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Section 5: Traits and Probability
Study Guide A
KEY CONCEPT
The inheritance of traits follows the rules of probability.
VOCABULARY
Punnett square
testcross
law of independent assortment
monohybrid cross
dihybrid cross
probability
MAIN IDEA: Punnett squares illustrate genetic crosses.
Identify what each of the numbered parts represents in the Punnett square below.
Then draw lines from each of the parents’ alleles to the corresponding alleles in
the offspring.
2. _______________________________________
A
A
a
AA
Aa
_______________________________________
1. _______________________________________________________________
3. __________________________________
a
Aa
aa
_______________________________________________________________
__________________________________
1. Circle the parts of the Punnett square that represent the parents’ alleles.
2. What are the possible genotypes of the offspring in this cross?
_________________________________________________
Fill in the blank with the word or phrase that best complete the following sentence.
3. Each parent contributes only one allele to the offspring, because the alleles
segregated during gamete formation, which is called ____________. During
this process, the ___________ chromosomes separated.
© Houghton Mifflin Harcourt Publishing Company
Holt McDougal Biology
Study Guide A
1
Meiosis and Mendel
Section 6: Meiosis and Genetic Variation
Study Guide A continued
MAIN IDEA: A monohybrid cross involves one trait.
4. You know a ratio is a comparison that tells how two or more things relate.
Therefore, a genotypic ratio is a comparison that tells the proportion of
offspring that have a particular ____________. A phenotypic ratio is a
comparison that tells the proportion of offspring that have a particular
___________.
5. What is the genotypic ratio of the offspring in Figure 5.3? __________
6. What is the phenotypic ratio of the offspring in Figure 5.4? ________
MAIN IDEA: A dihybrid cross involves two traits.
7. A dihybrid cross examines the inheritance of
a. two testcrosses.
b. four testcrosses.
c. two traits.
d. four traits.
8. Suppose an organism has the genotype AABb. Two types of gametes could
result from this allele combination: ____________ and _____________.
9. What is the phenotypic ratio that results from a dihybrid cross between two
organisms that are heterozygous for both traits? See Figure 5.5 for help.
__________________________
MAIN IDEA: Heredity patterns can be calculated with probability.
Circle the word or phrase that best completes the statement.
10. Probability predicts the average / exact number of occurrences, not the
average / exact number of occurrences.
11. To calculate the probability that two independent events will happen together,
divide / multiply the probability of each individual event.
Vocabulary Check
Circle the word or phrase that best completes the statement.
12. A testcross is done between an organism with a dominant / recessive
phenotype and an organism with a(n) known / unknown genotype.
13. In the law of independent assortment, allele pairs / gene pairs are independent.
© Houghton Mifflin Harcourt Publishing Company
Holt McDougal Biology
Study Guide A
2
Meiosis and Mendel
Section 6: Meiosis and Genetic Variation