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Chapter 5: Problem 26

In a certain plant, fruit is either red or yellow, and fruit shape is either oval or long. Red and oval are the dominant traits. Two plants, both heterozygous for these traits, were testcrossed, with the following results. Determine the location of the genes relative to one another and the genotypes of the two parental plants.

Short Answer

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Answer: The genotypes of the parental plants are RrOo, as they are heterozygous for both traits. Gene location can be determined through a testcross by examining the results for any deviations from the expected 1:1:1:1 ratio under independent assortment. If the results significantly deviate from the expected ratio, it indicates that the genes are linked and located close together on the same chromosome.

Step by step solution

01

Identify given information and symbols

: It's given that red and oval are the dominant traits. In order to express the dominant phenotype, at least one dominant allele (R, O) must be present. For simplicity, we will use the following letters for the traits: - R (dominant) = Red fruit - r (recessive) = Yellow fruit - O (dominant) = Oval fruit - o (recessive) = Long fruit Both plants are heterozygous for these traits, meaning they should have one dominant and one recessive allele for each trait. So the two parental plants' genotypes are: RrOo.
02

Perform the testcross

: A testcross is done by crossing the heterozygous plants (RrOo) with plants that are homozygous recessive for both traits (rroo). Parent 1: RrOo Parent 2: rroo To determine offspring phenotypes and genotypes, we will do a dihybrid cross.
03

Dihybrid cross

: We will do a dihybrid cross between RrOo and rroo for each trait separately. It might look complicated, but it is a simple 4x4 Punnett square. Trait: Color | R | r | | r | r | Trait: Shape | O | o | | o | o | Now, we combine them to find possible offspring of the testcross: 1. Rroo: Red and long 2. rroo: Yellow and long 3. RrOo: Red and oval 4. rrOo: Yellow and oval
04

Examine the testcross results

: We are given the testcross results. To analyze them, we have to look for any deviations from the 1:1:1:1 ratio that would normally occur under independent assortment. If the genes are not independently assorting, it indicates that they are linked and located close together on the same chromosome.
05

Determine the gene location and genotypes

: As we don't have the exact numbers of each offspring phenotype, we can only determine the expected ratio of genotypes assuming independent assortment. If the results significantly deviate from the expected ratio, then it might indicate gene linkage, and the genes are likely located near each other on the chromosome. However, without the testcross results, their specific location relative to each other cannot be determined precisely. The genotypes of the parental plants are RrOo as they are heterozygous for both traits.

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Most popular questions from this chapter

In this chapter, we focused on linkage, chromosomal mapping, and many associated phenomena. In the process, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions? (a) How was it established experimentally that the frequency of recombination (crossing over) between two genes is related to the distance between them along the chromosome? (b) How do we know that specific genes are linked on a single chromosome, in contrast to being located on separate chromosomes? (c) How do we know that crossing over results from a physi- cal exchange between chromatids? (d) How do we know that sister chromatids undergo recombination during mitosis? (e) When designed matings cannot be conducted in an organism (for example, in humans), how do we learn that genes are linked, and how do we map them?

In Drosophila, a cross was made between females-all expressing the three \(X\) -linked recessive traits scute bristles \((s c),\) sable body \((s),\) and vermilion eyes \((v)-\) and wild-type males. In the \(\mathrm{F}_{1},\) all females were wild type, while all males expressed all three mutant traits. The cross was carried to the \(\mathrm{F}_{2}\) generation, and 1000 offspring were counted, with the results shown in the following table. No determination of sex was made in the data. (a) Using proper nomenclature, determine the genotypes of the \(P_{1}\) and \(F_{1}\) parents. (b) Determine the sequence of the three genes and the map distances between them. (c) Are there more or fewer double crossovers than expected? (d) Calculate the coefficient of coincidence. Does it represent positive or negative interference?

Three loci, mitochondrial malate dehydrogenase that forms \(a\) and \(b(M D H a, M D H b),\) glucouronidase that forms 1 and 2 \((G U S 1, G U S 2),\) and a histone gene that forms \(+\) and \(-\left(H^{+},\right.\) \(\left.H^{-}\right),\) are located on chromosome \(\\# 7\) in humans. Assume that the \(M D H\) locus is at position \(35, G U S\) at position \(45,\) and \(H\) at position \(75 .\) A female whose mother was homozygous for \(M D H a, G U S 2,\) and \(H^{+}\) and whose father was homozygous for \(M D H b, G U S 1,\) and \(H^{-}\) produces a sample of 1000 egg cells. Give the genotypes and expected numbers of the various types of cells she would produce. Assume no chromosomal interference.

What possible conclusions can be drawn from the observations that in male Drosophila, no crossing over occurs, and that during meiosis, synaptonemal complexes are not seen in males but are observed in females where crossing over occurs?

Are mitotic recombinations and sister chromatid exchanges effective in producing genetic variability in an individual? in the offspring of individuals?
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