Exam 3: The Chromosome Theory of Inheritance

When setting up crosses to determine map distances,why do geneticists prefer to cross the hybrid individuals to individuals homozygous for the recessive alleles in the cross?

Which of the following was useful in the identification of the gene that causes cystic fibrosis (CF)?

Females heterozygous for the recessive second chromosome mutations pn,px,and sp are mated to a male homozygous for all three mutations.The offspring are as follows: pxspcn 1,461 pxsp+ 3,497 px+cn 1 px++ 11 +spcn 9 +sp+ 0 ++cn 3,482 +++ 10,000 Using the three-point cross method,determine which gene is in the middle (which has to be flipped).

Why does the formula for map distance between the two outside genes in a three-point testcross count the double crossovers twice whereas the formula for the distance between an outside gene and the middle gene counts the double crossovers only once?

Chromosomal interference is not uniform and may significantly vary within a region of a single chromosome.

In fruit flies,the recessive pr and cn mutations cause brown and bright-red eyes,respectively (wild-type flies have brick-red eyes).The double mutant pr cn combination has orange eyes.A female who has wild-type eyes is crossed to an orange-eyed male.Their progeny have the following distribution of eye colours: wild-type 8 brown 241 bright-red 239 orange 12 500 The mother of these progeny resulted from a cross between two flies from true breeding lines.What are the genotypes of these two lines?

In Drosophila,singed bristles (sn)and cut wings (ct)are both caused by recessive,X-linked alleles.The wild type alleles (sn⁺ and ct⁺)are responsible for straight bristles and intact wings,respectively.A female homozygous for sn and ct⁺ is crossed to a sn⁺ct male.The F₁ flies are interbred.The F₂ males are distributed as follows: snct 13 snc 36 sct 39 sc 12 What is the map distance between sn and ct?

Which of the following best describes a linkage map that contains a high density of genetic markers?

When considering the chi-square test,a p value of 0.05 is often considered significant but it is actually an arbitrary assignment of significance.

What use is genetic mapping? Give one example where it has been of value to society.