By the end of this lesson
- Use dominant, recessive, homozygous and heterozygous correctly.
- Distinguish genotype from phenotype.
- Infer possible genotypes from simple phenotypes.
BIG QUESTION
How can two organisms look alike while carrying different allele combinations?
A dominant phenotype can arise from homozygous or heterozygous genotypes.
Dominance describes heterozygote expression
A dominantDescribes an allele that is expressed in the phenotype even when only one copy is present (in a heterozygote). allele affects the stated phenotype when one copy is present. A recessiveDescribes an allele that is only expressed in the phenotype when two copies are present (homozygous). allele’s effect is seen in the simple model only when no dominant allele is present.
Dominance is a relationship between alleles for a defined phenotype, not a measure of frequency, strength or benefit.
Zygosity describes the allele pair
A homozygousHaving two identical alleles of a gene, such as BB or bb. genotype contains two copies of the same allele, such as AA or aa. A heterozygousHaving two different alleles of a gene, such as Bb. genotype contains two different alleles, such as Aa.
A heterozygote can carry a recessive allele without showing the recessive phenotype.
Genotype and environment contribute to phenotype
genotypeThe alleles an organism has for a particular characteristic, such as Bb. records allele combination; phenotypeThe observable characteristics of an organism, produced by its genotype and the environment. is the observable or measurable outcome. Many phenotypes also depend on environment and other genes.
In a complete-dominance model, a recessive phenotype identifies aa, but a dominant phenotype could be AA or Aa.
Core genetic terms
Use the exact term the evidence supports.
| Term | Meaning | Example |
|---|---|---|
| Dominant allele | Expressed in heterozygote. | A in Aa. |
| Recessive allele | Masked by dominant allele in heterozygote. | a in Aa. |
| Homozygous dominant | Two dominant alleles. | AA. |
| Heterozygous | Two different alleles. | Aa. |
| Homozygous recessive | Two recessive alleles. | aa. |
| Phenotype | Observed/measured characteristic. | Shows dominant trait. |
Language that earns precision
- dominant
- Allele expressed in the phenotype of a heterozygote in a stated inheritance model.
- recessive
- Allele whose effect is masked by a dominant allele in a heterozygote.
- homozygous
- Having two identical alleles at a locus.
- heterozygous
- Having two different alleles at a locus.
Common misconceptions
Claim: Dominant alleles are always common.
Correction: Dominance and population frequency are independent.
Claim: A carrier has no recessive allele.
Correction: A heterozygous carrier has one recessive allele but does not show the simple recessive phenotype.
Claim: Phenotype always reveals exact genotype.
Correction: A dominant phenotype may have AA or Aa genotype.
Match the answer to the command word
Use both allele symbols and define them first.
Recessive phenotype fixes genotype; dominant phenotype may not.
Recall, apply, then justify
Define homozygous, heterozygous, genotype and phenotype.
Hint
One precise statement for each term.
Suggested marking points
- Homozygous means having two identical alleles at a locus.
- Heterozygous means having two different alleles at a locus.
- Genotype is the allele combination.
- Phenotype is the observable or measurable characteristic.
Explain why an organism with a dominant phenotype may have two possible genotypes.
Hint
One or two dominant alleles.
Suggested marking points
- One dominant allele is sufficient for the phenotype.
- A homozygous dominant genotype has two dominant alleles.
- A heterozygous genotype has one dominant allele and gives the same simple phenotype.
A dominant phenotype is rare in a population. Explain why this is not contradictory.
Planning hint
Dominance is expression, not frequency.
What a strong design includes
- Dominance describes expression in heterozygotes.
- It does not determine how frequent an allele is.
- A dominant allele can be newly arisen or selected against.
- Few individuals may carry it.
- Anyone carrying one copy may still show the phenotype.
- Population history and selection determine frequency independently of dominance.