Short answerModerate1 mark
6(a)
In Mendel’s experiments, tall pea plants (T) are dominant over dwarf pea plants (t).
What is the phenotypic ratio and genotypic ratio of the above cross in (ii)?
Topic: Phenotypic and genotypic ratios
Previous-year question practice database
Answer
Exam answer
PHENOTYPIC RATIO = 3 TALL : 1 DWARF GENOTYPIC RATIO = 1 TT : 2 Tt : 1 tt
Explanation
ONE CROSS GIVES TWO DIFFERENT RATIOS, and confusing them is the commonest error in this chapter: PHENOTYPIC — what they LOOK like — 3 : 1 GENOTYPIC — what alleles they CARRY — 1 : 2 : 1
The reason they differ is that TT and Tt look IDENTICAL — both are tall, because T is dominant. So three of the four offspring appear tall although only one is pure tall.
Read the word in the question before answering. Compare the DIHYBRID ratio 9:3:3:1, which applies when two characters are followed together.
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