Passage
The fruit fly Drosophila melanogaster is frequently used as a model system to investigate neurological disorders. The genes Abl, Enabled (Ena) , and receptor protein tyrosine phosphatase (PTP) are involved in axonal guidance during neuronal development and are all believed to operate in the same pathway. To investigate interactions between PTP and either Ena or Abl, fly lines were developed in which mutations of each gene were easily identifiable by eye characteristics. In flies that would normally have white eyes, a dominant mutation that inactivates PTP results in red and white eyes; a dominant Ena mutation yields orange eyes (or red and orange if the PTP and Ena mutations are both present) ; and a dominant Abl mutation yields elongated eyes. Using these fly lines, the following experiments were conducted.Experiment 1Females heterozygous for PTP (eePp) were crossed with males heterozygous for Ena (Eepp) . F1 generation males and females, each with red and orange eyes, were then selected and crossed with each other to produce an F2 generation. Flies with two mutant Ena alleles (EE) , as well as those with two mutant PTP alleles and at least one mutant Ena allele (EePP) , did not survive past the larval stage.Experiment 2PTP and Abl are located on the same chromosome. Females with the PTP mutation on one copy of the chromosome and the Abl mutation on the other copy were crossed with wild-type males. In Drosophila, recombination occurs only in female gametes. Progeny with parental and recombinant genotypes are shown in Table 1. F1 males with red and white elongated eyes were then crossed with homozygous PTP mutant females. This produced an F2 generation in which 50% of both male and female progeny had elongated eyes.Table 1 Phenotypes of 500 F1 Flies
Like humans, male flies have a single X chromosome, whereas females have two X chromosomes. Unlike humans, female Drosophila can store sperm and fertilize multiple eggs over the course of several days after a single mating. Researchers ensured that females were virgins prior to setting up each cross to avoid contamination.
Adapted from Song JK, Giniger E, Desai CJ. The receptor protein tyrosine phosphatase PTP69D antagonizes Abl tyrosine kinase to guide axons in Drosophila. Mech Dev. 2008;125(3-4) :247-56.
-Which evidence in Table 1 suggests that PTP and Abl are located close together on the chromosome?

Question

Passage
The fruit fly Drosophila melanogaster is frequently used as a model system to investigate neurological disorders. The genes Abl, Enabled (Ena) , and receptor protein tyrosine phosphatase (PTP) are involved in axonal guidance during neuronal development and are all believed to operate in the same pathway. To investigate interactions between PTP and either Ena or Abl, fly lines were developed in which mutations of each gene were easily identifiable by eye characteristics. In flies that would normally have white eyes, a dominant mutation that inactivates PTP results in red and white eyes; a dominant Ena mutation yields orange eyes (or red and orange if the PTP and Ena mutations are both present) ; and a dominant Abl mutation yields elongated eyes. Using these fly lines, the following experiments were conducted.Experiment 1Females heterozygous for PTP (eePp) were crossed with males heterozygous for Ena (Eepp) . F1 generation males and females, each with red and orange eyes, were then selected and crossed with each other to produce an F2 generation. Flies with two mutant Ena alleles (EE) , as well as those with two mutant PTP alleles and at least one mutant Ena allele (EePP) , did not survive past the larval stage.Experiment 2PTP and Abl are located on the same chromosome. Females with the PTP mutation on one copy of the chromosome and the Abl mutation on the other copy were crossed with wild-type males. In Drosophila, recombination occurs only in female gametes. Progeny with parental and recombinant genotypes are shown in Table 1. F1 males with red and white elongated eyes were then crossed with homozygous PTP mutant females. This produced an F2 generation in which 50% of both male and female progeny had elongated eyes.Table 1 Phenotypes of 500 F1 Flies

Like humans, male flies have a single X chromosome, whereas females have two X chromosomes. Unlike humans, female Drosophila can store sperm and fertilize multiple eggs over the course of several days after a single mating. Researchers ensured that females were virgins prior to setting up each cross to avoid contamination.
Adapted from Song JK, Giniger E, Desai CJ. The receptor protein tyrosine phosphatase PTP69D antagonizes Abl tyrosine kinase to guide axons in Drosophila. Mech Dev. 2008;125(3-4) :247-56.
-Which evidence in Table 1 suggests that PTP and Abl are located close together on the chromosome?

Quizplus · Accepted Answer

11ecba2d_0ff4_35f1_a3bf_0f3ea905a410_MD0008_00 Homologous chromosomes can exchange genetic information in a process called crossing over, or recombination, during prophase I of meiosis. Crossing over can produce new combinations of alleles within a chromosome by moving some alleles of the maternal copy to the paternal copy and vice versa. New combinations of alleles are called recombinant, whereas combinations that already existed in the parent are called parental. If two genes are located close together on a chromosome (measured in centimorgans, or map units), they are relatively unlikely to be separated by a recombination event. This is because there is less distance between the two genes in which a recombination can occur. Therefore, substantially fewer progeny will have recombinant combinations than parental combinations. As a result, the recombinant progeny can be identified by the fact that there are fewer of them. Table 1 shows that only 43 of 500 flies (~9%) were recombinant. Given this small percentage of recombinant flies, it is likely that the PTP and Abl genes are close together on the chromosome. (Choice A) Recombination is the exchange of genes between homologous chromosomes, and always produces two recombinant chromosomes. Because each chromosome is equally likely to be passed on, recombinant genotypes always occur with equal frequency, regardless of genetic distance. (Choice C) According to the passage, recombination occurs only in female gametes. However, the lack of recombination in male gametes does not provide any information about the distance between genes on a chromosome. (Choice D) A high recombination frequency would indicate that genes are distant from, not close to, each other on the chromosome. Educational objective: Homologous chromosomes can exchange genetic material by recombining. Genes that are located close together on a chromosome have a lower probability of being separated by recombination than those that are far apart. Fewer progeny from a cross will have recombinant genotypes than will have parental genotypes.

Passage the Fruit Fly Drosophila Melanogaster Is Frequently Used as a a Model