Fig. 1 T t T t A farmer grows two varieties of tomato plant on a small farm. One variety is pure-breeding for tall stems and the other is pure-breeding for ...
Question 1 Report
| T | t | |
|---|---|---|
| T | ||
| t |
A farmer grows two varieties of tomato plant on a small farm. One variety is pure-breeding for tall stems and the other is pure-breeding for short stems. The allele for tall stems is dominant over the allele for short stems.
The farmer crossed a pure-breeding tall plant with a pure-breeding short plant. All the F1 offspring had tall stems.
(a) State what is meant by the term 'pure-breeding'. [1]
(b) Using the letter T for the dominant allele and t for the recessive allele:
(i) State the genotype of the tall parent plant. [1]
(ii) State the genotype of the short parent plant. [1]
(iii) Two F1 plants were crossed together. Complete the Punnett square in Fig. 1 to show the expected genotypes of the F2 offspring. [2]
(c) State the expected phenotypic ratio of tall to short plants in the F2 generation. [1]
(d) Suggest one advantage to the farmer of understanding how stem height is inherited in tomato plants. [1]
[Total: 7]
Answer Details
(a) Pure-breeding means an organism that always produces offspring with the same characteristic (phenotype) when crossed with another organism of the same genotype. [1] In genetic terms, the organism is homozygous for the trait in question, so it can only pass on one type of allele.
(b)(i) Genotype of the tall parent plant: TT [1]
The tall parent is pure-breeding, so it must be homozygous dominant.
(b)(ii) Genotype of the short parent plant: tt [1]
The short parent is pure-breeding for the recessive trait, so it must be homozygous recessive.
(b)(iii) The F1 plants have genotype Tt (heterozygous tall). When two F1 plants (Tt x Tt) are crossed, the completed Punnett square is: [2]
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
The gametes from each F1 parent are T and t. [1] The resulting genotypes are: TT, Tt, Tt and tt. [1]
(c) Expected phenotypic ratio of tall to short plants in the F2 generation: 3 tall : 1 short. [1]
Of the four possible offspring genotypes, three produce the tall phenotype (TT and two Tt, since T is dominant) and one produces the short phenotype (tt, homozygous recessive).
(d) One advantage to the farmer of understanding stem height inheritance: [1]
The farmer can select for shorter plants that are easier to support, manage and harvest, or plan crosses that will produce a predictable proportion of plants with the desired stem height. [1] This allows more efficient use of resources in the growing area.
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