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Chapter 7: Problem 5

Why is a 50 percent recovery of single-crossover products the upper limit, even when crossing over always occurs between two linked genes?

Short Answer

Expert verified
Answer: The maximum percentage of single-crossover products of linked genes that can be formed during meiosis is 50%.

Step by step solution

01

Understand the concept of linked genes

Linked genes are those present on the same chromosome and inherited together. These genes do not follow the typical Mendelian inheritance patterns, as their alleles are not distributed independently during meiosis.
02

Define crossing over

Crossing over is the process that occurs during meiosis, in which homologous chromosomes exchange genetic material between non-sister chromatids. This process contributes to genetic diversity among offspring.
03

Explain single-crossover products

In a single-crossover event, two linked genes exchange genetic material only once during meiosis. The outcome of a single crossover event can produce two types of chromatids: recombinant and parental. Recombinant chromatids show the combination of alleles that are not present in the parent cell, while parental chromatids carry the same combination of alleles as were present in the parent cell.
04

Understand the 50 percent recovery upper limit

When crossing over occurs between two linked genes, four chromatids are involved: two recombinant and two parental. The recombinant chromatids contain new combinations of alleles, whereas the parental chromatids retain the original combinations of alleles. Since only one of the two original chromatids will undergo crossing over, a maximum of 50 percent recombinant chromatids can be formed, leaving the other 50 percent as parental chromatids.
05

Explain why 50 percent is the upper limit for single-crossover products

It is essential to understand that the 50 percent upper limit is only for single-crossover products. In a single crossover event, the maximum percentage of recombinant chromatids formed is 50 percent. However, multiple crossover events can potentially happen between the same pair of linked genes and can further increase the percentage of recombinant chromosomes beyond 50 percent. For single-crossover products, the 50 percent recovery is the maximum because only one crossover event occurs, resulting in only one set of recombinant chromatids.

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Most popular questions from this chapter

Describe the cytological observation that suggests that crossing over occurs during the first meiotic prophase.

In Drosophila, a cross was made between females expressing the three X-linked recessive traits, scute bristles \((s c),\) sable body \((s)\) and vermilion eyes ( \(v\) ), and wild-type males. All females were wild type in the \(F_{1},\) 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 \(\mathrm{F}_{2}\) 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 distance between them. (c) Are there more or fewer double crossovers than expected? (d) Calculate the coefficient of coincidence; does this represent positive or negative interference?

Drosophila females homozygous for the third chromosomal genes pink eye (p) and ebony body (e) were crossed with males homozygous for the second chromosomal gene dumpy wings (dp). Because these genes are recessive, all offspring were wild type (normal). \(F_{1}\) females were testcrossed to triply recessive males. If we assume that the two linked genes ( \(p\) and \(e\) ) are 20 mu apart, predict the results of this cross. If the reciprocal cross were made (F1 males-where no crossing over occurs-with triply recessive females), how would the results vary, if at all?

Review the Chapter Concepts list on p. \(121 .\) Most of these center on the process of crossing over between linked genes. Write a short essay that discusses how crossing over can be detected and how the resultant data provide the basis of chromosome mapping.

A female of genotype \\[ \frac{a}{+++} \\] produces 100 meiotic tetrads. Of these, 68 show no crossover events. Of the remaining 32,20 show a crossover between \(a\) and \(b\), 10 show a crossover between \(b\) and \(c,\) and 2 show a double crossover between \(a\) and \(b\) and between \(b\) and \(c .\) Of the 400 gametes produced, how many of each of the eight different genotypes will be produced? Assuming the order \(a-b-c\) and the allele arrangement shown above, what is the map distance between these loci?
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