Describe homozygosity at a particular gene
Homozygosity describes identical alleles within one specified gene pair.
Core and Extended; Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Recall chromosomes, genes and alleles, meiosis, haploid gametes and fertilisation restoring two chromosome sets.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which pair is homozygous at its stated gene?
What you will learn
- Identify identical alleles at the model gene
- Separate homozygosity from dominance phenotype and whole-genome identity
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| TT | Homozygous for T at the model gene |
| tt | Homozygous for t at the model gene |
| Tt | Different alleles; heterozygous |
| Scope | One matching pair does not establish whole-genome identity |
Identify identical alleles at the model gene
Homozygous means having two identical alleles of a particular gene. Both TT and tt are homozygous in the supplied two-allele model because the two forms at that gene match. Identical describes the allele identities, not necessarily the visible appearance of an organism. Homozygous is a genotype description; dominance is a relationship between allele expression under a specified model.
A dominant homozygote and a recessive homozygote can therefore both be homozygous while producing different phenotypes. If T produces pigment whenever present and t produces absence only without T, TT shows pigment and tt does not. You cannot replace homozygous with dominant. Likewise, a dominant phenotype alone does not establish homozygosity because Tt can also show that phenotype.
Separate homozygosity from dominance phenotype and whole-genome identity
The phrase particular gene matters. One organism can have TT at the pigment gene and Aa at a separate model gene. The first allele pair is homozygous and the second is heterozygous. A record at one gene cannot establish that every allele pair in the genome matches. Two organisms sharing TT at one gene can still differ at many other genes and in environmental histories.
In the normal model, a TT individual contributes T at this gene to every gamete, while a tt individual contributes t. This follows from the identical pair and the stipulated one-allele gamete rule. It does not mean all gametes are identical across their entire genomes, or that copying and development can never vary in real organisms. Keep the conclusion focused on the specific gene and normal model assumptions.
Pause and explain
Which statement describes both TT and tt?
Worked example
Identify homozygous genotypes among TT, tt and Tt, and judge whether pigment alone proves homozygosity.
Show the worked solution
- Compare the two alleles within each genotype: TT matches and tt matches; Tt differs.
- Apply the stated expression rule. TT and Tt share pigment, so phenotype alone cannot identify an identical allele pair.
Answer TT and tt both have matching allele pairs. Under the supplied rule they produce different pigment phenotypes, while Tt can share TT’s phenotype without being homozygous.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain why TT and tt are both homozygous even though they can give different pigment phenotypes under the supplied dominance rule.
Give me a hint
Compare identities within each pair.
Compare my reasoning
- Check TT has two T alleles.
- Check tt has two t alleles.
- Keep phenotype expression separate.
Each genotype has two identical alleles of the model gene, so both are homozygous. Their allele forms differ between genotypes and can produce different phenotypes under the rule.
Look for these in your work
- Identical alleles within each pair
- Separates homozygosity from phenotype
State the normal gamete allele at this gene from a TT parent and from a tt parent. Does this establish that all their gametes are identical at every gene?
Give me a hint
Limit the gamete statement to the given allele pair.
Compare my reasoning
- Assign T from TT and t from tt.
- Identify the unspecified other genes.
TT contributes T and tt contributes t at this model gene. This does not establish whole-genome identity between all gametes because other genes are not followed in the record.
Look for these in your work
- Correct T and t contributions
- No whole-genome identity claim
Two organisms both have TT at one gene. One also has Aa at another gene, while the other has aa there. Explain what shared TT proves and what it does not.
Give me a hint
Compare the stated genes separately.
Compare my reasoning
- Identify the shared homozygous T genotype.
- Identify the different A-gene genotypes.
They share the homozygous TT genotype at the T gene. They are not shown to have identical overall genotypes; Aa and aa differ at the separate A gene.
Look for these in your work
- Limits shared homozygosity to T gene
- Identifies difference at A gene
Common mistakes
- Using homozygous as a synonym for dominant
- Inferring TT from a dominant phenotype alone
- Extending a one-gene record to the entire genome
What can you explain now?
Which statement describes both TT and tt?
Compare with the explanation
Both are homozygous at the model gene
Each pair contains identical alleles despite different forms between the pairs.
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Make it stick
Without your notes, identify identical alleles at the model gene and separate homozygosity from dominance phenotype and whole-genome identity. Use the supplied allele key and state what the evidence can establish.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.