Overview
This lecture covers core concepts in genetics, including inheritance, genetic crosses (monohybrid and dihybrid), dominance, sex linkage, and related exam questions for Leaving Cert Biology.
Variation and Inheritance
- A species is a group of similar organisms that can interbreed to produce fertile offspring.
- Variation in a species can be caused by environmental factors or inheritance (genes).
- Genetics is the study of how characteristics are passed from one generation to the next.
- Inherited characteristics are passed from parents (e.g., eye color); acquired characteristics are learned (e.g., playing an instrument).
Chromosomes, Genes, and Alleles
- Chromosomes are thread-like structures in the cell nucleus; humans have 23 pairs (46 total).
- Genes are sections of DNA on chromosomes that code for proteins.
- Alleles are different forms of a gene (e.g., brown or blue eye color).
- During reproduction, sperm and egg each contribute 23 chromosomes to form a zygote with 46 chromosomes.
Dominant and Recessive Traits
- Dominant alleles mask the effect of recessive alleles in heterozygotes.
- Brown eye color (B) is dominant to blue (b); only bb gives blue eyes.
- The physical expression is the phenotype, the genetic makeup is the genotype.
- Homozygous: two identical alleles (BB or bb); heterozygous: different alleles (Bb).
Mendel’s Laws and Genetic Crosses
- Mendel’s Law of Segregation: Each parent passes a random allele to offspring.
- Punnett squares are used to show possible gene combinations from parents.
- Typical monohybrid cross ratios: 3:1 (dominant:recessive) for heterozygous parents.
- Exam questions often ask for genotype/phenotype results and their ratios/percentages.
Incomplete Dominance and Codominance
- Incomplete dominance: neither allele is dominant; heterozygotes show an intermediate phenotype (e.g., red x white = pink).
- Codominance: both alleles contribute to the phenotype (e.g., red x white cow = roan).
- Phenotypic ratio for two heterozygotes: 1:2:1 (e.g., red:pink:white).
Dihybrid Crosses and Law of Independent Assortment
- Dihybrid crosses examine two traits at once; alleles of different genes assort independently.
- Example: Crossing BbSs x bbss gives four phenotype combinations (black/short, black/long, white/short, white/long).
Sex Determination and Sex Linkage
- Human sex is determined by X and Y chromosomes; males are XY, females XX.
- Sex-linked traits are usually on the X chromosome; males inherit X-linked traits from their mother.
- Example: Hemophilia is X-linked; Punnett squares show carrier mothers can have affected sons.
Exam Practice and Application
- Practice drawing Punnett squares for various crosses (monohybrid, dihybrid, sex-linked).
- Identify genotypes and phenotypes based on problem context.
- Apply Mendel’s laws to exam questions, including linkage and independent assortment.
Key Terms & Definitions
- Allele — a version of a gene.
- Phenotype — physical expression of genetic traits.
- Genotype — genetic makeup of an organism.
- Homozygous — having two identical alleles.
- Heterozygous — having two different alleles.
- Dominant — an allele that masks another’s effect.
- Recessive — an allele masked by a dominant one.
- Incomplete dominance — neither allele is dominant; heterozygote is intermediate.
- Codominance — both alleles are fully expressed.
- Sex linkage — gene located on a sex chromosome.
Action Items / Next Steps
- Review H1 notes in the genetics resource pack (Unit 2).
- Watch video tutorials for monohybrid and dihybrid crosses.
- Complete genetics quizzes to test understanding.
- Practice drawing and interpreting Punnett squares.