## "S" PROBLEM SOLUTIONS : S-5 to S-7

Pr. S-5: Drawing the Results of Crossing-over:

Your diagrams should be the "anaphase I" equivalent of the textbook diagrams 5.3 a & b and 5.12 a & d, which show the bivalents in meiotic metaphase I and the individual single-chromatid chromosomes in the gametes at the end of meiosis II. For clarity, you should always show all alleles by actual letters rather than by using "+" signs. That is, indicate the alleles of genes alpha. beta, and gamma as A and a, B and b, G and g.

Pr. S-6: Pea Chromosome #1 Map:

1. The "beta-delta" cross gives a recombination frequency of 2%, therefore gene beta is 2 cM to the left of gene delta.

2. Gene alpha is 3 cM from delta and 5 cM from beta, so gene alpha must be 3 cM to the right of gene delta.

3. Gene gamma is about 15 cM from alpha and about 20 cM from beta, so gene gamma is apporximately 15 cM to the right of gene alpha.

4. Gene epsilon is far away, but closer to gene gamma than to gene alpha. An exact cM distance cannot be determined from the data. What you can conclude is that the alpha-epsilon distance is greater than 50cM.

So, the map of pea chromosome #1 looks something like this:

. . . . beta..delta....alpha..........................gamma..........................................................epsilon. . . . . . .

Pr. S-7: Human Genome Recombination Map:

From the data presented in Table 1 of Kong et al., 2002:

a. The smallest human autosome is chromosome #21, which is 30 million base pairs long.
( Chromosome #22 is about the same size, at 31 million base pairs.)

b. The genetic length of human chromosome #21 was measured at about 50cM in an earlier study and at about 62 cM in this study.

c. The "sex-averaged recombination rate" is listed as about 2 cM per million base pairs. So, 1 cM is about a half million base pairs. Thus, two genes alpha and beta showing a 1% recombination frequency between them are about 500,000 base pairs apart.

d. From Problem S-1, an average size human gene is about 15,000 base pairs. So, if genes alpha and beta are average size and 1 cM apart, the distance between the two genes is about 33 times the size of each gene, as shown below (not to exact scale).

........ alpha ................................................................................................. beta ..........

e. For all of our autosomes, the "female" genetic length is greater than the "male" genetic length. Obviously, the physical lengths of the autosomes do not differ by gender. So, this means that the recombination frequency is greater in women than it is in men. That is, crossing-over must happen more readily in female cells going through meiosis (i.e., oogenesis) than it does in male cells going through meiosis (spermatogenesis).

f. For "recombination rate" defined as "cM per million base pairs", the number varies between about 1 and 2 for our autosomes. For the 12 largest chromosomes (#1 through #12), the number is pretty close to one.

For almost all of our chromosomes, the sex-averaged total genetic length is well over 100 cM, and even over 200cM for the largest ones. This means that crossing-over occurs at least once in essentially all of the bivalents that form in prophase I in all human meiotic divisions.