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NEL The Basis of Heredity 603

Section18.2

A test crossis the cross of an individual of unknown genotype to an individual with

a recessive genotype. The phenotypes of the F 1 generation of a test cross reveal whether

an individual with a dominant trait (such as a white ram) is homozygous or heterozygous

for the dominant allele. If a white ram is crossed with a black ewe and the observed phe-

notypic ratio is 1:1 black to white, then the genotype of the ram must be Ww(Figure 5).

If all the offspring are white, then the genotype of the white ram must be WW.

Test crosses are the simplest way of determining the genotype of an individual.

Sometimes, however, the parents are not available to test cross. When only information

about the phenotypes of the offspring of a cross is available, the genotypes and pheno-

types of the parents can be found by working backwards through a Punnett square.

Half of the offspring are

black and half are white.

All of the offspring

are white.

Ww Ww

Ww Ww

W W

w

w

W? × ww W? × ww

Ww ww

Ww ww

W w

w

w Figure 5

A test cross is a way of determining

if an individual with the dominant

trait is heterozygous or

homozygous.

A horticulture worker has seeds from a particular cross, but has no information about the

genotype or the phenotype of the parents. He plants and grows the offspring, and records the

traits of each offspring (Table 1). What was the genotype and phenotype of the parent plants?

Solution

Determine the observed phenotypic ratio of the progeny, rounding off if needed.



w

r

r

o

in

u

k

n

l

d

ed



List the possible genotypes for each phenotype, as shown in Table 2.

3



1

5472



1850

SAMPLE exercise 3

Table 2

Phenotype Genotype

round-seed peas RR orRr

wrinkled-seed peas rr

Table 1

Offspring phenotype Numbers

round-seed peas 5472

wrinkled-seed peas 1850

test crossthe cross of an

individual of unknown genotype to

an individual that is fully recessive

Factors that Contribute to

Genetic Variation

In this Audio Clip, you will hear

about the underlying mechanisms

that create genetic variation in the

offspring of sexually reproducing

individuals.

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