Basic Genetics Terms for Beginners
Quick Glossary on DNA, Genes, Chromosomes, Phenotypes & Genotypes
Jan 1, 2009
It can be difficult to know where to begin in genetics – some textbooks jump back and forth between different areas, which can be confusing to readers. Learning some terms at the beginning of study is a good start, even if the meanings aren’t fully grasped until the material is reread at a later date.
What are DNA and Genes?
DNA (deoxyribonucleic acid) is a molecule containing genetic instructions. It is found in the cells of organisms (living things). This genetic material is responsible for the traits that are passed on from one generation to the next.
Genes are meaningful parts of DNA that tell proteins what to do. For example, the eye color of a human depends on several genes, probably more than have currently been identified. The term allele refers to a gene variation (see the section What do Dominant and Recessive Mean? below).
Sex Chromosomes and Autosomal Chromosomes
Chromosomes are structures of tightly coiled DNA and proteins. Humans have 23 pairs of these chromosomes, half from each biological parent, in most of their cells. Having two copies of each chromosome in a cell is referred to as diploid, and one set (as is the case with human sex cells) is haploid.
The sex chromosomes are called X and Y (human females have two X chromosomes, males X and Y). The remaining non-sex chromosomes are called autosomal chromosomes. A specific position on a chromosome is called a locus.
What are Phenotypes and Genotypes?
These terms are used when considering physical traits and their associated genes. A phenotype is a trait that can be observed in an organism, and the genetic makeup responsible for a trait is the genotype. Some traits are monogenic, meaning one gene is involved, whereas others are polygenic, which means several genes are involved.
The article entitled It Runs in the Family on The Tech’s Understanding Genetics website acknowledges the saying, “Genotype determines phenotype”, which may be a quick and helpful aid to remembering the difference between the terms.
What do Dominant and Recessive Mean?
Basically, dominant genes are expressed over recessive genes. A good example is to look at when there are two versions of a gene (alleles) for a trait. If a mother has a straight hair line and the father has a dipped hairline (called “Widow’s peak”), their child will have a Widow’s peak. This is because this gene is dominant, and the straight line gene is recessive.
It’s important to note that this model applies only to some phenotypes, and is often incorrectly applied to others, such as eye color. Students are still being taught that two blue-eyed parents cannot have brown-eyed offspring, despite this being incorrect.
For more basic definitions of genes, DNA, proteins and chromosomes, read Introduction to the Terminology Used in Genetics. For a revision primer on molecules and human cells, read Useful Science Basics for Understanding Genetics.
Sources
DNA, Genes and Chromosomes, Virtual Genetics Education Centre, University of Leicster website, accessed: 1st January 2009, no author specified.
Inherited Human Traits: A Quick Reference, Genetics Science Learning Centre website, accessed: 1st January 2009, authors: Stark, L. et al.
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