Genetics Quiz: Mendelian Vs Non Mendelian Inheritance
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Mendelian Vs Non Mendelian InheritanceQuestion 1 of 20

A form of hereditary deafness is known to be an autosomal recessive condition. Two deaf individuals from different families have a child together, and, surprisingly, the child has normal hearing. Assuming there are no new mutations, what is the most likely genetic explanation?

The parents are homozygous for recessive mutations in different genes, and the child is heterozygous at both loci.
One of the parents' deafness was caused by environmental factors, not genetics.
The deafness trait shows incomplete penetrance, and the child did not express the phenotype.
The normal-hearing allele from one parent was dominant over the deafness allele from the other parent.
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Genetics Quiz

Genetics Quiz: Mendelian Vs Non Mendelian Inheritance

Practice Mendelian Vs Non Mendelian Inheritance in Genetics with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Mendelian Vs Non Mendelian Inheritance, giving you a quick way to practice the rules, question types, and explanations that matter most for Genetics.

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Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

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Question 1

A form of hereditary deafness is known to be an autosomal recessive condition. Two deaf individuals from different families have a child together, and, surprisingly, the child has normal hearing. Assuming there are no new mutations, what is the most likely genetic explanation?

  1. The parents are homozygous for recessive mutations in different genes, and the child is heterozygous at both loci. (correct answer)
  2. One of the parents' deafness was caused by environmental factors, not genetics.
  3. The deafness trait shows incomplete penetrance, and the child did not express the phenotype.
  4. The normal-hearing allele from one parent was dominant over the deafness allele from the other parent.

Explanation: When you encounter genetics problems where two affected individuals have an unaffected child, think about genetic heterogeneity - the phenomenon where mutations in different genes can cause the same phenotype. In this case, both parents are deaf due to autosomal recessive conditions, but they likely have mutations in different deafness genes. Let's say Parent 1 is homozygous recessive at gene A (aaBB) and Parent 2 is homozygous recessive at gene B (AAbb). Their child inherits one functional copy of each gene (AaBb), providing enough normal protein function for hearing. This complementation occurs because the child has at least one working copy of each gene needed for normal hearing. Looking at the wrong answers: Option B suggests environmental causes, but the question states this is hereditary deafness, ruling out non-genetic factors. Option C mentions incomplete penetrance, but this doesn't explain why two affected parents would have an unaffected child - incomplete penetrance typically works the opposite way. Option D incorrectly suggests simple dominance, but autosomal recessive traits require two copies of the mutant allele to be expressed, so a single normal allele wouldn't restore hearing unless we're dealing with different genes. The correct answer is A - the parents have recessive mutations in different genes, and their heterozygous child has normal hearing due to complementation. Study tip: Remember that identical phenotypes don't always mean identical genotypes. When genetics problems seem to violate expected inheritance patterns, consider whether multiple genes might be involved in the trait.

Question 2

Prader-Willi syndrome and Angelman syndrome are distinct genetic disorders caused by the same deletion on human chromosome 15. The resulting phenotype depends on whether the deletion is inherited from the father (Prader-Willi) or the mother (Angelman). This differential expression of a gene based on its parental origin is an example of:

  1. Sex-linked inheritance
  2. Mitochondrial inheritance
  3. Genomic imprinting (correct answer)
  4. Pleiotropy

Explanation: Genomic imprinting is an epigenetic phenomenon where certain genes are expressed in a parent-of-origin-specific manner. In this case, genes in the critical region on chromosome 15 are 'imprinted' or silenced in either the egg or the sperm. Consequently, the zygote has only one active copy. If the active copy inherited from the father is deleted, Prader-Willi syndrome results. If the active copy inherited from the mother is deleted, Angelman syndrome results. This is a classic example of imprinting, a non-Mendelian effect.

Question 3

In Labrador retrievers, coat color is determined by two genes. The B/b gene determines pigment color (B=black, b=brown). The E/e gene controls pigment deposition (E=deposition, e=no deposition). A dog with the genotype ee will be yellow regardless of its B/b genotype. A cross is made between two black labs, both with the genotype BbEe. What is the expected phenotypic ratio in their offspring?

  1. 9 black : 3 brown : 4 yellow (correct answer)
  2. 9 black : 3 yellow : 3 brown : 1 golden
  3. 12 black : 3 brown : 1 yellow
  4. 1 black : 1 brown : 1 yellow

Explanation: This is a case of recessive epistasis, where the homozygous recessive genotype at one locus (ee) masks the expression of the other locus (B/b). The dihybrid cross BbEe x BbEe produces genotypes in a 9 B_E_ : 3 B_ee : 3 bbE_ : 1 bbee ratio.

  • B_E_ will be black (9/16).
  • bbE_ will be brown (3/16).
  • B_ee and bbee will both be yellow because the ee genotype prevents pigment deposition (3/16 + 1/16 = 4/16). This results in a 9:3:4 phenotypic ratio.

Question 4

A researcher analyzes the inheritance of a metabolic disorder in a large family. A pedigree reveals that the disorder is passed from an affected mother to all of her children, both male and female. However, affected fathers never pass the disorder to any of their children. This pattern is a strong indicator of which non-Mendelian mode of inheritance?

  1. Autosomal recessive
  2. Y-linked
  3. Mitochondrial inheritance (correct answer)
  4. X-linked dominant

Explanation: Mitochondrial DNA (mtDNA) is inherited exclusively from the mother through the cytoplasm of the egg cell. Therefore, an affected mother will pass her mitochondria (and any mutations within the mtDNA) to all of her offspring. Since sperm contribute virtually no mitochondria to the zygote, affected fathers do not transmit mitochondrial traits to their children. This pattern of strict maternal inheritance is the defining characteristic of mitochondrial inheritance.

Question 5

In a hypothetical insect, wing color is determined by two independently assorting genes, A and B. A researcher performs a cross between two true-breeding strains, one with white wings (AABB) and another also with white wings (aabb). The resulting F1 generation all have red wings. When the F1 insects are self-crossed, the F2 generation exhibits a phenotypic ratio of 9 red-winged insects to 7 white-winged insects. This outcome is best explained by which non-Mendelian inheritance pattern?

  1. Dominant epistasis, where one dominant allele masks the effect of the other gene.
  2. Recessive epistasis, where a recessive genotype at one locus masks the effect of the other locus.
  3. Codominance, where heterozygotes for a single gene express both parental phenotypes.
  4. Complementary gene action, where at least one dominant allele from each gene is required for the red phenotype. (correct answer)

Explanation: The 9:7 ratio is a classic signature of complementary gene action, a form of epistasis. This occurs when two genes work in tandem to produce a phenotype. The expected dihybrid Mendelian ratio is 9:3:3:1. In this case, the genotypes A_B_ (9/16) produce red wings, while A_bb (3/16), aaB_ (3/16), and aabb (1/16) all produce white wings. The 3+3+1 genotypes are phenotypically indistinguishable, leading to a (9) : (3+3+1) or 9:7 ratio. This requires a dominant allele from both gene A and gene B to complete the pathway for red pigment.

Question 6

An allele responsible for an autosomal recessive disorder is analyzed. At the organismal level, heterozygotes are phenotypically normal, suggesting the allele is fully recessive. However, at the biochemical level, analysis of the relevant enzyme shows that heterozygotes produce exactly 50% of the enzyme activity seen in homozygous normal individuals. This discrepancy implies that:

  1. The concepts of dominance and recessiveness are absolute and do not vary by the level of analysis.
  2. The trait displays complete dominance at the organismal level but incomplete dominance at the biochemical level. (correct answer)
  3. The trait is actually codominant because two different levels of enzyme activity can be measured.
  4. The inheritance pattern is Mendelian at the organismal level but non-Mendelian at the biochemical level.

Explanation: Dominance relationships are defined by the phenotype of the heterozygote relative to the two homozygotes. This relationship can change depending on the level of observation. In this case, at the organismal level, the heterozygote is indistinguishable from the homozygous dominant, showing complete dominance. At the biochemical level, the heterozygote's phenotype (enzyme activity) is intermediate between the two homozygotes, which is the definition of incomplete dominance. This illustrates that dominance is a property of the phenotype, not the gene itself.

Question 7

In humans, the ABO blood group system is determined by three alleles: IA, IB, and i. A man with type A blood and a woman with type B blood have their first child, who has type O blood. What is the probability that their second child will have type AB blood?

  1. 0%
  2. 25% (correct answer)
  3. 50%
  4. 75%

Explanation: For a type A parent and a type B parent to have a type O (ii) child, both parents must be heterozygous, carrying the recessive i allele. Therefore, the man's genotype must be IAi and the woman's genotype must be IBi. A Punnett square for the cross IAi x IBi shows the following possible offspring genotypes: IAIB (type AB), IAi (type A), IBi (type B), and ii (type O). Each genotype has an equal probability of 1/4. Thus, the probability of their second child having type AB blood (IAIB) is 1/4 or 25%.

Question 8

A botanist crosses a true-breeding plant with purple flowers and long pollen grains with a plant that is true-breeding for red flowers and round pollen grains. All F1 offspring have purple flowers and long pollen. An F1 x F1 cross yields the following F2 progeny: 284 purple/long, 21 purple/round, 21 red/long, and 55 red/round. A chi-square test for independent assortment yields a p-value < 0.001. What is the most likely reason for the deviation from the expected 9:3:3:1 Mendelian ratio?

  1. The genes for flower color and pollen shape are genetically linked. (correct answer)
  2. The purple allele is epistatic to the pollen shape gene.
  3. The red flower and round pollen alleles are recessive lethal.
  4. The flower color gene exhibits incomplete dominance.

Explanation: The expected 9:3:3:1 ratio for a dihybrid cross assumes the two genes assort independently. The observed numbers show a significant overrepresentation of the parental phenotypes (purple/long and red/round) and an underrepresentation of the recombinant phenotypes (purple/round and red/long). This statistical deviation (confirmed by the low p-value) is the classic sign of genetic linkage, meaning the two genes are located close together on the same chromosome and tend to be inherited together more often than not.

Question 9

A cross between two heterozygous yellow mice (AYa x AYa) produces offspring in a ratio of 2 yellow mice to 1 agouti (wild-type) mouse. The homozygous dominant genotype (AYAY) is never observed among the live births. This distorted 2:1 ratio, instead of the expected Mendelian 3:1 ratio, is best explained by:

  1. Codominance between the AY and a alleles.
  2. A recessive lethal allele. (correct answer)
  3. Incomplete penetrance of the yellow allele.
  4. Epistasis with another gene.

Explanation: The AY allele is a recessive lethal allele. While it is dominant with respect to coat color (AYa is yellow), it is recessive with respect to viability. The expected genotypic ratio from an AYa x AYa cross is 1 AYAY : 2 AYa : 1 aa. However, the AYAY genotype is lethal and these embryos do not survive. Therefore, the observed ratio among living offspring is 2 yellow (AYa) : 1 agouti (aa).

Question 10

A researcher is studying a trait controlled by a single gene with two alleles, T and t. A standard Mendelian model predicts that a Tt x Tt cross should yield a 3:1 phenotypic ratio. However, the researcher consistently observes a 2:1 ratio. Which of the following non-Mendelian phenomena is the most plausible cause, assuming the experimental setup is correct?

  1. The T allele is codominant with the t allele.
  2. The homozygous dominant genotype (TT) is lethal. (correct answer)
  3. The trait exhibits 66% penetrance in the offspring.
  4. The t allele is epistatic to the T allele.

Explanation: A Tt x Tt cross is expected to produce genotypes in a 1 TT : 2 Tt : 1 tt ratio. If the T allele is dominant, this gives a 3 (TT and Tt) : 1 (tt) phenotypic ratio. If the homozygous dominant genotype (TT) is lethal, those individuals die before they can be counted. This removes one category from the genotypic ratio, leaving a ratio of 2 Tt : 1 tt among the surviving offspring. If T is dominant, this results in a 2:1 phenotypic ratio. Codominance or incomplete dominance would produce a 1:2:1 phenotypic ratio. Penetrance issues or epistasis would not typically result in this specific 2:1 ratio from a monohybrid cross.

Question 11

A single gene mutation in humans causes Marfan syndrome, which is characterized by a suite of seemingly unrelated symptoms, including disproportionately long limbs, scoliosis, heart valve defects, and dislocation of the eye lens. The ability of a single gene to influence multiple, distinct phenotypic traits is known as:

  1. Polygenic inheritance
  2. Variable expressivity
  3. Pleiotropy (correct answer)
  4. Incomplete penetrance

Explanation: Pleiotropy is the phenomenon where one gene influences two or more seemingly unrelated phenotypic traits. Marfan syndrome is a classic example, where a mutation in the FBN1 gene affects connective tissue throughout the body, leading to diverse symptoms. Polygenic inheritance is the opposite, where multiple genes contribute to a single trait. Variable expressivity refers to the range of severity of a phenotype, not the presence of multiple distinct traits. Incomplete penetrance refers to when an individual with the genotype for a trait does not express the trait at all.

Question 12

In some sheep, the allele for horned (H) is dominant in males but recessive in females. The allele for hornless (h) is recessive in males but dominant in females. A heterozygous male (Hh) is horned, while a heterozygous female (Hh) is hornless. This pattern of inheritance, where an allele's dominance is dependent on the sex of the individual, is known as:

  1. Sex-linked inheritance
  2. Sex-limited inheritance
  3. Sex-influenced inheritance (correct answer)
  4. Y-linked inheritance

Explanation: Sex-influenced traits are determined by autosomal genes, but their expression is modified by the sex hormones of the individual. As a result, the same heterozygous genotype (Hh) has a different phenotype in males (horned) versus females (hornless). This is different from sex-linked traits, where the gene is located on a sex chromosome (X or Y), and sex-limited traits, where expression is restricted to only one sex (e.g., lactation).

Question 13

Human height is a trait that shows continuous variation within the population, following a bell-shaped curve of distribution. This pattern is difficult to explain using simple Mendelian genetics. Which of the following provides the best explanation for the inheritance of human height?

  1. A single gene with multiple alleles determines the full range of human height.
  2. Pleiotropy, where a single height-determining gene also affects other traits.
  3. Incomplete dominance at a single genetic locus results in a blended height.
  4. Polygenic inheritance, where multiple genes contribute additively to the phenotype. (correct answer)

Explanation: Traits that exhibit continuous variation, such as height, weight, and skin color, are typically polygenic. This means they are influenced by the combined effects of many different genes (and environmental factors). Each gene contributes a small, additive effect to the final phenotype, leading to a continuous spectrum of outcomes that often approximates a normal (bell-shaped) distribution in a population. The other options describe patterns that result in discrete, not continuous, phenotypic categories.

Question 14

A dihybrid test cross is performed in fruit flies between a fly heterozygous for black body (b) and vestigial wings (vg) and a fly with a wild-type phenotype (gray body, normal wings). The genes are in coupling (cis) configuration in the heterozygous parent (BVg/bvg). The resulting offspring are counted: 820 gray body/normal wings, 804 black body/vestigial wings, 185 gray body/vestigial wings, and 191 black body/normal wings. What do these results suggest about the two genes?

  1. The genes assort independently, following Mendelian principles.
  2. The genes are linked, and the recombination frequency is approximately 50%.
  3. The genes are linked, and the recombination frequency is significantly less than 50%. (correct answer)
  4. The genes exhibit a lethal interaction, altering the expected phenotypic ratios.

Explanation: Independent assortment in a dihybrid test cross would produce a 1:1:1:1 ratio of the four phenotypes. The observed numbers (820:804:185:191) are a significant deviation from this. The parental phenotypes (gray/normal and black/vestigial) are far more common than the recombinant phenotypes (gray/vestigial and black/normal). This indicates the genes are linked on the same chromosome. The recombination frequency is calculated as (total recombinants / total offspring) × 100 = ((185 + 191) / (820 + 804 + 185 + 191)) × 100 = (376 / 2000) × 100 = 18.8%. This value is significantly less than the 50% recombination frequency expected for independently assorting genes.

Question 15

In a hypothetical insect, wing color is determined by two independently assorting genes, A and B. A researcher performs a cross between two true-breeding strains, one with white wings (AABB) and another also with white wings (aabb). The resulting F1 generation all have red wings. When the F1 insects are self-crossed, the F2 generation exhibits a phenotypic ratio of 9 red-winged insects to 7 white-winged insects. This outcome is best explained by which non-Mendelian inheritance pattern?

  1. Dominant epistasis, where one dominant allele masks the effect of the other gene.
  2. Recessive epistasis, where a recessive genotype at one locus masks the effect of the other locus.
  3. Codominance, where heterozygotes for a single gene express both parental phenotypes.
  4. Complementary gene action, where at least one dominant allele from each gene is required for the red phenotype. (correct answer)

Explanation: The 9:7 ratio is a classic signature of complementary gene action, a form of epistasis. This occurs when two genes work in tandem to produce a phenotype. The expected dihybrid Mendelian ratio is 9:3:3:1. In this case, the genotypes A_B_ (9/16) produce red wings, while A_bb (3/16), aaB_ (3/16), and aabb (1/16) all produce white wings. The 3+3+1 genotypes are phenotypically indistinguishable, leading to a (9) : (3+3+1) or 9:7 ratio. This requires a dominant allele from both gene A and gene B to complete the pathway for red pigment.

Question 16

A single gene mutation in humans causes Marfan syndrome, which is characterized by a suite of seemingly unrelated symptoms, including disproportionately long limbs, scoliosis, heart valve defects, and dislocation of the eye lens. The ability of a single gene to influence multiple, distinct phenotypic traits is known as:

  1. Polygenic inheritance
  2. Variable expressivity
  3. Pleiotropy (correct answer)
  4. Incomplete penetrance

Explanation: Pleiotropy is the phenomenon where one gene influences two or more seemingly unrelated phenotypic traits. Marfan syndrome is a classic example, where a mutation in the FBN1 gene affects connective tissue throughout the body, leading to diverse symptoms. Polygenic inheritance is the opposite, where multiple genes contribute to a single trait. Variable expressivity refers to the range of severity of a phenotype, not the presence of multiple distinct traits. Incomplete penetrance refers to when an individual with the genotype for a trait does not express the trait at all.

Question 17

A researcher analyzes the inheritance of a metabolic disorder in a large family. A pedigree reveals that the disorder is passed from an affected mother to all of her children, both male and female. However, affected fathers never pass the disorder to any of their children. This pattern is a strong indicator of which non-Mendelian mode of inheritance?

  1. Autosomal recessive
  2. Y-linked
  3. Mitochondrial inheritance (correct answer)
  4. X-linked dominant

Explanation: Mitochondrial DNA (mtDNA) is inherited exclusively from the mother through the cytoplasm of the egg cell. Therefore, an affected mother will pass her mitochondria (and any mutations within the mtDNA) to all of her offspring. Since sperm contribute virtually no mitochondria to the zygote, affected fathers do not transmit mitochondrial traits to their children. This pattern of strict maternal inheritance is the defining characteristic of mitochondrial inheritance.

Question 18

Human height is a trait that shows continuous variation within the population, following a bell-shaped curve of distribution. This pattern is difficult to explain using simple Mendelian genetics. Which of the following provides the best explanation for the inheritance of human height?

  1. A single gene with multiple alleles determines the full range of human height.
  2. Pleiotropy, where a single height-determining gene also affects other traits.
  3. Incomplete dominance at a single genetic locus results in a blended height.
  4. Polygenic inheritance, where multiple genes contribute additively to the phenotype. (correct answer)

Explanation: Traits that exhibit continuous variation, such as height, weight, and skin color, are typically polygenic. This means they are influenced by the combined effects of many different genes (and environmental factors). Each gene contributes a small, additive effect to the final phenotype, leading to a continuous spectrum of outcomes that often approximates a normal (bell-shaped) distribution in a population. The other options describe patterns that result in discrete, not continuous, phenotypic categories.

Question 19

A cross between two heterozygous yellow mice (AYa x AYa) produces offspring in a ratio of 2 yellow mice to 1 agouti (wild-type) mouse. The homozygous dominant genotype (AYAY) is never observed among the live births. This distorted 2:1 ratio, instead of the expected Mendelian 3:1 ratio, is best explained by:

  1. Codominance between the AY and a alleles.
  2. A recessive lethal allele. (correct answer)
  3. Incomplete penetrance of the yellow allele.
  4. Epistasis with another gene.

Explanation: The AY allele is a recessive lethal allele. While it is dominant with respect to coat color (AYa is yellow), it is recessive with respect to viability. The expected genotypic ratio from an AYa x AYa cross is 1 AYAY : 2 AYa : 1 aa. However, the AYAY genotype is lethal and these embryos do not survive. Therefore, the observed ratio among living offspring is 2 yellow (AYa) : 1 agouti (aa).

Question 20

An allele responsible for an autosomal recessive disorder is analyzed. At the organismal level, heterozygotes are phenotypically normal, suggesting the allele is fully recessive. However, at the biochemical level, analysis of the relevant enzyme shows that heterozygotes produce exactly 50% of the enzyme activity seen in homozygous normal individuals. This discrepancy implies that:

  1. The concepts of dominance and recessiveness are absolute and do not vary by the level of analysis.
  2. The trait displays complete dominance at the organismal level but incomplete dominance at the biochemical level. (correct answer)
  3. The trait is actually codominant because two different levels of enzyme activity can be measured.
  4. The inheritance pattern is Mendelian at the organismal level but non-Mendelian at the biochemical level.

Explanation: Dominance relationships are defined by the phenotype of the heterozygote relative to the two homozygotes. This relationship can change depending on the level of observation. In this case, at the organismal level, the heterozygote is indistinguishable from the homozygous dominant, showing complete dominance. At the biochemical level, the heterozygote's phenotype (enzyme activity) is intermediate between the two homozygotes, which is the definition of incomplete dominance. This illustrates that dominance is a property of the phenotype, not the gene itself.