Key Terms in Genetics

Section 1 of 7Genes and alleles

Gene

• Gene A gene is a short section of a DNA molecule that carries the code for making one protein.
• Genes sit at fixed positions on chromosomes, so every human has two copies of each gene, one inherited from each parent.

Allele

• Allele An allele is one particular version of a gene.
  • Different alleles of the same gene give different forms of a characteristic, such as tall or short stems in a pea plant.
• Alleles are written as single letters, for example B and b.

Two alleles per gene

• Two alleles per gene Because chromosomes come in pairs, each body cell carries exactly two alleles for a gene.
  • They may be identical or different.

Section 2 of 7Dominant and recessive alleles

Which allele is shown?

Dominant allele

• Dominant allele A dominant allele affects the phenotype even when only one copy is present.
• It is written as a capital letter, such as T.

Recessive allele

• Recessive allele A recessive allele only affects the phenotype when there is no dominant allele, that is, when two copies are present.
• It is written as the lower-case version of the same letter, such as t.

Masking

• Masking If a dominant and a recessive allele are present together, the dominant allele hides the effect of the recessive one,
  • although the recessive allele is still there and can be passed on.

Choosing letters

• Choosing letters Pick a letter whose capital and lower-case forms look clearly different, such as A/a or D/d.
  • Avoid C/c or S/s, which are easy to confuse.

Section 3 of 7Genotype and phenotype

Genotype

• Genotype The genotype is the combination of alleles an organism has for a gene, written as two letters such as TT, Tt or tt.
• You cannot see a genotype; it has to be worked out from breeding results or tests.

Phenotype

• Phenotype The phenotype is the observable characteristic that results from the genotype working together with the environment, for example tall stems or the ability to taste a bitter chemical.

Same phenotype, different genotype

• Same phenotype, different genotype Two organisms can look identical but have different genotypes.
  • In a dominant-recessive pair, both TT and Tt show the dominant phenotype.

Section 4 of 7Homozygous and heterozygous

Term	Meaning	Example genotype
Homozygous dominant	Two identical dominant alleles	TT
Homozygous recessive	Two identical recessive alleles	tt
Heterozygous	Two different alleles, one dominant and one recessive	Tt

Section 5 of 7Worked example: tasting a bitter chemical

1

Suppose the ability to taste a bitter chemical is controlled by one gene. T (taster) is dominant and t (non-taster) is recessive.

2

A person with genotype TT is homozygous dominant, so their phenotype is taster.

3

A person with genotype Tt is heterozygous. The dominant allele masks the recessive one, so they are also a taster.

4

A person with genotype tt is homozygous recessive. With no dominant allele present, they are a non-taster.

5

In an exam you may be given an unfamiliar characteristic, so learn the relationships between the terms rather than memorising this example.

Section 6 of 7Key terms

Term	Meaning
Gene	A section of DNA that codes for a protein and so controls a characteristic.
Allele	A different version of the same gene.
Genotype	The combination of alleles an organism has for a particular gene.
Phenotype	The observable characteristics of an organism.
Dominant	An allele that is expressed in the phenotype even if only one copy is present.
Recessive	An allele that is only expressed when two copies are present.
Homozygous	Having two identical alleles of a gene.
Heterozygous	Having two different alleles of a gene.

Section 7 of 7Exam technique

1

Using 'gene' and 'allele' as if they mean the same thing: a gene is the section of DNA, while alleles are the different versions of it.

2

Thinking a dominant allele is always the more common one or the 'stronger' one; dominant only describes how it is expressed when paired with another allele.

3

Writing that Tt is a 'recessive' organism; Tt is heterozygous and shows the dominant phenotype.

4

Mixing up genotype and phenotypea genotype is letters such as Tt, a phenotype is a description such as 'taster'.

5

Using letters like Cc or Ss in a genetic diagram, which makes it unclear which allele is dominant.