Pedigree Genetics Inferences X Linked Disorders Answer Key

Pedigree Genetics Inferences: X‑Linked Disorders Answer Key Understanding how traits are passed through families is a cornerstone of genetics education. When the trait in question is linked to the X chromosome, the inheritance pattern shows distinctive clues that can be read directly from a pedigree chart. This article walks you through the logic behind those clues, provides a step‑by‑step method for analyzing pedigrees, and includes a detailed answer key for several practice problems involving X‑linked recessive and X‑linked dominant disorders. By the end, you should be able to look at any pedigree and confidently state whether the disorder is X‑linked, autosomal, or another mode of inheritance, and you will know the genotypes of the individuals involved.

1. Why Pedigrees Matter for X‑Linked Traits

A pedigree is a visual family tree that uses standardized symbols to show relationships, phenotypes, and sometimes genotypes. For X‑linked disorders, the pedigree reveals patterns that are rarely seen with autosomal traits:

• Male‑to‑male transmission never occurs – a father cannot pass an X‑linked allele to his son because sons receive the Y chromosome from their father.
• Affected males often have unaffected fathers – the mutant allele must have come from the mother (who is usually a carrier).
• Female carriers may be asymptomatic – especially for recessive disorders, heterozygous females show little or no phenotype.
• If the disorder is dominant, affected females appear in every generation – because they need only one mutant X to express the trait.

Recognizing these signatures allows you to infer the mode of inheritance before diving into detailed genotype assignments.

2. Core Concepts: X‑Linked Recessive vs. X‑Linked Dominant

These differences shape the pedigree patterns you will look for.

3. Step‑by‑Step Procedure for Inferring X‑Linkage

Follow this checklist when you encounter a pedigree problem:

1. Count the sexes of affected individuals.

   • If only males are affected → strong suspicion of X‑linked recessive.
   • If both sexes are affected but males are more severely affected or appear more often → still consider X‑linked recessive with occasional carrier females showing mild symptoms.
   • If affected females appear in every generation and there is no male‑to‑male transmission → think X‑linked dominant.

2. Check for male‑to‑male transmission.

   • Presence of an affected father with an affected son rules out any X‑linked mode (points to autosomal or Y‑linked).

3. Examine the parental phenotypes.

   • For a recessive trait, an affected male must have a mother who is at least a carrier (could be affected if homozygous).
   • For a dominant trait, an affected child must have at least one affected parent (unless a new mutation occurred).

4. Look for skipped generations.

   • Recessive X‑linked traits often skip generations (carrier females pass the allele to sons).
   • Dominant X‑linked traits rarely skip generations unless penetrance is incomplete.

5. Assign provisional genotypes. - Use Xⁿ for the normal allele and Xᵐ for the mutant allele.

   • Males: XⁿY (normal) or XᵐY (affected).
   • Females: XⁿXⁿ (normal), XⁿXᵐ (carrier/affected depending on dominance), XᵐXᵐ (homozygous mutant).

6. Test consistency.

   • Walk through each mating in the pedigree, applying Mendelian rules for sex chromosomes.
   • If any contradiction appears, revisit your assumption (e.g., maybe the trait is autosomal recessive with sex‑biased expression).

7. State the conclusion.

   • Clearly label the inheritance mode and list the genotypes of all individuals (or note where they remain ambiguous).

4. Sample Problems and Answer Key

Below are three pedigree scenarios. Each includes a brief description, the pedigree in text form (you can sketch it on paper), and a detailed answer key. Try to solve them on your own before checking the solutions.

Problem 1 – Classic Hemophilia Pedigree

Description:
A family has a history of a bleeding disorder that affects only males. The maternal grandfather (I‑1) was unaffected. The maternal grandmother (I‑2) was a known carrier (she had a brother with the disease). Their daughter (II‑2) married an unaffected man (II‑1). They have three children: a son (III‑1) who is affected, a daughter (III‑2) who is unaffected, and another son (III‑3) who is unaffected.

Pedigree (text):
Generation I: I‑1 (♂, normal) – I‑2 (♀, carrier)
Generation II: II‑1 (♂, normal) – II‑2 (♀, carrier)
 Children: III‑1 (♂, affected), III‑2 (♀, normal), III‑3 (♂, normal)

Answer Key:

Explanation:
The disease appears only in males, never passes from father to son, and the mother of the affected son (II‑2) is a carrier—exactly the pattern expected for an X‑linked recessive disorder.

Problem 2 – X‑Linked Dominant Trait (Vitamin D‑Resistant Rickets)

Description:
A family shows a bone‑softening condition that appears in