their offspring, all of them have normal vision, which means they must have inherited two recessive alleles (mm). Offspring genotypes: - 5 normal children: mm