What 'autosomal' adds to the story
You inherit two copies of most genes — one from each parent — because you carry pairs of matching chromosomes. Of your 23 pairs, 22 are autosomes, the chromosomes that are the same in everyone regardless of sex. A trait is autosomal simply when its gene sits on one of these 22 pairs. That single fact has a big consequence: sons and daughters have an equal chance of inheriting it, because autosomes are passed on without regard to sex.
Within a pair, the two versions of a gene are its alleles. If both alleles are the same you are homozygous; if they differ you are heterozygous. Whether you actually *show* a trait depends on which allele wins out — and that is the difference between dominant and recessive.
Autosomal dominant: one copy is enough
An autosomal-dominant trait appears whenever you carry even one copy of the relevant dominant allele. A single dose is enough to show, so the trait does not skip generations: an affected person usually has an affected parent, and on average half their children inherit it. Examples in the catalog include Huntington disease and achondroplasia.
Cross: affected heterozygous parent (Aa) × unaffected parent (aa)
a a
---------------------
A | Aa | Aa | <- affected
---------------------
a | aa | aa | <- unaffected
---------------------
Children: 1 Aa : 1 aa = 50% affected, 50% unaffected
(A = dominant allele; one copy is enough to show the trait)Autosomal recessive: you need two
An autosomal-recessive trait shows only when both copies are the recessive allele — that is, only in homozygous individuals. A person with one recessive and one working allele is a healthy carrier: they do not show the trait but can pass the recessive copy on. This is why recessive conditions can seem to appear 'out of nowhere' — two unaffected carrier parents can have an affected child. Catalog examples include cystic fibrosis and sickle-cell disease.
Cross: two carrier parents (Aa × Aa)
A a
---------------------
A | AA | Aa |
---------------------
a | Aa | aa | <- only aa shows the trait
---------------------
Children: 1 AA : 2 Aa : 1 aa
-> 3/4 unaffected (but 2 of those are carriers)
-> 1/4 affected (aa)
Recurrence risk for each future child = 25%