Use the following information to answer questions: Genes that are located on different chromosomes assort independently. However, genes on the same chromosome can still separate from each other through a process called recombination. During meiosis, when four homologous chromatids form a tetrad, two of the chromatids can cross over, which causes them to break from their chromatid and reattach to the other chromatid. When this crossover event happens between non-sister chromatids that contain different alleles of a gene, recombination takes place and gives rise to genetic diversity.... Show more

Use the following information to answer questions:

Genes that are located on different chromosomes assort independently. However, genes on the same chromosome can still separate from each other through a process called recombination. During meiosis, when four homologous chromatids form a tetrad, two of the chromatids can cross over, which causes them to break from their chromatid and reattach to the other chromatid. When this crossover event happens between non-sister chromatids that contain different alleles of a gene, recombination takes place and gives rise to genetic diversity. Furthermore, the event can happen at more than one point between chromatids, resulting in multiple crossovers.

Evidence of crossovers can be seen microscopically by the detection of chiasmata. hese structures are thought to remain at the site of crossovers because the chromatids stay tangled together as the cell continues through meiosis. In addition to the detection of chiasmata, geneticists can identify crossover events by tracking the recombination of alleles on either side of the chromatid exchange. This tracking also allows the mapping of distance between genes. There is a direct correlation between the extent of recombination between two linked genes and their distance from each on a chromosome: genes that are far apart recombine frequently. The rates of recombination can also reveal the order of genes on a chromosome because single crossover events are much more likely to occur than multiple crossover events.

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MCAT Biological and Biochemical Foundations of Living Systems: Passage 17

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4 Questions

1. The two images below are of a chromosome pair, stained either with DAPI (to visualize the DNA) or HTP-3 (to visualize chromosomal axes), in wild-type C. elegans (top image) or in animals in which a protein that plays an important role in resolving crossovers, syp-1, was depleted (bottom image).

Based on the images, what is the difference in the number of chiasmata between the WT and syp-1–depleted animals?

The syp-1–depleted image has five more chiasmata than the WT image.The WT and syp-1–depleted images have the same number of chiasmata.The WT image has one more chiasmata than the syp-1–depleted image.The syp-1–depleted image has four more chiasmata than the WT image.

2. In a cross between a heterozygous Drosophila  female (AB/ab) and a homozygous male (ab/ab), the frequency of progeny is as follows: 37 AB/ab, 46 ab/ab, 11 aB/ab, 7 Ab/ab. Based on these frequencies, what is the distance in centimorgan (cM) between genes a and b?

4 cM7 cM11 cM18 cM

3. The table below shows the number of progeny from a cross between a Drosophila female that is heterozygous for three X-linked genes—a wing gene called crossveinless (cv), an eye gene called echinus (ec), and bristle gene called scute (sc)—with a male that has a mutant copy of each gene on his X chromosome. The genes are named for the phenotypes associated with their mutant forms. Because the wild-type (WT) genes are dominant, the female exhibits normal wings, eyes, and bristles; the male has the crossveinless, echinus, scute phenotypes. Based on the frequency of phenotypes among the progeny, what is the order of the genes on X chromosome?

cv-sc-ecec-sc-cvsc-ec-cvcv-ec-sc

4. During which stage of meiosis does crossover between homologous chromosomes occur?

Metaphase of meiosis IMetaphase of meiosis IIProphase of meiosis IProphase of meiosis II