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 mutation \(\left(b c d^{+} / b c d^{-}\right)\) and a homozygous male \(\left(b c d^{\left.-/ b c d^{-}\right)}\right.\) (a) How is it possible for a male homozygous for the mutation to exist? (b) Predict the outcome (normal vs, failed embryogenesis) in the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) generations of the cross described.

Maternal-effect mutations, such as bicoid, are inherited in a unique way. The phenotype of the offspring depends solely on the genotype of the mother, not on the offspring's own genotype. In this case, embryos lacking the bicoid protein product will not complete embryogenesis. The bicoid (bcd) mutation is recessive, meaning that an individual needs two copies of the mutated gene (bcd-/bcd-) to exhibit a mutant phenotype.

In the given cross, a heterozygous female (bcd+/bcd-) is crossed with a homozygous male (bcd-/bcd-). To determine the possible genotypes of their offspring, we can make a Punnett square. Using Punnett square: Parental genotypes: bcd+/bcd- (female) x bcd-/bcd- (male) | | bcd- | bcd- | |--------|--------|--------| | bcd+ | bcd+/bcd- | bcd+/bcd- | | bcd- | bcd-/bcd- | bcd-/bcd- | The possible offspring genotypes are: bcd+/bcd- and bcd-/bcd-.

As mentioned earlier, maternal-effect mutation's phenotype depends on the mother's genotype. In this case, the mother is heterozygous, with one functional (bcd+) allele. Hence, she will be able to produce the bicoid protein, which allows for normal embryogenesis. (a) A male homozygous (bcd-/bcd-) exists because despite having two mutated alleles, the mother of this male has at least one functional allele (bcd+) allowing normal embryogenesis to occur. (b) Since the phenotypic outcome solely depends on the mother's genotype and not the genotype of the offspring: \(\mathrm{F}_{1} \) generation: Regardless of the offspring's genotype, they all will have a normal embryogenesis because their mother has a functional allele (bcd+). Phenotypic ratio: 100% normal embryogenesis. To predict the outcome of the \(\mathrm{F}_{2}\) generation, let us assume an F1 offspring (bcd+/bcd-) mates with a male homozygous for the mutation (bcd-/bcd-). This case is similar to the original cross. F1: bcd+/bcd- (female) x bcd-/bcd- (male) | | bcd- | bcd- | |--------|--------|--------| | bcd+ | bcd+/bcd- | bcd+/bcd- | | bcd- | bcd-/bcd- | bcd-/bcd- | \(\mathrm{F}_{2} \) generation: Again, the mother in this case has a functional allele (bcd+), allowing for normal embryogenesis to occur for all offspring, regardless of their genotypes. Phenotypic ratio: 100% normal embryogenesis.