Principles of Inheritance and Variation Set-2

Q1. In a test cross between an $AaBb$ plant and $aabb$, the progeny counts observed are: parental types $AB=420$ and $ab=430$, recombinant types $Ab=70$ and $aB=80$. Calculate the recombination frequency between the two genes using $RF=\dfrac{R}{T}\times100\%$.

Q2. In an F2 dihybrid cross the expected phenotypic ratio is $9:3:3:1$. A cross produced $1600$ offspring with observed counts (in the same order as $9:3:3:1$): $870,\;330,\;310,\;90$. Using $\chi^2=\sum\dfrac{(O-E)^2}{E}$ and the critical value $\chi^2_{0.05,\,df=3}=7.815$, which statement is correct about these data?

Q3. Three genes $A$, $B$, $C$ lie in linear order with recombination frequencies $r_{AB}=10\%$ and $r_{BC}=20\%$. Approximate the expected number of gametes showing double recombination (crossovers in both intervals) among $10{,}000$ meioses, using the approximation $P_{double}\approx r_{AB}\times r_{BC}$.

Q4. In a three-point test cross ($AaBbCc\times aabbcc$) of $1000$ progeny the observed class counts are: parental $ABC=260$, parental $abc=270$, single recombinant (A–B only)$=200$, single recombinant (B–C only)$=250$, double recombinant$=20$. Using
$r_{AB}=\dfrac{single_{AB}+double}{N},\; r_{BC}=\dfrac{single_{BC}+double}{N},$
$expected\ double=r_{AB}\times r_{BC}\times N,\; C.O.C=\dfrac{observed\ double}{expected\ double},\; I=1-C.O.C,$
the interference $I$ is approximately:

Q5. Assertion (A): In a two-step biochemical pathway S $\xrightarrow{A}$ I $\xrightarrow{B}$ P, a recessive mutation $a_1$ ($a_1a_1$) that blocks the first step gives white fruit, a recessive mutation $b_1$ ($b_1b_1$) that blocks the second step gives yellow fruit, and the double mutant $a_1a_1\,b_1b_1$ is white; this shows $a_1$ is epistatic to $b_1$.
Reason (R): The upstream block caused by $a_1$ prevents formation of intermediate I, so absence of substrate for the B-encoded enzyme masks the phenotypic effect of $b_1$, making the upstream gene epistatic.

Q6. In a large randomly mating population in Hardy–Weinberg equilibrium, $9\%$ of individuals express a recessive trait. If $q^2=0.09$, what percentage of the population are heterozygous carriers ($2pq$)?

Q7. A dihybrid individual (genotype $A B / a b$) is testcrossed to $a b / a b$ and yields 1000 progeny with phenotypes: $AB = 420$, $ab = 400$, $Ab = 120$, $aB = 60$. Using $RF = \dfrac{\text{recombinants}}{\text{total}} \times 100$, the map distance between genes $A$ and $B$ is closest to:

Q8. Three recessive mutants (m1, m2, m3) showing the same phenotype are crossed pairwise. Results: m1 × m2 → wild-type progeny; m1 × m3 → mutant progeny; m2 × m3 → wild-type progeny. These results indicate the number of distinct genes mutated is:

Q9. Assertion (A): X‑linked recessive disorders are observed more frequently in human males than in females.
Reason (R): Random X‑chromosome inactivation (lyonization) in females produces mosaic expression of X‑linked genes.
Choose the correct option:

Q10. A female heterozygous for three linked loci (genotype $ABC/abc$) is testcrossed to $abc/abc$ producing 1000 progeny with these counts: $ABC=420$, $abc=380$, $aBC=90$, $Abc=60$, $ABc=20$, $abC=20$, $AbC=6$, $aBc=4$. Based on these data, which statement is correct? (Use map units = % recombination; coefficient of coincidence = observed DCO / expected DCO)