Decoding pedigrees: the six patterns of inheritance

Autosomes, gonosomes, genotype notation, and the six patterns you need to read a pedigree.

You have 46 chromosomes, in 23 pairs. 22 of those pairs are autosomes. The last pair is the sex chromosomes, also called gonosomes: XX if you're female, XY if you're male. Genes on autosomes follow one set of rules. Genes on the X or Y chromosome follow a different set. Most of the work in reading a pedigree is knowing which set applies.

This article covers the six inheritance patterns: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, Y-linked, and mitochondrial. For each one, you'll see the genotype notation, the rule that defines it, and an example tree.

Genotype notation

A gene can come in different versions, called alleles. By convention, the dominant allele gets a capital letter, like A. The recessive allele gets the same letter in lowercase, like a. Since you have two copies of each autosomal gene, one from each parent, your genotype is one of three combinations: AA, Aa, or aa.

AA and aa are homozygous. You have two copies of the same allele. Aa is heterozygous. You have one of each. For a dominant trait, both AA and Aa show the trait, since one dominant allele is enough. Only aa shows a recessive trait, since both copies have to be the recessive version.

For genes on the X chromosome, the notation changes for males. A female has two X chromosomes, so she's written the same way as an autosomal gene: XᴬXᴬ, XᴬXᵃ, or XᵃXᵃ. A male has one X and one Y, so he only has one allele to write down: XᴬY or XᵃY. This is called hemizygous, and it's the reason X-linked traits behave differently in men and women.

Autosomal dominant

One copy of the dominant allele is enough to show the trait. Genotype Aa or AA both produce the phenotype. Only aa is unaffected.

On a pedigree, this shows up as a trait that appears in every generation. An affected person almost always has an affected parent, since the allele has to come from somewhere. Two unaffected parents (aa and aa) cannot have an affected child, because neither parent has a dominant allele to pass down.

Huntington's disease, Marfan syndrome, and achondroplasia are all autosomal dominant.

Autosomal recessive

Two copies of the recessive allele are needed. Only genotype aa shows the trait. AA and Aa are both unaffected, but Aa carries the allele and can pass it on. This genotype, unaffected but carrying one recessive allele, is called a carrier.

On a pedigree, this is the pattern that explains an affected child with two unaffected parents. Both parents are Aa. Neither shows the trait. Each has a 50 percent chance of passing the recessive allele to a given child, so a child who receives it from both parents ends up aa, and affected, even though neither parent is.

Two affected parents (aa and aa) can only produce affected children, since neither parent has a dominant allele left to give.

Cystic fibrosis, sickle cell disease, and Tay-Sachs disease are all autosomal recessive. If you want to see a full pedigree worked through for one of these, head over to our article on cystic fibrosis.

X-linked recessive

The allele is on the X chromosome, and it's recessive. A female needs XᵃXᵃ to be affected. A male only needs XᵃY, since he only has one X chromosome to begin with.

This is why X-linked recessive conditions show up far more often in males than females. A male with even one copy of the recessive allele is affected. A female needs two, which is far less likely if the allele is rare.

The key rule for reading a pedigree: a father can never pass an X-linked allele to his son. He passes his Y chromosome to his son and his X chromosome to his daughters. So if you see an affected father and an affected son, with an unaffected mother, X-linked recessive inheritance is ruled out for that family. The son's X chromosome, and whatever allele is on it, came from his mother.

Duchenne muscular dystrophy, hemophilia A, and red green color blindness are all X-linked recessive. For the full pedigree pattern and genotype breakdown, see our article on Duchenne muscular dystrophy.

X-linked dominant

Same chromosome, different inheritance. Only one copy of the allele is needed, so XᴬXᵃ, XᴬXᴬ, and XᴬY are all affected if A is the dominant disease allele. This pattern is uncommon. Most X-linked conditions are recessive.

The pedigree signature: an affected father passes the trait to every one of his daughters, since they all receive his one X chromosome, and to none of his sons, since they receive his Y chromosome instead. An affected mother passes the trait to roughly half her children, regardless of sex, the same as autosomal dominant.

Y-linked

The allele is on the Y chromosome. Only males have a Y chromosome, so only males are ever affected. The Y chromosome passes from father to son intact, with no equivalent on the X to recombine with, so every son of an affected father is affected, with no exceptions.

This pattern is rare in humans. Few disease causing genes are located on the Y chromosome.

Mitochondrial

Mitochondria have their own DNA, separate from the chromosomes in the nucleus. You inherit your mitochondria from the cytoplasm of the egg cell, which means entirely from your mother.

An affected mother passes the trait to all her children, sons and daughters alike. An affected father passes it to none of his children, since sperm contribute essentially no mitochondria to the embryo. Leber hereditary optic neuropathy follows this pattern.

Reading a real pedigree

Textbook pedigrees are built to demonstrate one pattern cleanly. Real ones rarely are. A small family, six or seven people, often doesn't have enough relationships to rule out every pattern but one. That's not a flaw in the method. A pedigree only records what actually happened in one family, not every possible test case, and other factors, like recombination during meiosis, can occasionally make a pattern look less clean than the textbook version too.

To solve a real pedigree, apply the rules above directly: assume a pattern holds, then look for one relationship that contradicts it. If you find one, that pattern is out, no matter how well it fit everywhere else. Say you see a healthy father with an affected son. You already know what that means: the condition can't be Y-linked, since a Y-linked trait passes from an affected father to every son without exception.

For how these rules get turned into the actual checks the tool runs, see how a pedigree becomes an algorithm.