Overview
This lecture provides concise definitions of key genetics terms from Chapter 12, covering Mendel's experiments, patterns of inheritance, and genetic terminology.
Mendelian Genetics and Inheritance Patterns
- Mendel's experiments explored how traits are inherited across generations in pea plants.
- The blending theory (now disproven) proposed offspring are a blend of parental traits.
- Mendel established laws of dominance, segregation, and independent assortment to describe inheritance.
- Law of dominance: one trait can mask another in heterozygotes.
- Law of segregation: genes separate equally into gametes.
- Law of independent assortment: genes sort independently during gamete formation.
Genetic Variation and Expression
- Alleles are gene variants at the same chromosome location.
- Homozygous means having two identical alleles for a gene.
- Heterozygous means having two different alleles for a gene.
- Hemizygous indicates only one allele present for a gene (e.g., X-linked genes in males).
- Phenotype is the observable trait; genotype is the underlying genetic makeup.
- Dominant traits are expressed if at least one allele is present; recessive traits require two alleles to be expressed.
Complex Inheritance and Crosses
- Codominance: both alleles are fully expressed in heterozygotes.
- Incomplete dominance: heterozygotes show an intermediate phenotype.
- Epistasis: one gene masks or interferes with another's expression.
- Linkage: genes close together on a chromosome are inherited together.
- Sex-linked genes are located on sex chromosomes (typically X or Y).
- Dihybrid cross: parents differ in two traits; monohybrid cross: parents differ in one trait.
- Reciprocal cross: swapping male and female traits to determine inheritance patterns.
- Test cross: crossing a dominant phenotype with a homozygous recessive to determine genotype.
Probability and Genetic Tools
- Product rule: probability of two independent events equals the product of individual probabilities.
- Sum rule: probability of one of two mutually exclusive events is the sum of their probabilities.
- Punnett square: diagram predicting offspring genotype and phenotype ratios.
Key Terms & Definitions
- Allele — gene variation at the same locus.
- Autosomes — non-sex chromosomes.
- Blending theory of inheritance — outdated idea of blending parental traits.
- Codominance — expression of both alleles in heterozygotes.
- Continuous variation — traits show a range of values.
- Discontinuous variation — traits are distinct and separate.
- Epistasis — gene interaction where one gene masks another.
- Genotype — organism’s genetic identity.
- Hemizygous — only one allele present.
- Heterozygous — two different alleles for a gene.
- Homozygous — two identical alleles for a gene.
- Hybridization — breeding of different individuals for desired traits.
- Phenotype — observable characteristics.
- Punnett square — genetic cross diagram.
- Recessive/Dominant — trait expressed with two alleles/one allele.
- Sex-linked/X-linked — gene present on a sex chromosome.
- Test cross — cross to determine unknown genotype.
- Trait — heritable characteristic.
Action Items / Next Steps
- Review and memorize key terms for upcoming quizzes or exams.
- Practice drawing and interpreting Punnett squares.
- Read Chapter 12 Summary for reinforcing main concepts.