A wild population contains tall and short plants. In one cross, two parental tal

Question

A wild population contains tall and short plants. In one cross, two parental tall plants yield all tall progeny. In another experiment, two short plants are crossed and all progeny are short (cross 2). In the third experiment (cross number 3), two different short plants from cross 2 are crossed and 23 tall progeny and 77 short progeny are observed. Explain the observations, give the most probable genotypes of the parents and progeny of each cross, and identify which are dominant and recessive.

Cross 1.

Cross 2.

Cross 3.

Explanation / Answer

In cross 3, we obtain 77 short progeny and 23 tall progengy. So 77/(23+77) 75% of the plants have the short phenotype and the other 25% have the tall phenotype. This 3:1 phenotypic ratio is characteristic of a monohybrid cross, where the parents are heterozygous for a certain allele. Thus, the parents or the cross 3 progengy are heterozygotes. Since these parents were also short plants, this indicates that the allele coding for shortness must be dominant to the allele coding for tallness. Now let's look at each of the individual crosses to determine the most probable genotypes of both parents and offspring.

In cross 1, two tall plants are crossed to yield all tall plants. This is consistent with the fact that the allele for tallness is recessive. Both parents and all of the progeny must be homozygous recessive.

In cross 2, two short plants are crossed to yield all short plants. This too is consistent with the fact that the allele for shortness in dominant. Since the progeny of this cross are the parents of the cross 3 progeny, the cross 2 progeny must be heterozygous as explained before. Furthermore, since all of the cross 2 progeny are short plants some of which we know to be heterozygotes, then at least one parent must be heterozygous. We cannot know for sure whether the other parent is homozygous dominate or heterozygous. But it is more likely that one parent is heterozygous and the other homozygous dominant, since this ensures that no offspring express the recessive phenotype, tallness.

In cross 3, the parents are heterozygotes as already discussed. The genotypic ratio of a monohybrid cross is 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive. So all 23 of the tall progeny of cross 3 are homozygous recessive. Of the other 77 short progeny, about 1/3 of them are homozygous dominant and the other 2/3 are heterozygous.

So in conclusion:

S=shortness (dominant)

s=tallness (recessive)

cross 1 - both parents = ss

              all progeny = ss

cross 2 - parent 1 =Ss

             parent 2 = Ss or SS (most likely SS)

             progeny = If parent 2 is Ss, 1/3 are SS and 2/3 are Ss.

                            If parent 2 is SS (most likely) 1/2 are Ss and 1/2 are SS

cross 3 - both parents = Ss.

             progeny = 23 ss, ~(2/3)*77 Ss, ~(1/3)*77 SS.

Leave a comment if you have any questions.

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