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12. Non-Mendelian Genetics — Practice Questions 3
This chapter explores genetic inheritance patterns beyond Mendel’s laws, including codominance, incomplete dominance, polygenic traits, linked genes, environmental influences, and nonnuclear (mitochondrial) inheritance.
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(Multiple Choice — Click to Reveal Answer)
1. What does it mean when two genes are "linked"?
(A) They are located on different chromosomes
(B) They undergo independent assortment
(C) They are inherited together due to proximity on the same chromosome
(D) They have identical sequences
Answer
(C) — Linked genes are located close together on the same chromosome and tend to be inherited together.
2. What term describes when the same genotype can result in different phenotypes due to environmental factors?
(A) Incomplete dominance
(B) Genetic drift
(C) Phenotypic plasticity
(D) Codominance
Answer
(C) — Phenotypic plasticity occurs when environmental conditions influence the expression of a genotype.
3. A red-flowered plant crossed with a white-flowered plant produces pink-flowered offspring. What inheritance pattern is this?
(A) Complete dominance
(B) Codominance
(C) Incomplete dominance
(D) Polygenic inheritance
Answer
(C) — In incomplete dominance, the heterozygous phenotype is intermediate between the two homozygous forms.
4. What is a key feature of codominance?
(A) One allele is dominant and masks the other
(B) The heterozygote expresses both alleles equally
(C) The phenotype is a mix of the two alleles
(D) The recessive allele is not expressed
Answer
(B) — In codominance, both alleles are fully and equally expressed in the phenotype.
5. Which of the following is an example of a polygenic trait?
(A) Blood type
(B) Skin color
(C) Widow’s peak
(D) Attached earlobes
Answer
(B) — Polygenic traits, like skin color, result from the combined effect of multiple genes.
6. What pattern of inheritance is shown when only mothers pass a trait to all of their children?
(A) Autosomal dominant
(B) X-linked recessive
(C) Mitochondrial inheritance
(D) Y-linked inheritance
Answer
(C) — Mitochondrial DNA is inherited maternally, so only mothers pass it to their offspring.
7. In humans, why are males more likely to express sex-linked recessive traits?
(A) They inherit two X chromosomes
(B) They inherit mitochondrial DNA from their father
(C) They have only one X chromosome
(D) They inherit the gene from their Y chromosome
Answer
(C) — Males have only one X chromosome, so a single recessive allele on that chromosome will be expressed.
8. Which of the following is an example of environmental influence on gene expression in humans?
(A) Blood type
(B) Freckles developing after sun exposure
(C) Eye color
(D) Attached earlobes
Answer
(B) — UV light can stimulate melanin production, changing skin pigmentation — an example of phenotypic plasticity.
9. In fruit flies, the white-eye allele is X-linked and recessive. Which cross would result in all white-eyed males?
(A) XᴿXʷ × XʷY
(B) XʷXʷ × XᴿY
(C) XʷXʷ × XʷY
(D) XᴿXᴿ × XʷY
Answer
(B) — All sons will inherit the Xʷ from the mother and Y from the father, resulting in white eyes.
10. What is a typical phenotypic result of polygenic inheritance in a large population?
(A) Two distinct phenotypes
(B) Discrete ratios like 3:1
(C) A range of phenotypes forming a bell curve
(D) Traits expressed only in males
Answer
(C) — Polygenic traits produce a continuum of phenotypes, typically forming a normal distribution.
11. What does the term pleiotropy refer to in genetics?
(A) A gene that has multiple alleles
(B) A gene that affects multiple traits
(C) A trait controlled by multiple genes
(D) A trait inherited only through the mother
Answer
(B) — Pleiotropy occurs when one gene influences multiple, seemingly unrelated phenotypic traits.
12. Which of the following traits is most likely to be influenced by polygenic inheritance?
(A) Cystic fibrosis
(B) Hemophilia
(C) Eye color
(D) Blood type
Answer
(C) — Eye color results from the additive effects of multiple genes, making it polygenic.
13. A red cow and a white cow produce a calf with red and white patches. This is an example of:
(A) Incomplete dominance
(B) Polygenic inheritance
(C) Codominance
(D) Sex-linked inheritance
Answer
(C) — Codominance occurs when both alleles are expressed fully, as in red and white spotted fur.
14. A trait that shows a wide range of phenotypes is most likely controlled by:
(A) A single dominant gene
(B) A single recessive gene
(C) Codominance
(D) Multiple genes (polygenic)
Answer
(D) — Polygenic traits are responsible for continuous variation across a range of phenotypes.
15. Why can’t a father pass an X-linked trait directly to his son?
(A) Sons only inherit X chromosomes from their fathers
(B) Sons inherit the Y chromosome from their fathers
(C) X-linked traits are always dominant
(D) Males have no X chromosome
Answer
(B) — Sons inherit their X chromosome from their mother and the Y chromosome from their father.
16. What kind of inheritance results in a 1:2:1 phenotypic ratio in the offspring?
(A) Codominance or incomplete dominance
(B) X-linked dominant
(C) Polygenic trait
(D) Autosomal dominant
Answer
(A) — Codominance and incomplete dominance both produce a 1:2:1 phenotypic ratio in heterozygous crosses.
17. Which of the following best describes mitochondrial inheritance?
(A) Traits passed only from the father
(B) Traits passed only from the mother
(C) Traits controlled by nuclear DNA
(D) Traits found on autosomes
Answer
(B) — Mitochondria, and thus mitochondrial genes, are inherited maternally.
18. What is a carrier in the context of sex-linked inheritance?
(A) A male with a dominant allele
(B) A female with two recessive alleles
(C) A female with one dominant and one recessive allele
(D) A male with no alleles for the trait
Answer
(C) — A female with one recessive and one dominant allele for an X-linked trait is a carrier.
19. Which pair of genotypes can result in a child with type O blood?
(A) IAIB × IAIA
(B) IAIB × ii
(C) IAi × IBi
(D) IBIB × IAIA
Answer
(C) — If both parents carry the i allele, their child could inherit ii, which results in type O blood.
20. A person has genotype IAIB for blood type. What will be their phenotype?
(A) Type A
(B) Type B
(C) Type O
(D) Type AB
Answer
(D) — The IA and IB alleles are codominant, so both are expressed as AB blood type.
21. What is the role of environment in phenotypic plasticity?
(A) Environment alters genotype permanently
(B) Environment directly determines allele frequencies
(C) Environment can influence phenotype expression without changing genotype
(D) Environment has no effect on inheritance
Answer
(C) — In phenotypic plasticity, the genotype remains the same, but the phenotype varies depending on environmental conditions.
22. What is true of traits inherited through incomplete dominance?
(A) Recessive allele is not expressed
(B) Only dominant allele is visible
(C) The heterozygote has an intermediate phenotype
(D) Only one phenotype is observed
Answer
(C) — In incomplete dominance, the heterozygote expresses a phenotype between the two homozygous forms.
23. Which of the following best illustrates a non-Mendelian inheritance pattern?
(A) A cross that results in 3:1 offspring
(B) Inheritance of attached earlobes
(C) Variation in height across a population
(D) Inheritance of cystic fibrosis
Answer
(C) — Height is influenced by many genes and environmental factors, a hallmark of polygenic, non-Mendelian inheritance.
24. What is the expected ratio of phenotypes in a codominant cross between two heterozygotes?
(A) 3:1
(B) 1:1
(C) 1:2:1
(D) 2:1
Answer
(C) — Codominance produces a 1:2:1 phenotypic ratio: one for each homozygous form and one for the heterozygote.
25. A person expresses a trait differently depending on light exposure. This is an example of:
(A) Pleiotropy
(B) Incomplete dominance
(C) Environmental influence
(D) Polygenic inheritance
Answer
(C) — Environmental conditions like light can affect the way a gene is expressed in phenotypic plasticity.
26. A cross between two heterozygotes (Rr × Rr) results in a 1:2:1 phenotype ratio. What kind of inheritance does this represent?
(A) Polygenic inheritance
(B) Incomplete dominance
(C) Codominance
(D) Epistasis
Answer
(B) — In incomplete dominance, the heterozygote has a distinct intermediate phenotype, resulting in a 1:2:1 ratio.
27. A flower exhibits both red and white patches. Which type of inheritance explains this pattern?
(A) Complete dominance
(B) Codominance
(C) Incomplete dominance
(D) Polygenic inheritance
Answer
(B) — Codominance results in both alleles being expressed distinctly and simultaneously in the phenotype.
28. A woman with normal vision has a father who is colorblind. What is her genotype?
(A) XᴺXᴺ
(B) XᴺXᶜ
(C) XᶜXᶜ
(D) XᶜY
Answer
(B) — She must be a carrier (XᴺXᶜ) since she received the Xᶜ from her father and is not affected herself.
29. A trait is influenced by many genes and is also affected by nutrition. What is this an example of?
(A) Polygenic trait
(B) Codominance
(C) Environmental inheritance
(D) Mitochondrial inheritance
Answer
(A) — Traits like height and weight are polygenic and influenced by environmental factors like nutrition.
30. What is the expected phenotypic ratio from a dihybrid cross affected by recessive epistasis?
(A) 9:3:3:1
(B) 9:4:3
(C) 9:3:4
(D) 12:3:1
Answer
(C) — Recessive epistasis modifies the dihybrid 9:3:3:1 ratio to 9:3:4 due to one gene masking another.
31. Why does mitochondrial inheritance not follow Mendel’s laws?
(A) It is passed through chromosomes
(B) It follows a 3:1 ratio
(C) It is inherited only through the mother
(D) It only affects males
Answer
(C) — Mitochondrial DNA is inherited exclusively from the mother and does not segregate through meiosis like nuclear DNA.
32. A heterozygous individual for sickle cell (HbA/HbS) is resistant to malaria. What is this an example of?
(A) Pleiotropy
(B) Polygenic inheritance
(C) Heterozygote advantage
(D) Codominance
Answer
(C) — Heterozygote advantage provides a survival benefit (malaria resistance) without full disease expression.
33. Which of the following is a hallmark of pleiotropy?
(A) One gene controls one trait
(B) One gene affects multiple unrelated traits
(C) Many genes affect one trait
(D) One trait is affected only by environment
Answer
(B) — Pleiotropy refers to one gene having multiple phenotypic effects, as in Marfan syndrome.
34. In a cross involving codominant alleles, what is the phenotype of a heterozygote?
(A) Same as homozygous dominant
(B) A blend of both traits
(C) Both traits expressed fully and separately
(D) No trait is visible
Answer
(C) — In codominance, both alleles contribute fully and independently to the phenotype.
35. Which situation can lead to a deviation from the expected Mendelian ratios in offspring?
(A) Complete dominance
(B) Independent assortment
(C) Environmental influence on gene expression
(D) Simple monohybrid cross
Answer
(C) — Environmental factors can modify phenotypic outcomes, leading to unexpected ratios despite Mendelian genotypes.
36. Describe how codominance differs from incomplete dominance in terms of phenotype.
Answer
In codominance, both alleles are expressed fully and separately (e.g., AB blood type). In incomplete dominance, the heterozygous phenotype is a blend of both alleles (e.g., red × white = pink flowers).
37. What is X-inactivation, and how does it affect female mammals?
Answer
X-inactivation randomly silences one of the two X chromosomes in female cells, creating mosaic expression of X-linked genes (e.g., calico fur in cats).
38. How can linked genes affect inheritance patterns in dihybrid crosses?
Answer
Linked genes are inherited together more frequently than expected due to their proximity on the same chromosome, which reduces recombination frequency and alters Mendelian ratios.
39. Why do mitochondrial traits show maternal inheritance?
Answer
Because mitochondria are located in the cytoplasm of the egg, only mothers contribute mitochondrial DNA to offspring.
40. What is pleiotropy, and provide one example in humans.
Answer
Pleiotropy is when a single gene affects multiple traits. For example, the gene responsible for Marfan syndrome affects connective tissue, eyes, and the cardiovascular system.
41. Explain how polygenic inheritance leads to continuous variation in traits like height.
Answer
Multiple genes contribute small additive effects to the phenotype, resulting in a range of intermediate outcomes rather than distinct categories.
42. How does phenotypic plasticity demonstrate the interaction between genotype and environment?
Answer
Phenotypic plasticity shows that environmental conditions, such as temperature or pH, can alter how genes are expressed without changing the underlying DNA sequence.
43. Why are males more likely to express X-linked recessive disorders than females?
Answer
Males have only one X chromosome, so if it carries a recessive allele, they will express the trait because there's no second X to mask it.
44. What is heterozygote advantage? Give an example.
Answer
Heterozygote advantage occurs when heterozygous individuals have greater fitness than homozygotes. For example, carriers of the sickle cell trait (HbA/HbS) are resistant to malaria.
45. A plant’s flower color changes based on the pH of the soil. What does this illustrate?
Answer
This is phenotypic plasticity, where environmental conditions like soil pH influence gene expression and visible traits, such as hydrangea flower color.
46. What genetic principle explains why red and white fur appear together in the offspring of cattle?
Answer
This is codominance, where both alleles contribute to the phenotype independently and are fully expressed (e.g., roan coat).
47. What happens to the expected 9:3:3:1 dihybrid ratio if one gene exhibits recessive epistasis?
Answer
The ratio becomes 9:3:4 because the epistatic gene masks the effect of another gene when in the homozygous recessive state.
48. What is the inheritance pattern of a trait that is passed from mothers to all of their children, but never from fathers?
Answer
This describes mitochondrial inheritance, which is strictly maternal because sperm contribute no mitochondria to the zygote.
49. How does pleiotropy differ from polygenic inheritance?
Answer
Pleiotropy involves one gene affecting multiple traits, whereas polygenic inheritance involves multiple genes affecting a single trait.
50. Why are traits like intelligence or height difficult to predict using simple genetic crosses?
Answer
They are polygenic and influenced by many environmental factors, making their inheritance complex and non-Mendelian in nature.