9.1.4 Mendel's Conclusions: Segregation and Recombination

Question 1

traits

Answer 1

• The form of the trait that masks the other in the F1
  is dominant (S). The trait that disappears in the F1
  is recessive (s).
  • For a given trait, a parent can be either homozygous with two identical alleles or heterozygous with two different alleles.
  • According to Mendel’s law of segregation, a parent can only donate one allele of the two alleles per gamete.
  • Because of the law of segregation, when two heterozygotes are crossed, three different genotypes and two different phenotypes result. This process results in recombination.

Question 2

note

Answer 2

• In the experiment on the left, the true-breeding plants in the parental generation are homozygous (have two identical alleles for a given trait). The smooth-seed plant is designated SS and the wrinkled-seed plant is designated ss.
• The smooth-seed plant can only contribute an S allele and the wrinkled-seed plant can only contribute an s allele to the F1 offspring. The resulting F1 offspring are heterozygous (have two different alleles for a given trait), having the designation Ss. The F1 plants display the smooth-seed trait.
• The trait that masks the other in the F1 generation is the dominant trait. The one that disappears is the recessive trait
• The genotype is the allelic composition of the trait in
  question. The phenotype is the physical manifestation of the trait. The chart on the left shows all of the possible genotypes that may result in the F2 generation and the resulting phenotypes. The chance of two independent events can be determined by multiplying the probability of their separate occurrences together. Because each sperm or egg type has a 1/2 probability of occurring, the probability of each genotype occurring is 1/2 X 1/2 = 1/4. For example the probability of a sperm containing an S allele is 1/2. The probability of an egg containing an S allele is 1/2. The probability of the events co-occurring, resulting in an SS offspring, is 1/2 X 1/2 = 1/4.
• Three genotypes and two phenotypes are possible. Note that the SS and Ss genotypes have the same phenotype. The ratio of smooth to wrinkled seeds is 3:1. The genotypes occur in a 1:2:1 ratio.

Question 3

law of segregation

Answer 3

• According to the law of segregation, each parent can only donate one allele of the two per gamete. Because the F1 plants are heterozygous they can contribute two different kinds of gametes to the F2 offspring. The sperm may contain either an S or s allele. Similarly the egg may contain either an S or s allele. There is a 50% likelihood for each gametic contribution.
• For a heterozygous cross, there are four possible combinations (shown on the left). The probability for each combination occurring is 1/4 (= 1/2 X 1/2).

Question 4

In the following diagram, the F1 generation is

Answer 4

• 100% heterozygous

Question 5

Which of the following pairs of genotypes would have the same phenotypes?

Answer 5

• Ss & SS

Question 6

In the F2 of a monohybrid cross of two true-breeding strains of pea, one with smooth seeds and one with wrinkled seeds, what is the frequency of heterozygotes in the offspring?

Answer 6

• 0.50

Question 7

The term that means that the two alleles for a given trait are identical is:

Answer 7

• Homozygous

Question 8

Which of the following is homozygous dominant?

Answer 8

• NN

Question 9

A monohybrid cross of true-breeding strains can be represented by the following:
SS × ss
S = smooth-shelled, dominant
s = wrinkle-shelled, recessive
If this cross produced 4 offspring, how could their genetic makeup be represented?

Answer 9

• Ss, Ss, Ss, Ss

Question 10

Let B be the allele for brown eyes and b be the allele for blue eyes. If an individual is heterozygous for eye color, what is the probability that the gametes will contain the gene for blue eyes?

Answer 10

• 50% or 1 out of 2

Question 11

Which of the following statements is true?

Answer 11

• The dominant trait will be the phenotype of the individual if it has one dominant gene and one recessive gene.

Question 12

Which of the following pairs of terms are synonomous?

Answer 12

• Heterozygous = Hybrid

Question 13

In Mendel’s experiment where he crossed a recessive true-breeding plant with a dominant true-breeding plant, the F2 generation has three __________, but only two __________.

Answer 13

• genotypes, phenotypes

Question 14

What is the genetic makeup and frequency of the gametes produced by a heterozygous plant, Ss?

S = smooth shell
s = wrinkled shell

Answer 14

• 50% S, 50% s