2. An observant gardener finds that some of his bean plants have pubescent (hairy) leaves and others have glabrous (smooth) leaves. He crosses plants and obtains the results shown.
Cross Parents Offspring
1 pubescent x glabrous 56 pubescent, 61 glabrous
2 pubescent x pubescent 63 pubescent, 0 glabrous
3 glabrous x glabrous 0 pubescent, 44 glabrous
4 pubescent x glabrous 59 pubescent, 0 glabrous
5 pubescent x pubescent 122 pubescent, 41 glabrous
a. Explain these results genetically
Well, it seems like the presence or absence of hair on the leaves of the bean plants is controlled by a single gene. Let's call the presence of hair (pubescent) the dominant trait and the absence of hair (glabrous) the recessive trait.
In cross 1, one parent had pubescent leaves (genotype PP) and the other had glabrous leaves (genotype pp). The offspring had a combination of the two traits because they inherited one copy of the gene from each parent. The result of 56 plants with pubescent leaves (genotype Pp) and 61 plants with glabrous leaves (genotype pp) suggests that the presence of hair is dominant over its absence.
In cross 2, both parents had pubescent leaves (genotype Pp). This means that all the offspring inherited the dominant trait and had pubescent leaves (genotype PP).
In cross 3, both parents had glabrous leaves (genotype pp). As a result, all the offspring inherited the recessive trait and had glabrous leaves (genotype pp).
In cross 4, one parent had pubescent leaves (genotype Pp) and the other had glabrous leaves (genotype pp). However, all the offspring had pubescent leaves (genotype Pp) which suggests that the presence of hair is once again dominant.
Finally, in cross 5, both parents had pubescent leaves (genotype Pp). The result of 122 plants with pubescent leaves (genotype PP) and 41 plants with glabrous leaves (genotype Pp) suggests that there is some variability in the expression of the trait.
So, to summarize, the presence of hair on the bean plant leaves is controlled by a single gene where the presence of hair (pubescent) is dominant over its absence (glabrous).
To explain the results genetically, we need to understand the inheritance pattern of the pubescent and glabrous traits. In this case, the presence of the hair on the leaves (pubescence) is controlled by a single gene with two alleles: P for pubescent and p for glabrous.
From the results of the crosses, we can infer the following:
Cross 1: Pubescent x Glabrous
- Offspring: 56 pubescent, 61 glabrous
This suggests that the pubescent trait is dominant over the glabrous trait. The parent with pubescent leaves must be heterozygous (Pp), while the parent with glabrous leaves is homozygous recessive (pp).
Cross 2: Pubescent x Pubescent
- Offspring: 63 pubescent, 0 glabrous
Since both parents have pubescent leaves, all the offspring also have pubescent leaves. This indicates that the pubescent trait is solely determined by the presence of the dominant allele (P).
Cross 3: Glabrous x Glabrous
- Offspring: 0 pubescent, 44 glabrous
Both parents have glabrous leaves, and all the offspring also have glabrous leaves. This indicates that the glabrous trait is determined by the recessive allele (p).
Cross 4: Pubescent x Glabrous
- Offspring: 59 pubescent, 0 glabrous
The parent with pubescent leaves must be heterozygous (Pp), while the parent with glabrous leaves is homozygous recessive (pp). None of the offspring inherited the recessive allele (p), resulting in all of them having pubescent leaves.
Cross 5: Pubescent x Pubescent
- Offspring: 122 pubescent, 41 glabrous
Given that both parents have pubescent leaves, we can conclude that the glabrous trait can still occur in offspring when both parents are heterozygous (Pp). This means that the glabrous trait is recessive.
In summary, the pubescent trait (P) is dominant over the glabrous trait (p). When a plant inherits at least one dominant allele (P), it will have pubescent leaves. Only when a plant inherits two recessive alleles (pp) will it have glabrous leaves.
To explain the results genetically, we need to consider the inheritance pattern of the trait for hairy (pubescent) leaves versus smooth (glabrous) leaves in the bean plants. It appears that this trait is controlled by a single gene with two alleles, one for pubescent leaves (P) and one for glabrous leaves (p).
Based on the results of the crosses, we can deduce the following:
Cross 1: pubescent (Pp) x glabrous (pp)
- Offspring ratio: 56 pubescent, 61 glabrous
- This suggests that the offspring inherited one copy of the pubescent allele (P) from the pubescent parent and one copy of the glabrous allele (p) from the glabrous parent. The genotype of the offspring is a heterozygous (Pp) combination, resulting in some plants with pubescent leaves and some with glabrous leaves.
Cross 2: pubescent (Pp) x pubescent (Pp)
- Offspring ratio: 63 pubescent, 0 glabrous
- This result indicates that both parents contribute the pubescent allele (P) to all the offspring. The genotype of the offspring is homozygous (PP), resulting in plants with only pubescent leaves.
Cross 3: glabrous (pp) x glabrous (pp)
- Offspring ratio: 0 pubescent, 44 glabrous
- In this cross, both parents contribute the glabrous allele (p) to all the offspring. The genotype of the offspring is homozygous (pp), resulting in plants with only glabrous leaves.
Cross 4: pubescent (Pp) x glabrous (pp)
- Offspring ratio: 59 pubescent, 0 glabrous
- This result suggests that the pubescent allele (P) is dominant over the glabrous allele (p). The genotype of the offspring could be either heterozygous (Pp) or homozygous dominant (PP), resulting in plants with pubescent leaves.
Cross 5: pubescent (Pp) x pubescent (Pp)
- Offspring ratio: 122 pubescent, 41 glabrous
- Similar to Cross 2, both parents contribute the pubescent allele (P) to all the offspring. The genotype of the offspring is homozygous (PP) or heterozygous (Pp), resulting in plants with pubescent leaves. However, there are also some plants with glabrous leaves, which could be the result of either incomplete dominance or other genetic factors at play.
In summary, the trait for hairy (pubescent) versus smooth (glabrous) leaves in the bean plants seems to be controlled by a single gene with complete dominance, in which the pubescent allele (P) is dominant over the glabrous allele (p). The specific genotypes of the plants can vary depending on the parental alleles contributed during the cross, leading to different ratios of phenotypic expression in the offspring.