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12. Non-Mendelian Genetics — Practice Questions 2
This chapter focuses on non-Mendelian inheritance patterns including codominance, incomplete dominance, polygenic traits, environmental effects, linked genes, and nonnuclear inheritance.
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(Multiple Choice — Click to Reveal Answer)
1. Which term describes traits that are inherited via mitochondria rather than nuclear DNA?
(A) Polygenic traits
(B) Nonnuclear inheritance
(C) Linked inheritance
(D) Sex-linked inheritance
Answer
(B) — Nonnuclear inheritance refers to traits passed through organelles like mitochondria or chloroplasts, which are typically inherited from the mother.
2. Which of the following is most likely to be affected by phenotypic plasticity?
(A) Eye color
(B) Flower color of hydrangeas
(C) Blood type
(D) Colorblindness
Answer
(B) — Hydrangeas display different flower colors depending on soil pH, a clear example of phenotypic plasticity.
3. What inheritance pattern explains why a heterozygous individual expresses an intermediate trait between two homozygotes?
(A) Codominance
(B) Incomplete dominance
(C) Complete dominance
(D) Recessive linkage
Answer
(B) — In incomplete dominance, the heterozygote shows a blend of the two parental phenotypes.
4. In which of the following cases would you expect a 1:2:1 phenotypic ratio among offspring?
(A) A dihybrid cross
(B) Codominance
(C) Polygenic inheritance
(D) Sex-linked recessive inheritance
Answer
(B) — A 1:2:1 ratio often results from codominant or incompletely dominant alleles.
5. Which term best describes a trait influenced by the interaction of multiple genes?
(A) Pleiotropy
(B) Codominance
(C) Polygenic trait
(D) Nonnuclear trait
Answer
(C) — Polygenic traits are controlled by several genes, producing a range of phenotypes like height or skin tone.
6. Which of the following genotypes is most likely to result in a calico female cat?
(A) XᴮXᴮ
(B) XᴼXᴼ
(C) XᴮXᴼ
(D) XᴮY
Answer
(C) — Calico cats are heterozygous females for fur color genes on the X chromosome.
7. Why are males more likely to express X-linked recessive traits?
(A) They inherit two copies of the trait
(B) They inherit the Y chromosome from their mother
(C) They only have one X chromosome
(D) They lack mitochondrial DNA
Answer
(C) — Males have only one X chromosome, so a single recessive allele on it is expressed.
8. Which example shows the result of a polygenic trait?
(A) ABO blood type
(B) Presence of dimples
(C) Human height variation
(D) Red-green color blindness
Answer
(C) — Height is influenced by multiple genes, making it a polygenic trait.
9. Which of the following is an example of codominance?
(A) Red × White = Pink
(B) AB blood type
(C) Blue eyes from two heterozygous parents
(D) Curly hair from straight × curly
Answer
(B) — AB blood type expresses both A and B alleles equally, a hallmark of codominance.
10. What does the genotype XᴴXʰ indicate in terms of inheritance?
(A) Homozygous dominant
(B) Carrier female for an X-linked trait
(C) Male with hemophilia
(D) Female with hemophilia
Answer
(B) — A female with one dominant and one recessive X-linked allele is a carrier of the recessive trait.
11. Which of the following is an example of a non-Mendelian inheritance pattern?
(A) Trait controlled by one gene with two alleles
(B) Trait with 3:1 phenotypic ratio in F2 generation
(C) Trait influenced by multiple genes and environment
(D) Trait inherited via autosomal dominance
Answer
(C) — Traits influenced by multiple genes and environmental factors are examples of non-Mendelian inheritance.
12. What term refers to a single gene controlling more than one seemingly unrelated trait?
(A) Polygenic inheritance
(B) Incomplete dominance
(C) Pleiotropy
(D) Codominance
Answer
(C) — Pleiotropy occurs when one gene influences multiple phenotypic traits.
13. Which of the following conditions is inherited via mitochondrial DNA?
(A) Hemophilia
(B) Color blindness
(C) Leber’s hereditary optic neuropathy
(D) Huntington’s disease
Answer
(C) — Mitochondrial disorders like LHON are passed only through the maternal line.
14. In incomplete dominance, what is the phenotype of a heterozygote if red (R) and white (W) are alleles?
(A) Red
(B) White
(C) Red and white patches
(D) Pink
Answer
(D) — In incomplete dominance, the heterozygous phenotype is a blend of the two homozygous phenotypes.
15. A woman with type A blood and a man with type B blood can produce offspring with which blood types?
(A) A and B only
(B) A, B, AB, and O
(C) AB only
(D) A, B, and AB only
Answer
(B) — If both parents are heterozygous (IAi × IBi), all four blood types are possible.
16. What do polygenic traits typically show in a population?
(A) Two extreme phenotypes only
(B) Discrete categories
(C) Continuous variation
(D) Incomplete dominance
Answer
(C) — Polygenic traits result in a range of phenotypes, not distinct categories.
17. A female with genotype XʰXʰ and a male with XᴴY will have daughters who are:
(A) All carriers
(B) All affected
(C) Half affected
(D) All normal
Answer
(A) — Daughters will inherit one normal (Xᴴ) and one affected (Xʰ) allele and be carriers.
18. What is the term for the phenomenon where one gene masks the effect of another gene?
(A) Codominance
(B) Polygenic inheritance
(C) Epistasis
(D) X-linkage
Answer
(C) — Epistasis occurs when one gene affects the expression of a different gene.
19. Which type of inheritance does NOT follow Mendel’s rules of dominance and segregation?
(A) Monohybrid cross
(B) Linked gene inheritance
(C) Dihybrid cross
(D) Autosomal recessive
Answer
(B) — Linked genes are inherited together and do not always segregate independently.
20. What is the result of codominance in terms of phenotype?
(A) Only dominant allele expressed
(B) Intermediate phenotype
(C) Both alleles expressed equally
(D) Recessive allele always masked
Answer
(C) — In codominance, both alleles contribute equally and visibly to the phenotype.
21. What is one effect of X-inactivation in female mammals?
(A) Both X chromosomes are always active
(B) One X chromosome is randomly inactivated in each cell
(C) It results in fewer dominant traits
(D) It occurs only in males
Answer
(B) — X-inactivation silences one of the two X chromosomes in each cell randomly.
22. Which type of trait would most likely produce a bell-shaped curve when graphed across a population?
(A) Incomplete dominance
(B) Sex-linked trait
(C) Polygenic trait
(D) Codominant trait
Answer
(C) — Polygenic traits like height or skin color show continuous variation and typically form a bell curve.
23. What is the typical inheritance pattern of a trait that appears in every generation and affects both sexes equally?
(A) X-linked recessive
(B) Autosomal dominant
(C) Autosomal recessive
(D) Y-linked
Answer
(B) — Autosomal dominant traits generally appear in every generation and affect males and females equally.
24. Which trait is likely to show phenotypic plasticity?
(A) Blood type
(B) Eye color
(C) Hydrangea flower color
(D) Hairline shape
Answer
(C) — Hydrangea color changes based on soil pH, a classic example of environmental influence on phenotype.
25. In humans, how are mitochondrial traits inherited?
(A) From both parents
(B) From father only
(C) From mother only
(D) Randomly from either parent
Answer
(C) — Mitochondrial DNA is inherited exclusively from the mother through the egg cell.
26. In a cross between two heterozygotes showing incomplete dominance (Rr × Rr), what is the expected phenotypic ratio of the offspring?
(A) 1:2:1
(B) 3:1
(C) 9:3:3:1
(D) 2:1:1
Answer
(A) — In incomplete dominance, both genotype and phenotype ratios are 1:2:1, with one dominant, one recessive, and two intermediate phenotypes.
27. Which of the following scenarios demonstrates epistasis in genetic inheritance?
(A) A single gene influences multiple traits
(B) One gene masks the effect of another gene
(C) Both alleles are equally expressed
(D) Environmental factors change phenotype expression
Answer
(B) — Epistasis is when one gene suppresses or modifies the expression of another gene, altering expected Mendelian ratios.
28. A child has type O blood, but neither parent is type O. What are the most likely genotypes of the parents?
(A) IAIA and IBIB
(B) IAi and IBi
(C) IAIB and ii
(D) ii and ii
Answer
(B) — Each parent must carry a recessive i allele (IAi and IBi) to produce an offspring with the ii genotype (type O).
29. A plant's flower color is affected by both genetics and temperature. What kind of inheritance is this?
(A) Polygenic
(B) Phenotypic plasticity
(C) Sex-linked dominance
(D) Codominance
Answer
(B) — Phenotypic plasticity is when environmental conditions influence how a genotype is expressed as a phenotype.
30. In cats, a cross between a black female (XᴮXᴮ) and an orange male (XᴼY) results in calico females. What is the genotype of the calico female?
(A) XᴮXᴼ
(B) XᴮXᴮ
(C) XᴼXᴼ
(D) XᴼY
Answer
(A) — The heterozygous XᴮXᴼ female shows mosaic coloration due to X-inactivation, resulting in a calico pattern.
31. Which of the following best explains why mitochondrial disorders are not passed from father to child?
(A) Males destroy mitochondria during fertilization
(B) Only the egg contributes cytoplasm with mitochondria
(C) Mitochondria are only present in female cells
(D) Mitochondria are inherited from the Y chromosome
Answer
(B) — Since mitochondria are found in the egg's cytoplasm, only mothers pass them to offspring.
32. What type of phenotypic pattern would be expected in a polygenic trait across a population?
(A) Bimodal distribution
(B) Skewed dominant distribution
(C) Normal distribution (bell curve)
(D) All-or-nothing outcome
Answer
(C) — Polygenic traits tend to follow a bell-shaped curve because multiple genes contribute additive effects.
33. What is the result of crossing two individuals with genotypes IAIB × ii?
(A) All type AB
(B) 50% A, 50% B
(C) 100% type O
(D) 75% type B, 25% type A
Answer
(B) — IAIB × ii can produce IAi (type A) or IBi (type B) offspring.
34. Which of the following inheritance patterns results in a 9:3:4 phenotypic ratio?
(A) Dihybrid cross with codominance
(B) Dominant epistasis
(C) Recessive epistasis
(D) Polygenic trait
Answer
(C) — Recessive epistasis alters the typical 9:3:3:1 dihybrid ratio to 9:3:4 due to gene interactions.
35. In a certain bird species, feather color is affected by two genes. If one gene masks the other, what genetic principle is this an example of?
(A) Codominance
(B) Epistasis
(C) Pleiotropy
(D) Incomplete dominance
Answer
(B) — When one gene affects the expression of another, it is an example of epistasis.
36. Explain the genetic mechanism behind codominance using human blood types as an example.
Answer
In codominance, both alleles are equally and fully expressed. For example, in AB blood type, both IA and IB alleles are expressed on the surface of red blood cells.
37. How is incomplete dominance different from codominance in terms of phenotype expression?
Answer
In incomplete dominance, the heterozygous phenotype is a blend of both alleles (e.g., red × white = pink). In codominance, both traits appear independently (e.g., AB blood type).
38. Why do X-linked recessive disorders appear more frequently in males than in females?
Answer
Males have only one X chromosome, so a single recessive allele on that chromosome will be expressed, while females would need two copies.
39. What is phenotypic plasticity and give an example from plants?
Answer
Phenotypic plasticity is when the environment influences how a genotype is expressed. An example is hydrangea flower color changing with soil pH.
40. What is epistasis and how does it affect phenotypic ratios?
Answer
Epistasis occurs when one gene masks or alters the expression of another gene. It can modify expected Mendelian ratios, such as changing 9:3:3:1 to 9:3:4.
41. Describe how polygenic inheritance creates a range of phenotypes in a trait like skin color.
Answer
Polygenic traits are controlled by multiple genes, each contributing a small effect. The combined action of these genes results in a continuous spectrum of phenotypes.
42. What does pleiotropy mean in genetics? Provide an example.
Answer
Pleiotropy occurs when one gene influences multiple unrelated traits. For example, the gene responsible for Marfan syndrome affects the eyes, skeleton, and cardiovascular system.
43. Why are mitochondrial traits passed only through the mother?
Answer
Because the mitochondria are inherited from the egg cell's cytoplasm, and sperm do not contribute mitochondria to the embryo.
44. In a trait influenced by both genetics and the environment, how might identical twins show differences?
Answer
Even though twins share the same genotype, environmental factors such as diet or exposure can lead to differences in phenotype, demonstrating phenotypic plasticity.
45. What type of ratio would you expect in a dihybrid cross influenced by recessive epistasis?
Answer
The expected phenotypic ratio would be 9:3:4 instead of the classic 9:3:3:1, due to one gene masking another.
46. How does X-inactivation in females affect the expression of X-linked traits?
Answer
One X chromosome in each cell is randomly inactivated, so females can show mosaic expression of X-linked traits, such as calico fur in cats.
47. Give an example of a human trait that demonstrates incomplete dominance.
Answer
Sickle cell trait shows incomplete dominance. Heterozygous individuals (AS) have both normal and sickle-shaped red blood cells.
48. How can linked genes violate the law of independent assortment?
Answer
Linked genes are located close together on the same chromosome and tend to be inherited together unless crossing over occurs, thus not assorting independently.
49. Why do polygenic traits not follow simple Mendelian inheritance patterns?
Answer
Because they involve multiple genes contributing small additive effects, leading to continuous variation instead of discrete categories.
50. What is the significance of non-Mendelian inheritance patterns in human genetics?
Answer
Non-Mendelian patterns help explain the inheritance of complex traits and diseases that can't be predicted by Mendel’s laws alone, such as diabetes, cancer risk, or height.