4. Genetic Mapping and Linkage

Crossing Over and Recombinants

4. Genetic Mapping and Linkage

Crossing Over and Recombinants

1:59 minutes

Problem 36

Textbook Question

Divide a clean sheet of paper into four quadrants and draw one pair of homologous chromosomes in each quadrant. Draw the chromosomes with two sister chromatids each. The four sets of homologous pairs are identical. Label one chromosome of each pair with alleles A₁ and A₂ and the other member of each pair with the alleles A₁ and A₂. You are to illustrate a single crossover between the homologs in each quadrant and list the parental and recombinant chromosomes, but you are to illustrate four different ways the crossover can occur by involving different chromatids in each illustration.

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Start by dividing your paper into four quadrants. Label each quadrant (e.g., Quadrant 1, Quadrant 2, Quadrant 3, Quadrant 4) to keep track of the different crossover scenarios you will illustrate.

In each quadrant, draw a pair of homologous chromosomes. Each chromosome should consist of two sister chromatids, represented as two parallel lines. Ensure that the homologous chromosomes are paired together, and label one chromosome with alleles A₁ and A₂, and the other homologous chromosome with alleles A₁ and A₂.

Illustrate a single crossover event in each quadrant. For Quadrant 1, show a crossover between one chromatid from each homologous chromosome (e.g., the top chromatid of one homolog and the top chromatid of the other homolog). For Quadrant 2, show a crossover between the top chromatid of one homolog and the bottom chromatid of the other homolog. For Quadrant 3, show a crossover between the bottom chromatid of one homolog and the top chromatid of the other homolog. For Quadrant 4, show a crossover between the bottom chromatid of one homolog and the bottom chromatid of the other homolog.

After illustrating the crossover events, label the resulting chromatids in each quadrant as either parental or recombinant. Parental chromatids will retain the original allele combinations (A₁A₁ or A₂A₂), while recombinant chromatids will have new allele combinations (A₁A₂ or A₂A₁) due to the crossover.

List the parental and recombinant chromatids for each quadrant. For example, in Quadrant 1, identify which chromatids remain parental and which are recombinant after the crossover. Repeat this process for Quadrants 2, 3, and 4, ensuring that you clearly distinguish between the different crossover scenarios.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Homologous Chromosomes

Homologous chromosomes are pairs of chromosomes in a diploid organism that have the same structure and gene sequence but may carry different alleles. Each parent contributes one chromosome to the pair, resulting in two homologs that are similar but not identical. Understanding homologous chromosomes is crucial for grasping genetic variation and inheritance patterns during meiosis.

Sister Chromatids

Sister chromatids are identical copies of a single chromosome that are formed during DNA replication. Each chromosome consists of two sister chromatids joined at a region called the centromere. During cell division, these chromatids are separated into daughter cells, ensuring that each new cell receives an identical set of chromosomes, which is essential for maintaining genetic consistency.

Crossover and Genetic Recombination

Crossover is a process that occurs during meiosis where homologous chromosomes exchange segments of genetic material. This exchange leads to genetic recombination, resulting in new combinations of alleles in the offspring. Understanding crossover is vital for analyzing genetic diversity and the inheritance of traits, as it plays a key role in producing recombinant chromosomes that differ from the parental types.

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Textbook Question

In experiments published in 1918 that sought to verify and expand the genetic linkage and recombination theory proposed by Morgan, Thomas Bregger studied potential genetic linkage in corn (Zea mays) for genes controlling kernel color (colored is dominant to colorless) and starch content (starchy is dominant to waxy). Bregger performed two crosses. In Cross 1, pure-breeding colored, starchy-kernel plants (C1 Wx/C1 Wx) were crossed to plants pure-breeding for colorless, waxy kernels (c1 wx/c1 wx). The F₁ of this cross were test-crossed to colorless, waxy plants. The test-cross progeny were as follows:

In Cross 2, plants pure-breeding for colored, waxy kernels (C1 wx/C1 wx) and colorless, starchy kernels (c1 Wx/c1 Wx) were mated, and their F₁ were test-crossed to colorless, waxy plants. The test-cross progeny were as follows:

Taken together, are the results of these two experiments compatible with the hypothesis of genetic linkage? Explain why or why not.

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Textbook Question

Calculate the recombination frequency for each of the progeny groups.

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Textbook Question

For each set of test-cross progeny, determine whether genetic linkage or independent assortment is more strongly supported by the data. Explain the rationale for your answer.

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Textbook Question

The gene controlling the Xg blood group alleles (Xg⁺ and Xg⁻) and the gene controlling a newly described form of inherited recessive muscle weakness called episodic muscle weakness (EMWX) (Ryan et al., 1999) are closely linked on the X chromosome in humans at position Xp22.3 (the tip of the short arm). A male with EMWX who is Xg⁻ marries a woman who is Xg⁺ and they have eight daughters and one son, all of whom are normal for muscle function, the male being Xg⁺ and all the daughters being heterozygous at both the EMWX and Xg loci. Following is a table that lists three of the daughters with the phenotypes of their husbands and children.For each of the offspring, indicate whether or not a crossover was required to produce the phenotypes that are given.

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Multiple Choice

Which of the following gametes can be formed from the genotype AaBb if AB and ab are linked?

An experiment that was performed found the recombination frequency between two genes was 12.5%. What is the distance (in mapping units) between two genes?

Which of the following terms describes two copies of the same chromosome?

Genes E and H are syntenic in an experimental organism with the genotype EH/eh. Assume that during each meiosis, one crossover occurs between these genes. No homologous chromosomes escape crossover, and none undergo double crossover. Are genes E and H genetically linked? Why or why not? What is the proportion of parental gametes produced by meiosis?

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