Short answerModerate1 mark
7(a)
In a homozygous pea plant, axial flowers (A) are dominant over terminal flowers (a).
What is the phenotypic ratio and genotypic ratio of the above cross shown in (ii)?
Topic: Phenotypic and genotypic ratios
Previous-year question practice database
Answer
Exam answer
PHENOTYPIC RATIO = 3 AXIAL : 1 TERMINAL GENOTYPIC RATIO = 1 AA : 2 Aa : 1 aa
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
They differ because AA and Aa look IDENTICAL — both bear axial flowers, since A is dominant. So three of the four offspring appear axial although only one is PURE axial.
Read which ratio the question asks for. Compare the DIHYBRID ratio 9:3:3:1, which applies when two characters are followed together.
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