2.4.3Unit 2 — Multicellular Organisms
Key Terms in Genetics
5 min read 5 parts Lesson 17 of 39
Section 1 of 5Genes, Alleles and Gametes
Characteristics are passed on from parents to offspring in the form of genes carried on chromosomes.
Chromosome
- A thread-like structure of DNA found in the nucleus, carrying many genes arranged along its length.
Gene
- A short length of DNA on a chromosome that codes for a particular protein and so influences a characteristic.
Allele
- A different version of the same gene; a person has two alleles of each gene, one from each parent.
Gamete
- A sex cell such as an egg or a sperm, carrying one allele of each gene to the next generation.
Section 2 of 5Dominant and Recessive Alleles
Dominant and Recessive Alleles
Dominant allele
- Its characteristic appears in the phenotype even when only one copy is present, and it is written as a capital letter.
Recessive allele
- Its characteristic appears only when two copies are present and no dominant allele is there, written as a lowercase letter.
Choosing letters
- Select a letter whose capital and lowercase forms look clearly different, such as Tt or Dd, but not Cc or Ss.
Not always alike
- The two alleles inherited need not be the same, for example one allele for brown eyes and one for blue eyes.
Section 3 of 5Genotype and Phenotype
Genotype
- The combination of alleles an organism has for a characteristic, written using letters such as TT, Tt or tt.
Phenotype
- The observable characteristic of the organism, such as tall or short, resulting from genotype and the environment.
Homozygous
- Having two identical alleles of a gene, either homozygous dominant (TT) or homozygous recessive (tt).
Heterozygous
- Having two different alleles of a gene, such as Tt, where the dominant allele is expressed in the phenotype.
| Genotype | Description | Phenotype (T = tall, t = short) |
|---|---|---|
| TT | Homozygous dominant | Tall |
| Tt | Heterozygous | Tall |
| tt | Homozygous recessive | Short |
Section 4 of 5Generations and Family Trees
1
The P generation is the parental generation, the original pair of true-breeding organisms whose genotypes are known at the start of a cross.
2
The F₁ generation is the first set of offspring produced when the two P generation parents are crossed with each other.
3
The F₂ generation is the offspring produced when two individuals from the F₁ generation are crossed with each other.
4
A family tree uses squares for males and circles for females, with a horizontal line joining a couple and their children shown below.
5
Shading or colour marks affected individuals, so the pattern of inheritance of a characteristic can be followed through the generations.
Section 5 of 5Exam Checklist & Tips
2.4.3 – Key Points for the Exam
- Allele A different form of the same gene.
- Genotype The alleles an organism has for a characteristic.
- Phenotype The observable features that result from genotype and environment.
- Homozygous Two identical alleles for a gene.
- Heterozygous Two different alleles for a gene.
- F₂ generation Offspring of two F₁ individuals.
Common Mistakes
- Confusing genotype with phenotype; genotype is the letters, phenotype is the visible feature.
- Saying that a dominant allele is stronger or more common; it is simply expressed when present.
- Writing the recessive allele with a different letter from the dominant allele instead of the lowercase form.
- Using the word gene when allele is required; alleles are the versions of a gene.