Pedigree Practice Problems With Answers

Pedigree Practice Problems: Mastering the Art of Genetic Inheritance

Understanding pedigrees is crucial for anyone studying genetics. A pedigree chart visually represents the inheritance of a specific trait or disease across generations within a family. Analyzing these charts allows us to deduce inheritance patterns, predict the probability of offspring inheriting a particular trait, and even identify the mode of inheritance (autosomal dominant, autosomal recessive, X-linked dominant, or X-linked recessive). This article provides comprehensive pedigree practice problems with detailed answers, helping you master this essential skill in genetics. We'll cover various inheritance patterns and complexities to build a solid understanding.

Introduction to Pedigree Analysis

Before diving into the practice problems, let's review some key elements of pedigree analysis:

• Symbols: Pedigrees use standardized symbols. Squares represent males, circles represent females, shaded shapes indicate individuals expressing the trait, and unshaded shapes indicate individuals without the trait. Half-shaded shapes often represent carriers (individuals who carry the recessive allele but don't express the trait).

• Generations: Generations are typically numbered with Roman numerals (I, II, III, etc.), while individuals within a generation are numbered with Arabic numerals (1, 2, 3, etc.).

• Inheritance Patterns: Understanding the different modes of inheritance is crucial:

  • Autosomal Dominant: The trait appears in every generation, affecting males and females equally. Only one affected allele is needed to express the trait.

  • Autosomal Recessive: The trait may skip generations, affecting males and females equally. Two affected alleles are needed to express the trait. Carriers are common.

  • X-linked Dominant: Affected fathers pass the trait to all their daughters, while affected mothers pass the trait to approximately half their sons and daughters.

  • X-linked Recessive: More males are affected than females. Affected fathers do not pass the trait to their sons, but their daughters will be carriers. Affected mothers can pass the trait to both sons and daughters.

Pedigree Practice Problems with Answers

Now, let's tackle some practice problems. Remember to carefully analyze the pedigree charts, consider the inheritance patterns, and justify your answers.

Problem 1: Autosomal Recessive Inheritance

Scenario: The following pedigree shows the inheritance of albinism, an autosomal recessive trait.

[Insert a simple pedigree chart showing two unaffected parents having one albino child (a recessive trait), and a subsequent unaffected child. Several generations would help demonstrate the recessive inheritance pattern. Use appropriate symbols.]

Questions:

1. What is the genotype of the parents?
2. What is the probability of their next child having albinism?
3. Is it possible for two albino parents to have a child with normal pigmentation?

Answers:

1. The parents are both heterozygous carriers (Aa), possessing one normal allele (A) and one albino allele (a). They are phenotypically normal, but carry the recessive allele.

2. The probability of their next child having albinism (aa) is 25%. This is because the Punnett square for Aa x Aa yields the following genotypes: AA (25%), Aa (50%), aa (25%).

3. No, it is not possible. Two albino parents (aa x aa) can only produce albino offspring (aa).

Problem 2: Autosomal Dominant Inheritance

Scenario: The following pedigree shows the inheritance of a rare form of dwarfism, an autosomal dominant trait.

[Insert a pedigree chart showing an affected parent having affected and unaffected children with an unaffected spouse. Make sure the chart adequately demonstrates autosomal dominant inheritance.]

Questions:

1. What is the genotype of the affected parent?
2. What is the probability that their next child will have dwarfism?
3. Can two unaffected parents have a child with this form of dwarfism?

Answers:

1. Assuming complete penetrance (meaning the trait always manifests when the dominant allele is present), the affected parent is heterozygous (Dd). If the trait were fully penetrant and the parent was homozygous dominant (DD), then all their children would have shown dwarfism. The presence of unaffected children indicates the parent is heterozygous.

2. The probability of their next child having dwarfism is 50%. A Punnett square for Dd x dd (assuming the unaffected spouse is homozygous recessive) shows a 50% chance of Dd (affected) and 50% chance of dd (unaffected).

3. No. For an autosomal dominant trait, at least one parent must have the dominant allele to pass it on to their offspring.

Problem 3: X-linked Recessive Inheritance

Scenario: The following pedigree charts the inheritance of red-green color blindness, an X-linked recessive trait.

[Insert a pedigree chart showing an affected male and an unaffected female having children, illustrating the classic X-linked recessive pattern, with several generations to fully illustrate the pattern.]

Questions:

1. What is the genotype of the affected male?
2. What is the genotype of the unaffected female? (Assuming she is not a carrier)
3. What is the probability that their daughter will be a carrier?
4. What is the probability that their son will be colorblind?

Answers:

1. The affected male has the genotype X^cY, where X^c represents the allele for color blindness.

2. The unaffected female, if not a carrier, has the genotype X^CX^C, where X^C represents the normal allele.

3. The probability that their daughter will be a carrier (X^CX^c) is 100%. All daughters of an affected father will receive his X^c chromosome.

4. The probability that their son will be colorblind is 0%. Sons inherit their Y chromosome from their father and their X chromosome from their mother (X^C), so they cannot be colorblind.

Problem 4: A More Complex Pedigree

Scenario: The following pedigree illustrates the inheritance of a rare genetic disorder. The mode of inheritance is unclear.

[Insert a more complex pedigree chart with three generations, including instances of affected males and females, skipping generations in some branches, and showing affected individuals with both affected and unaffected parents.]

Questions:

1. Based on this pedigree, what is the most likely mode of inheritance? Justify your answer.
2. What are the possible genotypes of individuals II-2 and II-3?
3. What is the probability that individual III-1 will be affected?

Answers:

1. The most likely mode of inheritance is autosomal recessive. The trait skips generations, appearing in both males and females. The presence of unaffected parents with affected children strongly suggests a recessive pattern.

2. The genotypes of individuals II-2 and II-3 are likely to be heterozygous (Aa), as they are unaffected but have an affected child.

3. The probability of individual III-1 being affected depends on the genotypes of their parents (II-2 and II-3). If both parents are heterozygous (Aa), there’s a 25% chance that III-1 will inherit two recessive alleles (aa) and be affected.

Problem 5: Pedigree with Incomplete Penetrance

Scenario: This pedigree shows the inheritance of a trait with incomplete penetrance (meaning that an individual with the genotype for the trait may not always express the phenotype). The trait is autosomal dominant.

[Insert a pedigree chart illustrating an autosomal dominant trait but showing some individuals with the genotype but without the phenotype, to demonstrate incomplete penetrance.]

Questions:

1. Explain what incomplete penetrance means in this context.
2. Why might individual II-2 not exhibit the trait despite likely possessing the dominant allele?
3. What is the probability of individual III-1 exhibiting the trait?

Answers:

1. Incomplete penetrance means that even if an individual inherits the dominant allele responsible for the trait, they may not express the phenotype associated with that allele. Genetic background, environmental factors, or modifier genes can influence whether the dominant allele expresses itself.

2. Individual II-2 likely has the dominant allele but the expression of this gene is influenced by one of the factors mentioned above (genetic background, environment, or modifier genes) which causes them not to exhibit the phenotype of the trait.

3. To answer this accurately, we need to assess the penetrance (the percentage of individuals with the genotype who actually express the phenotype) from the pedigree chart. For example, if the penetrance is 80%, even if III-1 inherits the dominant allele, there's a 20% chance they will not express the trait. The calculation becomes more complex than a simple Mendelian probability because we are introducing the probability of expressing the phenotype given the genotype.

Conclusion

Analyzing pedigrees is a valuable skill for unraveling genetic inheritance patterns. By practicing with various pedigree charts and understanding the different modes of inheritance, you can develop your ability to interpret these charts accurately. Remember to always consider the possibility of incomplete penetrance or other complexities that can modify the straightforward Mendelian ratios. The problems presented here offer a foundation for further exploration of genetic inheritance and its intricacies. Continue practicing with more complex pedigrees to enhance your understanding and proficiency in pedigree analysis. The key lies in meticulous observation, careful consideration of inheritance patterns, and systematic application of probability rules. Through consistent practice, you can master the art of pedigree analysis and confidently interpret genetic information presented in this format.