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602 Chapter 18 NEL

Test Crosses

Wool producers often prefer sheep with white wool, since black wool tends to be brittle and

difficult to dye. Black sheep can be avoided by breeding only homozygous white rams.

However, the allele for white wool (W)is dominant over the allele for black wool (w),so

white rams can be heterozygous. How could a wool producer be sure that a white ram is

homozygous?

For the cross shown in Figure 3, what is the probability that an offspring will have a

phenotype of wrinkled seeds? Express the answer as a percent.

Solution

Since the allele for wrinkled seeds, r, is recessive, only offspring with a genotype rrwill

have wrinkled seeds. From the Punnett square, 1 of every 4 offspring are expected to

have this genotype, so the probability that an offspring will have wrinkled seeds is 25 %.

SAMPLE exercise 2

Practice

1. What is the phenotypic ratio of the cross in the Punnett square shown in
   Figure 4?

2.Using a Punnett square, determine the expected phenotypic ratio and

genotypic ratio for the progeny of a cross between a pea plant that is

homozygous for the white allele (r) for flower colour and a pea plant that is

homozygous for the red allele (R).

Rr

round

seed

r

Rr

R

Rr

gametes

RR Rr

Rr rr

g a m e t e s

round

seed

Figure 3

A Punnett square showing the results

of a cross between two heterozygous

plants with round seeds

Figure 4

Punnett square of a monohybrid cross between a homozygous tall pea and a

homozygous short pea

Tt

Tt Tt

t Tt

t

T T

WEBActivity

Case Study—Creating a Personal Profile

Some human genes determine visible traits that show an inheritance pattern that is similar to

that of Mendel’s garden peas. As a result, you can predict a person’s genotype for these traits

just by observing him or her. In this activity, you will use a list of some common dominant and

recessive traits, and use this information to create a profile of your own phenotype and

potential genotype.

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Genetics

In this Virtual Biology Laboratory,

you can assess data and perform

simulated crosses to explain the

inheritance of shell colour in

glyptodonts, an extinct relative of

the armadillo.

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+ EXTENSION

F 2 Ratios

This animation shows some of the

results of Mendel's crosses, which

you can then convert to

phenotypic ratios. How close are

the observed phenotypic ratios to

the predicted phenotypic ratio?

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+ EXTENSION