Biology (Holt)

Determining Unknown Genotypes

Animal breeders, horticulturists, and others involved in breeding

organisms often need to know whether an organism with a dominant

phenotype is heterozygous or homozygous for a trait. How do they

determine this? For example, how might a horticulturist determine

whether a pea plant with a dominant phenotype, such as yellow

seeds, is homozygous (YY) or heterozygous (Yy)? The horticulturist

could perform a test cross. In a ,an individual whose

phenotype is dominant, but whose genotype is not known, is crossed

with a homozygous recessive individual.

For example, a plant with yellow seeds but of unknown genotype

(Y?) is test-crossed with a plant with green seeds (yy).If all of the off-

spring produce yellow seeds, the offspring must be Yy.Thus, the

genotype of the “unknown” plant must be YY.If half of the offspring

produce yellow seeds and half produce green seeds, the genotype of

the unknown plant must be Yy.In reality, if the cross produces even

one plant that produces green seeds, the genotype of the unknown

parent plant is likely to be heterozygous. After performing a test

cross, the horticulturist can continue breeding the original plant

with more certainty of its genotype.

test cross

172 CHAPTER 8Mendel and Heredity

0100010110
011101010
0010010001001
1100100100010
0000101001001
1101010100100
0101010010010

Analysis

1. Determinewhat the letters
   at the top and side of each
   box represent.


2. Determinewhat the letters
   in each box represent.
   3. Calculate the genotypic
   and phenotypic ratios that
   would be predicted if the
   parent of the unknown
   genotype were homozygous
   for the trait (Figure B).

4.Critical Thinking

Predicting Outcomes

If half of the offspring have

white flowers, what is the

genotype of the plant with

purple flowers?

P

p

p

p P

p

p

P

Figure AHeterozygous (Pp) plant Figure BHomozygous (PP) plant

Is this purple

flowering pea

plant Ppor PP?

Analyzing a Test Cross

Background

You can use a test cross to determine whether a plant with pur-

ple flowers is heterozygous (Pp) or homozygous dominant

(PP). On a separate sheet of paper, copy the two Punnett

squares shown below, and fill in the boxes in each square.

2C 6D

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Topic: Breeding Texas

Livestock

Keyword: HXX4003