Principles of Inheritance and Variation Set-1

Q1. In a plant species alleles $R$ (red) and $r$ (white) show incomplete dominance so that heterozygote $Rr$ is pink. Two pink F$_1$ plants ($Rr$) are selfed to produce F$_2$. What fraction of the F$_2$ progeny will show the pink phenotype?

Q2. Genes $A$ and $B$ exhibit complementary interaction for flower colour: at least one dominant allele at both loci ($A\_$ and $B\_$) is required for purple pigment; any other genotype is white. A dihybrid cross $AaBb \times AaBb$ is performed. What fraction of the offspring will be purple?

Q3. A man with blood group $B$ and a woman with blood group $A$ have a child with blood group $O$. Assuming ABO alleles ($I^A$, $I^B$, $i$), what is the probability that their next child will have blood group $AB$?

Q4. Genes $A$, $B$ and $C$ lie in that linear order on a chromosome. Recombination frequency between $A$–$B$ is $10\%$ and between $B$–$C$ is $6\%$. In a sample of $10\,000$ test-cross progeny, $30$ double recombinants (recombination between both $A$–$B$ and $B$–$C$) are observed. What are the coefficient of coincidence ($C$) and the interference ($I$)?

Q5. Assertion (A): If a mother carries a pathogenic mitochondrial DNA mutation, all her children inherit the mutant mitochondrial genome but may show variable severity of disease. Reason (R): Mitochondrial inheritance is maternal and, due to heteroplasmy, different offspring receive different proportions of mutant and wild‑type mitochondria, producing variable expressivity among them.

Q6. In a large randomly mating population at Hardy–Weinberg equilibrium the frequency of individuals showing the recessive phenotype $aa$ is given as $q^2=0.09$. What percentage of the population are heterozygous carriers ($Aa$)?

Q7. In a test cross of a dihybrid heterozygote (AaBb) with the double recessive (aabb) the progeny phenotypes counted were: parental types (AB and ab) total = 400 and recombinant types (Ab and aB) total = 200. Using $r=\dfrac{\text{recombinants}}{\text{total}}$, what is the map distance between the two loci (in cM)?

Q8. Two unaffected parents have an affected child with an autosomal recessive disorder. Their unaffected daughter marries an unrelated man from the general population where the carrier frequency for the disease allele is $1\%$. What is the probability that their first child will be affected? (Use: probability that an unaffected sibling of an affected child is a carrier = $2/3$.)

Q9. In a test cross between a female heterozygote for three linked genes (ABC/abc) and a homozygous recessive tester (abc/abc) the following offspring counts were obtained: $ABC=360,\ abc=360,\ Abc=40,\ aBC=40,\ ABc=90,\ abC=90,\ AbC=10,\ aBc=10$. Based on these data, which of the following gives the most likely gene order and the map distances between A–B and B–C?

Q10. In a three-locus mapping experiment total offspring = 2000. Observed recombination frequencies are $r_{AB}=0.10$ and $r_{BC}=0.20$. The observed number of double crossover offspring is 10. Calculate the interference $I=1-\text{(coefficient of coincidence)}$, where coefficient of coincidence $= \dfrac{\text{observed double cross}}{\text{expected double cross}}$.