I'm having trouble with this problem, because I keep getting the same answer for two of the questions, which doesn't help me with part d. If someone could give me some hints as to how to solve this correctly. Also do how does the anothocyanin factor in? Do rr plants die?
In Asiatic cotton, a pair of factors R and r controls the [presence or absence, respectively, of anthocyanin pigmentation. Another gene, about 10 map units away from the R locus, controls chlorophyll production. The homozygous recessive genotype at this locus (yy) produces a yellow chlorophyll-deficient plant that dies early in the seedling stage. The heterozygote Yy is phonetically green and indistinguishable from the dominate homozygote YY. Testcrosses are not possible for the Y locus. When dihybrids are crossed together, calculate the expected phenotypic proportions among the seedlings when parents are (a) both coupling/cis phase (b) both repulsion/trans phase (c) one coupling/cis and one repulsion/trans phase. (d) which method produces greatest mortality?
To correctly solve this problem, let's break it down step by step.
First, let's understand the genetic factors involved:
1. The R gene: This gene controls anthocyanin pigmentation, where the dominant allele R produces pigmentation, and the recessive allele r inhibits pigmentation.
2. The Y gene: This gene controls chlorophyll production. The dominant allele Y produces chlorophyll, while the recessive allele y inhibits chlorophyll production, resulting in a yellow chlorophyll-deficient plant that dies early.
Now, let's analyze the different phases and their impact on the phenotypic proportions among the seedlings:
(a) Both coupling/cis phase:
In this phase, the R and Y alleles are present on the same chromosomes. When the dihybrids with genotype RrYy are crossed together, the expected phenotypic ratio among the seedlings is 9 green, pigmented: 3 green, unpigmented: 3 yellow, pigmented: 1 yellow, unpigmented. This ratio arises due to independent assortment of the R and Y alleles during meiosis.
(b) Both repulsion/trans phase:
In this phase, the R and Y alleles are present on different chromosomes. When the dihybrids with genotype RrYy are crossed together, the expected phenotypic ratio among the seedlings is 1 green, pigmented: 1 green, unpigmented: 1 yellow, pigmented: 1 yellow, unpigmented. This 1:1:1:1 ratio is due to the repulsion or trans phase.
(c) One coupling/cis and one repulsion/trans phase:
In this scenario, the expected phenotypic ratio will vary. Let's assume one parent is in the coupling/cis phase (RrYy) and the other parent is in the repulsion/trans phase (RrYy). The expected phenotypic ratio among the seedlings will be a mixture of the ratios obtained in parts (a) and (b). The exact ratio would depend on the proportion of coupling and repulsion gametes produced by the parents.
(d) Mortality of plants:
Based on the information given, the homozygous recessive genotype (rr) for the chlorophyll production gene (Y) results in a yellow, chlorophyll-deficient plant that dies early in the seedling stage. Therefore, the method that involves the production of rr genotype individuals would lead to the greatest mortality.
To solve the problem accurately, it would be essential to know the proportion of coupling and repulsion gametes produced by the parents in part (c), which is not provided. However, you can still calculate the expected phenotypic ratio assuming equal proportions of coupling and repulsion gametes.
I hope this explanation helps you understand how to approach and solve the problem.