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12. Non-Mendelian Genetics — Practice Questions
This chapter explores complex inheritance patterns beyond Mendel’s laws, including linked genes, polygenic traits, sex-linked inheritance, nonnuclear inheritance, and environmental effects on gene expression.
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(Multiple Choice — Click to Reveal Answer)
1. What is the term for genes that are located close together on the same chromosome and tend to be inherited together?
(A) Independent genes
(B) Linked genes
(C) Dominant genes
(D) Unlinked genes
Answer
(B) — Linked genes are physically close together on the same chromosome and tend to be inherited together.
2. In fruit flies, white eyes are X-linked recessive. If a red-eyed female and a white-eyed male are crossed, what eye color will all the daughters have?
(A) Red
(B) White
(C) Mixed
(D) Cannot be determined
Answer
(A) — All daughters inherit a red allele from the mother and a white allele from the father, so they are heterozygous and express the red-eyed dominant phenotype.
3. A trait that is affected by multiple genes is known as a:
(A) Polygenic trait
(B) Codominant trait
(C) Sex-linked trait
(D) Dominant trait
Answer
(A) — Traits controlled by more than one gene, such as skin color or height, are polygenic.
4. Which of the following is an example of phenotypic plasticity?
(A) Eye color from multiple alleles
(B) Hair color determined by sex-linked genes
(C) Hydrangea flower color changing with soil pH
(D) Color blindness in males
Answer
(C) — Phenotypic plasticity occurs when the environment alters the expression of a trait, such as soil pH affecting flower color.
5. A female carrier for hemophilia (X-linked recessive) has children with a normal male. What percentage of sons are expected to have hemophilia?
(A) 0%
(B) 25%
(C) 50%
(D) 100%
Answer
(C) — Sons have a 50% chance of inheriting the X-linked recessive allele from their carrier mother and will express the trait.
6. Which of the following best describes nonnuclear inheritance?
(A) Traits that skip generations
(B) Traits inherited through mitochondrial DNA
(C) Traits that are X-linked recessive
(D) Traits caused by multiple alleles
Answer
(B) — Nonnuclear inheritance refers to traits passed through organelles like mitochondria, which are inherited maternally.
7. What is the typical result of crossing two organisms heterozygous for three polygenic traits (e.g., AaBbCc × AaBbCc)?
(A) Only one phenotype is possible
(B) 3:1 ratio
(C) Continuous range of phenotypes
(D) Codominant inheritance
Answer
(C) — Polygenic traits produce a continuous spectrum of phenotypes due to additive gene effects.
8. Which of the following is more likely to affect males than females?
(A) Autosomal recessive trait
(B) Polygenic trait
(C) X-linked recessive disorder
(D) Mitochondrial disorder
Answer
(C) — X-linked recessive disorders are more common in males since they only have one X chromosome.
9. Which of the following would be inherited from the mother only?
(A) Y-linked gene
(B) Sex-linked gene
(C) Mitochondrial gene
(D) Dominant allele
Answer
(C) — Mitochondrial genes are inherited maternally because sperm do not contribute mitochondria to the zygote.
10. Which situation shows codominance?
(A) Red × white = pink
(B) Red × white = red and white patches
(C) Red × red = red
(D) Red × white = white only
Answer
(B) — Codominance occurs when both alleles are expressed simultaneously, such as red and white patches appearing together.
11. Which inheritance pattern produces a blending of traits in the heterozygote?
(A) Complete dominance
(B) Codominance
(C) Incomplete dominance
(D) X-linked inheritance
Answer
(C) — In incomplete dominance, the heterozygous phenotype is a blend of both parental traits.
12. A person with type AB blood has both A and B antigens. This is an example of:
(A) Complete dominance
(B) Incomplete dominance
(C) Codominance
(D) Multiple alleles
Answer
(C) — Both alleles are expressed fully and equally in codominance.
13. Traits like skin color and height, which are influenced by many genes, are called:
(A) Monogenic traits
(B) Polygenic traits
(C) Codominant traits
(D) Sex-linked traits
Answer
(B) — Polygenic traits involve the additive effect of multiple genes, creating a range of phenotypes.
14. Which genotype pair represents an individual heterozygous for a codominant trait?
(A) AA
(B) AB
(C) aa
(D) BB
Answer
(B) — In codominance, heterozygous individuals express both alleles (e.g., AB blood type).
15. What does the term “sex-linked” refer to?
(A) Traits found only in males
(B) Traits controlled by genes on autosomes
(C) Traits controlled by genes on sex chromosomes
(D) Traits found in mitochondria
Answer
(C) — Sex-linked traits are located on the X or Y chromosomes, often showing different patterns of inheritance in males and females.
16. What determines the inheritance of mitochondrial DNA?
(A) Father only
(B) Mother only
(C) Both parents
(D) X chromosome
Answer
(B) — Mitochondrial DNA is passed exclusively through the mother.
17. What inheritance pattern is observed when a trait skips generations and affects both sexes equally?
(A) Autosomal dominant
(B) Autosomal recessive
(C) X-linked dominant
(D) Y-linked
Answer
(B) — Autosomal recessive traits can skip generations and usually affect both sexes equally.
18. A female has a genotype XᴬXᵃ. If A is dominant and a is recessive, what is her phenotype?
(A) She has the recessive trait
(B) She has the dominant trait
(C) She has both traits
(D) She has a mitochondrial disorder
Answer
(B) — The dominant allele masks the recessive one, so the phenotype is dominant.
19. What is the main effect of polygenic inheritance on phenotypes?
(A) Few possible outcomes
(B) Random trait distribution
(C) Continuous variation
(D) No dominant or recessive traits
Answer
(C) — Polygenic traits result in a continuum of phenotypes, not distinct categories.
20. What is true about environmental effects on gene expression?
(A) Environment changes genotype
(B) Phenotype can be influenced by environment
(C) Dominant traits are immune to environment
(D) Only sex-linked traits are influenced
Answer
(B) — Environmental factors like temperature, nutrition, and light can influence gene expression and resulting phenotype.
21. Which inheritance pattern explains why men are more likely to be colorblind?
(A) Autosomal dominant
(B) X-linked recessive
(C) Y-linked dominant
(D) Mitochondrial
Answer
(B) — Males only have one X chromosome, so they are more likely to express X-linked recessive traits like colorblindness.
22. Which term refers to a single gene affecting more than one trait?
(A) Polygenic
(B) Epistasis
(C) Pleiotropy
(D) Codominance
Answer
(C) — Pleiotropy is when one gene influences multiple unrelated traits.
23. A female with genotype XᵃXᵃ marries a male with normal vision. What is the probability their sons will be colorblind?
(A) 0%
(B) 25%
(C) 50%
(D) 100%
Answer
(D) — All sons inherit one X from the mother. Since the mother has two recessive alleles, all sons will inherit Xᵃ and be colorblind.
24. Which of the following traits is NOT considered a non-Mendelian trait?
(A) Codominant blood type
(B) Incomplete dominance in flower color
(C) Independent assortment of height and color
(D) Polygenic inheritance of skin tone
Answer
(C) — Independent assortment follows Mendel's laws; the others do not fit strict Mendelian patterns.
25. Which best describes the pattern of inheritance for sickle cell anemia where heterozygotes have a mild advantage?
(A) Codominance
(B) Incomplete dominance
(C) Heterozygote advantage
(D) Pleiotropy
Answer
(C) — Heterozygotes for sickle cell have resistance to malaria, giving them a survival advantage in certain environments.
26. What is the result of a cross between two heterozygous individuals for an incomplete dominant trait (Rr × Rr)?
(A) 1:2:1 genotypic and phenotypic ratio
(B) 3:1 phenotypic ratio
(C) All offspring show dominant phenotype
(D) 9:3:3:1 ratio
Answer
(A) — In incomplete dominance, the heterozygote shows an intermediate phenotype, leading to a 1:2:1 ratio in both genotype and phenotype.
27. Which scenario demonstrates epistasis?
(A) Two alleles blend into one phenotype
(B) One gene suppresses the expression of another gene
(C) Both alleles are fully expressed
(D) A trait is determined by a single dominant allele
Answer
(B) — Epistasis occurs when one gene affects or masks the expression of another gene.
28. In cats, coat color is influenced by a sex-linked gene. A cross between a black female (XᴮXᴮ) and an orange male (XᴼY) produces tortoiseshell females. What type of inheritance is this?
(A) Autosomal dominance
(B) Incomplete dominance
(C) X-linked codominance
(D) Mitochondrial inheritance
Answer
(C) — The female heterozygote (XᴮXᴼ) expresses both alleles due to X-inactivation, leading to a mosaic (tortoiseshell) pattern — a form of X-linked codominance.
29. What is the expected phenotypic ratio for a dihybrid cross with recessive epistasis (AaBb × AaBb)?
(A) 9:3:3:1
(B) 9:3:4
(C) 12:3:1
(D) 1:2:1
Answer
(B) — Recessive epistasis modifies the standard 9:3:3:1 to 9:3:4.
30. A woman with type A blood and a man with type B blood have a child with type O blood. What are the genotypes of the parents?
(A) IAIA and IBIB
(B) IAIA and ii
(C) IAi and IBi
(D) IAIB and ii
Answer
(C) — Both parents must carry the i allele in order to produce a child with type O (ii).
31. A certain flower’s color is determined by a gene that is influenced by temperature. This is an example of:
(A) Codominance
(B) Environmental effect on phenotype
(C) Incomplete dominance
(D) Sex linkage
Answer
(B) — Phenotypic plasticity occurs when environmental factors affect gene expression.
32. What inheritance pattern results when a heterozygote expresses both traits equally and not as a blend?
(A) Codominance
(B) Incomplete dominance
(C) Dominance and recessiveness
(D) Epistasis
Answer
(A) — In codominance, both alleles are fully expressed, not blended (e.g., AB blood type).
33. Why do mitochondrial disorders often affect muscles and nervous tissue the most?
(A) These tissues receive fewer genes from the nucleus
(B) They undergo rapid cell division
(C) They require high amounts of energy from mitochondria
(D) They are inherited from the father
Answer
(C) — Mitochondrial DNA affects energy production, and high-energy tissues like muscles and nerves are most impacted.
34. Which of the following explains why males are more likely to express an X-linked recessive disorder?
(A) They receive two X chromosomes
(B) They are carriers of the gene
(C) They lack a second X to mask recessive alleles
(D) The gene is expressed only in females
Answer
(C) — Males have only one X chromosome, so any recessive allele on it is expressed without opposition.
35. In humans, skin color is affected by several genes. What type of inheritance is this?
(A) Codominance
(B) Polygenic inheritance
(C) Epistasis
(D) Sex linkage
Answer
(B) — Skin color is a classic example of polygenic inheritance, where multiple genes contribute to a continuous range of outcomes.
36. What is codominance? Provide an example from human genetics.
Answer
Codominance is when both alleles in a heterozygote are fully and simultaneously expressed. An example is the AB blood type, where both A and B antigens are present on red blood cells.
37. How does incomplete dominance differ from complete dominance in inheritance patterns?
Answer
In incomplete dominance, the heterozygote shows a blended phenotype between the two alleles, while in complete dominance, the dominant allele fully masks the recessive allele.
38. Explain how a male inherits an X-linked disorder even if both parents show no symptoms.
Answer
If the mother is a carrier (heterozygous) and passes the recessive X-linked allele to her son, he will express the disorder because he lacks a second X chromosome to mask it.
39. What is phenotypic plasticity and how can it affect gene expression?
Answer
Phenotypic plasticity is when an organism’s phenotype changes in response to environmental conditions, such as temperature or pH, without altering the underlying genotype.
40. Why are mitochondrial traits inherited only from the mother?
Answer
Because mitochondria are found in the cytoplasm of the egg cell and not passed on through the sperm, mitochondrial DNA is inherited exclusively from the mother.
41. How can polygenic traits lead to continuous variation in a population?
Answer
Since multiple genes contribute additively to a single trait, polygenic inheritance produces a wide range of intermediate phenotypes, like height or skin color.
42. Define pleiotropy and give one example in human disease.
Answer
Pleiotropy occurs when a single gene influences multiple traits. An example is Marfan syndrome, where one gene affects connective tissue, causing symptoms in the eyes, heart, and skeleton.
43. What is a carrier in X-linked recessive inheritance, and can a carrier express symptoms?
Answer
A carrier is a female who has one normal and one recessive X-linked allele. She typically does not express the trait but can pass it to offspring. In rare cases, X-inactivation can cause mild symptoms.
44. Describe how sex-linked traits are inherited differently in males and females.
Answer
Males have only one X chromosome, so X-linked recessive traits are more likely to be expressed. Females have two X chromosomes, so they must inherit two copies of the recessive allele to express the trait.
45. Why do traits with incomplete dominance or codominance not follow Mendel’s predicted 3:1 phenotype ratio?
Answer
Because the heterozygous phenotype is visibly distinct in both cases, the offspring show three different phenotypes in a 1:2:1 ratio rather than just dominant and recessive.
46. What determines the expression of a polygenic trait?
Answer
Polygenic traits are determined by the additive effects of multiple genes, with each allele contributing a small effect to the overall phenotype.
47. How does X-inactivation in females contribute to mosaicism in traits like tortoiseshell fur in cats?
Answer
X-inactivation randomly turns off one X chromosome in each cell of a female, leading to patches where different alleles are expressed depending on which X is active.
48. What does the term “epistasis” mean in genetics?
Answer
Epistasis refers to a situation where one gene masks or modifies the expression of another gene, often altering expected Mendelian ratios.
49. In what way does environment influence non-Mendelian traits more than Mendelian traits?
Answer
Non-Mendelian traits like polygenic traits are more sensitive to environmental inputs, meaning that phenotype can vary even with the same genotype.
50. Why is studying non-Mendelian inheritance important in understanding human traits and diseases?
Answer
Because many human traits and diseases do not follow simple Mendelian patterns, understanding non-Mendelian inheritance allows scientists to better predict, diagnose, and treat complex conditions.