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Chapter 4: Problem 53

From parents of genotypes \(A / A \cdot B / B\) and \(a / a \cdot b / b,\) a dihybrid was produced. In a testcross of the dihybrid, the following seven progeny were obtained: $$\begin{array}{c}A / a \cdot B / b, a / a \cdot b / b, A / a \cdot B / b, A / a \cdot b / b \\\a / a \cdot b / b, A / a \cdot B / b, \text { and } a / a \cdot B / b\end{array}$$ Do these results provide convincing evidence of linkage?

Short Answer

Expert verified
Yes, the excess of parental types over recombinant types suggests linkage, but the small sample size weakens the evidence.

Step by step solution

01

Define the Parental Genotypes

The parental genotypes given are \( A/A \cdot B/B \) and \( a/a \cdot b/b \). The dihybrid produced from these parents will have the genotype \( A/a \cdot B/b \).
02

Explain Testcross Principles

In a testcross, the dihybrid \( A/a \cdot B/b \) is crossed with a double recessive \( a/a \cdot b/b \) to reveal the genotypes present in the gametes produced by the dihybrid. If the genes are unlinked, we expect a 1:1:1:1 ratio of progeny genotypes.
03

Count the Progeny Types

The progeny genotypes observed are: \( A/a \cdot B/b \) (3 times), \( a/a \cdot b/b \) (2 times), \( A/a \cdot b/b \) (1 time), \( a/a \cdot B/b \) (1 time). There are a total of 7 progeny.
04

Analyze the Progeny Distribution

Expected distribution under independent assortment is equal frequencies of each recombinant and parental type (1:1:1:1). Here, the parental genotypes (\( A/a \cdot B/b \) and \( a/a \cdot b/b \)) appear 5 times, while the recombinant genotypes (\( A/a \cdot b/b \) and \( a/a \cdot B/b \)) appear only 2 times.
05

Determine Evidence of Linkage

The deviation from the expected 1:1:1:1 ratio, with more parental types than recombinant types, suggests linkage. However, with only 7 progeny, this is a small sample size, reducing the statistical power to draw a strong conclusion.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Dihybrid Cross
A dihybrid cross involves studying the inheritance of two different traits controlled by two different pairs of alleles. In this exercise, we start with two parent genotypes: one parent with genotype \(A/A \cdot B/B\) and the other with \(a/a \cdot b/b\). The offspring from this cross is a dihybrid with the genotype \(A/a \cdot B/b\).Understanding a dihybrid cross helps in predicting the offspring's genotype based on the combination of alleles from both parents. It provides insight into how traits controlled by different genes can be passed down from one generation to the next.
Testcross
A testcross is an experimental mating test used to determine the underlying genotype of an individual exhibiting a dominant trait. It involves crossing the individual with another individual that is homozygous recessive for the traits being studied.In this specific case, the dihybrid \(A/a \cdot B/b\) is crossed with a double recessive \(a/a \cdot b/b\). The testcross is particularly useful to uncover genetic linkages as the phenotypic outcomes directly correlate with the genotypic combinations of alleles produced by the dihybrid parent.
Independent Assortment
Independent assortment is a fundamental principle of genetics stating that genes for different traits are distributed to sex cells (& gametes) independently of one another. This principle underlies Mendel鈥檚 laws of inheritance. For the progeny in this exercise, if the two genes are assorting independently, we expect a 1:1:1:1 ratio of all possible progeny genotypes. However, the observed distribution deviated from this expectation, suggesting some interaction between the genes. If the genes were truly assorting independently, the frequency of parental and recombinant types would be equal.
Recombinant Frequency
Recombinant frequency is a measure of how often crossing over occurs between two genes during gamete formation. It is calculated by counting the number of recombinant offspring divided by the total number of offspring, often expressed as a percentage.In the case of this exercise, the observed recombinant genotypes appeared less frequently than would be expected if the genes were assorting independently. With more parental types \((A/a \cdot B/b \) and \( a/a \cdot b/b)\) than recombinant types \((A/a \cdot b/b \) and \( a/a \cdot B/b)\), this pattern suggests some form of linkage between the genes, affecting the typical expectation ratios.

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Most popular questions from this chapter

In the plant Arabidopsis, the loci for pod length (L, long; 1, short) and fruit hairs ( \(H,\) hairy; \(h,\) smooth) are linked 16 m.u. apart on the same chromosome. The following crosses were made: (i) \(L H / L H \times l h / l h \rightarrow F_{1}\) (ii) \(L h / L h \times l H / l H \rightarrow F_{1}\) If the \(\mathrm{F}_{1}\) 's from cross i and cross ii are crossed, a. what proportion of the progeny are expected to be \(l h / l h ?\) b. what proportion of the progeny are expected to be \(L h / l h ?\)

In corn, the cross \(W W\) ee \(F F \times w w E E f f\) is made. The three loci are linked as follows Assume no interference. a. If the \(F_{1}\) is testcrossed, what proportion of progeny will be ww ee ff? b. If the \(F_{1}\) is selfed, what proportion of progeny will be ww ee ff?

If \(A / A \cdot B / B\) is crossed with \(a / a \cdot b / b\) and the \(F_{1}\) is testcrossed, what percentage of the testcross progeny will be \(a / a \cdot b / b\) if the two genes are (a) unlinked; (b) completely linked (no crossing over at all); (c) \(10 \mathrm{m}\).u. apart; (d) \(24 \mathrm{m} .\) u. apart?

The \(A\) locus and the \(D\) locus are so tightly linked that no recombination is ever observed between them. If \(A d /\) \(A d\) is crossed with \(a D / a D\) and the \(F_{1}\) is intercrossed, what phenotypes will be seen in the \(\mathrm{F}_{2}\) and in what proportions?

A geneticist mapping the genes \(A, B, C, D,\) and \(E\) makes two 3 -point testcrosses. The first cross of pure lines is $$A / A \cdot B / B \cdot C / C \cdot D / D \cdot E / E \times a / a \cdot b / b \cdot C / C \cdot d / d \cdot E / E$$ The geneticist crosses the \(\mathrm{F}_{1}\) with a recessive tester and classifies the progeny by the gametic contribution of the \(\mathrm{F}_{1}\) $$\begin{array}{lr}A \cdot B \cdot C \cdot D \cdot E & 316 \\\a \cdot b \cdot C \cdot d \cdot E & 314 \\\A \cdot B \cdot C \cdot d \cdot E & 31 \\\a \cdot b \cdot C \cdot D \cdot E & 39 \\\A \cdot b \cdot C \cdot d \cdot E & 130 \\\a \cdot B \cdot C \cdot D \cdot E & 140 \\\A \cdot b \cdot C \cdot D \cdot E & 17 \\\a \cdot B \cdot C \cdot d \cdot E & \frac{13}{1000}\end{array}$$ The second cross of pure lines is \(A / A \cdot B / B \cdot C / C \cdot D / D\) \(E / E \times a / a \cdot B / B \cdot c / c \cdot D / D \cdot e / e\) The geneticist crosses the \(\mathrm{F}_{1}\) from this cross with a recessive tester and obtains The geneticist also knows that genes \(D\) and \(E\) assort independently. a. Draw a map of these genes, showing distances in map units wherever possible. b. Is there any evidence of interference?
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