Overview
This lecture explains how genes determine traits, covers Mendelian inheritance, Punnett squares, test crosses, and explores non-Mendelian genetic patterns such as incomplete dominance, codominance, multiple alleles, sex linkage, and lethal alleles.
Genotype and Phenotype
- Diploid organisms have two versions (alleles) of each gene, one from each parent.
- A genotype is the genetic makeup (the alleles present); a phenotype is the observable trait.
- Homozygous individuals have two identical alleles; heterozygous have two different alleles for a trait.
Dominant and Recessive Alleles
- Dominant alleles mask recessive ones; only homozygous recessive individuals show the recessive trait.
- Genotypes use uppercase letters for dominants (e.g., V) and lowercase for recessives (v).
- Diseases and traits in humans may follow dominant or recessive inheritance patterns.
Monohybrid Crosses and Punnett Squares
- A monohybrid cross involves parents differing in one trait.
- A Punnett square predicts genotype and phenotype ratios of offspring.
- F2 generation from heterozygous parents shows a 1:2:1 genotypic and 3:1 phenotypic ratio.
Test Crosses and Pedigree Analysis
- A test cross reveals if a dominant-phenotype organism is homozygous or heterozygous by crossing it with a homozygous recessive.
- In humans, pedigrees are used instead of test crosses to analyze inheritance patterns.
Non-Mendelian Inheritance
- Incomplete dominance: heterozygotes display an intermediate phenotype (e.g., pink snapdragons).
- Codominance: both alleles are equally expressed (e.g., MN blood types).
- Multiple alleles: more than two alleles exist for a gene in the population, leading to complex dominance patterns (e.g., rabbit coat color).
- Lethal alleles: certain allele combinations can cause death (recessive or dominant lethal patterns).
Sex-Linked Inheritance
- X-linked genes are located on the X chromosome, often affecting males more than females.
- Females can be carriers for X-linked recessive traits; males with one mutant X will express the trait.
- Human examples include color blindness and hemophilia.
Key Terms & Definitions
- Allele — a variant form of a gene.
- Genotype — the genetic makeup of an organism (allele combination).
- Phenotype — the observable traits of an organism.
- Homozygous — having two identical alleles for a gene.
- Heterozygous — having two different alleles for a gene.
- Dominant — an allele that masks another in heterozygotes.
- Recessive — an allele masked by a dominant allele in heterozygotes.
- Punnett Square — a chart predicting genetic cross outcomes.
- Test Cross — crossing an unknown genotype individual with a recessive one to determine the unknown.
- Incomplete Dominance — heterozygote shows intermediate phenotype.
- Codominance — heterozygote expresses both alleles distinctly.
- Multiple Alleles — more than two possible alleles for a gene in a population.
- Sex-linked/X-linked — gene located on a sex chromosome.
- Lethal Allele — an allele that may cause death when in a certain genotype.
Action Items / Next Steps
- Practice drawing and interpreting Punnett squares for monohybrid crosses.
- Review examples of non-Mendelian inheritance patterns (e.g., incomplete dominance, codominance).
- Study human pedigrees illustrating sex-linked and autosomal inheritance.