Mendel's explanation of the
The Laws of Dominance & Segregation
hypothesized that seed colour and other phenotypic traits
are controlled by alternative forms of an inherited "element" (which we now
call a gene) in the parental
plants. Each plant receives one factor (now
called an allele)
from each parent. The yellow plants have two copies of the "yellow"
factor (Y) and the
green plants two copies of the "green" factor (y). The F1 plants
receive one Y or one y factor from either
parent, and are thus Yy. Since the plants are
yellow like the YY
parent, the Y factor can be said to dominate the y factor in determining
the appearance of the seeds. That is, the heterozygous Yy combination resembles the
homozygous YY combination,
rather than the homozygous yy combination.
In gamete formation, the factors segregate (separate),
so that each gamete carries only one or the other. One-half
(1/2) of the female gametes are Y, 1/2 are y,
and the same is true for the male gametes.
of two independent
events occurring together is the product of their
independent occurrence (the "and" rule). Then, the
probability that any F2
plant will receive a Y
from the female and a
Y from the male is
(1/2) x (1/2) = 1/4, and the probability of a y from both parents is
also (1/2) x (1/2) = 1/4.
The probability of two alternative events
occurring together is the sum
of their separate occurrence (the "or" rule). Then, there are two ways of
obtaining a Yy
plant: either Y from the female and y from the male, or y from the female and Y from the male: (1/2) x
(1/2) + (1/2) x (1/2) = 1/2
Finally, the probability of a plant being
either YY or Yy is 1/4 + 1/2 = 3/4,
and the probability of yy
remains 1/4. The expected ratio of yellow ("Y-") to green ("yy") plants is 3/4 : 1/4 or 3:1, as observed.
Mendel showed that these ratios were
obtained for seven separate traits in peas, including seed
colour and shape, pod shape, and plant form.
Figure ©2002 by Griffiths et al.; all text material ©2013 by Steven