A man with brown eyes married a brown-eyed woman. Their children turned out to be brown-eyed.
A man with brown eyes married a brown-eyed woman. Their children turned out to be brown-eyed. In the second case, the twin brother of the first man married a woman with blue eyes, but their children also turned out to be brown-eyed. Determine the genotypes of the man, his twin brother, both women and children in the first and second marriage. Give a cytological rationale for the crossing results. Brown eye color is the dominant feature.
Let’s designate the gene that causes the development of brown eyes in humans as K, then the gene for blue eyes will be designated as k.
A woman from her brother’s family has a genotype kk homozygous for the recessive gene, because the homozygosity of the genotype is a necessary condition for the phenotypic realization of a recessive trait (blue eyes). It produces eggs of the same type k and is capable of transmitting only the gene for blue eyes to its offspring.
Since all children married to a brown-eyed man have brown eyes, it should be concluded that this man is homozygous for the brown-eyed gene – KK. It produces the same type of spermatozoa K. All offspring have a heterozygous Kk genotype.
Since this man is the twin brother of a man from the first family, who also has only brown-eyed children, it should be assumed that the brothers are identical twins who are formed from one egg and one sperm and, as a result, have the same KK genotype homozygous for the brown eyes gene.
The genotype of a brown-eyed woman from her first marriage can be both homo- and heterozygous /
A homozygous brown-eyed KK woman produces the same type of KK eggs. In a marriage with the same KK man, all offspring are genotypically and phenotypically uniform and are represented by brown-eyed KK homozygotes.
A heterozygous brown-eyed female Kk produces two types of eggs – K and k. The offspring is uniform in phenotype (has brown eyes), and includes genotypes of two species – heterozygous children (Kk) – 50%, and children homozygous for the brown eyes gene (KK) – 50%.
