Chapter 4 Quiz

"A sweet pea plant of genotype A/A . B/B is crossed to one that is a/a . b/b to produce a dihybrid F1. One cell of one F1 individual (genotype A/a . B/b) goes through meiosis and produces four gametes of the following genotypes: A . b, A . b, a .B and a .B. What can be concluded regarding the linkage relationship between the two genes?"

"Genes A and B could be linked or unlinked; the result is not indicative of either, because a single meiosis could produce 100% recombinant gametes in either case."

"A diploid plant is a trihybrid for flower color (gene ÒFÓ), leaf size (gene ÒLÓ) and seed weight (gene ÒSÓ); its phenotype includes red flowers, large leaves, and heavy seeds. This plant is crossed to a tester plant (which has white flowers, small leaves, and light seeds). The progeny is as follows:" What can be concluded about the linkage relationships of the three genes in question?

"Genes F and S are linked, while L assorts independently"

"In Drosophila, the genes crossveinless-c and Stubble are linked, about 7 map units apart on chromosome 3. cv-c is a recessive mutant allele of crossveinless-c (cv-c+ is wild type), while Sb is a dominant mutant allele of Stubble (Sb+ is wild type). A dihybrid female Drosophila with genotype cv-c Sb+/cv-c+Sb is testcrossed (to cv-c Sb+/ cv-c Sb+). The proportion of phenotypically wild-type individuals in the progeny of the testcross will be:"

0.035

"The maize genes sh and bz are linked, 40 map units apart. If a plant sh+bz/sh bz+ is selfed, what proportion of the progeny will be sh bz/sh bz?"

0.04

In one short chromosome arm the average frequency of crossovers per meiosis was measured to be 0.5. What proportion of meioses will have no crossovers at all in this region?

0.6

"In a cross of A B X a b (the loci are linked) in a meiosis in which there has been a four-strand double crossover between the loci, the percent of gametes (from that single meiocyte) that will be recombinant for A and B will be:"

100%

The molecular process which results in the formation of recombinant chromosomes is initiated by:

a double-stranded break in one of the chromatids

"A plant of genotype C/C ; d/d is crossed to c/c ; D/D, and the F1 is testcrossed (to c/c ; d/d). If the genes in question are unlinked, the percentage of double homozygous recessive individuals in the offspring of the testcross will be:"

about 25%