A gene controls the production of chlorophyll in lettuce plants. The recessive allele of this gene blocks chlorophyll formation which prevents seedling development. Plants heterozygous for this gene were cross bred and 1000 of their seeds were planted. Only about 750 developed into mature plants. What percentage of these mature plants would be heterozygous for this gene?

Answer:

10) D

Let ‘T’ be the dominant allele and let ‘t’ be the recessive allele.

                              T                                  t

                T          TT                               Tt

                t            Tt                                 tt

We know that recessive allele prevents seed development, which is why all seeds which had ‘tt'(1/4×1000 = 250) are dead. The remaining 750 seeds developed.
Out of these 750 seeds, some have ‘TT’ whilst others have ‘Tt’

From the table, we can see that 2 out of three plants are Tt whilst 1/3 are TT.

Number of heterozygous plants in the 750 mature plants = 2/3 = 66.666..% ≈ 67%