Mendel's explanation of the monohybrid cross:

The Laws of Dominance & Segregation

Mendel
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. Mendel started with two true-breeding lines of
plants in the **P** (**Parental**) generation, with
either **yellow **or **green **seed coats. 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). Their offspring (the
**first filial **or F_{1}
generation of plants) receive one Y or one y
factor from either parent, and are thus uniformly Yy. Since these **Yy **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. Classically, **Y** "*masks*"
**y**.

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.

The probability of two independent events occurring together is the product of their independent occurrence (the "and" rule). Then, when two F_{1}
**Yy **plants are crossed, the probability that any
second-generation F_{2}
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 parent and y from the male parent, 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 results and ratios were obtained for seven separate traits in peas, including seed colour and shape, pod shape, and plant form.

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.

The probability of two independent events occurring together is the product of their independent occurrence (the "and" rule). Then, when two F

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 parent and y from the male parent, or y from the female and Y from the male:

Finally, the probability of a plant being either YY or Yy is

Mendel showed that these results and 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 ©2016 by Steven
M. Carr