Each person carries two copies of most genes, one from each parent, on 23 pairs of chromosomes. Which copies a child inherits and how they work together shape many traits and diseases.
Genes and alleles
A gene is a stretch of DNA that codes for a product, and an allele is one version of that gene. Two identical alleles make a person homozygous; two different ones make them heterozygous.
Dominant and recessive
A dominant allele shows its effect with only one copy. A recessive allele shows only when both copies are recessive, so carriers are usually healthy.
Sex-linked inheritance
Males have one X and one Y chromosome, so a recessive gene on the X shows in males with just one copy. That is why conditions like hemophilia mostly affect males.
Beyond single genes
Most common traits, such as height, and common diseases, such as type 2 diabetes, come from many genes plus the environment.
| Pattern | Who is affected | Example |
|---|---|---|
| Autosomal dominant | Every generation; 50 percent risk per child of an affected parent | Huntington disease |
| Autosomal recessive | Often skips generations; 25 percent risk per child of two carriers | Cystic fibrosis |
| X-linked recessive | Mostly males, passed on by carrier mothers | Hemophilia A |
At the bedside
Draw a three-generation pedigree to spot a pattern of inherited disease. A strong family history, such as early heart disease or cancer in close relatives, can change screening for your patient.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).