Predict genetic crosses with a visual Punnett square grid. Supports monohybrid, dihybrid, codominance, incomplete dominance, and X-linked inheritance. Perfect for GCSE and A-Level Biology.
Single gene cross — dominant vs recessive
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A Punnett square is a diagram used to predict the possible genotypes and phenotypes of offspring from a genetic cross. It was invented by British geneticist Reginald Punnett in the early 1900s.
The square works by placing one parent's gametes (sex cells) across the top and the other parent's gametes down the left side. Each cell in the grid represents one possible offspring, showing the combination of alleles it would inherit.
The genetic makeup of an organism — the alleles it carries (e.g. Bb, IAIB)
The observable trait or characteristic (e.g. brown eyes, Type AB blood)
A variant of a gene. Each organism inherits two alleles per gene, one from each parent
An allele that is expressed when at least one copy is present. Written as UPPERCASE (e.g. B)
An allele only expressed when two copies are present (homozygous). Written as lowercase (e.g. b)
Having two identical alleles for a gene (e.g. BB or bb)
Having two different alleles for a gene (e.g. Bb). Also called a hybrid
An organism heterozygous for a recessive allele — appears normal but can pass the allele to offspring
Single gene cross. The classic 3:1 ratio occurs when both parents are heterozygous.
Tt × Tt → 3 tall : 1 short
Two gene cross with independent assortment. Produces 16 possible offspring in a 4×4 grid.
RrYy × RrYy
Both alleles fully expressed. The classic example is ABO blood types with three alleles.
IAi × IBi → A:B:AB:O
The heterozygote shows a blended intermediate phenotype. Genotypic ratio = phenotypic ratio.
RW × RW → 1:2:1
Gene on the X chromosome. Males are more commonly affected because they have only one X.
XHXh × XHY
Practice with these GCSE and A-Level style genetics problems:
Two heterozygous tall pea plants (Tt × Tt) are crossed. Predict the offspring ratios.
Gametes: T, t from each parent
Grid: TT, Tt, Tt, tt
Genotypic ratio: 1 TT : 2 Tt : 1 tt
Phenotypic ratio: 3 tall : 1 short (75% : 25%)
A black mouse (Bb) is crossed with a white mouse (bb). What are the expected offspring ratios?
Gametes: B, b from Bb parent; b from bb parent
Grid: Bb, bb
Phenotypic ratio: 1 black : 1 white (50% : 50%)
RrYy × RrYy (round yellow × round yellow). Predict the 9:3:3:1 ratio.
4 gamete types each: RY, Ry, rY, ry
4×4 = 16 cells in the grid
9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green
Phenotypic ratio: 9:3:3:1
A Type A heterozygous parent (IAi) crosses with a Type B heterozygous parent (IBi).
P1 gametes: IA, i | P2 gametes: IB, i
Grid: IAIB, IAi, IBi, ii
Type AB (25%), Type A (25%), Type B (25%), Type O (25%)
Phenotypic ratio: 1:1:1:1 (all four blood types equally likely)
A carrier mother (XHXh) × unaffected father (XHY). What are the offspring probabilities?
Mother gametes: XH, Xh | Father gametes: XH, Y
Grid: XHXH, XHXh, XHY, XhY
Daughters: 50% XHXH (unaffected), 50% XHXh (carrier)
Sons: 50% XHY (unaffected), 50% XhY (affected)
25% chance of an affected son. 0% affected daughters.
Avoid these frequent errors when solving genetic cross questions in exams:
Genotype is the alleles (e.g. Bb). Phenotype is the observable trait (e.g. brown eyes). Two different genotypes (BB and Bb) can give the same phenotype.
Always state clearly whether you are describing the genotype or phenotype. Label your answers.
A common error is writing "Bb" as a gamete. Each gamete carries only ONE allele per gene. The gametes of Bb are B and b, not Bb.
Remember: gametes are haploid. They carry one allele per gene, not two.
Students often forget that a female can be a carrier (X^H X^h) — she appears unaffected but carries the recessive allele and can pass it to her sons.
In X-linked problems, always check if any daughters are carriers (heterozygous on the X chromosome).
The 3:1 ratio only occurs when both parents are heterozygous (Aa × Aa). Other crosses give different ratios: AA × Aa gives 100% dominant, Aa × aa gives 1:1.
Always draw the Punnett square. Don't assume a ratio — work it out.
For AaBb, the gametes are AB, Ab, aB, ab (not Aa, Bb). Each gamete takes one allele from EACH gene, not both alleles from one gene.
Use the FOIL method for dihybrid gametes: combine one allele from gene 1 with one allele from gene 2.
In codominance, BOTH alleles are fully expressed (e.g. red AND white patches on a cow). In incomplete dominance, there is a BLEND (e.g. pink flowers).
Codominance = both show. Incomplete dominance = blend. They are different inheritance patterns.
A Punnett square is a diagram that predicts the genotypes and phenotypes of offspring from a genetic cross. One parent's gametes go across the top, the other's down the side, and each cell shows a possible offspring.
The 3:1 phenotypic ratio occurs when two heterozygous parents are crossed (Aa × Aa). 3 out of 4 offspring show the dominant trait and 1 out of 4 shows the recessive trait.
A test cross involves crossing an organism of unknown genotype with a homozygous recessive organism (aa). If any offspring show the recessive trait, the unknown parent must be heterozygous (Aa).
In codominance, both alleles are fully expressed (e.g. AB blood type). In incomplete dominance, the heterozygote shows a blended intermediate phenotype (e.g. pink from red × white).
Males have one X and one Y chromosome (XY). Since the gene is on the X, they only need one copy of the recessive allele to be affected. Females (XX) need two copies.
A carrier is heterozygous for a recessive allele. They appear phenotypically normal but can pass the allele to offspring. In X-linked inheritance, only females can be carriers.
Take one allele from each gene pair. For AaBb: combine A or a with B or b to get AB, Ab, aB, ab (4 gametes). For AABb: AB and Ab (2 gametes, since gene 1 can only give A).
Yes! This covers all genetics topics: monohybrid, dihybrid, codominance, incomplete dominance, and X-linked inheritance with step-by-step solutions and interactive learning.
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