Capital = dominant, lowercase = recessive (e.g. Bb, BB, bb)
Enter 4 letters: e.g. BbTt, BBTt, BbTT
A Punnett square is a diagram used to predict the possible genetic outcomes of a cross between two parent organisms. Developed by geneticist Reginald Punnett in 1905, it remains a fundamental tool in genetics education from GCSE through to university level.
The principle is straightforward: each parent has two alleles for each gene, and they pass one allele randomly to each offspring. The Punnett square shows every possible combination, and the frequency of each cell represents the probability of that outcome.
Genes exist in different forms called alleles. In classical genetics:
Example: B = brown eyes (dominant), b = blue eyes (recessive). A person with genotype Bb has brown eyes because B masks the effect of b. Only a person with genotype bb will have blue eyes.
When two heterozygous (Bb) parents are crossed, the expected offspring genotype ratio is 1 BB : 2 Bb : 1 bb. Since BB and Bb both produce the dominant phenotype, the phenotype ratio is 3 dominant : 1 recessive (written 3:1). This is one of the most tested ratios in GCSE biology.
| Cross | Genotype Ratio | Phenotype Ratio |
|---|---|---|
| BB × BB | All BB | All dominant |
| BB × Bb | 1 BB : 1 Bb | All dominant |
| BB × bb | All Bb | All dominant |
| Bb × Bb | 1 BB : 2 Bb : 1 bb | 3 dominant : 1 recessive |
| Bb × bb | 1 Bb : 1 bb | 1 dominant : 1 recessive |
| bb × bb | All bb | All recessive |
Not all alleles follow simple dominant-recessive patterns. Two important exceptions are codominance and incomplete dominance.
In codominance, both alleles are fully expressed simultaneously in the heterozygote. The ABO blood group system is the classic example taught at GCSE. When both IA and IB alleles are present, the person has blood type AB — both antigens are produced. Neither allele masks the other.
Another example is the coat colour of Shorthorn cattle: a cross between red (RRRR) and white (RWRW) produces roan cattle (RRRW) which have a mixture of red and white hairs.
In incomplete dominance, the heterozygote shows a phenotype intermediate between the two homozygous parents. For example, in snapdragons, crossing a red-flowered plant (RR) with a white-flowered plant (rr) produces pink-flowered offspring (Rr). The pink is not dominant; it is a blend of both alleles' effects.
Sex-linked traits are controlled by genes found on the sex chromosomes (X or Y). In humans, females have two X chromosomes (XX) and males have one X and one Y (XY). The Y chromosome carries very few genes, so males express whatever alleles they inherit on their single X chromosome.
Red-green colour blindness is caused by a recessive allele on the X chromosome (Xb). Females need two copies of the allele (XbXb) to be colour blind, while males only need one (XbY). This is why colour blindness is much more common in males (approximately 8% of males vs 0.5% of females in the UK).
A female with genotype XBXb is called a carrier — she has normal vision but can pass the Xb allele to her children. On average, half of her sons will be colour blind and half of her daughters will be carriers.
A dihybrid cross examines the inheritance of two genes simultaneously. Mendel's Law of Independent Assortment states that alleles for different genes assort independently during gamete formation (assuming the genes are on different chromosomes).
When two heterozygous parents are crossed for two traits (AaBb × AaBb), the offspring phenotype ratio is 9:3:3:1:
A test cross is a standard genetic technique to determine whether an organism showing the dominant phenotype is homozygous dominant (BB) or heterozygous (Bb). The organism is crossed with a homozygous recessive individual (bb). If any offspring show the recessive phenotype, the tested parent must be Bb. If all offspring show the dominant phenotype, the parent is likely BB (though a small sample size could be misleading).
The Hardy-Weinberg principle states that allele and genotype frequencies in a population remain constant from generation to generation in the absence of evolutionary influences. The equation is: p² + 2pq + q² = 1, where p is the frequency of the dominant allele, q is the frequency of the recessive allele, p² represents homozygous dominant frequency, 2pq represents heterozygous frequency, and q² represents homozygous recessive frequency. This principle is widely used in population genetics to estimate carrier frequencies for genetic disorders.
A Punnett square is a grid diagram used to predict the possible genotypes of offspring from a genetic cross between two parents. It was developed by Reginald Punnett in the early 20th century. Each row and column of the grid represents an allele from one parent, and each cell shows a possible offspring genotype. Each cell in a completed Punnett square represents a 25% probability for a two-allele cross.
A monohybrid cross examines the inheritance of one trait (one gene locus), producing a 2×2 Punnett square with four cells. A dihybrid cross examines two traits simultaneously, producing a 4×4 Punnett square with 16 cells. The classic dihybrid cross of two double-heterozygous parents (AaBb × AaBb) gives a 9:3:3:1 phenotype ratio, assuming independent assortment.
A 3:1 phenotype ratio appears when two heterozygous parents (Aa × Aa) are crossed. The offspring are 1 AA : 2 Aa : 1 aa in genotype, but since the dominant allele (A) masks the recessive (a), three-quarters show the dominant phenotype and one-quarter show the recessive phenotype, giving a 3:1 ratio. This ratio was first described by Gregor Mendel in his pea plant experiments in the 1860s.
Codominance occurs when both alleles are fully expressed in a heterozygous individual, with neither being dominant over the other. The ABO blood group system is the classic example: when a person inherits both the IA and IB alleles, they have blood type AB because both A and B antigens are produced on red blood cell surfaces. This is different from incomplete dominance, where the heterozygote shows an intermediate phenotype.
A test cross involves crossing an organism showing the dominant phenotype (but with unknown genotype) with a homozygous recessive individual (aa). If any offspring show the recessive phenotype, the unknown parent must be heterozygous (Aa). If all offspring show the dominant phenotype, the parent is likely homozygous dominant (AA). Test crosses are widely used in plant and animal breeding programmes.
Sex-linked traits are controlled by genes on the sex chromosomes (X or Y). Most sex-linked traits are X-linked recessive, such as colour blindness and haemophilia. Females (XX) need two copies of the recessive allele to show the trait; males (XY) only need one copy because they have no second X chromosome to mask it. Carrier females (XBXb) are unaffected but can pass the recessive allele to their sons, who will then be affected.
Punnett squares predict probability ratios, not exact counts. Each conception is an independent random event. Just as flipping a coin four times does not guarantee exactly two heads, a couple with a 1-in-4 risk of having an affected child may have 0, 1, 2, 3, or 4 affected children from four pregnancies. The ratios only become reliable averages with very large numbers of offspring, as demonstrated by Mendel's large-scale pea plant experiments.