Fundamentals of Inheritance and Meiosis

Overview

• Topic: Principles of inheritance (standard D3.2), focused on sexually reproducing eukaryotes.
• Covers: meiosis/gametes, zygote ploidy, alleles, Mendel’s methods, genotype vs phenotype, dominance, Punnett squares, mutations, and phenotypic plasticity.
• Purpose: Summarize core concepts and definitions for study.

Meiosis, Gametes, And Ploidy

• Meiosis produces haploid (n) gametes: eggs (female) and sperm (male).
• Fusion of gametes forms a diploid (2n) zygote.
• Diploid cells contain homologous chromosome pairs, one from each parent.
• Homologous chromosomes have the same genes in same locations, but may carry different alleles.

Mendel And Experimental Design

• Gregor Mendel used pea plants for controlled inheritance studies.
• He prevented self-pollination (removed anthers), hand-pollinated chosen plants, and covered flowers to avoid outside pollen.
• Generations named: P (parental), F1 (first filial), F2 (second filial, from crossing F1 individuals).
• Large sample sizes and controlled crosses revealed inheritance patterns.

Alleles And Genotype

• Allele: different version of a gene (sequence variants).
• Genotype: combination of alleles an individual has for a gene (e.g., Rr).
• New alleles originate via mutation (base-sequence changes).

Homozygous, Heterozygous, Dominance

• Homozygous: two identical alleles (e.g., AA or aa).
• Heterozygous: two different alleles (e.g., Aa).
• Dominant allele: expressed in phenotype whenever present.
• Recessive allele: expressed only when dominant allele is absent.
• Phenotypic outcomes:
  • Homozygous dominant → dominant phenotype.
  • Homozygous recessive → recessive phenotype.
  • Heterozygous → dominant phenotype (recessive masked).

Phenotype And Environmental Influence

• Phenotype: observable/physical trait; results from genotype and environment.
• Some traits are strictly genetic (e.g., blood type).
• Many traits are influenced by both genes and environment (e.g., height depends on nutrition).
• Some phenotypes arise solely from environment (e.g., tattoos).

Punnett Squares And Probability

• Punnett square visualizes possible offspring genotypes from parental gametes.
• Parental genotypes are represented by the alleles present in their gametes along the square edges.
• Each cell simulates fertilization of one parental gamete with the other.
• Results show possible genotypes and allow probability estimates, not guarantees.
• For single-trait crosses use a 2x2 grid; multiple traits require larger grids.

Phenotypic Plasticity And Gene Expression

• Phenotypic plasticity: reversible phenotype changes due to altered gene expression.
• Genotype remains unchanged while expression patterns vary.
• Cells can activate or silence genes in response to environment.
• Example: Skin darkening from UV exposure increases melanin via gene expression changes.
• Emphasizes continuity (genotype stable) and change (phenotype variable).

Key Terms And Definitions

• Haploid: one chromosome set (n).
• Diploid: two chromosome sets (2n).
• Homologous chromosomes: chromosome pair with same genes.
• Allele: gene variant.
• Genotype: allele combination at a locus.
• Phenotype: physical expression of traits.
• Homozygous: identical alleles.
• Heterozygous: different alleles.
• Dominant allele: expressed when present.
• Recessive allele: expressed only when dominant absent.
• Mutation: source of new alleles.
• Phenotypic plasticity: reversible phenotype change via gene expression.

Action Items / Next Steps

• Practice constructing and interpreting Punnett squares for single and multiple traits.
• Identify examples of traits determined purely genetically versus environmentally influenced.
• Review Mendel’s experimental setup and generation naming (P, F1, F2).
• Study examples of phenotypic plasticity and mechanisms of gene activation/silencing.