The genotype of an offspring is the combination of alleles it inherits from both parents, one allele from each. For a single gene, this means every offspring carries two copies: one from the egg and one from the sperm. Which specific combination an offspring receives depends on the parents’ genotypes, and predicting the possibilities comes down to a few straightforward patterns.
How Alleles Combine in Offspring
Every parent has two alleles for a given gene, but only passes one of them to each offspring. The offspring’s genotype is simply the pairing of whichever allele it received from each parent. If a gene has a dominant allele (B) and a recessive allele (b), three genotypes are possible: BB (homozygous dominant), Bb (heterozygous), and bb (homozygous recessive).
A homozygous parent (BB) can only contribute one type of allele: B. A heterozygous parent (Bb) has a 50/50 chance of passing on either B or b. This randomness is what creates variation among siblings, even though they share the same two parents.
Using a Punnett Square to Predict Genotypes
A Punnett square is a simple grid that maps out every possible allele combination in offspring. You place one parent’s possible gametes across the top and the other parent’s along the side, then fill in the boxes. Each box represents an equally likely offspring genotype.
When two heterozygous parents (Bb × Bb) are crossed, the Punnett square produces four boxes: BB, Bb, bB, and bb. That gives a genotypic ratio of 1 BB : 2 Bb : 1 bb. In percentage terms, 25% of offspring are expected to be homozygous dominant, 50% heterozygous, and 25% homozygous recessive. This 1:2:1 ratio is the foundational pattern for a single-gene cross between two carriers.
Other parental combinations produce different ratios. If one parent is homozygous dominant (BB) and the other is heterozygous (Bb), all offspring will show the dominant trait, but half will be BB and half Bb. If one parent is homozygous dominant and the other homozygous recessive (BB × bb), every offspring will be heterozygous (Bb) with no variation at all.
Dihybrid Crosses: Two Traits at Once
When you track two genes simultaneously, the Punnett square expands to 16 boxes. For two heterozygous parents (SsYy × SsYy), each parent can produce four gamete types: SY, Sy, sY, and sy. The resulting 16 combinations produce nine distinct genotypes.
The phenotypic ratio in this classic dihybrid cross is 9:3:3:1. Breaking down the genotypes in more detail: SSYY appears at a frequency of 1/16, SSYy and SsYY each at 2/16, and SsYy at 4/16. On the recessive side, SSyy is 1/16, Ssyy is 2/16, ssYY is 1/16, ssYy is 2/16, and ssyy is 1/16. The key principle is that each gene still follows its own 1:2:1 ratio independently; the two ratios just multiply together.
Incomplete Dominance and Codominance
Not all genes follow the simple dominant/recessive pattern. In incomplete dominance, the heterozygous offspring shows a blended phenotype. The classic example is flower color: crossing a red-flowered plant with a white-flowered plant produces pink heterozygous offspring. When two of those pink plants are crossed, the genotypic ratio is still 1:2:1, but now all three genotypes look different: 1 red, 2 pink, 1 white. The genotypic and phenotypic ratios match perfectly because the heterozygote has its own distinct appearance.
Codominance works similarly in terms of ratios, but instead of blending, both alleles are fully expressed at the same time. Human MN blood type is a good example. A person with one M allele and one N allele doesn’t show a blend; they express both M and N markers on their red blood cells. Crossing two heterozygous (MN) individuals yields the same 1:2:1 genotypic ratio, with offspring showing M, MN, and N blood types.
ABO Blood Type as a Real-World Example
Blood type inheritance is one of the most commonly searched examples of offspring genotypes because it involves multiple alleles and two types of dominance at once. The ABO gene has three alleles: A, B, and O. Both A and B are dominant over O, but A and B are codominant with each other. A child receives one allele from each parent, giving six possible genotypes (AA, AO, BB, BO, AB, OO) and four blood types (A, B, AB, O).
If both parents are type A with genotype AO, their children could be AA (type A), AO (type A), or OO (type O) in a 1:2:1 ratio. If one parent is AO and the other is BO, the possible offspring genotypes are AB, AO, BO, and OO, each with a 25% chance. This is why two parents who are neither type O can still have a type O child.
Sex-Linked Traits Change the Pattern
Genes located on the X chromosome follow different rules for sons and daughters. Males have one X and one Y chromosome, so they carry only a single copy of any X-linked gene. Females have two X chromosomes and therefore two copies.
For an X-linked recessive trait like red-green color blindness, a carrier mother (one normal allele, one affected allele) paired with an unaffected father produces these outcomes: each son has a 50% chance of being affected, because he either inherits the X with the normal allele or the X with the affected allele. Each daughter has a 50% chance of being a carrier and a 0% chance of being affected, because she always receives a normal X from her father.
When an affected father has children with an unaffected, non-carrier mother, none of his sons will be affected (they get his Y chromosome, not his X). All of his daughters will be carriers, since they inherit his single affected X. This “diagonal” transmission pattern, where affected grandfathers pass the trait through carrier daughters to affected grandsons, is the hallmark of X-linked recessive inheritance. Red-green color blindness follows this pattern, affecting roughly 10% of men but only about 1% of women.
Using a Test Cross to Determine Unknown Genotypes
If an organism shows a dominant trait, you can’t tell by looking whether its genotype is homozygous (BB) or heterozygous (Bb). A test cross solves this by mating the unknown individual with a homozygous recessive (bb) partner. The recessive parent can only contribute b alleles, so the offspring’s phenotypes directly reveal what the unknown parent contributed.
If the unknown parent is BB, every offspring will be Bb and show the dominant trait. If the unknown parent is Bb, roughly half the offspring will be Bb (dominant phenotype) and half will be bb (recessive phenotype). Seeing even a single recessive offspring confirms the unknown parent was heterozygous. This same logic scales to multiple genes: crossing with a fully recessive tester reveals exactly which allele combinations the unknown parent can produce.
Traits Controlled by Many Genes
Simple Punnett squares work well for traits controlled by one or two genes, but many traits in humans are polygenic, meaning dozens of genes contribute to the outcome. Eye color is a well-known example. Researchers once thought it followed a straightforward dominant/recessive pattern where brown was dominant to blue, but that model turned out to be far too simple. Two genes on chromosome 15 play the largest role, but at least nine other genes also contribute. Their combined effects produce the full spectrum of eye colors.
This is why two blue-eyed parents can occasionally have a brown-eyed child, something impossible under the old single-gene model. For polygenic traits, predicting an offspring’s exact genotype isn’t practical with a Punnett square. Instead, the offspring’s outcome falls somewhere along a range influenced by many small genetic contributions from both parents.
De Novo Mutations: Genotypes Neither Parent Carries
Occasionally, an offspring carries a genotype that doesn’t match either parent’s DNA. These are de novo mutations, new genetic changes that arise spontaneously during the formation of sperm or egg cells, or very early in embryonic development. On average, each person carries about 63 new mutations not found in either parent, corresponding to a rate of roughly 1.2 new mutations per 100 million DNA letters per generation. The vast majority of these changes have no noticeable effect, but a small fraction can cause genetic conditions that appear for the first time in a family with no prior history.