Genetics and Heredity — Hard Practice Quiz

A Biology cheat sheet for Genetics and Heredity — every key formula with its symbols defined — plus a hard-level practice quiz to test recall.

Practice quiz

1. In pea plants, Tall ($T$) is dominant to short ($t$), and Purple flowers ($P$) are dominant to white flowers ($p$). A true-breeding tall, purple-flowered plant is crossed with a true-breeding short, white-flowered plant. An $F_1$ plant from this cross is then test-crossed. What is the expected phenotypic ratio of the offspring from this test cross?

   • A. $1$ Tall Purple : $1$ Tall White : $1$ Short Purple : $1$ Short White
   • B. $9$ Tall Purple : $3$ Tall White : $3$ Short Purple : $1$ Short White
   • C. $3$ Tall Purple : $1$ Short White
   • D. $1$ Tall Purple : $1$ Short White

   Answer: A. $1$ Tall Purple : $1$ Tall White : $1$ Short Purple : $1$ Short White

2. A woman is a carrier for an X-linked recessive disorder. She marries a man who does not have the disorder. What is the probability that their first child, regardless of sex, will be affected by the disorder?

   • A. $0\%$
   • B. $25\%$
   • C. $50\%$
   • D. $75\%$

   Answer: B. $25\%$

3. In snapdragons, flower color exhibits incomplete dominance: $R_1R_1$ is red, $R_1R_2$ is pink, and $R_2R_2$ is white. Plant height is Mendelian, with Tall ($T$) dominant to short ($t$). If a pink, tall plant (heterozygous for height) is self-pollinated, what proportion of the offspring will be white and tall?

   • A. $\frac{1}{16}$
   • B. $\frac{3}{16}$
   • C. $\frac{1}{8}$
   • D. $\frac{3}{8}$

   Answer: B. $\frac{3}{16}$

4. In Labrador retrievers, coat color is determined by two genes. Gene $B$ (black) is dominant to $b$ (brown). Gene $E$ (pigment deposition) is dominant to $e$ (no pigment, resulting in yellow regardless of $B/b$). A black Labrador with genotype $BbEe$ is crossed with a yellow Labrador with genotype $bbee$. What is the expected phenotypic ratio of their offspring?

   • A. $1$ Black : $1$ Brown : $2$ Yellow
   • B. $3$ Black : $1$ Brown : $4$ Yellow
   • C. $1$ Black : $1$ Yellow
   • D. $1$ Black : $1$ Brown : $1$ Yellow

   Answer: A. $1$ Black : $1$ Brown : $2$ Yellow

5. Two genes, $A$ and $B$, are located on the same chromosome. A dihybrid individual with genotype $AaBb$ (where $A$ and $B$ are linked on one chromosome, and $a$ and $b$ on the homologous chromosome) produces gametes. If the recombination frequency between gene $A$ and gene $B$ is $20\%$, what are the expected frequencies of the gametes $AB$, $Ab$, $aB$, and $ab$?

   • A. $AB = 40\%$, $Ab = 10\%$, $aB = 10\%$, $ab = 40\%$
   • B. $AB = 25\%$, $Ab = 25\%$, $aB = 25\%$, $ab = 25\%$
   • C. $AB = 10\%$, $Ab = 40\%$, $aB = 40\%$, $ab = 10\%$
   • D. $AB = 30\%$, $Ab = 20\%$, $aB = 20\%$, $ab = 30\%$

   Answer: A. $AB = 40\%$, $Ab = 10\%$, $aB = 10\%$, $ab = 40\%$

6. Consider an autosomal recessive disorder. A pedigree shows an affected individual (II-1) who marries an unaffected individual (II-2) from outside the family. If individual II-2 has no family history of the disorder (assume homozygous dominant), what is the probability that their child (III-1) will be a carrier for the disorder?

   • A. $0\%$
   • B. $25\%$
   • C. $50\%$
   • D. $100\%$

   Answer: D. $100\%$

7. A man with blood type A and a woman with blood type B have a child with blood type O. If they have another child, what is the probability that this child will have blood type AB?

   • A. $0\%$
   • B. $25\%$
   • C. $50\%$
   • D. $75\%$

   Answer: B. $25\%$

8. A couple has a child diagnosed with Klinefelter's Syndrome ($XXY$). Which parent experienced nondisjunction during meiosis, and in which meiotic division could this have occurred to produce an $XXY$ zygote?

   • A. Mother, Meiosis I or Meiosis II; Father, Meiosis I
   • B. Mother, Meiosis I only; Father, Meiosis II only
   • C. Mother, Meiosis II only; Father, Meiosis I only
   • D. Mother, Meiosis I or Meiosis II; Father, Meiosis II

   Answer: A. Mother, Meiosis I or Meiosis II; Father, Meiosis I

9. A plant with dominant phenotypes for two traits (e.g., Tall and Purple flowers) is test-crossed. The offspring show four phenotypes in roughly equal proportions: Tall Purple, Tall White, Short Purple, Short White. What was the genotype of the original dominant-phenotype parent, and what principle of Mendel's is best illustrated by these results?

   • A. $TTPp$; Law of Dominance
   • B. $TtPP$; Law of Segregation
   • C. $TtPp$; Law of Independent Assortment
   • D. $TTPP$; Law of Dominance

   Answer: C. $TtPp$; Law of Independent Assortment

10. In a certain insect, a reciprocal cross is performed for a trait. When a pure-breeding red-eyed female is crossed with a white-eyed male, all $F_1$ offspring have red eyes. When a pure-breeding white-eyed female is crossed with a red-eyed male, all $F_1$ females have red eyes, and all $F_1$ males have white eyes. Based on these results, what is the mode of inheritance for eye color?

    • A. Autosomal dominant
    • B. Autosomal recessive
    • C. X-linked dominant
    • D. X-linked recessive

    Answer: D. X-linked recessive