Worksheet - Pedigree Practice Problems 2012 | PDF | Dominance ... - Free Printable

Problem Analysis:

The task involves analyzing a pedigree chart for an autosomal recessive disorder. We need to identify genotypes, confirm the recessive nature of the trait, and answer specific questions about the individuals in the pedigree.

#### Key Symbols in Pedigree Charts:
- Square: Male
- Circle: Female
- Filled Symbol (Black): Affected individual
- Unfilled Symbol (White): Unaffected individual
- Shaded Half: Carrier (heterozygous)

#### Autosomal Recessive Disorder Characteristics:
1. The affected individual must have two copies of the recessive allele (e.g., `aa`).
2. Unaffected parents can be carriers (heterozygous, `Aa`) if they have an affected child.
3. If both parents are carriers, there is a 25% chance of having an affected child (`aa`).

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Solution:

#### a. How many generations are presented in this pedigree?

Count the rows of individuals from top to bottom:
- Generation 1: Individuals 1 and 2 (top row)
- Generation 2: Individuals 1–5 (second row)
- Generation 3: Individuals 1–9 (third row)
- Generation 4: Individuals 1–6 (bottom row)

Answer: There are 4 generations.

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#### b. We know that the trait is a recessive disorder. What clues are provided in the pedigree that would allow you to confirm this conclusion?

Clues for identifying a recessive disorder:
1. Affected individuals have unaffected parents: For example, individual 4 in Generation 3 is affected (`aa`), but both parents (individuals 3 and 4 in Generation 2) are unaffected. This indicates that the parents are carriers (`Aa`).
2. Unaffected individuals with affected offspring: For example, individuals 3 and 4 in Generation 2 are unaffected but have an affected child (individual 4 in Generation 3). This confirms that the parents are carriers (`Aa`).
3. Presence of carriers: Unaffected individuals who have at least one affected parent or sibling are likely carriers (`Aa`).

Answer: Clues include:
- Affected individuals (e.g., 4 in Generation 3) have unaffected parents.
- Unaffected parents (e.g., 3 and 4 in Generation 2) have an affected child.
- Presence of unaffected individuals who must be carriers to explain the inheritance pattern.

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#### c. Shade in half of the circle (or square) for all the individuals that are hybrid in the pedigree.

Hybrid individuals are carriers (`Aa`). To identify them:
1. Affected individuals (`aa`): These cannot be carriers since they have two recessive alleles.
2. Unaffected individuals with at least one affected parent or sibling: These must be carriers (`Aa`).

- Generation 1:
- Individual 1: Unaffected, no affected children → Likely `AA`.
- Individual 2: Affected (`aa`).

- Generation 2:
- Individual 1: Unaffected, no affected children → Likely `AA`.
- Individual 2: Unaffected, has an affected child (4) → Carrier (`Aa`).
- Individual 3: Unaffected, has an affected child (4) → Carrier (`Aa`).
- Individual 4: Unaffected, has an affected child (8) → Carrier (`Aa`).
- Individual 5: Unaffected, no affected children → Likely `AA`.

- Generation 3:
- Individual 1: Unaffected, no affected parents or siblings → Likely `AA`.
- Individual 2: Unaffected, no affected parents or siblings → Likely `AA`.
- Individual 3: Unaffected, no affected parents or siblings → Likely `AA`.
- Individual 4: Affected (`aa`).
- Individual 5: Unaffected, no affected parents or siblings → Likely `AA`.
- Individual 6: Unaffected, no affected parents or siblings → Likely `AA`.
- Individual 7: Unaffected, no affected parents or siblings → Likely `AA`.
- Individual 8: Affected (`aa`).
- Individual 9: Affected (`aa`).

- Generation 4:
- Individual 1: Unaffected, no affected parents → Likely `AA`.
- Individual 2: Unaffected, no affected parents → Likely `AA`.
- Individual 3: Unaffected, no affected parents → Likely `AA`.
- Individual 4: Affected (`aa`).
- Individual 5: Affected (`aa`).
- Individual 6: Unaffected, no affected parents → Likely `AA`.

Carriers (`Aa`):
- Generation 2: Individuals 2, 3, 4.
- Generation 3: None.

Shade half of the circles/squares for individuals 2, 3, and 4 in Generation 2.

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#### d. What is the genotype of the 6th individual in the fourth generation?

- Individual 6 in Generation 4 is unaffected.
- Both parents (individuals 5 and 6 in Generation 3) are affected (`aa`).
- Since both parents are `aa`, they can only pass the recessive allele (`a`) to their offspring.
- Therefore, individual 6 in Generation 4 must be homozygous recessive (`aa`).

Answer: The genotype is `aa`.

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#### e. Explain why some individuals can have multiple genotypes. Provide an example.

Some individuals can have multiple possible genotypes because their exact genotype cannot be determined with certainty based on the information provided in the pedigree. This ambiguity arises when:
1. The individual is unaffected but could be either homozygous dominant (`AA`) or heterozygous (`Aa`).
2. The presence of affected relatives does not provide enough information to conclusively determine the carrier status.

Example:
- Consider individual 2 in Generation 2. This individual is unaffected and has an affected child (individual 4 in Generation 3). Based on this information, individual 2 must be a carrier (`Aa`). However, without additional information (e.g., genotypes of other family members), we cannot definitively rule out the possibility that individual 2 is homozygous dominant (`AA`), although it is less likely given the presence of an affected child.

Answer: Some individuals can have multiple genotypes because their exact carrier status cannot be determined with certainty from the pedigree alone. For example, individual 2 in Generation 2 could be either `AA` or `Aa`.

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Final Answers:

1. a. There are 4 generations.
2. b. Clues include affected individuals with unaffected parents and unaffected parents with affected children.
3. c. Shade half of the circles/squares for individuals 2, 3, and 4 in Generation 2.
4. d. The genotype of the 6th individual in the fourth generation is `aa`.
5. e. Some individuals can have multiple genotypes because their carrier status cannot be definitively determined. Example: Individual 2 in Generation 2 could be either `AA` or `Aa`.

Boxed Final Answer:
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