Punnett Square Calculator for Sex-Linked Traits
Build a Punnett square for autosomal, X-linked, or Y-linked inheritance. Choose each parent's genotype to see offspring genotypes, phenotypes, and sex ratios with probabilities.
Punnett Square Calculator (Sex-Linked)
Build a Punnett square for autosomal, X-linked, or Y-linked traits. Pick each parent's genotype to see every possible offspring genotype with its probability.
Punnett square
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
Genotype ratios
Phenotype ratios
Documentation
A Punnett square is a grid that shows every combination of alleles two parents can pass to a child, and how likely each one is. This calculator builds the square for autosomal traits and for sex-linked traits carried on the X or Y chromosome.
How a Punnett square works
Each parent carries two alleles for a gene. An allele is one version of a gene. A capital letter marks the dominant allele and the matching lowercase letter marks the recessive allele, so A and a are two versions of the same gene.
When a parent makes egg or sperm cells, the two alleles separate, and each cell gets one of them. The square lists one parent's possible cells along the top and the other parent's down the side. Each box is one possible pairing, and every box is equally likely.
Genotype and phenotype
The genotype is the pair of alleles a child inherits, such as AA, Aa, or aa. The phenotype is the visible trait.
A dominant allele shows up whenever it is present, so AA and Aa both give the dominant trait. The recessive trait appears only with two recessive alleles, aa.
Example: Aa × Aa
Two parents both carry Aa. Each can pass A or a.
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
The genotype ratio is 1 AA : 2 Aa : 1 aa. The phenotype ratio is 3 dominant : 1 recessive, because AA and Aa look the same.
Two gene pairs
With two gene pairs, each parent makes four kinds of cells and the square has 16 boxes. An AaBb × AaBb cross gives the classic 9 : 3 : 3 : 1 phenotype ratio. Three gene pairs give a 64-box square.
X-linked inheritance
Some genes sit on the X chromosome. A mother has two X chromosomes, so she has two alleles. A father has one X and one Y, so he has only one allele for that gene.
Because a son has a single X, whichever allele he inherits from his mother is the one he shows. There is no second copy to mask it. This is called hemizygous, and it is why X-linked recessive conditions such as red-green colour blindness and haemophilia are more common in males.
A daughter inherits one X from each parent. She shows a recessive X-linked trait only if both of her X chromosomes carry the recessive allele. If she has one recessive allele and one dominant allele, she is a carrier: she does not show the trait but can pass it on.
Carrier mother and unaffected father
A carrier mother is X^A X^a and an unaffected father is X^A Y. Among their children:
- half the daughters are carriers (X^A X^a), half are not (X^A X^A)
- half the sons are affected (X^a Y), half are not (X^A Y)
No daughter is affected, because every daughter gets a dominant X^A from her father.
Y-linked inheritance
A Y-linked gene sits on the Y chromosome, which only fathers pass on and only to sons. Every son of a father who carries the trait inherits it, and no daughter ever does.
Frequently asked questions
Why do sons get X-linked traits more often?
A son has only one X chromosome. A single recessive allele on it is enough for the trait to appear. A daughter would need the recessive allele on both of her X chromosomes.
Can a father pass an X-linked trait to his son?
No. A father gives his son a Y chromosome, not an X. Sons get their X only from their mother.
What does carrier mean?
A carrier has one copy of a recessive allele but does not show the trait. Carriers can pass the allele to their children.
Are Punnett square results guaranteed?
No. The ratios are probabilities for each child, not a promise about a family. Every child has the same chance, no matter what the earlier children inherited.
Does the square assume the genes are independent?
Yes. Squares with more than one gene pair assume the genes sort independently, which holds when they sit on different chromosomes or far apart on the same one.