Genetics Answering Technique

The complete structure

1. Define the key: state which letter represents which allele, and which is dominant and which is recessive (e.g. Let T represent the allele for tall and t represent the allele for short).
2. State parental phenotypes and genotypes (e.g. Parents: tall (Tt) x tall (Tt)).
3. Determine and write the gametes, each inside a circle, splitting each parent's genotype into single alleles.
4. Draw the Punnett square, placing one parent's gametes across the top and the other's down the side, and fill in every box.
5. List the offspring genotypes and their corresponding phenotypes.
6. State the genotype ratio and the phenotype ratio clearly at the end.

Worked example following the structure

Question: two tall pea plants, both heterozygous, are crossed. Determine the phenotype ratio of the offspring.

Key: let T = allele for tall (dominant), t = allele for short (recessive).

Parents: tall (Tt) x tall (Tt).

Gametes: (T) (t) from each parent.

Second worked example: a test cross

Question: a pea plant with round seeds could be RR or Rr, where round (R) is dominant to wrinkled (r). It is crossed with a wrinkled-seeded plant to reveal its genotype.

Set out the answer for the case where the round parent is heterozygous.

Key: let R = allele for round seed (dominant), r = allele for wrinkled seed (recessive).

Parents: round (Rr) x wrinkled (rr).

Gametes: (R) (r) from the round parent; (r) (r) from the wrinkled parent.

Third worked example: working backwards from a ratio

Question: in guinea pigs, black coat (B) is dominant to white coat (b). Two black guinea pigs are crossed and produce offspring in an approximate ratio of 3 black : 1 white.

State the genotypes of both parents and explain, using the answer structure, how you reached them.

Reasoning: a white offspring must be bb, so it received one b allele from each parent. Each black parent therefore carries a hidden b allele and must be heterozygous (Bb).

Key: let B = allele for black coat (dominant), b = allele for white coat (recessive).

Parents: black (Bb) x black (Bb).

Gametes: (B) (b) from each parent.

Command words to watch for

• 'State the genotype', write the letter pair only (e.g. Tt).
• 'Determine the phenotype ratio', count boxes, group by observable trait, and write as a ratio.
• 'Explain', give the biological reasoning (e.g. why a trait skips a generation), not just the ratio.
• 'Complete the genetic diagram', fill in every gamete and every box, showing full working, not just the final answer.

Marking points markers look for

Markers typically award separate marks for the key/definition of alleles, correct parental genotypes, gametes correctly shown in circles, a correctly completed Punnett square, and the final ratio. Missing any one step, even with a correct final ratio, can lose marks.

Practice problems

1. In tomatoes, red fruit (R) is dominant to yellow fruit (r). A heterozygous red plant is crossed with a yellow plant. Set out the full genetic diagram and state the phenotype ratio of the offspring.
2. A homozygous tall pea plant (TT) is crossed with a short plant (tt). Write the key, parental genotypes, gametes and the offspring genotype, then explain why the short trait reappears only in a later generation.
3. Two black mice, each heterozygous (Bb), are crossed, where black (B) is dominant to brown (b). State the genotype ratio and the phenotype ratio of the offspring, showing every step of the answer.

Answers to the practice problems

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions