9.5.1 What Is a Dominant Gene? Intermediate Inheritance

Question 1

To understand the Mendelian concepts of dominant and recessive factors, we need to examine the underlying biochemical basis:

Answer 1

• DNA makes RNA

- RNA makes proteins

Question 2

review

Answer 2

• Many of Mendel’s contemporaries believed that traits present in the parents blended together like paint in the
  offspring.
  • In experimental crosses involving intermediate traits, Mendel’s results showed that alleles segregate from one another and don’t blend together like paint.

Question 3

intermediate inheritance

Answer 3

• In intermediate inheritance, the heterozygote genotype has a unique phenotype that is intermediate to the two parental phenotypes; neither allele dominates.
• At first glance, the case of intermediate inheritance on the left appears to support the blending theory. When a
  red-flowered plant is crossed with a white-flowered plant, the F1 offspring have pink flowers.
• Can the results be explained in terms of Mendelian
  inheritance?
• If the red and white colors of the parental generation blended together like paint in the F1 generation, we can make a prediction: Crossing pink F1 plants should yield all pink plants in the F2 generation.
• If flower color is inherited in a Mendelian fashion, then the pink flowers are actually heterozygotes, each having one red and one white allele. A cross involving two heterozygotes is predicted to result in three genotypes—CrCr, CrCw, and CwCw—and three phenotypes—red, pink, and white flowers.
• Red, pink, and white flowers appear in the F2 generation, supporting a Mendelian mode of inheritance.

Question 4

note

Answer 4

• In performing his experimental crosses, Mendel determined that the expression of one form of a trait can dominate the expression of another form of a trait.
• Modern genetics has determined the biochemical basis for dominant and recessive alleles. Dominance involves coding genes that produce proteins. A recessive allele may not produce a protein and may appear to be masked by the dominant allele.

Question 5

codominance

Answer 5

• In codominance, both alleles of a heterozygote are distinctly expressed.
• An example is coat color in cattle. Cattle with red coats are crossed with white producing the color “roan”. A close look at the hairs reveals that some are red and some are white. Thus, both phenotypes are expressed in the heterozygote.

Question 6

Which of these explains what occurs during intermediate inheritance?

Answer 6

• Heterozygotes show a unique phenotype intermediate to the homozygous phenotypes

Question 7

Pink flowers are a result of blending. Which of the following is the genotype of a pink flower?

Answer 7

• CrCw

Question 8

Which of the statements concerning incomplete dominance is not true?

Answer 8

• Incomplete dominance is evidence for the blending hypothesis.

Question 9

You conduct a cross of snapdragons:

CRCWTt × CRCWTt

Where:
CR	= blending red flower color
CW	= blending white flower color
T	= dominant tall plants
t	= recessive short plants

The two traits segregate independently. What is the expected frequency of pink short plants?

Answer 9

• 1/8

Question 10

Suppose that you have recently inherited your grandparents’ ranch in New Mexico. After moving there, you find that it is in shambles. Sensing your plight, a fellow rancher donates a roan bull and a white cow to your establishment, and tells you to establish a strain of true-breeding red cattle. What steps do you need to take to accomplish this task?

Answer 10

• Cross roan females and males from the F1 generation.

Question 11

Which of the following is a result of blending?

Answer 11

• The phenotypic ratio equals the genotypic ratio.

Question 12

A snapdragon shows incomplete dominance for flower color. Red- and white-flowered plants with genotypes CrCr and CwCw are crossed. The F1 generation is then crossed. What is the phenotypic ratio of the F2 generation?

Answer 12

• 1/4 red, 2/4 pink, 1/4 white

Question 13

A single gene with three alleles determines coat color in a species of mice. The alleles B and Y are codominant and allele w is recessive to both. For a population of mice, how many phenotypes are possible if all possible combinations of alleles are represented?

B = brown hair

Y = yellow hair

w = white hair

Answer 13

• Four

Question 14

Which of the following represents the central dogma of genetics?

Answer 14

• DNA makes RNA makes Protein

Question 15

You conduct a cross of snapdragons:

CRCWTt × CRCWTt

Where:
CR	= blending red flower color
CW	= blending white flower color
T	= dominant tall plants
t	= recessive short plants

The two traits segregate independently. What is the expected frequency of pink tall plants?

Answer 15

• 3/8