Solutions For An Introduction to Genetic Analysis Chapter 17 Problem 63P

A plant homozygous for all the 5 recessive alleles is crossed with a wild type plant, which is homozygous for all the five dominant alleles. The F₁ progeny express the dominant alleles. When the F₁ is backcrossed to the parent with the recessive alleles, the progeny details are observed:

• Colored, green, starchy, smooth, tall – 360

• White, bronze, waxy, shrunk, dwarf – 355

• Colored, bronze, waxy, shrunk, dwarf – 40

• White, green, starchy, smooth, tall – 46

• Colored, green, starchy, smooth, dwarf – 85

• White, bronze, waxy, shrunk, tall – 84

• Colored, bronze, waxy, shrunk, tall – 8

• White, green, starchy, smooth, dwarf – 9

• Colored, green, waxy, smooth, tall – 7

• White, bronze, starchy, shrunk, dwarf – 6

The details of cross are as follows:

Parents: c bz wx sh d/c bz wx sh d x C Bz Wx Sh D/C Bz Wx Sh D

The F₁ progeny: C Bz Wx Sh D/ c bz wx sh d

The backcross: C Bz Wx Sh D/ c bz wx sh d x c bz wx sh d/c bz wx sh d

a) We find out the number of recombinants per pair of genes:

• C-Bz:

The number of plants which show a phenotype other than colored aleurone with green leaves and white aleurone with bronze leaves are treated as recombinants. Recombinants occur only after crossovers occur.

Hence, the number of crossovers for this region would be:

=40+46+8+9

=103.

The distance in m.u. observed is 10.3 while the value given in table is 12.

• Bz-Wx:

The number of plants which show a phenotype other than green leaves with starchy seeds and bronze leaves with waxy seeds are treated as recombinants. Recombinants occur only after crossovers occur.

Hence, the number of crossovers for this region would be:

=7+6

=13.

The distance in m.u. observed is 1.3 while the value given in table is 8.

• Wx-Sh:

The number of plants which show a phenotype other than starchy and smooth seeds and waxy and shrunken seeds are treated as recombinants. Recombinants occur only after crossovers occur.

Hence, the number of crossovers for this region would be:

=7+6

=13.

The distance in m.u. observed is 1.3 while the value given in table is 10.

• Sh-D:

The number of plants which show a phenotype other than tall plants with smooth seeds and dwarf plants with shrunken seeds are treated as recombinants. Recombinants occur only after crossovers occur.

Hence, the number of crossovers for this region would be:

=85+84+8+9

=186.

The distance in m.u. observed is 18.6 while the value given in table is 20.

We see a reduction in the map units for two of the above cases. This suggests an inversion. The inversion most likely involves the Bz, Wx and Sh genes.

b) With the above different loci, more number of possible classes is missing. They are:

• Four single-crossover classes resulting from crossover in inverted region

• Eight double-crossover classes involving the inverted and non-inverted region

• Triple crossovers and higher classes

The 10 classes shown are the only ones viable. These classes involved a single crossover outside the inverted region or a double crossover within the inverted region.

c) The origin of each class with its frequency is given below:

Class 1 – parental – the frequency increased due to non-viability of some crossovers

Class 2 – parental – the frequency increased due to non-viability of some crossovers

Class 3 – crossing over between C and Bz - Expected frequency

Class 4 – crossing over between C and Bz – expected frequency

Class 5 – crossing over between Sh and D – expected frequency

Class 6 - crossing over between Sh and D – expected frequency

Class 7 – double crossover between C and Bz and between Sh and D –expected frequency

Class 8 – double crossover between C and Bz and between Sh and D –expected frequency

Class 9 - double crossover between Bz and Wx and between Wx and Sh – expected frequency

Class 10 – double crossover between Bz and Wx and between Wx and Sh – expected frequency

d) One test for my hypothesis could be cytological verification. By observing chromosomes at meiotic pairing, we can find out whether the hypothesis holds true or not. Genetically, these genes of wild type could be mapped and the altered relationships observed.