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Biology · 2024 · 3 marks
CBSE 2024 · Region 3 · Set 1 · Q25
A population of snakes lived in a desert with brown sand. Study the drawings given below showing the change in the population from 'one' to 'two'…
A population of snakes lived in a desert with brown sand. Study the drawings given below showing the change in the population from 'one' to 'two' over time and answer the question that follows. Brown snakes and Grey snakes are represented by alleles A/a (Dominant/recessive).
(a)
If the frequency of the recessive trait is $\displaystyle 9$% in population-one, work out the frequency of homozygous dominant and heterozygous dominant snakes.
(b)
Name the mechanism of evolution that must have operated so that population-two evolved from population-one.
Marking-scheme solution
(a)
Hardy – Weinberg Equilibrium.
p2 + 2pq + q2 = $\displaystyle 1$
AA + 2Aa + aa = $\displaystyle 1$
-Since Frequency of grey snakes in the population = $\displaystyle 9$% = $\displaystyle 9$ = ⋅$\displaystyle 09$
q2 = ⋅$\displaystyle 09$
q = $\displaystyle 0$⋅$\displaystyle 3$
-Since p + q = $\displaystyle 1$
p = $\displaystyle 1$ – q
p = $\displaystyle 1$ – $\displaystyle 0$⋅$\displaystyle 3$
-p = $\displaystyle 0$⋅$\displaystyle 7$
-The frequency of homozygous dominant (AA) is equal to P2 = $\displaystyle 0$⋅$\displaystyle 49$
Or the % frequency of homozygous dominant = $\displaystyle 49$% -
The frequency of heterozygous dominant (Aa) is equal to
2pq = $\displaystyle 2$ × $\displaystyle 0$⋅$\displaystyle 7$ × $\displaystyle 0$⋅$\displaystyle 3$ = $\displaystyle 0$⋅$\displaystyle 42$ Or % frequency of Aa = $\displaystyle 42$%
(b)
Natural selection
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