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Chapter 11
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Introduction to Genetics
Solving Heredity Problems
You may want to refer students to Chapter 11 in
the textbook before performing this investigation.
Time required: 40 minutes
Introduction
Inheritable characteristics of organisms are passed from parents to
offspring by genes. Four terms are used to describe organisms
genetically. Genotype describes an organism’s genetic makeup.
Genotypes made up of like alleles are homozygous; those made up of
unlike alleles are heterozygous. Phenotype describes an organism’s
appearance and is based on the organism’s genotype.
In this investigation, you will solve two different heredity
problems. The first problem is concerned with the color of hair in
guinea pigs. The second problem is concerned with codominance.
Problem
What are the possible genotypes and phenotypes of offspring
produced by parent organisms with known characteristics?
Pre-Lab Discussion
Read the entire investigation. Then, work with a partner to answer the
following questions.
1. In pea plants, the allele for purple flower color is dominant. The
allele for white flower color is recessive. Write the genotype of a pea
plant that is heterozygous for flower color. What two genotypes
might a pea plant homozygous for flower color have?
Students should include one capital letter and one lowercase letter in their heterozygous genotype, such
as Pp. For the homozygous genotypes, students should write one that includes two capital letters and one
that includes two lower case letters, such as PP and pp.
2. Predict the phenotype of a pea plant with the genotype Pp for
flower color. (Hint: Flower color in pea plants is completely
dominant.)
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The pea plant would produce purple flowers.
3. What is the difference between alleles that are codominant and
those that are completely dominant?
An allele that is completely dominant will mask a recessive allele, and a heterozygous organism’s phenotype
will reflect only the dominant allele. Alleles that are codominant will both appear in the phenotype of a
heterozygous organism’s phenotype.
Biology Laboratory Manual B/Chapter 11 101
4. What do the boxes in a Punnett square represent? How will you use
the boxes to calculate genotypic ratios?
Each box represents a possible combination of alleles. To calculate a genotypic ratio, students should list the
different genotypes represented in all four boxes of a completed Punnett square, tally the number of each
genotype represented, and write the figures as a ratio, reduced to reflect lowest common terms—for example,
2 : 2 would be reduced to 1 : 1.
5. For a given Punnett square, will the genotypic ratio always be the
same as the phenotypic ratio? Explain your answer.
No. For example, in heterozygotes for completely dominant alleles, the phenotype will reflect only the dominant
allele. So for a cross Pp Pp, the phenotypic ratio would be 3 purple-flowering plants : 1 white-flowering plant,
but the genotypic ratio would be 1 PP : 2 Pp : 1 pp.
Procedure
1. In guinea pigs, the allele for black fur (B) is dominant over the allele
for white fur (b). Fill in the Punnett square in Figure 1 to determine
the possible genotypes and phenotypes of offspring produced from
the cross between a homozygous black guinea pig and a
heterozygous black guinea pig.
Phenotype: black
Genotype: homozygous
B
Phenotype: black
Genotype: heterozygous
b
B
Genotype:
Genotype:
Phenotype:
Phenotype:
Genotype:
Genotype:
Phenotype:
Phenotype:
Figure 1
2. In cattle, codominance of the allele for a red coat (R) and the allele
for a white coat (W) results in offspring with a roan coat (RW); that
is, a coat with both red and white hairs. Fill in the Punnett square in
Figure 2 to determine the possible genotypes and phenotypes of
offspring produced from the cross between a roan cow and a white
bull.
102 Biology Laboratory Manual B/Chapter 11
© Prentice-Hall, Inc.
B
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Phenotype:
R
W
Phenotype:
W
W
Genotype:
Genotype:
Phenotype:
Phenotype:
Genotype:
Genotype:
Phenotype:
Phenotype:
Figure 2
Analysis and Conclusions
1. Calculating What is the genotypic ratio in Figure 1?
1 BB : 1 Bb
2. Calculating What is the phenotypic ratio in Figure 1?
4 black coat : 0 white coat
3. Drawing Conclusions Is it possible to produce a white guinea pig
by crossing a homozygous black guinea pig and a heterozygous
black guinea pig? Explain your answer.
No, because the homozygous black guinea pig will produce only B alleles. The B allele will mask any b allele
contributed by the heterozygous guinea pig because the B allele is completely dominant over the b allele in
guinea pigs.
4. Calculating What is the genotypic ratio in Figure 2?
1 RW : 1 WW
5. Calculating What is the phenotypic ratio in Figure 2?
1 roan coat : 1 white coat
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6. Using Tables What would the genotypes and phenotypes of the
offspring be from the cross between a roan cow and a roan bull?
Draw a Punnett square below to support your answer.
Punnett squares should show the cross RW RW. Genotypes: 1 RR, 2 RW, 1 WW, Phenotypes: 1 red, 2 roan,
1 white.
Biology Laboratory Manual B/Chapter 11 103
7. Is it possible for two organisms to have different genotypes but the
same phenotype? Explain your answer.
Yes, if one organism has a homozygous dominant genotype and another organism has a heterozygous
dominant genotype, both organisms will display the dominant trait.
Going Further
Animal and plant breeders often keep careful records about the
phenotypes of the organisms they raise—often in the form of a
pedigree. Find examples of pedigrees and trace the inheritance of
traits from one generation to the next. Why do you think pedigrees
and careful records might be important to breeders?
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Students can read about pedigrees in Section 14-1 of the textbook. In addition, college
textbooks, the American Kennel Association, and other breeders’ organizations are good
sources of pedigrees and breeding lineages. Students should realize that because some
phenotypes can reflect more than one genotype, records are needed in order to deduce
an individual’s probable genotype.
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