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Section 1

1. A dihybrid cross of two plants is done and the seed pod shape is examined. The seed stock was pure one always made triangular pods and the other always made ovoid shaped pods. The pod shape is known to be controlled by two independently assorting genes. The resulting F2 progeny has 6% ovoid shaped pods and 56% triangular shaped pods and two new pod shapes which are present equally at 19%. Suggest a hypothesis that is consistent with these data. Support the hypothesis and demonstrate how other hypothesizes are not consistent with the data.

2. One of the X chromosomes in a particular Drosophila female had a normal gene order  (yvfB) for the recessive alleles yellow body color (y), vermillion eyes (v) and forked bristles (f) as well as the dominate Bar eye mutation (B). Her other X chromosome carried wild type alleles of all four genes but the region including the y+, v+ and f+ was inverted (fvy  but not the B+). This female is crossed to a wild male. The table below (see Table 1) shows the resulting male offspring from this cross. describe why there are no male offspring with the allele combinations: yvf+, y+v+f, yv+f+ or y+vf.

Table X-linked Drosophila cross 114 f1 males scored

genotype            # male offspring
yvf B                           48
y+v+f+B+                    45
yvf B+                        11
y+v+f+B                      8
y+vf+B+                      1
yv+fB                          1        

A Hfr strain of E. coli is mated with an F- strain of E. coli. Neither the Hfr strain nor the recipient strain could ferment lactose. The Hfr bacteria had a mutation that made the lac promoter non-functional and the recipient strain has mutant lacZ gene. The mating is interrupted at 10, 20 and 30 minutes. At 10 minutes no lac+ colonies are isolated but at 20 minutes out of 100 leu+ cells there are 5 lac+ and at 30 minutes there are 12 lac+ colonies. Suggest a hypothesis that is consistent with these data. Support the hypothesis and demonstrate how other hypothesizes are not consistent with the data.

4. Assume you have a calico female cat that has mated numerous times with your black male cat. The litters had orange and black males as well as calico and black females. All of these offspring are equally probable (expected ratio 1:1:1:1) because the color genes are X-linked and the sex ratio is 50%. Because you are aware of this you always assume orange kittens are male. However one of the orange kittens you placed with a friend has kittens. Discuss the possible genetic explanations. You may assume no orange males were able to mate with your calico cat.

Section 2

1. The experiments used to produce the data present here were done with 6 species of Brassica plants: B. carinata, B. naptus, B. nigra, B. oleracea, B. campestris. The karyotype information for some members of this family and several crosses is compiled in Table 2. Using the data in second Table below support or refute the hypothesis that B. juncea is a tetraploid plant. If you refute this hypothesis suggest and defend a different ploidy for B. juncea.

2. The data in third Table below is from a drosophila cross between a triply heterozygous female (sc+, cw+, e+/sc, cw, e) and a homozygous recessive male (sc, cw, e/sc, cw, e). The map in Figure shows a model of the gene order and arrangement in the female. Defend or refute the map (model) based on the data. If you refute the map, then suggest a map that is consistent with the data.

Table chromosome numbers and pairing in Brassica crosses
Species or f1 cross    Chromosome number    Number of chromosome pairs    Number of unpaired chromosomes
B. juncea                               36                                  18                                          0
B. carinata                             34                                  17                                          0
B. naptus                               38                                  19                                          0
B. juncea X B. nigra                 26                                   8                                          10
B. naptus X B. campestris         29                                  10                                          9
B. carinata X B. oleracea          26                                   9                                           8
B. juncea X B. oleracea            27                                   0                                          27
B. carinata X B. campestris       27                                   0                                          27
B. naptus X B. nigra                27                                    0                                          27

Table  Drosophila Cross data 1000 f1 progeny scored for 3 linked loci

Trait                                                              number
Wild type(+, +, +)                                               35
Scarlet eyes(sc)                                                455
Ebony body (e)                                                   8
Curly wings(cw) and ebony body                          465
Curly wings and scarlet eyes                                 7
Scarlet eyes, curly wings and ebony body               30

Figure : Genetic map for sc, cw and e genes

  + 7.3cM     cw  1.5cM      e

        sc           +              +

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  • Category:- Biology
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