Mendelian Genetics Calculators

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Mendelian genetics is the study of genetic inheritance patterns based on Gregor Mendel's experiments with pea plants (1856–1863). Mendel formulated two fundamental laws: the Law of Segregation (alleles of a gene separate during gamete formation, so each gamete carries only one allele) and the Law of Independent Assortment (alleles of different genes are distributed independently to gametes — when genes are on different chromosomes). Key Mendelian concepts include: dominance (A_ phenotype = dominant; aa = recessive), codominance, incomplete dominance, the test cross, and characteristic F2 ratios (3:1 monohybrid; 9:3:3:1 dihybrid). Exceptions include linkage, epistasis, and pleiotropy.

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Mendel's Two Laws

Law of Segregation: The two alleles at a locus separate during gamete formation (meiosis I); each gamete carries exactly one allele. Foundation: monohybrid cross (Aa × Aa → 1:2:1 genotype; 3:1 phenotype ratios). Law of Independent Assortment: Alleles at different loci separate independently during gamete formation (when on different chromosomes). Foundation: dihybrid cross (AaBb × AaBb → 9:3:3:1 phenotype ratio). Not applicable when genes are linked (same chromosome, close together).

Types of Dominance

  • Complete dominance: Aa looks like AA (heterozygote = dominant phenotype)
  • Incomplete dominance: Aa = intermediate phenotype; F2 ratio 1:2:1
  • Codominance: both alleles expressed in Aa; e.g., blood type AB, MN; F2 ratio 1:2:1
  • Overdominance: Aa has higher fitness than either homozygote; e.g., HbA/HbS in malaria

Exceptions to Mendel

Linkage: genes on same chromosome; do not assort independently. Epistasis: one gene masks effect of another → modified ratios (12:3:1; 9:3:4; 9:7; 15:1). Pleiotropy: one gene affects multiple phenotypic traits. Incomplete penetrance: genotype not always expressed as phenotype. Variable expressivity: same genotype expressed differently in different individuals.

Glossary

Law of Segregation
Alleles of a gene separate during meiosis; each gamete carries one allele; produces 1:2:1 genotype and 3:1 phenotype ratios in F2 of monohybrid Aa×Aa; Mendel's first law.
Law of Independent Assortment
Alleles at different loci on different chromosomes segregate independently; produces 9:3:3:1 F2 ratio in dihybrid AaBb×AaBb; Mendel's second law; violated by genetic linkage.
Epistasis
Gene interaction where one gene masks or modifies another's expression; produces modified dihybrid F2 ratios (12:3:1 dominant; 9:3:4 recessive; 9:7 complementary; 15:1 duplicate dominant).

Frequently Asked Questions

Law of Segregation: each individual carries two alleles for each gene; these alleles separate during meiosis so that each gamete contains only one allele. The alleles are randomly and equally distributed to gametes → each allele has a 50% chance of going to any given gamete. Tested by: monohybrid cross (Aa × Aa → 1 AA : 2 Aa : 1 aa = 3:1 phenotype ratio). Law of Independent Assortment: alleles at different loci (on different chromosomes) separate independently of each other during gamete formation. Tested by: dihybrid cross (AaBb × AaBb → 9:3:3:1 phenotype ratio). Limitation: applies only to unlinked genes (on different chromosomes or far apart on the same chromosome).

Complete dominance: one allele (A) completely masks the other (a) in the heterozygote → Aa looks identical to AA. F2 phenotype ratio: 3 dominant : 1 recessive. Most classic Mendelian traits (pea flower color, seed shape). Incomplete dominance: neither allele fully dominant → heterozygote Aa shows an intermediate phenotype. F2 phenotype ratio: 1 AA : 2 Aa : 1 aa = 1:2:1. Example: Antirrhinum (snapdragon) RR = red, rr = white, Rr = pink. Codominance: both alleles are fully expressed simultaneously in the heterozygote — not a blend, but both traits visible. F2 phenotype ratio: 1:2:1. Examples: ABO blood type (AB type expresses both A and B antigens); MN blood groups; roan coat color (intermingled red and white hairs).

Epistasis: one gene (epistatic gene) masks or modifies the expression of another gene (hypostatic gene). It modifies the expected 9:3:3:1 dihybrid ratio. Types: Dominant epistasis (12:3:1): A_ masks B expression; only aaB_ is different from A_ categories. Example: yellow (A_) vs. colored coat in Labrador retrievers — A_ is epistatic to B (chocolate/black determination). Recessive epistasis (9:3:4): aa masks B; both aa genotypes look the same. Example: coat color — certain aa genotypes eliminate color regardless of B genotype. Complementary genes (9:7): both A and B required for phenotype — only A_B_ shows phenotype; all others look alike. Duplicate dominant (15:1): A_ or B_ alone sufficient; only aabb shows recessive phenotype.

Linkage: genes on the same chromosome do not assort independently; recombination frequency < 50% creates non-Mendelian ratios; discovered by Morgan (1910). Sex-linkage: genes on the X chromosome show different patterns in males (XY) vs. females (XX); males are hemizygous → X-linked recessives expressed in males more often. Incomplete penetrance: not all individuals with a genotype show the expected phenotype (e.g., BRCA1 mutation and cancer risk: high but not 100%). Variable expressivity: same genotype shows different degrees of expression in different individuals. Anticipation: disease becomes more severe or has earlier onset in each generation (trinucleotide repeat expansion disorders: Huntington's, myotonic dystrophy). Genomic imprinting: gene expression depends on whether allele is maternal or paternal in origin.