Derive the possible gamete alleles before constructing a monohybrid cross
Derive one-allele gamete records before combining the contributions.
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 gamete record fits a normal TT parent at this gene?
What you will learn
- Separate the two parental alleles into normal gamete contributions
- State the probability assumptions used for a cross
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| TT parent | Gamete allele type T |
| Tt parent | Gamete allele types T and t |
| tt parent | Gamete allele type t |
| Standard square | Equal probability of the T/t alternatives is a stated model assumption |
Separate the two parental alleles into normal gamete contributions
Begin a monohybrid cross with the parental genotypes at the specified autosomal gene. A normal gamete carries one allele at that gene. TT supplies T, tt supplies t, and Tt can supply T or t. The one-allele record does not mean the gamete contains only one gene: it is a focused record of one gene within its complete haploid chromosome set.
In the standard model, the two alternatives from a Tt parent are equally likely to be transmitted. This half-and-half assumption lets the rows and columns of a two-by-two Punnett square have equal probability. State the assumption when interpreting the square. It describes probabilities over possible events, not a guarantee that each small batch of gametes or offspring has exactly equal counts.
State the probability assumptions used for a cross
Each offspring genotype combines one gamete contribution from each parent. For Tt crossed with tt, the first parent can supply T or t, while the second supplies t. The possible offspring pairs are Tt and tt. Writing the whole Tt parental genotype inside every gamete would incorrectly preserve both alleles in that contribution and prevent a valid one-gene cross.
Repeated labels may be used to make a two-by-two square, such as t above both columns for a tt parent. Those duplicate labels do not create a second allele type. A one-column display can also list distinct outcomes if its probabilities are handled correctly. Distinguish distinct allele types, repeated equal-probability positions and the final genotype combinations before calculating a ratio.
Pause and explain
What are the possible gamete allele types from Tt?
Worked example
List the possible gamete types and offspring genotypes for Tt × tt.
Show the worked solution
- Tt contributes T or t. The homozygous tt parent supplies t; repeating it in two columns does not create a second allele type.
- Combine T with t to obtain Tt and t with t to obtain tt.
Answer Tt × tt: possible gametes T/t from the first parent and t from the second. Combining them gives Tt or tt; repeated t columns in a square repeat one allele type.
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Test this topic →From guided practice to a new situation
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.
List the possible normal gamete allele types from TT, Tt and tt.
Give me a hint
Follow one allele from each parental pair.
Compare my reasoning
- Assign T from TT.
- Assign T or t from Tt.
- Assign t from tt.
TT: T; Tt: T or t; tt: t, at the tracked gene.
Look for these in your work
- Correct homozygous contributions
- Correct two heterozygous alternatives
For Tt × tt, name each parent’s possible gamete alleles and the possible offspring genotypes.
Give me a hint
Combine one contribution from each parent.
Compare my reasoning
- List T or t from the first parent.
- List t from the second.
- Combine T+t and t+t.
First parent T or t; second parent t. The possible offspring genotypes are Tt and tt.
Look for these in your work
- Correct gamete alternatives
- Correct Tt and tt outcomes
A two-by-two square places t above both columns for a tt parent. A student says that parent makes two different gamete alleles. Explain the error and the standard probability assumption for the Tt parent.
Give me a hint
Repeated positions are not different allele identities.
Compare my reasoning
- Identify t in both columns as the same form.
- Distinguish the T and t alternatives from Tt.
- State equal likelihood without a sample guarantee.
The tt parent supplies only the allele type t; two columns repeat it. The model gives the Tt parent’s T and t contributions equal probability, which does not guarantee equal counts in every small sample.
Look for these in your work
- One t allele type despite duplicate columns
- Equal-probability assumption with sample limit
Common mistakes
- Putting Tt together inside one normal gamete
- Treating duplicated t labels as two different allele forms
- Assuming half-and-half probability guarantees equal counts in every small sample
What can you explain now?
What are the possible gamete allele types from Tt?
Compare with the explanation
T and t
A gamete receives one of the two alternative forms.
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Make it stick
Without your notes, separate the two parental alleles into normal gamete contributions and state the probability assumptions used for a cross. 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.