9.5.1 What Is a Dominant Gene? Intermediate Inheritance Flashcards
To understand the Mendelian concepts of dominant and recessive factors, we need to examine the underlying biochemical basis:
- DNA makes RNA
- RNA makes proteins
review
- 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.
intermediate inheritance
- 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.
note
- 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.
codominance
- 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.
Which of these explains what occurs during intermediate inheritance?
- Heterozygotes show a unique phenotype intermediate to the homozygous phenotypes
Pink flowers are a result of blending. Which of the following is the genotype of a pink flower?
- CrCw
Which of the statements concerning incomplete dominance is not true?
- Incomplete dominance is evidence for the blending hypothesis.
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?
- 1/8
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?
- Cross roan females and males from the F1 generation.
Which of the following is a result of blending?
- The phenotypic ratio equals the genotypic ratio.
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?
- 1/4 red, 2/4 pink, 1/4 white
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
- Four
Which of the following represents the central dogma of genetics?
- DNA makes RNA makes Protein
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?
- 3/8