Why children resemble their parents
Children tend to look like their parents because they receive instructions from them. These instructions are carried in DNA, and the sections of DNA that influence a trait are called genes. Each parent contributes half of a child’s genetic material, so a child is a mix of both, and not identical to either.
Chromosomes and genes
Human cells normally hold 46 chromosomes, arranged as 23 pairs. Chromosomes are long molecules of DNA packed with proteins, and each carries many genes. For each pair, one chromosome comes from the mother and the other from the father. Eggs and sperm are made by a special kind of cell division that halves the chromosome number, so each carries 23. When they combine at fertilization, the full set of 46 is restored.
Because of this, a person has two copies of most genes, one from each parent. The structure of DNA is covered in the separate article on DNA structure.
What alleles are
Different versions of the same gene are called alleles. For example, a gene involved in pigmentation might have several versions that lead to slightly different outcomes. You inherit two alleles for the gene, one from each parent. If they are the same, you are homozygous for that gene. If they differ, you are heterozygous.
Dominant and recessive
Often one allele has a stronger visible effect than another. The allele whose effect shows up when paired with a different allele is called dominant, and the one that is hidden in that case is recessive. A recessive trait shows only when a person has two copies of the recessive allele.
The idea comes from Gregor Mendel, an Austrian monk who bred pea plants in the 1850s and 1860s and published his results in 1866. He tracked traits such as flower color and seed shape and found regular patterns in how they passed to the next generation. His work was largely overlooked until around 1900.
Dominant does not mean “more common” or “better”. It only describes how two alleles interact. Some dominant traits are rare in the population, and some recessive ones are common.
A worked example
Using Mendel’s convention, suppose a pea plant gene has a dominant allele A (purple flowers) and a recessive allele a (white flowers). Take two parent plants that each carry one of each, written Aa. Each parent passes on either A or a, with equal chance. The four equally likely combinations are:
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
That gives one AA, two Aa and one aa. Because A is dominant, AA and Aa plants both look purple, and only aa plants are white. So on average three out of four offspring would be purple and one out of four white, a ratio of 3 to 1. These are probabilities for each offspring, not a guarantee. A family of four plants might not follow the ratio exactly, in the same way four coin flips might not give exactly two heads.
Not every trait follows this pattern
The single-gene pattern is a useful starting point, but it describes only a minority of human traits. Others work differently:
- Incomplete dominance: the offspring looks like a blend, such as pink flowers from red and white parents.
- Codominance: both alleles show up at once. The ABO system in blood types is a standard example, where a person can have both A and B markers.
- Polygenic traits: many genes each contribute a small effect. Height and skin color are examples, which is why they vary continuously instead of falling into a few categories.
- Sex-linked traits: genes on the X or Y chromosome follow different inheritance patterns in males and females.
Genes and environment
Even for traits with a strong genetic component, the environment plays a part. Height is influenced by genes but also by nutrition and health in childhood. Many traits, from disease risk to some aspects of personality traits, are shaped by an interplay between genes and experience, and separating the two is difficult. Statements that a single gene “causes” a complex trait are almost always oversimplified.
Why siblings differ
Siblings share the same parents but not the same combination of genes. Each egg or sperm gets a different mix of the parent’s chromosomes, and chromosome pairs also swap segments in a process called recombination before the cells divide. The result is a very large number of possible combinations. Identical twins are the exception: they come from one fertilized egg that splits, and share nearly all of their DNA.
Mutations and variation
Occasionally DNA changes when it is copied, creating a new allele. Most such changes have little effect, some are harmful, and a few may be helpful in a given environment. These changes are the raw material on which natural selection acts, which is how Charles Darwin’s ideas about evolution later came to be combined with genetics. Over generations, shifting allele frequencies underlie human adaptation and other examples of ongoing evolution.
A note on health
This is general educational information, not medical or genetic advice. People with questions about inherited conditions in their family can speak to a doctor or a genetic counselor, since individual cases differ.