Overview
This lecture covers sex-linked (X-linked) disorders, how they're inherited, how to calculate inheritance probabilities using Punnett squares, and guidelines for explaining and writing about this topic.
Chromosome Basics
- Humans have 23 chromosome pairs: 22 pairs of autosomes and 1 pair of sex chromosomes (gonosomes).
- Sex chromosomes are X and Y; females are XX, males are XY.
- X and Y chromosomes are not homologous; the X is larger and contains more genes.
- Males are more susceptible to X-linked disorders due to only having one X chromosome.
X-Linked Disorders
- X-linked disorders are caused by genes on the X chromosome.
- Red-green color blindness is a common X-linked recessive disorder, meaning the affected allele is lowercase (e.g., x^r).
- Females need two copies of the recessive allele (x^r x^r) to be affected, while males need only one (x^r Y).
Punnett Square Calculation
- Use superscript notation (e.g., X^H, X^h for hemophilia) and indicate if individuals are male (XY) or female (XX).
- Example: Unaffected father (X^R Y) and carrier mother (X^R X^r) produce children with varying genotypes and phenotypes.
- Females can be unaffected (X^R X^R), carriers (X^R X^r), or affected (X^r X^r); males are unaffected (X^R Y) or affected (X^r Y).
- Hemophilia is another X-linked disorder; similar Punnett square approach is used.
Interpreting Results
- Group genotypes and phenotypes by sex and presence/absence of disorder.
- Do not describe carriers as a phenotype; "carrier" is only a genotype.
- Calculate probabilities based on all children or by sex, as specified in the question.
Constructing Written Explanations
- State both parents' genotypes clearly.
- Explain that males inherit their X from their mother and Y from their father; females get one X from each parent.
- Males are more affected because they have only one X chromosome and no masking effect from a second X.
Key Terms & Definitions
- Autosomes — The 22 non-sex chromosome pairs in humans.
- Gonosomes (Sex Chromosomes) — Chromosome pair (X and Y) determining biological sex.
- X-Linked Disorder — Genetic disorder carried on the X chromosome.
- Genotype — The genetic makeup or alleles present (e.g., X^R X^r).
- Phenotype — Observable traits; for sex-linked disorders, do not list "carrier" as a phenotype.
- Carrier — An individual (typically female) with one normal and one affected allele, not showing physical symptoms.
- Recessive Allele — Only causes disorder if present in two copies in females or one copy in males.
- Dominant Allele — Masks the effect of a recessive allele.
Action Items / Next Steps
- Practice creating and interpreting Punnett squares for X-linked disorders.
- Review key terminology and definitions from this lecture.
- Pay close attention to exam questions about inheritance patterns and probability calculations.