Mendelian Genetics - Quick Reference

Gregor Mendel

  • Gregor Johann Mendel (1822–1884), father of genetics; monk and priest; pea plant experiments (Pisum sativum) on trait transmission; 1866 paper; work later recognized in 1900s; foundational to genetics; established Mendelian or transmission genetics with laws of segregation and independent assortment.

Alleles

  • Alleles are different versions of a gene that determine specific traits (e.g., flower color, seed shape).
  • Each organism inherits two alleles for each trait (one from each parent).
  • Alleles can be dominant or recessive; dominant masks the effect of a recessive allele in heterozygotes.
  • Notation for a gene with two alleles: AA, Aa, aa (genotypes).

Genotype and Phenotype

  • Genotype: organism's complete set of heritable genes; specific allele combo (e.g., AA, Aa, aa).
  • Phenotype: observable traits resulting from genotype and environment.
  • Environment can influence how genes are expressed.
  • Example genotype notations: AA, Aa, aa.

Dominant and Recessive Alleles

  • Dominant allele expresses its trait even when paired with a different allele (e.g., A in Aa).
  • Recessive allele expresses its trait only when two copies are present (aa).
  • If one dominant and one recessive allele are present, the dominant trait is expressed.

Homozygous and Heterozygous

  • Homozygous: same alleles for a gene (e.g., RR, rr).
  • Heterozygous: different alleles (e.g., Rr).

Punnett Square

  • Tool to predict possible genotypes and phenotypes of offspring from parental alleles.
  • Example (monohybrid cross): cross between two heterozygotes Aa imes Aa
    • Offspring genotypes: 1:2:1 ( AA: Aa: aa )
    • Offspring phenotypes: 3:1 (dominant:recessive)
  • Gametes from each parent: A, a; combine to predict outcomes.

Mendel's Laws

  • Law of Segregation: for a given trait, the two alleles separate into gametes during meiosis; each gamete carries one allele; fertilization restores the pair.
  • Law of Independent Assortment: alleles of different genes assort independently during gamete formation if the genes are on different chromosomes or far apart on the same chromosome.
  • Law of Dominance: in a heterozygote, the dominant allele determines the phenotype; the recessive allele is masked.
  • Law of Inheritance (overview): traits are transmitted from parents to offspring through alleles carried on genes.

Monohybrid Cross and Ratios

  • Example: tall (T) is dominant over dwarf (t).
  • Cross: TT imes tt
    ightarrow ext{all } Tt ext{ (phenotype: tall)}
  • Self-cross of F1 (Tt imes Tt):
    • Genotypic ratio: 1:2:1 ( TT: Tt: tt )
    • Phenotypic ratio: 3:1 (tall:dwarf)
  • Notation: tall = T (dominant), dwarf = t (recessive).

Dihybrid Cross and 9:3:3:1

  • Cross two heterozygotes for two traits (e.g., seed shape R/r and seed color Y/y): RrYy imes RrYy
  • Possible gametes from each parent: RY, Ry, rY, ry
  • Phenotypic ratio: 9:3:3:1 (Round/Yellow : Round/Green : Wrinkled/Yellow : Wrinkled/Green)
  • Genotype combinations follow from combining the four gamete types.

Eye Color Genetics (example)

  • Brown eye allele is dominant: B; blue eye allele is recessive: b.
  • Genotypes: BB, Bb, bb
  • Phenotypes: brown if at least one B; blue if bb only.

Quick Takeaways

  • Mendel established the basic unit of heredity (genes) and how they segregate and assort.
  • Genotype vs. phenotype differentiate genetic makeup from observable traits.
  • Dominant vs. recessive determine phenotype in heterozygotes.
  • Punnett squares help predict offspring genotypes/phenotypes.
  • Ratios to know: monohybrid 3:1 (phenotype) and 1:2:1 (genotype); dihybrid 9:3:3:1.
  • Law of Segregation, Law of Independent Assortment, Law of Dominance are foundational to predicting inheritance patterns.