Mendelian Genetics & Punnett Square Calculator
Generate Punnett square ratios for genetic crosses, calculate offspring phenotype probabilities, and work through monohybrid and dihybrid inheritance problems.
Mendelian Genetics Guide
Mendel's Laws
Mendel's First Law (Segregation): each organism carries two alleles for each gene; these separate during gamete formation so each gamete carries only one allele. Mendel's Second Law (Independent Assortment): genes on different chromosomes are inherited independently of each other (dihybrid crosses give 9:3:3:1 phenotype ratio when both parents are AaBb heterozygotes). These laws apply to genes on different chromosomes (unlinked genes) and form the foundation of classical genetics. Linked genes (
The Punnett Square
A Punnett square systematically shows all possible offspring genotypes. For Aa × Aa: top row A, a (parent 1 gametes); left column A, a (parent 2 gametes). Four cells: AA, Aa, Aa, aa. Genotype ratio: 1AA : 2Aa : 1aa. Phenotype ratio (complete dominance): 3 dominant : 1 recessive. Key cross ratios to memorise: homozygous dominant × homozygous recessive → all Aa (100% dominant phenotype). Aa × Aa → 3:1 phenotype ratio. Aa × aa → 1:1 ratio (test cross — used to determine unknown genotype). Carrier ×
Codominance and Incomplete Dominance
Codominance: both alleles are fully expressed in the heterozygote. Example: blood group AB (IA and IB alleles both expressed), Ayrshire cattle coat colour (red and white coat both show as roan). Heterozygote shows a different phenotype from both homozygotes. Incomplete dominance: heterozygote shows an intermediate phenotype. Example: red snapdragon × white snapdragon → pink heterozygote (RR red, Rr pink, rr white). Unlike codominance, the individual allele products are not separately visible — t
Genetic Counselling Applications
Punnett squares are used in genetic counselling to estimate risk of inherited conditions. Autosomal recessive (cystic fibrosis, sickle cell): carrier × carrier → 1/4 risk of affected child, 2/3 of unaffected children are carriers. Autosomal dominant (Huntington's disease): affected (Hh) × unaffected (hh) → 1/2 risk of affected child. X-linked recessive (haemophilia, colour blindness): carrier mother × normal father → sons 1/2 affected, daughters all unaffected (but 1/2 carriers). These are proba
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