Question

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.

Short answer

Answer: A male homozygous for the bicoid mutation can exist because despite having two mutated alleles, their mother had at least one functional allele (bcd+), allowing normal embryogenesis to occur. For the F1 and F2 generations, we can predict 100% normal embryogenesis in both cases since the mothers in each generation have a functional allele (bcd+), allowing for normal embryogenesis to occur for all offspring, regardless of their genotypes.

Step-by-step solution

Understand the maternal-effect mutation and its inheritance pattern

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.

Determine the possibilities of offspring genotypes from the given cross

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-.

Predict the phenotype of each offspring genotype based on the maternal-effect mutation inheritance pattern

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.

Key concepts

Bicoid Mutation

The bicoid mutation is a fascinating example of a maternal-effect mutation, where the mother's genotype dictates the phenotype of her offspring. In the case of the bicoid mutation, embryos that do not possess the functional bicoid protein cannot complete the process of embryogenesis. A key characteristic of the bicoid mutation is that it is recessive. This means the mutant phenotype, which leads to abnormal embryogenesis, only appears in individuals that inherit two mutated gene copies (bcd-/bcd-). This unique genetic setup makes understanding bicoid mutation essential when studying genetics related to development and inheritance.

Genotype Inheritance

In the illustrated scenario, understanding genotype inheritance provides deep insights into the possible outcomes of the cross between certain genotypes. Specifically, a heterozygous female for the bicoid mutation (bcd+/bcd-) is crossed with a homozygous male (bcd-/bcd-). When considering the potential genotypes of their offspring, a simple tool such as a Punnett square can be used to visualize the possibilities.

• A Punnett square helps predict the potential genetic combinations: bcd+/bcd- (one functional and one mutant allele) and bcd-/bcd- (both mutant alleles).
• In this cross, offspring can only inherit either a heterozygous bcd+ or homozygous bcd- configuration.

Despite the genotypic variety, it's crucial to remember that in maternal-effect mutations like bicoid, it is the mother's genotype that ultimately determines the developmental outcome of the offspring.

Embryogenesis Outcome

The outcome of embryogenesis in the context of the bicoid mutation heavily depends on the mother's genotype. The maternal-effect nature of this mutation implies that the presence of at least one functional bcd+ allele in the mother is sufficient to ensure normal embryogenesis. This attribute explains why even when the offspring themselves may carry the bcd-/bcd- genotype, they still undergo regular development if their mother possesses at least one functional allele (bcd+).

• In the F1 generation, all offspring undergo normal embryogenesis due to the heterozygous condition (bcd+/bcd-) of the mother.
• Similarly, in the F2 generation, despite potential genotype variations, normal embryogenesis occurs for all offspring because their F1 mother carries a functional bcd+ allele.

The concept of maternal-effect mutation underscores the importance of the maternal genotype in determining the development outcome in embryos, overriding the offspring's own genotype when such mutations are involved.