Chromosomes
A chromosome is a long, coiled molecule of DNA associated with proteins, found in the nucleus of a cell. Human body cells have 46 chromosomes arranged in 23 homologous pairs. One chromosome in each pair comes from the mother and one from the father.
Homologous chromosomes are the same size, carry the same genes in the same order, but may carry different alleles of those genes. Gametes (sex cells) have half the chromosome number (23 in humans), which is restored to the full diploid number at fertilisation.
Genes
A gene is a short section of DNA on a chromosome that codes for a specific protein. The sequence of bases in the DNA determines the sequence of amino acids in the protein. Since proteins control cell activities (as enzymes, structural components, etc.), genes ultimately control the characteristics of an organism.
Each chromosome carries hundreds or thousands of genes. The position of a gene on a chromosome is called its locus. The same gene is always found at the same locus on a particular chromosome.
Alleles
Alleles are different forms of the same gene. They arise through mutation, changes in the base sequence of the DNA. For example, the gene for eye colour may have a blue allele and a brown allele. Because chromosomes come in homologous pairs, each individual has two alleles for every gene, one on each chromosome.
If both alleles are the same, the individual is homozygous (e.g. BB or bb). If the alleles are different, the individual is heterozygous (e.g. Bb). A dominant allele is expressed in the phenotype when one or two copies are present. A recessive allele is only expressed when two copies are present (homozygous recessive).
Key genetic vocabulary
| Term | Definition |
|---|---|
| Genotype | The combination of alleles an organism has (e.g. Bb) |
| Phenotype | The observable characteristic (e.g. Brown eyes) |
| Homozygous | Two identical alleles (BB or bb) |
| Heterozygous | Two different alleles (Bb) |
| Dominant | Allele expressed in the phenotype when one or two copies present |
| Recessive | Allele expressed only when homozygous (two copies) |
These terms must be used correctly in all genetic diagram questions. Using "gene" when you mean "allele" is a very common error that loses marks.