Overview
This lecture covers non-Mendelian inheritance patterns, focusing on sex-linked, sex-influenced, and sex-limited traits, their genetic mechanisms, and example problems using Punnett Squares.
Review of Non-Mendelian Genetics
- Incomplete dominance: the dominant allele does not completely mask the recessive, resulting in an intermediate phenotype.
- Codominance: both alleles are expressed equally in the phenotype.
- Multiple alleles: a trait controlled by a single gene with more than two allele options.
Chromosomes and Sex Determination
- Humans typically have 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes (XX or XY).
- Males: 44 autosomes + XY; females: 44 autosomes + XX.
- Sex is determined by sperm's sex chromosome (Y = male, X = female); 50% chance for each.
Sex-Linked Traits
- Sex-linked traits are controlled by genes on sex chromosomes, mostly on the X or Y chromosome.
- X-linked traits affect both sexes but are more common in males due to only one X chromosome.
- Y-linked traits (e.g., hairy ears) occur only in males.
Color Blindness Example (X-linked)
- Color blindness is caused by a recessive gene on the X chromosome.
- Females need two recessive alleles to be colorblind; carriers have one recessive allele.
- Males with one recessive X are colorblind since they have only one X.
- Punnett Square analysis: normal female x colorblind male = 50% carrier females, 50% normal males.
Hemophilia Example (X-linked)
- Hemophilia is a blood disorder caused by a recessive gene on the X chromosome.
- Carrier females have one hemophilia allele; hemophilic females have two.
- Males with a hemophilia allele are affected.
- Punnett Square: carrier female x normal male yields 25% each of normal female, carrier female, normal male, and hemophilic male.
Sex-Influenced Traits
- Traits influenced by autosomes but affected by sex hormones (e.g., baldness).
- Baldness is recessive and expressed with one allele in males (due to testosterone), but requires two alleles in females.
- Example: heterozygous female x homozygous bald male; genotypes differ in baldness expression by sex.
Sex-Limited Traits
- Traits expressed in only one sex despite both sexes carrying the genes (e.g., lactation in female cattle).
- The gene for lactation is dominant but only visible in females.
Key Terms & Definitions
- Incomplete Dominance — inheritance where heterozygotes show an intermediate phenotype.
- Codominance — both alleles are fully expressed in the phenotype.
- Multiple Alleles — more than two allele options for a single gene.
- Sex-Linked Trait — trait controlled by genes on sex chromosomes.
- X-Linked Trait — gene located on the X chromosome.
- Y-Linked Trait — gene located on the Y chromosome.
- Sex-Influenced Trait — autosomal trait affected by sex hormones.
- Sex-Limited Trait — trait expressed in only one sex.
Action Items / Next Steps
- Practice Punnett Square problems for each type of inheritance.
- Review key examples: color blindness, hemophilia, baldness, and lactation.
- Read textbook section on non-Mendelian inheritance patterns.