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?

Verified step by step guidance

1. Understand the concept of maternal-effect genes: Maternal-effect genes are genes whose products are provided by the mother and are required for early embryonic development. The phenotype of the offspring is determined by the genotype of the mother, not the offspring itself.
2. Recognize that the bicoid (bcd) gene is a maternal-effect gene: The bicoid protein is essential for embryogenesis, and its absence leads to failure in development. However, the genotype of the offspring does not directly affect its survival; it is the genotype of the mother that determines whether the offspring receives the necessary bicoid protein.
3. Analyze the genotype of the male homozygous for the mutation (bcd⁻/bcd⁻): A male homozygous for the bcd⁻ mutation can exist if his mother was heterozygous (bcd⁺/bcd⁻). The heterozygous mother would produce enough bicoid protein to support embryogenesis, allowing the male offspring to survive despite being homozygous for the mutation.
4. Consider the implications of the maternal-effect gene: Since the bicoid protein is provided by the mother, the survival of the offspring depends on whether the mother has at least one functional bcd⁺ allele. The offspring's genotype (bcd⁺/bcd⁻ or bcd⁻/bcd⁻) does not affect its ability to survive embryogenesis as long as the mother provides the necessary protein.
5. Conclude that the male homozygous for the mutation exists because his heterozygous mother (bcd⁺/bcd⁻) provided sufficient bicoid protein during embryogenesis, allowing him to develop and survive despite his genotype.

Key Concepts

Maternal Effect Genes

Maternal effect genes are genes expressed in the mother that influence the development of the offspring. The products of these genes, such as proteins or RNAs, are deposited in the egg and play crucial roles in early embryonic development. In the case of bicoid, the protein is essential for establishing the anterior-posterior axis in the embryo, and its absence leads to developmental failure.

Recessive Alleles

Recessive alleles are variants of a gene that do not manifest their traits in the presence of a dominant allele. In the context of the bicoid mutation, the bcd⁻ allele is recessive, meaning that an individual must inherit two copies of this allele (homozygous) to express the associated phenotype. This explains how a male homozygous for the bicoid mutation can exist if he inherits the recessive allele from both parents.

Genetic Crosses and Inheritance Patterns

Genetic crosses involve mating individuals with known genotypes to study inheritance patterns of traits. In this scenario, a heterozygous female (bcd⁺/bcd⁻) can produce offspring with varying genotypes, including homozygous recessive males (bcd⁻/bcd⁻). Understanding these patterns helps explain how recessive traits can appear in offspring even when one parent does not express the trait.