Overview
Genetics is the study of how traits are inherited from one generation to the next. Every person begins as a single cell carrying a complete set of instructions in its DNA. This lesson traces how those instructions, organized into genes, produce observable traits, and how simple rules let us predict the traits of offspring. Gregor Mendel worked out these patterns in pea plants long before DNA was known, and his principles still explain much of human inheritance.
From DNA to Trait
DNA is organized into genes, each a segment that typically codes for a protein. Proteins do the work that produces a trait (for example, an enzyme that makes a pigment). A person carries two copies of most genes, one on each member of a homologous chromosome pair. The alternative versions of a gene are called alleles.
- Genotype is the pair of alleles a person carries (written with letters, such as Aa).
- Phenotype is the resulting observable trait.
Dominant and Recessive Alleles
A dominant allele (capital letter) is expressed even when only one copy is present. A recessive allele (lowercase) is expressed only when two copies are present. This produces three genotype categories:
| Genotype | Term | Phenotype (if A is dominant) |
|---|---|---|
| AA | Homozygous dominant | Dominant trait |
| Aa | Heterozygous (carrier) | Dominant trait |
| aa | Homozygous recessive | Recessive trait |
A heterozygous person shows the dominant trait but can still pass on the hidden recessive allele, which is why they are called a carrier.
Punnett Squares
A Punnett square predicts the probability of each offspring genotype. For a cross of two heterozygotes (Aa x Aa):
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
The genotype ratio is 1 AA : 2 Aa : 1 aa, giving a phenotype ratio of 3 dominant : 1 recessive. In other words, each child of two carriers has a 25 percent chance of showing the recessive trait.
Autosomal vs. X-Linked Inheritance
Genes on the 22 pairs of non-sex chromosomes (autosomes) affect males and females equally. Genes on the sex chromosomes follow different rules.
- Autosomal recessive — Two carrier parents (each Aa) have a 25 percent chance of an affected child with each pregnancy. Cystic fibrosis follows this pattern: a defective chloride channel gene causes thick mucus in the lungs and pancreas.
- X-linked recessive — Because males have only one X chromosome (XY), a single recessive allele on that X is expressed. Females (XX) would need two copies to be affected but can be unaffected carriers. This is why hemophilia and red-green color blindness are far more common in males.
Disease Examples
Sickle cell disease is autosomal recessive and caused by a single base change in the beta-globin gene, altering one amino acid in hemoglobin. Under low-oxygen conditions the abnormal hemoglobin polymerizes, distorting red cells into a sickle shape that blocks small vessels. Notably, carriers (heterozygotes) are largely healthy and gain some protection against malaria, which helps explain why the allele persists.
Beyond Simple Mendelian Rules
Not all traits follow one-gene, two-allele patterns. In incomplete dominance the heterozygote shows a blended phenotype; in codominance, such as the AB blood type, both alleles are fully expressed. Many traits like height are polygenic, influenced by many genes plus the environment.
Clinical relevance
Genetic principles underlie everyday clinical practice. Pedigree analysis and carrier screening let clinicians estimate the recurrence risk of conditions such as cystic fibrosis and sickle cell disease for a couple planning a family. Knowing that a disorder is X-linked recessive explains why it may skip through unaffected female carriers and appear in their sons. Newborn screening programs test for treatable inherited conditions in the first days of life, and understanding a patient’s genotype increasingly guides drug dosing and targeted therapy in the growing field of pharmacogenomics.