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⁻).
Predict the outcome (normal vs. failed embryogenesis) in the F₁ and F₂ generations of the cross described.

Verified step by step guidance

1. Step 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. Step 2: Analyze the parental genotypes. The female parent is heterozygous (bcd⁺/bcd⁻), meaning she has one functional allele (bcd⁺) and one mutant allele (bcd⁻). The male parent is homozygous for the mutant allele (bcd⁻/bcd⁻), meaning he contributes no functional bicoid protein.
3. Step 3: Predict the F₁ generation outcome. Since the maternal genotype determines embryogenesis, the heterozygous female (bcd⁺/bcd⁻) will provide sufficient bicoid protein for embryogenesis in her offspring, regardless of their genotype. Therefore, all F₁ embryos will develop normally.
4. Step 4: Predict the F₂ generation outcome. To determine the F₂ generation, consider the genotypes of the F₁ individuals. If an F₁ female is heterozygous (bcd⁺/bcd⁻) and mates with an F₁ male (bcd⁻/bcd⁻), the maternal genotype of the F₂ offspring will determine their embryogenesis. If the F₂ female is homozygous mutant (bcd⁻/bcd⁻), she will not provide functional bicoid protein, leading to failed embryogenesis in her offspring.
5. Step 5: Summarize the outcomes. In the F₁ generation, all embryos develop normally due to the maternal heterozygous genotype. In the F₂ generation, embryogenesis will fail for offspring of homozygous mutant (bcd⁻/bcd⁻) females, while offspring of heterozygous (bcd⁺/bcd⁻) females will develop normally.

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. The bicoid gene is a classic example, where the presence of the bicoid protein in the egg is crucial for proper embryonic development. If the mother is homozygous for a recessive mutation, the offspring will inherit the effects of that mutation, leading to developmental failures.

Recessive alleles

Recessive alleles are variants of a gene that do not manifest their traits in the presence of a dominant allele. In the case of the bicoid mutation, the bcd⁻ allele is recessive, meaning that an individual must inherit two copies of this allele (bcd⁻/bcd⁻) to exhibit the associated phenotype. Understanding how recessive alleles function is essential for predicting the outcomes of genetic crosses.

Punnett square and genetic crosses

A Punnett square is a tool used to predict the genotypic and phenotypic outcomes of genetic crosses. By organizing the alleles of the parents, one can visualize the possible combinations in the offspring. In this scenario, analyzing the cross between a heterozygous female and a homozygous recessive male will help determine the likelihood of normal versus failed embryogenesis in the F₁ and F₂ generations.