The role of chance

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The Role of Chance

Now we come to a key point in the study of genetics: The way the chromosomes segregate during meiosis is entirely random. It is impossible to predict which genes will migrate to which cells.

Let’s complete meiosis by dividing these two interim daughter cells once again. (Note: 'daughter' doesn't imply the sex of the gamete, it is simply a term for the products of cell division.)

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ch11b.gif (12776 bytes)

These are the possible combinations of chromosomes in the spores. Because the spores divide by mitosis, these combinations will be passed down to the daughter cells, the gametes.

Remember that the production of egg and sperm takes place in different parts of the plant body, so each can have any one of the combinations above. Now suppose fertilization takes place, and egg and sperm are united. Again, chance plays a role.

Which particular egg and which particular sperm unite during fertilization is also entirely random.

There is no way to predict which reproductive cells will unite.

There are nine possible combinations of chromosomes in a zygote resulting from union of any two of these egg and sperm. Here are the nine, along with which expression of the traits they’ll exhibit:

	Seed Color	Seed Coat
YYRR	yellow	round
YYRr	yellow	round
YYrr	yellow	wrinkled
YyRR	yellow	round
YyRr	yellow	round
Yyrr	yellow	wrinkled
yyRR	green	round
yyRr	green	round
yyrr	green	wrinkled

You can see that eight out of the nine will express one or both dominant traits. Which of the combinations of genes results in a plant that exhibits the recessive traits, green seeds and wrinkled seed coat? Only the last one, yyrr.

Brown or Blue?
This information is applicable not only to plants, but to human traits as well. For example, brown eye color is a dominant trait (B), and blue is recessive (b). There are 3 possible combinations: BB, Bb, and bb. Anyone with the combination BB or Bb will have brown eyes. Only people with bb will have blue eyes. Can two blue-eyed parents have a brown-eyed child? No, because the dominant trait, B, is not present.

There are also a number of human diseases that are the expression of recessive genes. Huntington’s disease and sickle cell anemia are two examples. If a person has one dominant and one recessive gene for either of these diseases, they are "carriers" of the disease but will show no symptoms. Only people with two recessive genes will have the diseases. If two carriers mate, there is a one in four chance that their offspring will have two recessive genes, and therefore have the disease.