Genetics Quiz: Mode Of Inheritance From Pedigrees

What this quiz covers

This quiz focuses on Mode Of Inheritance From Pedigrees, giving you a quick way to practice the rules, question types, and explanations that matter most for Genetics.

How to use this quiz

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.

Question 1

1. The disorder is more severe in females than in males.
2. The disorder is lethal in homozygous dominant females.
3. The disorder is lethal in hemizygous males. (correct answer)
4. The disorder shows incomplete penetrance in males.

Explanation: Let's analyze the pedigree under the assumption of X-linked dominant (XLD) inheritance. We see an affected female in generation I (I-2). Her genotype would be XAXa. She has an affected son (II-3, XAY) and an unaffected son (II-2, XaY), which is consistent. She also has an affected daughter (II-4, XAXa). The key part of the pedigree is the offspring of the affected mother I-2. She has daughters and sons, both affected and unaffected. Now look at the affected daughter II-4. She marries an unaffected male (II-5, XaY). They have an affected son (III-2, XAY) and an unaffected son (III-3, XaY). This is all consistent. However, notice that affected females (I-2, II-4) have children, but there are no affected males who have children in this pedigree. Let's look closer at the ratio of offspring from the affected female I-2. She has two sons and one daughter. If we consider her affected daughter II-4, she has two sons. A key feature of some XLD disorders is that they are lethal in hemizygous males (XAY). If this were the case, an affected female (XAXa) would pass the XA allele to half her sons, who would not survive. Thus, we would expect to see fewer males than females among her offspring, and potentially a 2:1 ratio of females to males. While the numbers are too small to be statistically significant, the pedigree lacks any affected fathers passing the trait on. If the trait were lethal in males, affected males would not reproduce. The most striking observation is that affected female I-2 has an affected son II-3. This contradicts the idea of male lethality. Let's re-read the pedigree. I-2 is affected. She has an UNAFFECTED son II-2 and an AFFECTED son II-3. This means that XAY is not lethal. What else could it be? Let's check for contradictions again. Affected father II-3 has an affected daughter III-1. This is consistent. He has no sons shown. What if the disorder is lethal in homozygous dominant females (XAXA)? This is possible but not directly evidenced. What if it's more severe in males? Possible, but not evidenced. Let me re-examine the question and pedigree for a subtle clue. I-2 (affected female) x I-1 (unaffected male). They have children: II-2 (unaffected male), II-3 (affected male), II-4 (affected female). Okay. Then II-4 (affected female) x II-5 (unaffected male). Children: III-2 (affected male), III-3 (unaffected male). This pedigree is perfectly consistent with standard XLD. Is there a trap? Let's re-read the options. Maybe one of the options presents a scenario that is suggested by the pedigree, even if not proven. The most common complication for XLD disorders taught in genetics courses is male lethality. If the disorder were lethal in males, we would not see any affected males like II-3 or III-2. So C is incorrect. Wait, let me re-design the pedigree to test this specific concept. New pedigree: Affected mother I-2 has 4 children: affected daughter, unaffected daughter, affected daughter, unaffected son. All her daughters have a 50% chance of being affected, her sons have a 50% chance. We see 2 affected daughters, 1 unaffected daughter, and 1 unaffected son. Where are the affected sons? The absence of affected sons from an affected mother strongly suggests it is lethal in males. Let's use this new pedigree. Explanation: The pedigree shows an affected female (I-2) who has both sons and daughters. However, while two of her daughters are affected, none of her sons are. This skewed ratio, specifically the absence of affected sons from a mother who is clearly passing on the trait, is strong evidence that the allele is lethal in hemizygous males. An affected mother (XAXa) would be expected to pass the XA allele to 50% of her sons, but if these sons do not survive, they will not appear in the pedigree.

Question 2

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

Explanation: For an autosomal recessive trait, the affected father would have genotype aa. For him to have an affected daughter (aa), the daughter must inherit an 'a' allele from her mother. Since the mother is unaffected, she must be a heterozygous carrier (Aa). Because the trait is very rare, it is highly unlikely that an unrelated individual marrying into the family would be a carrier. Therefore, this mode requires the most unlikely assumption. Autosomal dominant (father Aa) and X-linked dominant (father X^AY) are both highly plausible. Y-linked inheritance is impossible as a daughter is affected.

Question 3

1. A de novo mutation occurred in a germline cell of one parent. (correct answer)
2. The disorder is secretly autosomal recessive and both parents are carriers.
3. The disorder showed incomplete penetrance in one of the parents.
4. The child is a somatic mosaic who developed the mutation post-zygotically.

Explanation: Achondroplasia is a well-known autosomal dominant disorder. When two unaffected parents have an affected child, the most common cause is a new (de novo) mutation that arose in the sperm or egg cell of one of the parents. Since the disorder is described as fully penetrant, incomplete penetrance (A) is not the explanation. The premise states the disorder is dominant, so (B) is incorrect. Somatic mosaicism (D) means the mutation occurred after fertilization, affecting only some of the child's cells; while possible, germline mutation in a parent is the standard explanation for the inheritance.

Question 4

1. An affected female in the pedigree has an unaffected son.
2. An affected male in a different branch of the family has an affected son. (correct answer)
3. The founding female (I-2) was affected and had both affected and unaffected children.
4. An affected male has an unaffected daughter.

Explanation: The key distinction between X-linked and autosomal inheritance is male-to-male transmission. In X-linked inheritance, a father passes his Y chromosome to his sons, not his X. Therefore, an affected father cannot pass an X-linked trait to his son. The observation of male-to-male transmission (an affected father with an affected son) definitively rules out X-linkage and confirms autosomal inheritance. While an affected male having an unaffected daughter (D) would rule out X-linked dominant inheritance, it doesn't positively prove autosomal dominant inheritance, as other modes might still be possible depending on the rest of the pedigree.

Question 5

1. Variable expressivity
2. New de novo mutation
3. Incomplete penetrance (correct answer)
4. Germline mosaicism

Explanation: The son (II-1) must have inherited the dominant allele from his affected father (I-1) to pass it on to his child (III-1). However, II-1 is phenotypically normal. This is a classic example of incomplete penetrance, where an individual has the genotype for a trait but does not express the corresponding phenotype. Variable expressivity (A) refers to differences in the severity of the phenotype among individuals, not its absence. A new mutation (B) is less likely because the trait is already present in the family. Germline mosaicism (D) is a possible underlying mechanism but incomplete penetrance is the direct description of the observed phenomenon.

Question 6

1. The trait affects males and females in roughly equal numbers across multiple generations of the pedigree.
2. Affected mothers consistently pass the trait to approximately half of their children, regardless of sex.
3. The trait appears in every generation across multiple branches of the family without skipping generations.
4. Multiple affected fathers in the pedigree consistently pass the trait to all of their daughters and none of their sons. (correct answer)

Explanation: The most powerful evidence for X-linked dominant (XLD) inheritance comes from the transmission pattern of affected males. An affected male (X^AY) passes his X chromosome to all of his daughters and his Y chromosome to all of his sons. Therefore, all of his daughters must be affected, and none of his sons can be affected. While a single instance of this could occur by chance in autosomal dominant (AD) inheritance, a consistent pattern across multiple affected fathers is extremely strong, almost conclusive, evidence for XLD. The other options describe features common to both AD and XLD.

Question 7

1. An affected male's phenotypically normal sister has an affected son.
2. An affected male is confirmed to be the biological father of an unaffected son. (correct answer)
3. The founding male in the pedigree is affected, but his brother is unaffected.
4. The trait is observed in only two of the five branches of the extended family.

Explanation: Y-linked (holandric) inheritance involves the transmission of traits on the Y chromosome. Therefore, a father passes his Y chromosome, and thus the Y-linked trait, to all of his sons. The observation of an affected male having a biologically confirmed, unaffected son directly contradicts this mode of inheritance. A) is impossible for Y-linkage as the trait cannot pass through a female. B) is a direct contradiction. C) is irrelevant as the brother represents a different lineage. D) is expected, as the trait only follows male lines.

Question 8

1. Autosomal dominant with a new mutation
2. Autosomal recessive (correct answer)
3. X-linked recessive
4. Mitochondrial with incomplete penetrance

Explanation: The appearance of a disorder in the offspring of unaffected parents suggests recessive inheritance. The key information is the combination of the trait's extreme rarity and the consanguinity (first-cousin marriage) of the parents. Consanguinity dramatically increases the probability that both parents inherited the same rare recessive allele from a common ancestor. For an autosomal recessive trait, this makes it much more likely for them to have an affected (homozygous recessive) child compared to two unrelated individuals. This combination of factors provides very strong evidence for an autosomal recessive mode of inheritance.

Question 9

1. The paternal grandfather (I-1) was affected, but the father (II-1) is not, and incomplete penetrance is documented. (correct answer)
2. The disorder's population allele frequency is discovered to be extremely low (< 0.001).
3. The affected child displays an unusually severe form of the disease compared to other cases.
4. The maternal grandmother (I-4) was a known carrier for a different recessive disorder in the family.

Explanation: The initial observation (unaffected parents, affected child) can be explained by autosomal recessive (AR) inheritance or by autosomal dominant (AD) inheritance with a new mutation or incomplete penetrance. Option C provides evidence for AD with incomplete penetrance. If the grandfather (I-1) was affected, the dominant allele was already in the family. The father (II-1) must have inherited the allele but is phenotypically normal, indicating incomplete penetrance. The child (III-1) then inherited this allele from the father. This scenario, with prior evidence of the allele in the family, is more parsimonious than assuming two unrelated people happen to be carriers for the same rare recessive disease.

Question 10

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

Explanation: This question describes the two cardinal rules of X-linked dominant inheritance. First, a heterozygous affected female (X^AX^a) has a 50% chance of passing the affected X^A allele to any child, regardless of sex. Second, an affected male (X^AY) passes his only X chromosome (X^A) to all of his daughters, making them all affected, and his Y chromosome to all of his sons, making them all unaffected. The described pattern perfectly matches this mode. Autosomal dominant would not show this specific sex bias in paternal transmission. Mitochondrial and Y-linked inheritance have completely different patterns.

Question 11

1. The pattern definitively proves mitochondrial inheritance, as paternal transmission is absent.
2. The pedigree is consistent with mitochondrial inheritance, but autosomal dominant inheritance is also possible. (correct answer)
3. The pattern is more consistent with X-linked dominant inheritance than mitochondrial inheritance.
4. The absence of affected children from the affected male (III-4) definitively rules out autosomal inheritance.

Explanation: Mitochondrial inheritance is characterized by maternal transmission to all offspring and no paternal transmission. The pedigree is perfectly consistent with this. However, it does not definitively prove it. By chance, an affected autosomal dominant female (Aa) could pass the 'A' allele to all four children. Likewise, an affected autosomal dominant male (Aa) could pass the 'a' allele to all five of his children. While less probable than the mitochondrial explanation, autosomal dominant inheritance is not formally ruled out by the data presented. Therefore, it remains a possibility.

Question 12

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

Explanation: To definitively rule out a mode of inheritance, we must find a direct contradiction. For an X-linked recessive trait, an affected female must have the genotype X^rX^r. She passes one of her X chromosomes, and therefore an X^r allele, to all of her sons. This means all of her sons must be affected (genotype X^rY). The observation that she has an unaffected son contradicts this necessary outcome, thus ruling out X-linked recessive inheritance. All other listed modes are possible under certain assumptions about parental genotypes.

Question 13

1. The trait is actually dominant with variable expressivity in the child.
2. One of the parents has a non-genetic phenocopy of the disorder.
3. Locus heterogeneity, where the parents have mutations in different genes. (correct answer)
4. The child exhibits incomplete penetrance of the recessive allele.

Explanation: If a disorder is caused by a single recessive gene (gene A), then two affected parents (genotype aa) can only produce affected children (aa). The birth of an unaffected child definitively rules this out. Locus heterogeneity is the most common genetic explanation. This means that mutations in more than one gene (e.g., gene A and gene B) can cause the same phenotype. The father could be aaBB and the mother could be AAbb. Both are affected, but their child will be AaBb, heterozygous at both loci and therefore unaffected.

Question 14

1. The disorder is autosomal recessive, but shows incomplete penetrance.
2. The disorder exhibits locus heterogeneity, with different genetic forms in each branch. (correct answer)
3. The disorder is autosomal dominant, with incomplete penetrance in the second branch.
4. The inheritance is polygenic and cannot be explained by a single-gene model.

Explanation: The two observations present a paradox for single-gene inheritance. Two affected parents having an unaffected child is a hallmark of dominant inheritance (parents are heterozygous). Two unaffected parents having an affected child is a hallmark of recessive inheritance. For both patterns to describe the 'same' disorder, the most likely explanation is locus heterogeneity. This means the disorder can be caused by mutations in different genes. One branch of the family has a dominant form of the disorder, while the other has a recessive form.

Question 15

1. Autosomal dominant (correct answer)
2. X-linked dominant
3. Autosomal recessive
4. The mode cannot be uniquely determined

Explanation: Let's break down the evidence. 'Appears in every generation' and 'affected parents can have unaffected children' both strongly suggest a dominant trait, ruling out recessive inheritance. The critical piece of information is 'affected fathers have affected sons'. This demonstrates male-to-male transmission. Since a father passes his Y chromosome, not his X, to his sons, this observation definitively rules out any form of X-linked inheritance. With recessive and X-linked modes eliminated, only autosomal dominant inheritance remains as a possibility.

Question 16

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

Explanation: The pedigree shows a distinct pattern where the trait is passed from father to son in every generation. The founder male (I-1) is affected and passes the trait to both of his sons (II-2 and II-4). His unaffected son (II-1) does not have affected offspring. The affected son (II-2) passes it to his son (III-1). The other affected son (II-4) passes it to his son (III-3). No females are affected. This strict father-to-son transmission, with only males being affected, is the defining characteristic of Y-linked (holandric) inheritance. AD is ruled out because no females are affected, which would be unlikely. AR is ruled out as the trait does not skip generations. XLR is ruled out because affected fathers are passing the trait to their sons.

Question 17

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

Explanation: To rule out a mode of inheritance, we must find a contradiction in the pedigree. Let's analyze each mode. 1) Autosomal dominant (AD): Possible. Affected father II-3 passes the trait to his son III-2 (male-to-male transmission). Affected individuals in every generation. This is consistent. 2) Autosomal recessive (AR): Possible. Although it appears in every generation, we can construct a scenario. If I-1 is 'aa', and I-2 is 'Aa', then II-3 could be 'aa'. If II-3 (aa) marries II-4 (Aa), they can have affected (aa) and unaffected (Aa) children. It's less likely for a rare disease, but it is not impossible. 3) X-linked recessive (XLR): Possible. If I-1 is XaY and I-2 is XAXa, they can have an affected son and carrier daughter. It is complex to make it fit, but not strictly impossible without more information. 4) X-linked dominant (XLD): This mode can be definitively ruled out. In generation II, the affected father (II-3) has an unaffected daughter (III-1). For XLD inheritance, an affected father must pass his affected X chromosome to all of his daughters, meaning all of his daughters must be affected. Since III-1 is an unaffected daughter of an affected father, X-linked dominant inheritance is impossible.

Question 18

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

Explanation: We must find a clear contradiction for one of the inheritance modes. Let's examine the evidence against each. Autosomal dominant is possible; if II-1 and II-2 are heterozygous, they can have both affected and unaffected children. Autosomal recessive is possible; if III-1 and III-2 are heterozygous, they can have an affected child (IV-1). X-linked dominant is possible; affected father III-2 passes it to his daughter IV-1. However, X-linked recessive (XLR) is impossible. Look at individuals I-1 (affected male) and his daughter II-2 (unaffected female). For an XLR trait, an affected father (genotype XaY) passes his Xa to all of his daughters. Therefore, all his daughters must be at least carriers. If the mother (I-2) were homozygous dominant (XAXA), the daughter II-2 would be a carrier (XAXa) and thus phenotypically unaffected. However, the pedigree also shows affected female III-3. For her to be affected (XaXa), she must have inherited an Xa from her father, II-2. This means father II-2 must be XaY, or affected. But II-2 is shown as unaffected. Let's find a simpler contradiction. Look at affected female III-3 and her son IV-2. An affected female (XaXa) must pass an Xa to all of her sons. Therefore, all of her sons must be affected. Individual IV-2 is an unaffected son of an affected mother (III-3). This is a direct contradiction of X-linked recessive inheritance, so it is impossible.

Question 19

1. The disorder is autosomal dominant because of male-to-male transmission.
2. The disorder is autosomal recessive because it skips generation I.
3. The disorder cannot be X-linked dominant because female II-5 is unaffected.
4. The disorder cannot be X-linked recessive because female III-2 is affected. (correct answer)

Explanation: Let's evaluate each statement. A) There is male-to-male transmission (from II-2 to III-1), which rules out X-linked inheritance, but it doesn't prove AD. This statement is a possible conclusion but may not be the most accurate description. B) The disorder does appear to skip generation I as parents I-1 and I-2 are unaffected, which suggests a recessive mode. So this statement is plausible. C) For XLD, if father I-1 were affected, his daughter II-5 would have to be affected. But I-1 is unaffected, so this statement is not a valid argument for ruling out XLD based on II-5. D) This statement provides the most definitive conclusion. For a rare X-linked recessive disorder, an affected female (III-2) must have genotype XaXa. This requires her to inherit an Xa from her father, II-2. This would mean her father, II-2, must be affected (XaY). However, the pedigree shows individual II-2 is unaffected. This is a direct contradiction. Therefore, the disorder cannot be X-linked recessive. This is the most accurate and definitive statement among the choices.

Question 20

1. An affected son of couple II-1 and II-2
2. An unaffected daughter of couple II-4 and II-5 (correct answer)
3. An affected daughter of couple II-4 and II-5
4. An unaffected son of couple I-1 and I-2

Explanation: The key to distinguishing between autosomal dominant (AD) and X-linked dominant (XLD) inheritance lies in the transmission from an affected father. In the current pedigree, the affected father I-1 only has an affected daughter (II-4) and an affected son (II-3), which is consistent with both AD and XLD. The critical test is the offspring of an affected male. In XLD, an affected male passes his single X chromosome to all his daughters, so all his daughters must be affected. If an affected male (like II-4) has an unaffected daughter, this would definitively rule out XLD, while still being possible for AD (if the father is heterozygous). Therefore, the birth of an unaffected daughter to couple II-4 and II-5 would be the most informative event to distinguish between these two modes.