Genetics Quiz
Practice Recurrence Risk From Pedigrees in Genetics with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.
Question 1 / 17
0 of 17 answered
A woman's father has an autosomal dominant condition that is 100% penetrant. The woman has undergone genetic testing and is confirmed to not carry the familial mutation. She and her partner, who has no family history of the condition, have a child. What is the risk that their child will have the condition?
This quiz focuses on Recurrence Risk From Pedigrees, giving you a quick way to practice the rules, question types, and explanations that matter most for Genetics.
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.
A woman's father has an autosomal dominant condition that is 100% penetrant. The woman has undergone genetic testing and is confirmed to not carry the familial mutation. She and her partner, who has no family history of the condition, have a child. What is the risk that their child will have the condition?
Explanation: The woman's father is affected with an autosomal dominant condition, so her prior risk of inheriting the mutation was 50%. However, the genetic test result supersedes the risk assessment based on the pedigree. The test has confirmed she does not carry the mutation. Therefore, she cannot pass it on to her children. Assuming the test is accurate and there are no other sources of the mutation (like a de novo mutation in the child, which is a negligible background risk), the risk for her child to inherit this specific familial condition from her is 0%. The partner's negative family history further supports that the risk is effectively zero.
A male (II-3) is affected with Leber hereditary optic neuropathy (LHON), a mitochondrial disorder. His sister (II-2) is also affected. Their mother (I-1) is affected, while their father (I-2) is not. What is the approximate recurrence risk for the offspring of the affected male (II-3)?
Explanation: Mitochondrial disorders are inherited exclusively through the maternal line. Mitochondria, which contain their own DNA, are present in the cytoplasm of the egg cell but are generally not found in the head of the sperm that fertilizes the egg. Therefore, a mother passes her mitochondrial DNA to all of her offspring (both male and female). A father does not pass his mitochondrial DNA to any of his offspring. Since individual II-3 is male, his children will not inherit his mitochondria, and thus will not inherit his mitochondrial disorder. The recurrence risk is effectively 0% (or equivalent to the background population risk).
A man is affected with a form of retinitis pigmentosa that follows an autosomal dominant inheritance pattern. He marries a woman who is also affected with retinitis pigmentosa, but her form is due to a mutation in a different gene and is autosomal recessive. They are concerned about the risk for their children. What is the probability that their first child will be affected with retinitis pigmentosa?
Explanation: This problem involves locus heterogeneity, where mutations in different genes cause the same phenotype. Let Gene A be the autosomal dominant form and Gene B be the autosomal recessive form.
Huntington's disease is an autosomal dominant disorder with age-dependent penetrance. By age 55, penetrance is 50%. A 30-year-old woman's father is 55 years old and unaffected. However, the woman's paternal grandfather died of Huntington's disease. Assuming no new mutations, what is the probability that the woman's first child will inherit the disease-causing allele?
Explanation: This problem involves a two-step risk calculation with a Bayesian update.
A phenotypically normal couple has a child with achondroplasia, a fully penetrant autosomal dominant disorder. Subsequently, they have a second child, who is also affected with achondroplasia. Which of the following is the most appropriate recurrence risk to quote this couple for their next pregnancy?
Explanation: While a single affected child with unaffected parents suggests a de novo (new) mutation with a very low recurrence risk, having two affected children makes two independent de novo events extremely improbable. The most likely biological explanation is germline mosaicism in one of the parents. This means a proportion of the parent's germ cells (sperm or eggs) carry the mutation, while their somatic cells do not, so they are phenotypically normal. The recurrence risk is therefore substantial, but dependent on the unknown fraction of mutant gametes. It is not 50% (which would imply one parent is heterozygous in all cells) or 25% (the risk for recessive disorders). Empirical data for such situations in AD disorders like achondroplasia suggest a recurrence risk in the range of 5-15%.
A male (II-3) is affected with Leber hereditary optic neuropathy (LHON), a mitochondrial disorder. His sister (II-2) is also affected. Their mother (I-1) is affected, while their father (I-2) is not. What is the approximate recurrence risk for the offspring of the affected male (II-3)?
Explanation: Mitochondrial disorders are inherited exclusively through the maternal line. Mitochondria, which contain their own DNA, are present in the cytoplasm of the egg cell but are generally not found in the head of the sperm that fertilizes the egg. Therefore, a mother passes her mitochondrial DNA to all of her offspring (both male and female). A father does not pass his mitochondrial DNA to any of his offspring. Since individual II-3 is male, his children will not inherit his mitochondria, and thus will not inherit his mitochondrial disorder. The recurrence risk is effectively 0% (or equivalent to the background population risk).
A man is affected with a form of retinitis pigmentosa that follows an autosomal dominant inheritance pattern. He marries a woman who is also affected with retinitis pigmentosa, but her form is due to a mutation in a different gene and is autosomal recessive. They are concerned about the risk for their children. What is the probability that their first child will be affected with retinitis pigmentosa?
Explanation: This problem involves locus heterogeneity, where mutations in different genes cause the same phenotype. Let Gene A be the autosomal dominant form and Gene B be the autosomal recessive form.
A woman's brother is affected with an X-linked recessive disorder. The woman and her partner are unaffected. They have three healthy sons. She is now pregnant with her fourth child. What is the probability that this child (sex unknown) will be affected?
Explanation: When tackling X-linked inheritance problems, you need to work through the genetics systematically, considering both the woman's possible genotype and the probability of each pregnancy outcome. Since the woman's brother has an X-linked recessive disorder, their mother must be at least a carrier (she passed the recessive allele to her son). This means the woman has a 50% chance of being a carrier herself. Let's call the normal allele X and the recessive allele x. If the woman is a carrier (Xx), each pregnancy has these outcomes: 25% normal female (XX), 25% carrier female (Xx), 25% normal male (XY), and 25% affected male (xY). So there's a 25% chance of an affected child per pregnancy. The probability that this fourth child will be affected equals: P(woman is carrier) × P(affected child | woman is carrier) = 21×41=81 Wait - but we have additional information! The woman already had three healthy sons. If she were a carrier, each son had a 50% chance of being affected. Having three healthy sons makes it less likely she's a carrier. Using Bayesian reasoning: P(carrier | three healthy sons) = (1/2)(1/2)3+(1/2)(1)3(1/2)(1/2)3=1/16+1/21/16=91 Therefore: 91×41=361 Answer choice A (1/9) represents the updated probability she's a carrier but ignores the 1/4 pregnancy risk. Answer choice B (1/18) and D (1/8) reflect calculation errors or incomplete reasoning. Remember: previous offspring outcomes provide valuable information that updates genetic probabilities - don't ignore this data in inheritance calculations.
A woman's brother is affected with an X-linked recessive disorder. The woman and her partner are unaffected. They have three healthy sons. She is now pregnant with her fourth child. What is the probability that this child (sex unknown) will be affected?
Explanation: When tackling X-linked inheritance problems, you need to work through the genetics systematically, considering both the woman's possible genotype and the probability of each pregnancy outcome. Since the woman's brother has an X-linked recessive disorder, their mother must be at least a carrier (she passed the recessive allele to her son). This means the woman has a 50% chance of being a carrier herself. Let's call the normal allele X and the recessive allele x. If the woman is a carrier (Xx), each pregnancy has these outcomes: 25% normal female (XX), 25% carrier female (Xx), 25% normal male (XY), and 25% affected male (xY). So there's a 25% chance of an affected child per pregnancy. The probability that this fourth child will be affected equals: P(woman is carrier) × P(affected child | woman is carrier) = 21×41=81 Wait - but we have additional information! The woman already had three healthy sons. If she were a carrier, each son had a 50% chance of being affected. Having three healthy sons makes it less likely she's a carrier. Using Bayesian reasoning: P(carrier | three healthy sons) = (1/2)(1/2)3+(1/2)(1)3(1/2)(1/2)3=1/16+1/21/16=91 Therefore: 91×41=361 Answer choice A (1/9) represents the updated probability she's a carrier but ignores the 1/4 pregnancy risk. Answer choice B (1/18) and D (1/8) reflect calculation errors or incomplete reasoning. Remember: previous offspring outcomes provide valuable information that updates genetic probabilities - don't ignore this data in inheritance calculations.
A woman's father has an autosomal dominant condition that is 100% penetrant. The woman has undergone genetic testing and is confirmed to not carry the familial mutation. She and her partner, who has no family history of the condition, have a child. What is the risk that their child will have the condition?
Explanation: The woman's father is affected with an autosomal dominant condition, so her prior risk of inheriting the mutation was 50%. However, the genetic test result supersedes the risk assessment based on the pedigree. The test has confirmed she does not carry the mutation. Therefore, she cannot pass it on to her children. Assuming the test is accurate and there are no other sources of the mutation (like a de novo mutation in the child, which is a negligible background risk), the risk for her child to inherit this specific familial condition from her is 0%. The partner's negative family history further supports that the risk is effectively zero.
Huntington's disease is an autosomal dominant disorder with age-dependent penetrance. By age 55, penetrance is 50%. A 30-year-old woman's father is 55 years old and unaffected. However, the woman's paternal grandfather died of Huntington's disease. Assuming no new mutations, what is the probability that the woman's first child will inherit the disease-causing allele?
Explanation: This problem involves a two-step risk calculation with a Bayesian update.
A phenotypically normal couple has a child with achondroplasia, a fully penetrant autosomal dominant disorder. Subsequently, they have a second child, who is also affected with achondroplasia. Which of the following is the most appropriate recurrence risk to quote this couple for their next pregnancy?
Explanation: While a single affected child with unaffected parents suggests a de novo (new) mutation with a very low recurrence risk, having two affected children makes two independent de novo events extremely improbable. The most likely biological explanation is germline mosaicism in one of the parents. This means a proportion of the parent's germ cells (sperm or eggs) carry the mutation, while their somatic cells do not, so they are phenotypically normal. The recurrence risk is therefore substantial, but dependent on the unknown fraction of mutant gametes. It is not 50% (which would imply one parent is heterozygous in all cells) or 25% (the risk for recessive disorders). Empirical data for such situations in AD disorders like achondroplasia suggest a recurrence risk in the range of 5-15%.
The pedigree provided shows a family with an X-linked dominant disorder. Individual II-2 is an affected male. He and his unaffected partner, II-1, are planning a family. What is the probability that their first child will be an unaffected daughter?
Explanation: For an X-linked dominant disorder, an affected male (genotype XDY) will pass his Y chromosome to all of his sons and his X chromosome (XD) to all of his daughters. His partner is unaffected (genotype XdXd).
An autosomal dominant disorder is characterized by 80% penetrance. In the pedigree shown, individual II-1 is affected. His son, III-2, is clinically unaffected at age 40. III-2 and his partner, III-3, who has no family history of the disorder, are expecting a child (IV-1). What is the probability that IV-1 will be affected by the disorder?
Explanation: This problem requires using Bayesian analysis to update the probability that the unaffected father (III-2) carries the disease allele.
In the pedigree for an X-linked recessive disorder, individual II-2 is a carrier. She has three children with her unaffected partner, II-1: an affected son (III-1), an unaffected son (III-2), and an unaffected daughter (III-3). This daughter, III-3, wants to know the probability that she is a carrier. What is this probability?
Explanation: This question tests whether a student can correctly apply basic Mendelian principles without getting confused by extraneous information.
The pedigree shows a family with an autosomal dominant condition with 90% penetrance. Individual III-1 is the first affected member in this family. His parents, II-1 and II-2, are unaffected. What is the approximate recurrence risk for III-1's unaffected sister, III-2, to have an affected child?
Explanation: This question combines concepts of de novo mutation, germline mosaicism, and incomplete penetrance.
The pedigree below shows a family with a trait inherited in a Y-linked fashion. Individual III-1 is affected. He and his partner, III-2, have one unaffected daughter, IV-1. What is the probability that their next pregnancy results in an affected male?
Explanation: Y-linked inheritance means the gene for the trait is on the Y chromosome.