The maternal-effect mutation bicoid (bcd) is recessive. In the absence of the bicoid protein product, embryogenesis is not completed. Consider a cross between a female heterozygous for the bicoid alleles (bcd⁺/bcd⁻) and a male homozygous for the mutation (bcd⁻/bcd⁻).

How is it possible for a male homozygous for the mutation to exist?

Key Concepts

Maternal Effect Genes

Maternal effect genes are genes expressed in the mother that influence the development of the offspring, regardless of the offspring's own genotype. In the case of bicoid, the mother's genotype determines the presence of the bicoid protein in the egg, which is crucial for proper embryonic development. If the mother is heterozygous, she can produce bicoid protein, allowing normal development even if the offspring inherits a recessive mutation.

Recessive Alleles

Recessive alleles are variants of a gene that do not manifest their traits in the presence of a dominant allele. In this scenario, the bicoid mutation (bcd⁻) is recessive, meaning that an individual must inherit two copies of the recessive allele to express the associated phenotype. However, if the mother carries a dominant allele (bcd⁺), she can still produce functional bicoid protein, which can rescue the development of her offspring.

Genotype and Phenotype

Genotype refers to the genetic constitution of an organism, while phenotype is the observable physical or biochemical characteristics resulting from the genotype and environmental influences. In this question, the male homozygous for the bicoid mutation (bcd⁻/bcd⁻) can exist because he inherits two recessive alleles from his parents, but his phenotype is influenced by the maternal genotype, which can provide the necessary bicoid protein for embryonic development.