The bicoid gene is a coordinate maternal–effect gene. If loss of bicoid function in the egg leads to lethality during embryogenesis, how are females homozygous for bicoid produced? What is the phenotype of a male homozygous for bicoid loss-of-function alleles?

Key Concepts

Maternal Effect Genes

Maternal effect genes are genes expressed in the mother that produce products (like mRNA or proteins) which are deposited into the egg. These products influence the development of the embryo, regardless of the embryo's own genotype. In the case of the bicoid gene, it is crucial for establishing the anterior-posterior axis in the embryo, and its loss can lead to severe developmental issues.

Homozygosity and Lethality

Homozygosity refers to having two identical alleles for a particular gene. In the context of the bicoid gene, females can be homozygous for bicoid loss-of-function alleles because they inherit one allele from each parent. However, if the mother is homozygous for a non-functional bicoid allele, the resulting embryos will lack the necessary bicoid protein, leading to lethality during embryogenesis.

Phenotype of Loss-of-Function Alleles

The phenotype of an organism refers to its observable characteristics, which can be influenced by its genotype. In males homozygous for bicoid loss-of-function alleles, the phenotype would typically be normal, as males do not rely on maternal bicoid for their development. However, they would not contribute functional bicoid to their offspring, which could lead to developmental issues in the next generation if the female is also homozygous for the loss-of-function allele.