Worksheet for problem set #7 February 27 2006

 

1.

a) how are these traits inherited?

Treat each trait separately- add up the proportions for eye color: rosy and red. What are the observed proportions of offspring for each eye color?

 

F2 ratio of _________= predictable ____________________ dominant:recessive.

 

Therefore, the trait ____________ eyes is ___________________ recessive.

Do the same for body color.

 

F2 ratio of _________= ____________________.

 

Therefore, the trait ___________________ body color is ___________________ recessive.

 

Note: with a sex-linked F2 cross, if the male carries the recessive trait, expect a ratio of

 

________________; if the male carries the dominant trait, expect a ratio of

 

__________.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2.

Testcross: an individual of unknown genotype is crossed with another individual with a homozygous recessive genotype for the trait in question.

Three-point testcross: a cross between an individual heterozygous at three linked loci with an individual homozygous recessive at the three loci.

With a three-point cross, the order of the three genes can be determined, plus double crossovers (DCO) can be detected, allowing the calculation of map distances.

 

Parental genotypes always have the _____________ progeny (nonrecombinant progeny), DCOs always have the _____________.

 

figure 7.12 here

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Note that in the DCOs, the outer two alleles are the same as in the

 

____________________, but the middle allele is different. From this we can figure the order of the genes.

 

In a triple cross, two kinds of progeny are produced: those that are heterozygous,

 

displaying the _____________ trait, and those that are homozygous, displaying

 

the ________________ trait (you get these two classes for each locus or trait- so that you expect 23=8 classes of progeny.

 

Shortcut: usually, the testcross progeny are not represented by the complete genotype, e.g.

e+z      rather, we list only the alleles expressed in the phenotype, e.g.  e+z

eyz

 

To make a map of the genes, we need information about where and how often crossing over occurs. The info that we need for mapping comes entirely from the

 

_________________ parent (the _______________ parent may cross over, but since the alleles are the same, we don’t gain any relevant information from this).

 

Map distances are based on the ______________________________. Add up the number of recombinant progeny (including both SCO and DCO), divide by the total number of progeny, and multiply by 100.