Overview
This lecture covers sex-linked, sex-influenced, and sex-limited traits as non-Mendelian forms of genetic inheritance, including examples and Punnett Square analyses.
Review of Non-Mendelian Genetics
- Incomplete dominance is when the dominant allele does not fully mask the recessive, creating an intermediate phenotype.
- Co-dominance occurs when both alleles are equally expressed in the phenotype.
- Multiple alleles refer to a gene with more than two allele forms.
Chromosomes and Sex Determination
- Humans have 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes.
- Males have XY sex chromosomes; females have XX.
- Sex of offspring is determined by the sperm's sex chromosome (X or Y), resulting in a 50% chance for male or female offspring.
Sex-Linked Traits
- Sex-linked traits are controlled by genes on sex chromosomes, mostly the X chromosome (X-linked).
- X-linked traits include color blindness and hemophilia; Y-linked traits (e.g., hairy ears) only affect males.
- Sex-linked traits are usually recessive and more common in males since they have only one X chromosome.
Example: Color Blindness (X-linked Trait)
- Female genotypes: XX (normal), XcX (carrier, normal vision), XcXc (color blind).
- Male genotypes: XY (normal), XcY (color blind).
- Carrier females can pass the trait to offspring; males need only one Xc to be affected.
- Punnett Square analysis: normal female × colorblind male yields 50% carrier females and 50% normal males.
Example: Hemophilia (X-linked Trait)
- Hemophilia gene is inherited similarly to color blindness.
- Carrier female × normal male: 25% normal female, 25% carrier female, 25% normal male, 25% hemophilic male.
Sex-Influenced Traits
- Traits affected by sex hormones but found on autosomes.
- Example: baldness—requires one recessive allele in males to be expressed but two in females.
- Males are more likely to show baldness due to higher testosterone levels.
Example: Baldness (Sex-Influenced Trait)
- Genotypes: BB (not bald), Bb (not bald female, bald male), bb (bald both sexes).
- Heterozygous female × homozygous bald male: offspring's baldness depends on genotype and sex.
Sex-Limited Traits
- Traits expressed in only one sex, despite genes being present in both.
- Example: lactation occurs only in female cattle, even if males carry the gene.
Key Terms & Definitions
- Incomplete Dominance — neither allele is fully dominant, producing an intermediate trait.
- Co-dominance — both alleles are equally and visibly expressed.
- Multiple Alleles — more than two allele forms exist for a gene.
- Sex Chromosomes — chromosomes that determine an organism's sex (X and Y).
- Sex-Linked Traits — traits controlled by genes located on sex chromosomes.
- X-Linked Traits — traits associated with genes on the X chromosome.
- Y-Linked Traits — traits associated with genes on the Y chromosome.
- Sex-Influenced Traits — autosomal traits influenced by an individual’s sex hormones.
- Sex-Limited Traits — traits expressed exclusively in one sex.
Action Items / Next Steps
- Practice Punnett Square problems involving sex-linked and sex-influenced traits.
- Review definitions and examples of non-Mendelian inheritance patterns.