Quiz · Medium
Practice monohybrid and dihybrid crosses, blood types, sex-linked traits, pedigrees and Hardy-Weinberg problems, with the working shown for every answer.
Two heterozygous pea plants (Aa × Aa) are crossed. A is completely dominant. What is the expected phenotypic ratio of the offspring?
1 : 2 : 1
3 dominant : 1 recessive
1 dominant : 1 recessive
9 : 3 : 3 : 1
The Punnett square gives AA, Aa, Aa, aa. Three of four show the dominant trait, so 3 : 1. The 1 : 2 : 1 ratio is the genotypic ratio, a common mix-up.
A heterozygous plant (Aa) is crossed with a homozygous recessive plant (aa). What fraction of offspring show the recessive phenotype?
1/2
1/4
3/4
0
Aa × aa gives Aa, Aa, aa, aa: half are aa. This is the classic test cross result for a heterozygous parent.
A tall pea plant of unknown genotype is crossed with a short (tt) plant. About half of the offspring are short. What is the tall parent's genotype?
TT
tt
Tt
It cannot be determined from this cross
Short offspring (tt) must get a t from each parent, so the tall parent carries t and is Tt. If it were TT, every offspring would be Tt and tall.
In the cross AaBb × AaBb (genes assort independently), what fraction of offspring are aabb?
1/4
1/16
3/16
9/16
Multiply the separate probabilities: P(aa) = 1/4 and P(bb) = 1/4, so 1/4 × 1/4 = 1/16. This is the single double-recessive box in the 9 : 3 : 3 : 1 square.
In the cross AaBb × AaBb, what fraction of offspring show BOTH dominant traits?
3/16
1/16
3/4
9/16
P(dominant for A) = 3/4 and P(dominant for B) = 3/4, so 3/4 × 3/4 = 9/16. 3/4 is the answer for just one of the traits.
What phenotypic ratio do you expect from the dihybrid test cross AaBb × aabb?
9 : 3 : 3 : 1
1 : 1 : 1 : 1
3 : 1
1 : 2 : 1
AaBb makes four gamete types (AB, Ab, aB, ab) in equal numbers, and aabb only gives ab, so each offspring phenotype appears 1/4 of the time. If you see a big departure from 1 : 1 : 1 : 1, the genes are probably linked.
Two parents are AaBbCc. Assuming independent assortment, what is the probability of an aabbcc offspring?
1/16
1/8
3/64
1/64
Each gene gives a 1/4 chance of the homozygous recessive. Product rule: (1/4)³ = 1/64. Drawing a 64-box Punnett square is never necessary.
In snapdragons, red (RR) × white (WW) gives all pink (RW) flowers. If two pink plants are crossed, what ratio is expected?
1 red : 2 pink : 1 white
3 red : 1 white
All pink
1 pink : 1 white
This is incomplete dominance, so each genotype has its own phenotype. RW × RW gives 1 RR : 2 RW : 1 WW, which shows up directly as 1 red : 2 pink : 1 white.
One parent has genotype I^A i and the other I^B i. Which blood types are possible in their children?
A and B only
AB only
A, B and AB only
A, B, AB and O
The square gives I^A I^B (AB), I^A i (A), I^B i (B) and ii (O), each with probability 1/4. People often miss type O, but both parents carry a hidden i allele.
A father with type AB blood and a mother with type O blood have a child. What is the probability that the child has type O blood?
25%
0%
50%
100%
The father gives I^A or I^B and the mother gives i, so children are I^A i (type A) or I^B i (type B), 50% each. An ii child is impossible because the father has no i allele.
Hemophilia is X-linked recessive. A carrier mother (X^H X^h) and an unaffected father (X^H Y) have a son. What is the probability the son has hemophilia?
1/4
0
1
1/2
A son gets his Y from his father and one of his mother's X chromosomes, X^H or X^h, with equal chance. So 1/2 of sons are affected. The 1/4 figure is the chance for any child before you know the sex.
Why are X-linked recessive disorders such as red-green color blindness more common in males?
The Y chromosome carries the disease allele
Males inherit their X chromosome from their father
Males have only one X chromosome, so a single recessive allele is expressed
Recessive alleles are passed only to sons
Males are hemizygous for X-linked genes: there is no second X to mask a recessive allele. Males get their X from their mother, not their father.
Which of Mendel's laws says that alleles of different genes go into gametes independently of one another?
Law of segregation
Law of independent assortment
Law of dominance
Hardy-Weinberg principle
Independent assortment (shown by dihybrid crosses) is about different genes, and it holds for genes on different chromosomes or far apart on one. Segregation is about the two alleles of one gene separating into different gametes.
Two genes are 12 map units (centimorgans) apart. In a test cross, what percentage of offspring are expected to be recombinant?
12%
88%
6%
24%
By definition, 1 map unit = 1% recombination frequency, so 12 map units means about 12% recombinant offspring. The other 88% are parental types.
In a population in Hardy-Weinberg equilibrium, 16% of individuals show a recessive trait. What is the frequency of heterozygotes?
0.24
0.48
0.36
0.16
, so and . Heterozygotes = . 0.36 is , the homozygous dominant frequency.
Which of these would push a population OUT of Hardy-Weinberg equilibrium?
Natural selection favoring one allele
Random mating
A very large population
No migration in or out
Hardy-Weinberg assumes no selection, no mutation, no gene flow, random mating and a very large population. The other three options are assumptions of the model, not violations.
Down syndrome (trisomy 21) most often results from:
A point mutation on chromosome 21
Nondisjunction during meiosis
Crossing over between sister chromatids
An X-linked recessive allele
When chromosome 21 fails to separate in meiosis, one gamete gets two copies; fertilization then gives three. It is a change in chromosome number, not a mutation in one gene.
In a pedigree, two unaffected parents have an affected daughter. Which mode of inheritance best fits?
Autosomal recessive
Autosomal dominant
X-linked recessive
Y-linked
Both parents must be carriers, which fits autosomal recessive. A dominant trait would show in at least one parent, and an X-linked recessive daughter needs an affected father.
