AP Biology: Sex?Linked Traits (X?Linked Recessive and Dominant) and Pedigree Analysis

Sex?Linked Traits (X?Linked Recessive and Dominant) and Pedigree Analysis

Concept Summary

• X-linked recessive traits: Traits caused by recessive alleles on the X chromosome; males (XY) express the trait if they inherit the allele, while females (XX) require two copies.
• X-linked dominant traits: Traits caused by dominant alleles on the X chromosome; expressed in both males and females with just one copy, but often more severe in males.
• Pedigree analysis: A diagram used to track inheritance patterns of traits across generations, identifying modes of inheritance (e.g., autosomal vs. sex-linked).
• Hemizygosity: Males (XY) have only one X chromosome, making them hemizygous for X-linked genes; they cannot be carriers for X-linked recessive traits.
• Carrier status: Females heterozygous for X-linked recessive traits are carriers; they do not express the trait but can pass it to offspring.

Core Questions

WHAT (definitional)

Q: What is an X-linked recessive trait? A: A trait caused by a recessive allele on the X chromosome, requiring two copies in females (XX) but only one in males (XY) for expression. Trap/Clarification: Males cannot be carriers for X-linked recessive traits—they either have the trait or do not.

Q: What is a pedigree? A: A family tree diagram showing the inheritance of a trait across generations, used to determine inheritance patterns (e.g., autosomal vs. sex-linked). Trap/Clarification: Squares = males, circles = females; shaded = affected, half-shaded = carrier (for recessive traits).

WHY (causal/explanatory)

Q: Why are X-linked recessive traits more common in males? A: Males (XY) need only one copy of the recessive allele to express the trait, while females (XX) require two copies. Trap/Clarification: Females can be carriers (heterozygous) and pass the trait to sons without expressing it themselves.

Q: Why is X-linked dominant inheritance often more severe in males? A: Males lack a second X chromosome to potentially mask the dominant allele’s effects, leading to full expression. Trap/Clarification: Affected fathers (XY) pass the trait to all daughters (XX) but no sons (XY).

HOW (process/application)

Q: How do you determine if a pedigree shows X-linked recessive inheritance? A: Look for: (1) More affected males than females, (2) no father-to-son transmission, (3) carrier mothers passing the trait to ~50% of sons. Trap/Clarification: Affected females must have an affected father and a carrier/affected mother.

Q: How do you calculate the probability of an X-linked recessive trait in offspring? A: Use a Punnett square: (1) Assign parental genotypes (e.g., XX × XY), (2) determine gamete possibilities, (3) calculate probabilities for each offspring genotype. Trap/Clarification:* Remember males inherit the Y from the father and X from the mother—X-linked traits skip generations via carrier females.

CAN (conditions/possibilities)

Q: Can a father pass an X-linked recessive trait to his son? A: No; fathers pass the Y chromosome to sons, so X-linked traits cannot be transmitted father-to-son. Trap/Clarification: Sons inherit X-linked traits only from their mother’s X chromosome.

Q: Under what conditions can a female express an X-linked recessive trait? A: If she inherits two copies of the recessive allele (XX), typically from a carrier mother (XX) and an affected father (XY). Trap/Clarification: Rare cases include X-inactivation skewing or Turner syndrome (XO).

Quick Facts & Traps

• Fact: X-linked dominant traits show vertical transmission (affected individuals in every generation) and affected fathers pass the trait to all daughters.
• Trap: Assuming all X-linked traits are recessive-Reality: X-linked dominant traits exist (e.g., hypophosphatemic rickets).
• Fact: Carrier females for X-linked recessive traits have a 50% chance of passing the allele to sons (who will express it) and daughters (who become carriers).
• Trap: Confusing autosomal recessive with X-linked recessive-Reality: Autosomal traits affect males/females equally; X-linked traits show sex bias.
• Fact: X-inactivation (Lyonization) in females can lead to mosaic expression of X-linked traits (e.g., calico cats).
• Trap: Forgetting that X-linked dominant traits can be lethal in males-Reality: Some (e.g., Rett syndrome) are often fatal in males in utero.

Rapid-Fire True/False

• Statement: A male with an X-linked recessive trait will always pass it to his daughters. Answer: TRUE Why the common mistake happens: Students assume males pass the trait to sons (they don’t; sons get the Y chromosome).

• Statement: If a mother is a carrier for an X-linked recessive trait, her sons have a 50% chance of being affected. Answer: TRUE Why the common mistake happens: Students forget that sons inherit the mother’s X chromosome and the father’s Y, so the mother’s genotype determines the son’s phenotype.

• Statement: X-linked dominant traits skip generations. Answer: FALSE Why the common mistake happens: Students confuse dominant inheritance (vertical transmission) with recessive (which can skip generations).