Biology · Heredity, Variation & Evolution
DNA, genes, chromosomes and hereditary information
Imagine two matching positions on a pair of chromosomes. One position carries a sequence version called A and the other a version called a. The labels are useful only after we say what they describe: versions at the same locus, not two different visible traits.
The earlier lessons on biological scales and cells provide useful background: recognise a cell, its DNA and the difference between an individual and a population. The DNA and nucleotide explanation in Chemistry of Life is optional support. You need fractions such as one half and one quarter; the worked cases explain the calculations before using them. Detailed molecular copying, Mendelian inheritance and evolutionary inference return in later chapters.
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Distinguish a DNA sequence, a gene, an allele and a chromosome without treating a visible trait as a piece of DNA.
Begin with a sequence, then its use
DNA is a molecule whose order of nucleotide bases carries hereditary information. A cell can copy that order and use parts of it in making RNA. For protein-coding genes, RNA participates in making a protein; other genes have functional RNA products. A gene is therefore a DNA region that specifies an RNA or protein product, rather than a synonym for a trait or for every stretch of DNA.
A protein or RNA can contribute to a process that influences a trait. The route includes cellular activity, development and surrounding conditions. The letter A on our drawing names a sequence version; it does not itself mean 'tall', 'strong' or 'better'.
A genome includes an organism's genetic material as a whole, including DNA beyond protein-coding genes. Knowing a short sequence or a genotype at one locus is a small piece of that inventory, not a complete description of an organism.
Locate a gene on a chromosome
In a eukaryotic cell, a nuclear chromosome consists of a long DNA molecule associated with proteins before replication. That DNA carries many regions, including genes and regulatory sequences. Replication produces two associated DNA copies, called sister chromatids; the later division lesson explains their separation.
Homologous chromosomes in a diploid set carry corresponding loci, although their sequence versions can differ. A locus is a position or region in the genome. At our marked locus, A and a are alleles: alternative sequence versions at that location. AA, Aa and aa describe the two versions carried in this one-locus diploid model.
Not every organism or cell carries two copies of each chromosome. Haploid cells carry one chromosome set; chromosome number and organisation vary across organisms. Some hereditary DNA in eukaryotes is also found in organelles such as mitochondria. Use the paired drawing as a stated example, not a universal inventory of life.
Two versions at the same chromosome position
Read the diagram in words
A and a occupy the same marked locus on two homologous chromosomes. The genotype there is Aa; each chromosome also contains other DNA regions.
| Homologue | Whole chromosome interval | Focal allele and locus interval | Other DNA intervals |
|---|---|---|---|
| Homologue 1 | 0–100 | A: 28–36 | 8–16; 55–66; 78–87 |
| Homologue 2 | 0–100 | a: 28–36 | 8–16; 55–66; 78–87 |
Relative chromosome position: 0–100
Genotype Aa
Other DNA regions
Keep the labels at their own scale
Genotype describes genetic composition, often specified at a particular locus. Phenotype describes an observable or measurable characteristic. A gene, its allele and the resulting trait are connected concepts with different jobs.
Dominance concerns the phenotype of a heterozygote compared with the two homozygotes for a specified trait under stated conditions. If Aa resembles AA for that trait, A is described as dominant in that comparison. This says nothing by itself about which allele is common, which appeared first or which increases reproductive success.
Many traits involve multiple genes and environmental influences. A one-locus diagram teaches transmission clearly; it does not justify assigning one gene to every complex characteristic.
Worked example
Read the same locus twice
Our two unreplicated homologous chromosomes each show one marked locus. The first carries A and the second a, so the genotype at that locus is Aa. There are two alleles represented, but both labels refer to the same locus. We have not drawn a second gene called 'a', measured a trait, or established dominance. To predict a phenotype, we would need further information about the locus, its products and the conditions.
Check your understanding
Does Aa necessarily mean an individual has two different genes for two different traits?
Show reasoning
No. In this model A and a are alternative versions at the same locus, one on each homologous chromosome.
If A is dominant for a particular trait, must it be the more common allele in the population?
Show reasoning
No. Dominance describes a heterozygote's phenotype for that trait. Frequency describes how often an allele occurs.
Why does knowing a genotype at one locus not give a complete phenotype prediction?
Show reasoning
Other loci, cellular processes, development and environmental conditions may also influence the trait. A prediction needs a stated genotype-to-phenotype relationship.
What to carry forward
Follow DNA regions and sequence versions before making claims about traits.
References
Resources supporting this lesson’s explanations and examples.
- Talking Glossary — Genes, alleles and inheritance terminologyNational Human Genome Research InstituteScientific glossaryTalking Glossary — Genes, alleles and inheritance terminologyAlleleChromosomeGenotypePhenotypePolygenic traitMutationGenomeLocusMitochondrial DNA
- Biology 2e — Characteristics and Traits (2018)Mary Ann Clark, Matthew Douglas and Jung Choi · OpenStaxTextbook chapter