Learning Objectives:

Know the following terms:

Dominant
Recessive
Hybrid

Know why a cross between two hybrids can give rise to two kinds of offspring

I gave an example of a population of living organisms with two discrete traits, like seed color in pea plants. Some pea plants have green seeds, some plants have pea yellow seeds.

Compare what would happen if I took out all the green seeded plants to what would happen if I took out all the yellow seeded plants. The real issue is what happens if I cross two yellow seeded plants vs what happens if I cross two green seeded plants. This kind of thing puzzled plant breeders for a long time.

The observation has always been that green seeded plants always produced green seeded offspring, while yellow seeded plants produced either 1) all yellow seeded offspring or 2) 3/4 yellow seeded and 1/4 green seeded; although until Mendel came along, no-one actually counted the offspring to observe the ratios. The question become; why does this happen.

animal and plant breeders have always known that if you cross two pure lines (lines that breed true; cross individuals within those lines and the offspring are always the same) you get offspring called HYBRIDS. Hybrids have the genetic basis of each of the traits of the parents. Cross a pure line of green seeded plants with a pure line of yellow seeded plants and you get a hybrid that has yellow seeds. Because the hybrid has yellow seeded, the yellow seed color trait is DOMINANT to the green seed color trait, which is RECESSIVE, because the green seed color trait does not appear in the hybrid. Cross two of the yellow seeded hybrids, and you get 3/4 yellow seeded offspring and 1/4 green seeded offspring.

Thus if a trait is dominant, a cross of two hybrids with the dominant trait can produce offspring with the recessive trait. So if we take out all the green seeded plants, we still have yellow seeded plants that are hybrids, and crosses of two of the hybrids can again produce green seeded plants. However, if all the yellow seeded plants are removed, none of the green seeded plants that are left can be hybrids, and the population will breed true for the green seeded trait.

There is a genetic explanation for this as well. Sexually reproducing diploid organisms, like plants and like humans, each get one copy of each gene from each parent. The hybrid yellow plant got the green copy of the color gene from it's green seeded parent and the yellow copy of the color gene (same gene, different copy as the green) from its yellow seeded parent. The two copies of the color gene in the hybrid were :

yellow/green

The parents were from pure lines. The copies of the color gene for the green seeded plant were:

green/green

The copies of the color gene for the yellow seeded parent were:

yellow/yellow.

When the green and yellow seeded parent reproduced, the produced gametes with one copy of each gene. All the gametes from the yellow seeded parent had the yellow copy of the color gene and all the gametes of the green seeded parent had the green copy of the color gene. Thus the hybrid gets a copy of each. Because the yellow shows up, the yellow is dominant.

Now, if we cross two hybrids, each having the yellow copy and the green copy of the color gene, there is a chance that an offspring will get the green copy of the gene from each parent. The chance is actually 1/4.

The chance is 1/4 because for each parent in a hybrid cross, 1/2 the gametes will have the yellow copy and 1/2 the gametes will have the green copy. 1/2 X 1/2 = 1/4.

The only way to be green seeded is for both copies of the color gene to be the green copy. Thus when you cross green seeded plants you only get green seeded plants.

If you have only green seeded plants in a population, you can get nothing but green seeded offspring.

If you have nothing but yellow seeded plants, you can get green seeded offspring if some of the yellow seeded plants are hybrids.