Key Terms in Genetics
Biology: Cambridge International A Level, topic 16.2.1
Key Terms in Genetics
You need to learn some key terms in order to understand genetics. These are:
Genes and alleles
- A gene is a section of DNA located at a particular site on a DNA molecule, called its locus.
- The base sequence of each gene carries the genetic information that determines the sequence of amino acids in a protein.
- Alleles are the different variations of a gene.
- Diploid organisms have two alleles (one on each chromosome).
Genotype and phenotype
- A genotype is an organism’s underlying genetic makeup.
- The genotype consists of both physically visible and non-expressed alleles.
- A phenotype is the observable traits expressed by an organism.
- The phenotype is determined by the interaction between its genetic constitution and the environment.
Allele interactions
- Dominant alleles are always expressed in the phenotype of an organism.
- Recessive alleles are only expressed in the phenotype if there are two copies of the allele.
- Co-dominant alleles are both expressed in the heterozygote.
Homozygote and heterozygote
- Homozygotes are organisms with two copies of the same allele.
- Homozygotes can be homozygous dominant (both alleles are dominant) or homozygous recessive (both alleles are recessive).
- Heterozygotes are organisms that have different alleles for a gene.
Test yourself on Key Terms in Genetics
What is the term given for a gene that has two copies of the same allele?
Which of the following best describes alleles?