Explain that genes are segments of DNA located on chromosomes, and distinguish between genes, alleles, genotype, and phenotype (Ohio's Learning Standards for Science, Biology, B.H.1).

What this topic is asking

Ohio standard B.H.1 states that "genes are segments of DNA located on chromosomes." Ohio's Biology EOC turns this into items on how DNA is organized into chromosomes and on the vocabulary that the rest of genetics depends on: gene, allele, genotype, and phenotype. Getting these terms right is the foundation for Mendelian genetics and Punnett squares, so the EOC tests them both directly and as the language of harder problems.

From DNA to chromosomes

A single human cell holds about two meters of DNA, far too much to sit loose in the nucleus. The DNA is therefore coiled and packaged into structures called chromosomes. When a cell is about to divide, the chromosomes condense tightly enough to be seen under a microscope; the rest of the time the DNA is more loosely arranged so its genes can be read.

Each chromosome carries many genes, lined up along its length. A gene is a segment of the DNA that codes for a particular product, so it influences a particular trait. This is the relationship Ohio standard B.H.1 wants: chromosome (the whole coiled DNA molecule), gene (a segment of it), all the way down to the base sequence.

Genes and alleles

The difference between a gene and an allele is a favorite EOC distinction.

• A gene is the segment of DNA for a trait, for example the gene for flower color.
• An allele is a version of that gene, for example a "red" allele and a "white" allele.

Different alleles arise because the DNA sequence of a gene can differ slightly between individuals (a difference that originally comes from a mutation). Most body cells carry two alleles of each gene, one on each chromosome of a homologous pair.

Homologous chromosomes

Body cells are diploid: their chromosomes come in matching pairs called homologous chromosomes. One member of each pair came from the mother and one from the father. The two homologous chromosomes carry the same genes in the same order, but they may carry different alleles of some of those genes, which is why you can inherit, say, a red allele from one parent and a white allele from the other. (Meiosis, which separates these pairs into gametes, is covered in meiosis and genetic variation.)

Genotype and phenotype

These two terms run through every genetics problem.

• The genotype is the set of alleles an organism carries for a gene, written as letters (RR, Rr, or rr).
• The phenotype is the observable trait that results (red flowers, white flowers).

A genotype is homozygous when the two alleles are the same (RR or rr) and heterozygous when they differ (Rr). A dominant allele (written as a capital letter) shows its effect even when only one copy is present; a recessive allele (lower case) shows only when two copies are present.

Try this

Q1. State the difference between a gene and an allele. [2]

• Cue. A gene is a segment of DNA on a chromosome that codes for a trait; an allele is a particular version of that gene.

Q2. A plant has the genotype TT. State whether it is homozygous or heterozygous and explain why. [2]

• Cue. It is homozygous, because its two alleles for the gene are the same (both T).