P generation crosses
female
YY
YY
YY
male
YY
yy
yy
yy
1. Female gametes are being provided by a
plant that has the dominant, yellow alleles
(YY); male gametes are being provided by
a plant that has the recessive, green alleles
(yy).
yy
P generation
Y
2. The cells of the pea plants that give rise to
gametes start to go through meiosis.
y y
Y Y
y
Y
female
gametes
y
male
gametes
y
y
Y
Yy
Yy
Y
Yy
Yy
Yy
Yy
possible
outcomes in
fertilization
Yy
Yy
3. The two alleles for pea color, which lie on
separate homologous chromosomes,
separate in meiosis, yielding gametes that
each bear a single allele for seed color. In
the female, each gamete bears a Y allele;
in the male, each bears a y allele.
4. The Punnett square shows the possible
combinations that can result when the male
and female gametes come together in the
moment of fertilization. (If you have trouble
reading the Punnett square, see Figure
11.5b). The single possible outcome in this
fertilization is a mixed genotype, Yy.
5. Because Y (yellow) is dominant over y
(green), the result is that all the offspring in
the F1 generation are yellow, because they
all contain a Y allele.
F1 generation
How to read a Punnett square
female gametes
p
P
p
pp
1. A p gamete from the male
combines with a p gamete from
the female to produce an
offspring of pp genotype (and
white color).
P
male gametes
male gametes
P
female gametes
p
P
p
Pp
2. A p gamete from the male
combines with a P gamete
from the female to produce
an offspring of Pp genotype
(and purple color).
David Krogh, A BRIEF GUIDE TO BIOLOGY, 1/e © 2007 Pearson Prentice Hall, Inc.
Figure 11.5