13. Mendelian Genetics

Monohybrid Crosses

Open Question
The blending inheritance hypothesis proposed that the genetic material from parents is mixed in the offspring. As a result, traits of offspring and later descendants should lie between the phenotypes of parents. Mendel, in contrast, proposed that genes are discrete and that their integrity is maintained in the offspring and in subsequent generations. Suppose the year is 1890. You are a horse breeder who has just read Mendel's paper. You don't believe his results, however, because you often work with cremello (very light-colored) and chestnut (reddish-brown) horses. You know that when you breed a cremello individual from a pure-breeding line with a chestnut individual from a pure-breeding line, the offspring are palomino—meaning they have an intermediate (golden-yellow) body color. What additional cross would you do to test whether Mendel's model is valid in the case of genes for horse color? According to his model, what offspring phenotype frequencies would you get from your experimental cross? Explain why your cross would test Mendel's model versus blending inheritance.
Multiple Choice
Which of the following is the genotypic ratio of offspring created from a cross of two heterozygous parents for a single gene?
a) 2:2
b) 3:1
c) 1:2:1
d) 1:3
Multiple Choice
In a single gene cross between a homozygous dominant parent and a homozygous recessive parent, which generation is always completely heterozygous?
a) F1 generation.
b) F2 generation.
c) F3 generation.
d) P generation.
Multiple Choice
Which of the following single gene crosses would always result in all offspring with the dominant phenotype?
a) tt x tt.
b) Tt x Tt.
c) TT x tt.
d) Tt x tt.
Multiple Choice
Assume tall (T) is completely dominant to dwarf (t) in a certain species of plant. If a homozygous dominant individual is crossed with a homozygous dwarf, the offspring will __________.
Multiple Choice
Pea plants are tall if they have the genotype TT or Tt, and they are short if they have genotype tt. A tall plant is mated with a short plant. Which outcome below would indicate that the tall parent plant was heterozygous?
Multiple Choice
If an organism that is homozygous dominant is crossed with a heterozygote for that trait, the offspring will be __________.
Open Question
DRAW IT A pea plant heterozygous for inflated pods (Ii) is crossed with a plant homozygous for constricted pods (ii). Draw a Punnett square for this cross to predict genotypic and phenotypic ratios. Assume that pollen comes from the ii plant.
Open Question
Two fruit flies with eyes of the usual red color are crossed, and their offspring are as follows: 77 red-eyed males, 71 ruby-eyed males, 152 red-eyed females. The allele for ruby eyes isa. autosomal (carried on an autosome) and dominant.b. autosomal and recessive.c. sex-linked and dominant.d. sex-linked and recessive.