Biology Now, 2e

122 ■ CHAPTER 07 Patterns of Inheritance

GENETICSGENETICS

shows all possible ways that two alleles can be

brought together through fertilization. To create

a Punnett square showing how a trait is inher-

ited, list the alleles of the male genotype across

the top of the grid, writing each unique allele

just once. List the alleles of the female genotype

along the left edge of the grid, again writing each

unique allele only once. In the case of Mendel’s

cross of the F 1 generation, the male genotype

(Pp) is mated with the female genotype (also Pp).

Next, fill in each box (or “cell”) in the grid

by combining the male allele at the top of each

column with the female allele listed at the begin-

ning of each row. The Punnett square shows all

four ways in which the two alleles in the sperm

can combine with the two alleles found in the

eggs. The four genotypes shown within the

Punnett square are all equally likely outcomes

of this cross.

Using the Punnett square method, we can

predict that ¼ of the F 2 generation is likely to

have genotype PP, ½ to have genotype Pp, and

¼ to have genotype pp. Because the allele for

purple flowers (P) is dominant, plants with

PP or Pp genotypes have purple flowers, while

plants with pp genotypes have white flowers.

Therefore, we predict that ¾ (75 percent) of the

F 2 generation will have purple flowers and ¼ (25

percent) will have white flowers—a 3:1 (¾:¼)

ratio of phenotypes. This prediction is very close

to the actual results that Mendel obtained. Of a

total 929 F 2 plants that Mendel raised, 705 (76

percent) had purple flowers and 224 (24 percent)

had white flowers.

Results like these supported Mendel’s first

law, the law of segregation, which, in modern

terms (Mendel did not know about DNA),

states that the two alleles of a gene are sepa-

rated during meiosis, the specialized type of

cell division during sexual reproduction that

was discussed in Chapter 6, and end up in

different gametes—egg or sperm cells. One

of the two alleles is found on one of the chro-

mosomes in a homologous pair, and the other

allele is found on the other chromosome in the

pair (see Figure 7.2). Remember, homologous

chromosome pairs are partitioned into separate

daughter cells during meiosis I. Mendel’s law of

segregation can be used to predict how a single

trait will be inherited.

You can try this on your own by making a

Punnett square to predict the ratio of black

The F 1 offspring of

PP × pp plants all

have genotype Pp.

Egg and sperm can

combine in four

possible ways in

the F 2 generation.

The Punnett square

method predicts 3

purple-flowered

offspring for every

1 white-flowered

offspring, a 3:1 ratio.

Each egg and each

sperm produced by

the F 1 plants has a

50% chance of

receiving a P allele

and a 50% chance

of receiving a

p allele.

×

Gametes

Sperm

Eggs

Pp

PP Pp

Pp pp

P p P

P

p

p

P p

Pp

Punnett square

F 1 generation

F 2 generation

Figure 7.5

Punnett squares predict the offspring of genetic crosses

Q1: Why did Mendel’s entire F 1 generation look the same?

Q2: The phenotype ratio in the F 2 generation is 3:1 purple-to-white

flowers. What is the genotype ratio?

Q3: Draw a Punnett square for a genetic cross of two heterozygous,

black-coated dogs. What are the phenotype and genotype ratios of

their offspring?

in which one parent bred true for purple flowers

(PP) and the other bred true for white flowers

(pp). The first generation of offspring of a genetic

cross is called the F 1 generation (“F” is for “filial,”

a word that refers to a son or daughter). When

the individuals of the F 1 generation are crossed

with each other, the resulting offspring are said

to belong to the F 2 generation. Mendel allowed

the F 1 -generation pea plants to self-fertilize to

produce the F 2 generation.

We can predict the results of an experimen-

tal cross by using a grid-like diagram called a

Punnett square (Figure 7.5). A Punnett square