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?

Verified step by step guidance

1. Understand the concept of maternal-effect genes: Maternal-effect genes, like bicoid, are genes whose products (mRNA or proteins) are deposited into the egg by the mother. These products are essential for early embryonic development. The genotype of the mother determines the phenotype of the offspring, regardless of the offspring's own genotype.
2. Explain how females homozygous for bicoid are produced: Females homozygous for bicoid loss-of-function alleles can be produced if their mother is heterozygous for the bicoid gene. A heterozygous mother (Bicoid+/Bicoid-) can provide sufficient bicoid gene product to the egg, allowing the embryo to develop normally, even if the embryo itself is homozygous for the loss-of-function allele (Bicoid-/Bicoid-).
3. Discuss the phenotype of a male homozygous for bicoid loss-of-function alleles: Since bicoid is a maternal-effect gene, the phenotype of the male embryo depends on the genotype of the mother. If the mother is heterozygous (Bicoid+/Bicoid-), she can provide enough bicoid product to the egg, and the male embryo will develop normally, even if it is homozygous for the loss-of-function allele (Bicoid-/Bicoid-).
4. Clarify why homozygous bicoid loss-of-function alleles in the mother lead to embryonic lethality: If a female is homozygous for the bicoid loss-of-function allele (Bicoid-/Bicoid-), she cannot produce functional bicoid gene product to deposit into her eggs. As a result, all her offspring, regardless of their genotype, will fail to develop properly and die during embryogenesis.
5. Summarize the key points: Females homozygous for bicoid loss-of-function alleles can develop if their mother is heterozygous and provides sufficient bicoid product. Males homozygous for bicoid loss-of-function alleles can develop normally if their mother provides the necessary bicoid product. However, a homozygous bicoid loss-of-function mother cannot produce viable offspring.

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 homozygous for bicoid loss-of-function alleles can produce viable eggs because they carry one functional allele inherited from their father. However, if these eggs are fertilized, the resulting embryos will lack bicoid function, 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. A male homozygous for bicoid loss-of-function alleles would typically exhibit a normal male phenotype since bicoid is not required for male development. However, if bicoid is essential for certain aspects of development, such as fertility or viability, these males may exhibit reduced fitness or reproductive success.