Question

2.) A female poodle with green eyes (recessive) and yellow spots on her tongue (recessive) was bred with a male poodle who was the wild-type phenotype but hybrid genotype for those same two traits. In a series of litters, this breeding pair gave birth to the following 85 puppies: 39 Normal eyes, normal tongue 36 Green eyes, yellow-spot tongue 6 Green eyes, normal tongue 4 Normal eyes, yellow-spot tongue a.) If these two genes were unlinked, given the way the cross was constructed, what ratio would you expect to see among the four categories of offspring shown here? b.) Which categories of dogs are "parental types" and which are recombinant types? c.) Are these two genes linked? If so, how far are they from each other (in CM)? d.) Would a non-crossover (NCO) and a double-crossover (DCO) event look different from each other in this cross? Why or why not? e.) If these two genes were very tightly linked, just 0.1 CM away from each other, what is the most likely outcome of these 85 puppies in terms of the categories above?

Answer 1

(a) Let, the genotype for green eye (recessive) is gg.

      the genotype for yellow spots on tongue (recessive) is yy

              the genotype for wild type (hybrid) is Gg.

      the genotype for yellow spots on tongue (hybrid) is Yy .

Therefore, the genotype of female poodle is ggyy and the genotype of male poodle is GgYy.

Male gametes: GY, Gy, gY and gy.

Female gametes: gy.

The cross over results in Normal eyes, normal tongue = GgYy

                                 Green eyes, yellow-spot tongue = ggyy

                                 Green eyes, normal tongue = ggYy

                                 Normal eyes, yellow-spot tongue = Ggyy

   GgYy : ggyy : ggYy : Ggyy = 1 : 1 : 1 : 1

(b) In linkage, the offspring contain higher number of parental genotype than the recombinants.

       So, the parental types are Normal eyes, normal tongue(39) and Green eyes, yellow-spot tongue (36)

          and the recombinants are Green eyes, normal tongue (6) and Normal eyes, yellow-spot tongue(4).

(c) Yes, these two genes are linked.

     Map distance = Number of recombinant offspring / Total number of offspring * 100

                              = 10 / 85 * 100

                              = 11.76 cM

The distance betweeen the two genes is 11.76 cM.

(d) A non-cross over (NCO) and a double- cross over (DCO) event look similar.

    Because in Non-cross over let the gene G and g are close to each other, after one crossing over g is replaced with y and with one more crossing over, g is again placed towards G.

(e) Map distance = Number of recombinant offspring / Total number of offspring * 100

                         0.1 = Number of recombinant off spring / 85 * 100

        Number of recombinant offspring = 0.1 * 85 / 100

                                                                    = 8.5 / 100

                                                                   = 0.085

So, the value suggests that it will be very difficult for the genes to separate from each other during crossing over. Thus the offspring will be

43Normal eyes, normal tongue

42Green eyes, yellow-spot tongue

0Green eyes, normal tongue

0Normal eyes, yellow-spot tongue

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