suo pericentric inversion, the centromeres are contained within the inverted region. Consequently, the chromosomes that have engaged in crossing over separate in the normal fashion, without the creation of a bridge (Figure 17-29). However, the crossover produces chromatids that contain a duplication and a deletion for different parts of the chromosome. In this case, if a gamete carrying a crossover chromosome is fertilized, the zygote dies because of gene imbalance. Again, the result is that only noncrossover chromatids are present in viable prog- eny. Hence, the RF value of genes within a pericentric inver- Paracentric inversions can lead to deletion products A Paracentric- inversion heterozygote A D CBE Pairing Crossover in loop sion also is zero. Inversions affect recombination in another way, too. Inver- sion heterozygotes often have mechanical pairing problems in the region of the inversion. The inversion loop causes a large distortion that can extend beyond the loop itself. This distor- tion reduces the opportunity for crossing over in the neigh- boring regions. Let us consider an example of the effects of an inversion on recombinant frequency. A wild-type Drosophila specimen from a natural population is crossed with a homozygous recessive laboratory stock dp en /dp cn. (The dp allele encodes dumpy wings and cn encodes cinnabar eyes. The two genes are known to be 45 map units apart on chromosome 2.) The F, generation is wild type. When an F, female is crossed with the recessive parent, the progeny are Segregation A B C DE A B C D E Acentric fragment (lost) D Dicentric bridge breaks randomly ++/ dp cn 246 dumpy cinnabar dp cn / dp cn 250 wild type ADCBE ADCB E 5 dumpy dp + / dp cn 7 cinnabar + en / dp cn ABC DE Normal product A Авср B Deletion product FIGURE 17-28 A crossover in the loop of a paracen- tric-inversion heterozygote gives rise to chromosomes Deletion product containing deletions. AD CBE LounchPad ANIMATED ART: Chromosome rearrangements: meiotic behavior of paracentric inversions Inversion product

suo pericentric inversion, the centromeres are contained within the inverted region. Consequently, the chromosomes that have engaged in crossing over separate in the normal fashion, without the creation of a bridge (Figure 17-29). However, the crossover produces chromatids that contain a duplication and a deletion for different parts of the chromosome. In this case, if a gamete carrying a crossover chromosome is fertilized, the zygote dies because of gene imbalance. Again, the result is that only noncrossover chromatids are present in viable prog- eny. Hence, the RF value of genes within a pericentric inver- Paracentric inversions can lead to deletion products A Paracentric- inversion heterozygote A D CBE Pairing Crossover in loop sion also is zero. Inversions affect recombination in another way, too. Inver- sion heterozygotes often have mechanical pairing problems in the region of the inversion. The inversion loop causes a large distortion that can extend beyond the loop itself. This distor- tion reduces the opportunity for crossing over in the neigh- boring regions. Let us consider an example of the effects of an inversion on recombinant frequency. A wild-type Drosophila specimen from a natural population is crossed with a homozygous recessive laboratory stock dp en /dp cn. (The dp allele encodes dumpy wings and cn encodes cinnabar eyes. The two genes are known to be 45 map units apart on chromosome 2.) The F, generation is wild type. When an F, female is crossed with the recessive parent, the progeny are Segregation A B C DE A B C D E Acentric fragment (lost) D Dicentric bridge breaks randomly ++/ dp cn 246 dumpy cinnabar dp cn / dp cn 250 wild type ADCBE ADCB E 5 dumpy dp + / dp cn 7 cinnabar + en / dp cn ABC DE Normal product A Авср B Deletion product FIGURE 17-28 A crossover in the loop of a paracen- tric-inversion heterozygote gives rise to chromosomes Deletion product containing deletions. AD CBE LounchPad ANIMATED ART: Chromosome rearrangements: meiotic behavior of paracentric inversions Inversion product

Human Heredity: Principles and Issues (MindTap Course List)

11th Edition

ISBN:9781305251052

Author:Michael Cummings

Publisher:Michael Cummings

Chapter6: Cytogenetics: Karyotypes And Chromosome Aberrations

Section: Chapter Questions

Problem 3QP: A colleague e-mails you saying that she has identified an interesting chromosome variation at 21q13....

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