These eight original questions follow the lessons in inheritance, and the plants and insects are invented. Try each one before opening the answer, and draw the Punnett square for every cross.
Questions
Q1. Tall (T) is dominant over short (t). State the phenotype of a plant with genotype Tt and explain in one sentence.
Answer
Tall. The dominant allele T masks the recessive t when both are present.
Q2. Write the gametes of a Tt plant and of a tt plant.
Answer
Tt makes T and t gametes. tt makes only t gametes. Each gamete carries one allele.
Q3. Cross Tt × Tt. State the genotype and phenotype ratios.
Answer
Offspring TT, Tt, Tt, tt. Genotype ratio 1 TT : 2 Tt : 1 tt. Phenotype ratio 3 tall : 1 short.
Q4. A tall plant is crossed with a short plant and gives half tall and half short offspring. What is the tall parent’s genotype?
Answer
Tt. The short parent is tt and makes only t gametes. A 1:1 ratio means the tall parent makes T and t gametes, so it is heterozygous.
Q5. A student writes the gametes of Rr as “Rr and Rr”. Correct it.
Answer
A gamete carries one allele, so the gametes of Rr are R and r.
Q6. Red (R) is dominant over white (r). Cross Rr × Rr. What is the probability that one offspring is white?
Answer
1 in 4, or 25%. It is the chance for each offspring and does not guarantee one white per four.
Q7. In the invented insect, white eyes (Xʳ) are recessive and X-linked. Cross XᴿXʳ × XᴿY. State the outcome for sons.
Answer
Sons are XᴿY or XʳY, so half have red eyes and half have white eyes.
Q8. Why are recessive X-linked traits seen more in males in the invented insect?
Answer
A male has one X, so a single recessive allele on it shows. A female has two X chromosomes and needs two recessive alleles to show the trait.
If you got these wrong
Q1 and Q2 point to using allele, genotype and phenotype terminology. Q3 to Q6 need completing monohybrid crosses.
Q7 and Q8 go with interpreting sex-linked inheritance. To practise what crosses and pedigrees can and cannot show, try inheritance evidence rather than family assumptions.
Use the simple inheritance cross explorer to repeat crosses and the mistake log to track your errors. If you want a teacher to go through your answers, see online one-to-one Biology tuition or the one-hour trial class (from RM50).