Genetic Crosses and Punnett Squares: GCSE Biology Guide

Gregor Mendel discovered in the 1860s that traits are inherited as discrete units — what we now call alleles. A Punnett square is a simple grid that predicts the probability of offspring inheriting particular combinations of alleles from two parents. This tool is a core GCSE biology skill and underpins all of classical genetics.

What are genes, alleles, and genotypes?

A gene is a section of DNA on a chromosome that codes for a particular characteristic (such as eye colour or blood group). Most genes exist in two or more alternative forms called alleles.

Term | Definition | Example
Gene | A sequence of DNA coding for one characteristic | Gene for pea seed colour
Allele | An alternative form of a gene | Yellow (Y) or green (y)
Genotype | The combination of alleles an organism carries | YY, Yy, or yy
Phenotype | The observable characteristic produced by the genotype | Yellow seeds or green seeds
Dominant allele | Expressed in the phenotype whenever present (capital letter) | Y (yellow)
Recessive allele | Only expressed when two copies are present (lower-case letter) | y (green)
Homozygous | Both alleles are the same | YY or yy
Heterozygous | Alleles are different | Yy

A carrier is an individual who is heterozygous for a recessive condition — they carry one copy of the recessive allele and can pass it to offspring, but do not show the condition themselves.

How do you draw a Punnett square?

A Punnett square is a 2×2 grid used to work out the possible genotypes (and probabilities) of offspring from two parents.

Step-by-step method

Step 1 — Write the genotypes of both parents. Step 2 — Work out the gametes each parent can produce (one allele per gamete, because gametes are haploid from meiosis). Step 3 — Draw the grid. Write one parent's gametes across the top; the other parent's gametes down the side. Step 4 — Fill in each box by combining the allele from the column with the allele from the row. Step 5 — Read off the ratio of genotypes and phenotypes.

Worked example: crossing two heterozygous yellow pea plants (Yy × Yy)

Parent genotypes: Yy × Yy

Gametes:

Parent 1: Y or y

Parent 2: Y or y

Punnett square:

| Y | y
Y | YY | Yy
y | Yy | yy

Results:

YY: 1 (homozygous dominant — yellow)

Yy: 2 (heterozygous — yellow)

yy: 1 (homozygous recessive — green)

Genotype ratio: 1 YY : 2 Yy : 1 yy Phenotype ratio: 3 yellow : 1 green

This 3:1 phenotype ratio is the classic result of a heterozygous cross and was what Mendel observed.

What about crosses involving a recessive disorder?

Worked example: cystic fibrosis

Cystic fibrosis (CF) is caused by a recessive allele. Let F = normal allele, f = cystic fibrosis allele.

Two carrier parents (Ff × Ff):

| F | f
F | FF | Ff
f | Ff | ff

Results:

FF: 1 (unaffected, not a carrier)

Ff: 2 (unaffected, carrier)

ff: 1 (affected with cystic fibrosis)

Probability of an affected child = 1/4 = 25% Probability of a carrier child = 2/4 = 50%

Note: probabilities describe the chance for each individual pregnancy — they do not guarantee that in a family of four, exactly one child will have CF.

How does sex determination work in humans?

Human body cells contain 46 chromosomes in 23 pairs. One of these pairs is the sex chromosomes:

Females: XX

Males: XY

Egg cells always carry an X chromosome (eggs are produced by XX females). Sperm cells carry either X or Y:

| X (egg) | X (egg)
X (sperm) | XX (female) | XX (female)
Y (sperm) | XY (male) | XY (male)

The probability of a male or female offspring is 50% for each conception. The sex of the child is determined by whether the fertilising sperm carries X or Y — the mother's egg always contributes X.

What is a test cross?

If an organism shows a dominant phenotype, you cannot tell from appearance alone whether it is homozygous dominant (e.g. YY) or heterozygous (Yy). A test cross crosses the unknown individual with a homozygous recessive organism (yy):

If all offspring show the dominant phenotype: parent was homozygous dominant (YY × yy → all Yy)

If approximately half show dominant and half show recessive: parent was heterozygous (Yy × yy → ½ Yy, ½ yy)

Test crosses were used by Mendel to work out the genotypes of his pea plants and remain a standard genetic tool today.

What is incomplete dominance?

In simple dominance, one allele is fully dominant over the other. In incomplete dominance, the heterozygote shows an intermediate phenotype:

Example — snapdragon flower colour:

RR: red flowers

rr: white flowers

Rr: pink flowers (blend of both)

The Punnett square method is identical; the key difference is that the heterozygous genotype produces a third phenotype. The genotype ratio (1:2:1) and phenotype ratio (1:2:1) are the same in this case.

Frequently asked questions

What is the difference between a genotype and a phenotype?

A genotype is the actual combination of alleles an organism carries for a particular gene — for example, Yy for a heterozygous yellow pea. A phenotype is the observable characteristic expressed as a result — for example, yellow seeds. The same phenotype can arise from different genotypes: both YY and Yy peas produce yellow seeds. Knowing the phenotype does not tell you the genotype unless you know which allele is dominant and which is recessive.

What does a 3:1 ratio mean in a Punnett square?

A 3:1 phenotype ratio is the expected result when two heterozygous individuals (carriers) for a trait with simple dominance are crossed. Three-quarters of offspring are expected to show the dominant phenotype (including both homozygous dominant and heterozygous individuals) and one-quarter the recessive phenotype. It is important to understand that these are probabilities, not guarantees — in a small family, the actual numbers may differ considerably from the predicted ratio.

What is the probability of having a daughter?

For each pregnancy, the probability of the child being a daughter is approximately 50%, or 1 in 2. This is because males produce equal numbers of X-bearing and Y-bearing sperm, and fertilisation by an X sperm produces a female (XX) while Y sperm produces a male (XY). The previous children a couple has had do not affect the probability for the next pregnancy — each conception is an independent event.

What is a recessive disorder and who is a carrier?

A recessive disorder is caused by a recessive allele that is only expressed when two copies are present (homozygous recessive, e.g. ff for cystic fibrosis). A carrier is someone who has one copy of the recessive allele and one copy of the dominant allele (heterozygous, e.g. Ff) — they do not show the disorder but can pass the recessive allele to their children. Two carrier parents have a 25% chance of having an affected child, a 50% chance of having a carrier child, and a 25% chance of having a homozygous dominant (unaffected, non-carrier) child.

For Socratic GCSE biology with Professor Darwin — tracing genetics from organism to chromosome to allele — visit aitutors.me.