Mendelian genetics

Mendelian Genetics: Definitions & Basic Facts

Generation Terms for Genetic Crosses

  • Parental (P0):

    • Initial individuals crossed.

    • Often true-breeding individuals (homozygous).

  • First Filial (F1):

    • Offspring from the initial hybridization.

    • If P0 is true-breeding for different traits, F1 is heterozygous.

  • Second Filial (F2):

    • Offspring from crossing F1 individuals.

    • A mix of genotypes and phenotypes; some resemble P0 individuals, others may have mixed traits.

Gene vs Allele

  • Gene:

    • A stretch of DNA on a chromosome coding for RNA influencing a trait.

    • Example: Starch Branching Enzyme gene (sbe1).

  • Allele:

    • Variant forms of a gene differing at precise sequence locations.

    • Dominant R allele produces functional enzyme in round peas.

    • Recessive r allele, with DNA insertion, results in no functional enzyme and wrinkled peas.

    • May also differ due to mutations in regulatory regions of the gene.

Alleles in Populations

  • Populations can have many alleles for a single gene.

  • Example: Hundreds of alleles exist for some immune system genes.

  • A diploid individual can possess at most two alleles per gene (one from each chromosome).

Dominance

  • Incomplete vs. complete dominance:

    • A diploid organism with two different alleles expresses only the dominant allele in phenotype.

    • Recessive alleles are “hidden” unless no dominant allele is present.

    • Phenotype of heterozygous individuals matches that of homozygous dominant individuals.

  • Mendel’s observation:

    • Cross of green-seed plant with true-breeding yellow-seed plants produced all yellow seeds.

    • Cross yellow-seed offspring with each other reintroduced green seeds, indicating yellow-seed allele dominance.

Basic Mendelian Ratios (Must Know)

  • 3 : 1:

    • Expected F2 phenotype ratio for monohybrid cross with complete dominance.

  • 1 : 2 : 1:

    • Expected F2 genotype ratio for monohybrid cross.

    • Also expected phenotype ratio for monohybrid with incomplete dominance.

  • 9 : 3 : 3 : 1:

    • Expected F2 phenotype ratio for dihybrid cross with complete dominance.

  • 1 : 1:

    • Expected phenotype and genotype ratio for a testcross when the parent with dominant phenotype is heterozygote.

    • All one phenotype in a monohybrid cross indicates dominant allele; testcross reveals homozygous or heterozygous parent traits.