Gene mutations in plants can produce changes in stomatal density. One such mutat
ID: 10789 • Letter: G
Question
Gene mutations in plants can produce changes in stomatal density. One such mutation is in a gene called ‘tmm’ (too many mouths) which causes an increase in stomatal density as illustrated in Figure 3. This is a recessive condition that leads to stomata developing in groups (b) instead of as single stoma (a)
(i)
Briefly, in the context of tmm, what is meant by the term ‘recessive condition’
(ii)
Draw the mating diagram of a genetic cross between two plants which are heterozygous carriers of the defective tmm gene.
(iii)
Use your mating diagram to predict the genotypic and phenotypic ratios of the offspring.
Explanation / Answer
(i) tmm is a recessive condition because plants must be homozygous for the trait (i.e. have two tmm alleles) to display the phenotype of having increased stomatal density. Thus, heterozygous plants (wild-type/tmm) are not affected because the "too many mouths" trait is overshadowed by the wild-type trait. (ii) WT/tmm X WT/tmm: WT tmm -------------------------- WT |WT/WT WT/tmm tmm |WT/tmm tmm/tmm WT/tmm are heterozygous carriers. (iii) Genotypic ratio of offspring: 1 WT/WT : 2 WT/tmm : 1 tmm/tmm Phenotypic ratio: 3 Normal stomatal density : 1 increased stomatal density (too many mouths phenotype)
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