Genes, Alleles, Chromosomes, and DNA

Genes are sections of DNA that code for traits, alleles are the different versions of a gene, and chromosomes are the structures of DNA and protein that carry the genes in the nucleus. In brief: humans have 46 chromosomes in 23 pairs, and each pair has one chromosome from each parent. For each gene a person has two alleles, and the combination (genotype) together with the environment produces the observable trait (phenotype). An allele can be dominant, which shows in the phenotype when present, or recessive, which shows only when two copies are present.

Heredity is the passing of traits from parents to offspring, and variation is the differences among individuals. This guide defines the basic vocabulary of genetics, explains how genes, chromosomes and DNA are related, shows how genotype and phenotype differ, works through an example with a human trait and corrects common mistakes.

What are heredity and variation?

Heredity, or inheritance, is the transmission of characteristics from parents to offspring through genes. Children resemble their parents, but they are never identical to them or to their siblings (except identical twins). This variation has two sources: genetic differences, which come from the mixing of genes in sexual reproduction, and environmental differences, such as diet and exercise, which affect how genes are expressed.

Some traits are almost fully genetic, such as blood group. Others, such as height or weight, are influenced by both genes and environment. Variation can be continuous, with a range of values like height, or discontinuous, with distinct categories like blood group.

How are DNA, genes and chromosomes related?

DNA (deoxyribonucleic acid) is the molecule that carries genetic information. It consists of two strands twisted into a double helix, with a sequence of four bases: adenine (A), thymine (T), cytosine (C) and guanine (G).

A gene is a section of DNA with a sequence of bases that codes for a specific protein, or sometimes for an RNA molecule. The protein then affects a trait. For example, a gene codes for an enzyme in the pigment pathway that gives color to the eye. The human genome has roughly 20 000 protein-coding genes.

A chromosome is a long molecule of DNA wound around proteins called histones, carrying hundreds to thousands of genes. Chromosomes are visible under the microscope when a cell divides. Humans have 46, arranged in 23 pairs. Of these, 22 pairs are autosomes, and one pair is the sex chromosomes (XX in females, XY in males). The two chromosomes of a pair are homologous chromosomes: they carry the same genes in the same order, but may carry different alleles. One member of each pair comes from the mother and the other from the father.

The hierarchy runs from the genome down: a nucleus contains chromosomes, each chromosome contains one DNA molecule, and genes are sections along that molecule. A gene’s position on a chromosome is its locus.

What are alleles?

Alleles are different forms of the same gene. They are located at the same locus on homologous chromosomes. They arise by mutation, a change in the DNA sequence, and usually differ by one or a few bases. For example, the gene for the shape of pea seeds has an allele for round seeds and an allele for wrinkled seeds.

• A dominant allele produces its effect in the phenotype even when only one copy is present. Dominant alleles are written with a capital letter, such as R.
• A recessive allele shows its effect only when two copies are present. It is written with a lowercase letter, such as r.

Dominant does not mean “better” or “more common”. For example, polydactyly (extra fingers) is caused by a dominant allele, but is rare. Many genes have more than two alleles in a population, such as the ABO blood group gene, which has three, but each person has only two.

What are genotype and phenotype?

The genotype is the combination of alleles that an individual has for a gene, such as Rr. The phenotype is the observable characteristic that results, such as round seeds. The phenotype depends on the genotype and often on the environment: a plant with genes for tall growth may stay short if it lacks nutrients.

What do homozygous and heterozygous mean?

• Homozygous: the two alleles of a gene are the same. RR is homozygous dominant, and rr is homozygous recessive.
• Heterozygous: the two alleles are different, as in Rr. A heterozygous person with a recessive disorder allele who does not show the disorder is called a carrier.

Genetics vocabulary at a glance

Worked example: reading genotypes and phenotypes

In humans, the ability to roll the tongue into a U shape is often taught as a simple dominant trait (allele T), and non-rollers have two recessive alleles (tt). In reality, the inheritance is more complex, but this model is useful for practice.

1. Genotype TT: homozygous dominant, so the person is a roller.
2. Genotype Tt: heterozygous. The dominant allele T is expressed, so the person is a roller and carries the hidden recessive allele t.
3. Genotype tt: homozygous recessive, so the person is a non-roller.

Now suppose a roller has a non-roller parent. The non-roller must be tt and gives a t allele to the child. The child is a roller, so it has at least one T. The child’s genotype must therefore be Tt, with T from the other parent. This shows how phenotype can be used to work out genotypes. A non-roller with two roller parents is also possible, if both parents are Tt and each passes on t.

How did genetics develop?

Gregor Mendel, an Augustinian friar, did experiments on pea plants in the 1850s and 1860s and described the rules of inheritance, but his work was largely ignored until it was rediscovered in 1900. Walter Sutton and Theodor Boveri suggested around 1902 that genes are carried on chromosomes. Thomas Hunt Morgan confirmed this using fruit flies in the 1910s. In 1953 James Watson and Francis Crick described the structure of DNA, and the Human Genome Project published a draft human genome in 2001, with a complete sequence reported in 2022.

Which mistakes are common?

Mistake 1: Using gene and allele as synonyms

A gene is a segment of DNA that affects a trait. An allele is one of the versions of that gene.

Mistake 2: Saying that dominant alleles are the most common

Dominance describes how an allele is expressed, not how frequent it is in the population.

Mistake 3: Mixing up genotype and phenotype

Genotype is the alleles (Rr). Phenotype is the visible result (round seeds).

Mistake 4: Believing that a recessive trait skips the carrier

Carriers (heterozygotes) have the recessive allele but show the dominant phenotype. They can pass the allele on.