9.3.1 Segregation and Independent Assortment

Question 1

Segregation and Independent Assortment

Answer 1

• Mendel’s first law of segregation explains the mechanism underlying the inheritance of a single trait (as determined by the monohybrid cross).
• Mendel performed dihybrid crosses to examine the mechanism underlying the inheritance of two traits.
• Mendel’s experimental results indicated that recombination had occurred. He concluded that the alleles for the two different traits segregated independently of one another. He called the phenomenon the law of independent assortment.

Question 2

crosses

Answer 2

• A monohybrid cross is a breeding experiment that uses parent varieties differing in a single trait. A dihybrid cross uses varieties differing in two traits.
• In the cross on the left, the two traits are seed color and seed shape. The alternate forms of seed color are yellow and green. The alternate forms of seed shape are round and wrinkled. Prior experiments indicated that yellow was dominant to green and round was dominant to wrinkled.
• Both plants are true breeding, meaning that the alleles for each trait are homozygous.

Question 3

note

Answer 3

• F1 offspring are heterozygous for both traits. Recombination is the formation of a new gamete combination that differs from the gametes that formed the parent.
• Question: What would the expected genotypes and
  phenotypes of the offspring be if no recombination occurred in the parental gametes?
• Prediction: If no recombination occurred, each F1 plant
  could produce only two kinds of gametes, YR and yr.
• To determine the genotypic and phenotypic frequency of the F2 offspring, construct a Punnett square. If no recombination in the F1 gametes occurs, a ratio of 3/4 smooth and yellow to 1/4 wrinkled and green peas should appear.
• If the prediction of no recombination is correct, out of 556 plants, 417 (3/4) will be yellow/round, and 139 (1/4) will be wrinkled/green.
• Mendel actually observed four phenotype combinations, not two:
• 9/16 yellow/round =315
• 3/16 yellow/wrinkled = 108
• 3/16 green/round = 101
• 1/16 green/wrinkled = 32
• Mendel’s experimental results indicated that recombination had occurred. He concluded that the alleles for the two different traits segregated independently of one another. He called this phenomenon the law of independent assortment.

Question 4

In pea plants, yellow seeds (Y) are dominant to green seeds (y) and smooth seeds (S) are dominant to wrinkled seeds (s). If two plants with the genotype YySs are crossed, what are the phenotypes of the offspring?

Answer 4

9/16 yellow smooth seeds
3/16 yellow wrinkled seeds
3/16 green smooth seeds
1/16 green wrinkled seeds

Question 5

In a particular strain of peas, yellow (Y) is the dominant seed color and green (y) is the recessive. The dominant seed type is smooth (S) and the recessive type is wrinkled (s). A true-breeding yellow, wrinkled pea plant crossed with a true-breeding green, smooth plant will produce offspring of what genotypic ratios?

Answer 5

• all YySs

Question 6

Monohybrid crosses evaluate the inheritance patterns of __________ trait(s), while dihybrid crosses evaluate __________ trait(s).

Answer 6

• one, two

Question 7

Which one of the following best describes recombination?

Answer 7

• In the formation of gametes, genes for different traits are sorted randomly

Question 8

Which one of the following is a true breeding genotype?

Answer 8

• YYrr

Question 9

An organism that is heterozygous for two independently segregating traits can produce how many different kinds of gametes?

Answer 9

• 4