The language of genetics
The language of genetics
- Genetics has precise terms you must use exactly.
- Get these right and every cross becomes easy.
- Let's build them up in groups.
Genes and alleles
- a gene is a length of DNA that codes for a protein; its position is its locus.
- an allele is one version of a gene.
- a dominant allele shows its effect with only one copy; a recessive allele needs two copies.
- codominant alleles both show together.
Practice
An allele is:
An allele is a version of a gene; its position on the chromosome is the locus.
Practice
A recessive allele:
A recessive allele is only expressed when homozygous; a dominant allele shows with a single copy.
Genotype and phenotype
- the genotype is the alleles an organism has.
- the phenotype is the features you can see.
- homozygous = the two alleles are the same; heterozygous = they are different.
Practice
The phenotype is:
Phenotype = what you can see; genotype = the alleles present.
Practice
An organism that is heterozygous for a gene has:
Heterozygous = two different alleles; homozygous = two the same.
The test cross
- A test cross finds an unknown genotype.
- You cross the organism with the recessive homozygote.
- The offspring reveal whether the unknown was homozygous or heterozygous.
Practice
A test cross finds an unknown genotype by crossing it with:
Crossing with the recessive homozygote reveals whether the unknown carries a recessive allele.
You've got it
Key idea
- gene (codes a protein) at a locus; an allele is a version of it
- dominant shows with one copy; recessive needs two; codominant both show
- genotype = alleles you have; phenotype = features you see
- homozygous (same) vs heterozygous (different); test cross with the recessive homozygote finds an unknown genotype