Question

In Mirabilis jalapa plant's cross (incomplete dominance) What is the probability of getting 2 red (RR) 2 pink (Rr) and 1 white (rr) ?

Answer 1

Incomplete dominance is when a dominant allele, or form of a gene, does not completely mask the effects of a recessive allele, and the organism’s resulting physical appearance shows a blending of both alleles. It is also called semi-dominance or partial dominance. One example is shown in roses. The allele for red color is dominant over the allele for white color, but heterozygous roses, which have both alleles, are pink. Note that this is different from codominance, which is when both alleles are expressed at the same time.
Many genes show complete dominance. This means that if an individual is heterozygous for a particular gene, the dominant allele will completely mask the recessive allele. Many of the properties that the Austrian monk Gregor Mendel studied in his famous pea plants were controlled by genes that showed complete dominance. For example, the dominant flower color was purple, and the recessive color was white. Plants that were heterozygous were also purple, since purple was the dominant allele, even though they also had the white allele. A plant only had white flowers if it was homozygous for the recessive allele, which means that it had two copies of that allele. (This is also why two purple plants sometimes produced white ones; a proportion of the offspring received two recessive alleles.)

Why does incomplete dominance occur? As we have seen, it does not always occur with flower color; roses (and tulips, carnations, and snapdragons, among others) show incomplete dominance, but Mendel’s pea plants showed complete dominance. Incomplete dominance can occur because neither of the two alleles is fully dominant over the other, or because the dominant allele does not fully dominate the recessive allele. This results in a phenotype that is different from both the dominant and recessive alleles, and appears to be a mixture of both.