Overview
This lecture covers genetic concepts, including the structure and function of genes and chromosomes, types of inheritance patterns such as incomplete dominance, codominance, genetic linkage, multiple alleles, sex-linked traits, epistasis, extra-nuclear inheritance, and polygenic traits.
Genes, Chromosomes, and Their Location
- Traits are observable characteristics inherited from parents.
- Chromosomes, found in the cell nucleus, carry genetic instructions.
- Genes are segments of DNA located on chromosomes that code for traits.
- The specific location of a gene on a chromosome is called its locus.
- Alleles are different versions of a gene at the same locus.
- Homologous chromosomes are pairs with the same genes at the same loci.
- DNA is composed of bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).
- Genes have organized, non-random locations (loci) on chromosomes.
- Body hierarchy: Body → Tissues → Cells → Nucleus → Chromosomes → Genes → DNA Molecule → DNA Helix → DNA Bases.
Non-Mendelian Genetics: Incomplete Dominance & Codominance
- Genetics is the study of heredity.
- Phenotype is an organism's observable traits.
- Homozygotes have two identical alleles; heterozygotes have two different alleles.
- Incomplete dominance: heterozygotes show a blend (e.g., red × white = pink).
- Codominance: both alleles are fully shown (e.g., red × white = roan with both color patches).
- RW × RW crosses yield a 1:2:1 genotype ratio (25% RR, 50% RW, 25% WW).
Genetic Linkage & Multiple Alleles
- Linked genes are close together on the same chromosome and inherited together.
- Homologous recombination during gamete formation can separate linked genes.
- Multiple alleles: Traits controlled by more than two alleles (e.g., human blood type: IA, IB, i).
- Blood type genotypes: IAIA/IAi (A), IBIB/IBi (B), IAIB (AB), ii (O).
- Only ii genotype results in blood type O.
Sex-Linked Traits & Related Disorders
- Sex-linked traits are controlled by genes on X or Y chromosomes.
- X-linked traits mainly affect males; Y-linked traits only affect males.
- Only females can be carriers of X-linked disorders.
- Common X-linked disorders: Color-blindness and hemophilia.
- Y-linked traits (e.g., hypertrichosis) pass from father to son.
- Use Punnett squares to determine risks for offspring.
Epistasis, Extra-Nuclear Inheritance, Polygenic Traits
- Epistasis: One gene masks another's effect (epistatic masks, hypostatic gets masked).
- Extra-nuclear inheritance is controlled by genes in mitochondria/chloroplasts and inherited maternally.
- Polygenic traits are controlled by multiple genes and show continuous variation (e.g., skin color, height).
Key Terms & Definitions
- Trait — Observable or inherited characteristic.
- Chromosome — Structure carrying genes in the nucleus.
- Gene — Segment of DNA coding for a trait.
- Locus — Specific gene location on a chromosome.
- Allele — Variant form of a gene.
- Homologous chromosomes — Chromosome pair with same gene loci.
- Phenotype — Observable characteristics.
- Incomplete dominance — Blending of parental traits.
- Codominance — Both parental traits appear fully.
- Genetic linkage — Genes inherited together due to close proximity.
- Multiple alleles — More than two possible alleles for a trait.
- Sex-linked trait — Trait controlled by a gene on a sex chromosome.
- Epistasis — Gene masking the effect of another gene.
- Extra-nuclear inheritance — Inheritance via organelle DNA.
- Polygenic trait — Trait influenced by several genes.
Action Items / Next Steps
- Review and memorize key definitions and mnemonics.
- Practice Punnett squares for inheritance variety.
- Complete any assigned matching, true/false, and situational questions.
- Study diagrams showing gene-chromosome relationships and inheritance patterns.
- Prepare for quizzes by reviewing strategic tips and reference tables.