Mendelian Genetics & Punnett Square Calculator

Mendelian Genetics Guide

What do I need to know about 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 (genes located close together on the same chromosome) are inherited together more often than not, breaking the law of independent assortment — the closer two genes sit on a chromosome, the less likely crossing over is to separate them, which is the basis of genetic linkage mapping.

What's the key thing to understand about 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 × carrier (Aa × Aa, both unaffected but carrying a recessive allele): 1/4 affected, 1/2 carrier, 1/4 unaffected homozygous dominant — the classic 3:1 phenotype ratio, but with the crucial distinction that two-thirds of the unaffected offspring are themselves carriers, relevant for genetic counselling of recessive conditions like cystic fibrosis.

What should I know about 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 — the blended appearance arises because both alleles are expressed but neither product alone reaches the concentration needed to produce the full-intensity phenotype — unlike codominance (such as AB blood type, where both A and B antigens are fully and separately visible on red blood cells at once), incomplete dominance produces a genuinely intermediate, merged trait.

What should I know about 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 probabilities per pregnancy, not fixed outcomes — each conception is an independent event, so having one affected child doesn't reduce or increase the risk for the next. Genetic counsellors use pedigree analysis alongside these Mendelian ratios to estimate risk and, increasingly, alongside direct genetic testing to confirm carrier or affected status rather than relying on probability alone.