CLEP Biology Mendelian Inheritance Patterns 5 — Questions and Answers
Question 1: In a pedigree analysis, an X-linked recessive trait is suspected. Which observation would MOST strongly support X-linked (rather than autosomal) recessive inheritance?
- Affected males are more frequent than affected females and all affected females have affected fathers (Correct answer)
- The trait skips exactly one generation in every family
- Affected individuals appear in equal numbers in both sexes
- Two affected parents always produce all affected offspring
Correct answer: Affected males are more frequent than affected females and all affected females have affected fathers
X-linked recessive traits predominantly affect males, and an affected female (X^a X^a) must have inherited X^a from an affected father.
Question 2: Polygenic inheritance differs from single-gene Mendelian inheritance in that polygenic traits:
- Show a continuous range of phenotypes influenced by multiple gene loci and the environment (Correct answer)
- Always show 3:1 ratios in F2 crosses
- Are controlled by a single locus with many alleles
- Cannot be measured quantitatively
Correct answer: Show a continuous range of phenotypes influenced by multiple gene loci and the environment
Polygenic traits like height or skin color result from additive effects of multiple loci, producing a bell-shaped distribution of phenotypes.
Question 3: During meiosis I, homologous chromosome pairs align at the metaphase plate and separate independently of other pairs. This physical process is the cellular basis of:
- Mendel's law of independent assortment (Correct answer)
- Mendel's law of segregation
- Crossing over and recombination
- Genetic drift in small populations
Correct answer: Mendel's law of independent assortment
The random, independent orientation of homologous pairs at metaphase I is the cellular mechanism behind independent assortment.
Question 4: A cross of AaBbCc × AaBbCc (three independently assorting loci) produces what fraction of offspring with the genotype aaBbcc?
- 1/32 (Correct answer)
- 1/16
- 1/64
- 3/64
Correct answer: 1/32
P(aa) = 1/4, P(Bb) = 2/4 = 1/2, P(cc) = 1/4; product = 1/4 × 1/2 × 1/4 = 1/32.
Question 5: Which of the following is an example of a trait exhibiting sex-influenced inheritance (not X-linked)?
- Pattern baldness, which is autosomal but expressed as dominant in males and recessive in females (Correct answer)
- Red-green color blindness, which is carried on the X chromosome
- Hemophilia A, which primarily affects males due to X-linkage
- Mitochondrial myopathy, transmitted only through mothers
Correct answer: Pattern baldness, which is autosomal but expressed as dominant in males and recessive in females
Pattern baldness is autosomal but sex hormones alter the dominance relationship — the same genotype expresses differently depending on sex.
Question 6: A trait shows a 13:3 ratio in an F2 dihybrid cross. This modified ratio arises from a form of:
- Dominant epistasis where A_ masks B locus and produces a 12:3:1 collapsed to 13:3 (Correct answer)
- Recessive epistasis producing a 9:7 collapsed ratio
- Codominance at both loci
- Complementary gene action producing a 9:7 ratio
Correct answer: Dominant epistasis where A_ masks B locus and produces a 12:3:1 collapsed to 13:3
Dominant epistasis: A_ suppresses B locus (9 A_B_ + 3 A_bb = 12 epistatic class) + 1 aaBB combined with 2 aaBb gives 13:3 when categories merge.
Question 7: Gregor Mendel's conclusion that hereditary factors exist as discrete units (rather than blending) was supported by which key observation from his experiments?
- Recessive traits reappeared unchanged in the F2 generation after disappearing in F1 (Correct answer)
- All offspring of a cross always resembled one parent exactly
- Offspring traits were always intermediate between the two parents
- F1 plants could not reproduce with each other
Correct answer: Recessive traits reappeared unchanged in the F2 generation after disappearing in F1
The reappearance of recessive traits in F2 in their original, unblended form proved that hereditary factors remain discrete and are not lost or diluted.
CLEP Biology Mendelian Inheritance Patterns 5
In a pedigree analysis, an X-linked recessive trait is suspected.
Which observation would MOST strongly support X-linked (rather than autosomal) recessive inheritance?