Principles of Inheritance and Variation Set-4

Q1. In a testcross of an F1 dihybrid (AaBb × aabb) the progeny counts were: AB = 420, ab = 380, Ab = 110 and aB = 90 (total 1000). Using $d=\dfrac{\text{recombinants}}{\text{total}}\times100$, estimate the map distance between genes A and B (in cM).

Q2. In a three-point mapping experiment the observed recombination frequencies are $r_{AB}=12\%$ and $r_{BC}=8\%$. Observed double crossover frequency is $0.4\%$. Calculate the interference $I$ where $I=1-\dfrac{\text{observed double cross}}{\text{expected double cross}}$ and expected double cross $=r_{AB}\times r_{BC}$ (use fractions for $r$).

Q3. A woman has an affected brother with an X‑linked recessive disorder while both her parents are phenotypically normal. She marries an unaffected man. Their first two sons are unaffected. What is the probability that their third son will be affected?

Q4. Assertion (A): A disorder caused by a mutation in mitochondrial DNA typically shows maternal inheritance and may display variable severity among siblings (heteroplasmy).
Reason (R): The variability among offspring arises because mitochondria are inherited equally from both parents, producing a mixture of mutant and wild‑type mtDNA in each embryo.

Q5. In a testcross for three linked genes A, B and C the progeny counts (gametic classes from F1) are: ABC = 420, abc = 400, ABc = 60, abC = 55, Abc = 35, aBC = 20, AbC = 5, aBc = 5 (total 1000). Determine the most likely gene order and recombination distances between adjacent genes (in cM).

Q6. In a cross between two heterozygous individuals $Aa\times Aa$, what is the probability that among three independent offspring exactly two will show the recessive phenotype $aa$?

Q7. A plant heterozygous in coupling phase $AB/ab$ is testcrossed with $ab/ab$ producing $1000$ progeny of which $820$ are parental types and $180$ are recombinants. What is the map distance between $A$ and $B$ (in cM) and the probability that two progeny chosen at random (without replacement) are both recombinants?

Q8. In a cross $AaBb\times AaBb$ where homozygous $bb$ exhibits recessive epistasis and masks the effect of $A$, two F2 plants that display the dominant phenotype (i.e. have at least one dominant allele at each locus, denoted $A\_B\_$) are chosen at random and crossed. What is the probability that an offspring from this cross will show the epistatic phenotype ($bb$)?

Q9. Genes $A$ and $B$ are linked on the same chromosome with recombination frequency $r$ (where $0\le r\le1$). A heterozygote in coupling phase $AB/ab$ is testcrossed with $ab/ab$. If homozygous $bb$ is epistatic and masks the effect of $A$, what fraction of the testcross progeny will display the epistatic phenotype ($bb$) as a function of $r$?

Q10. Assertion (A): A pathogenic mutation in mitochondrial DNA ($mtDNA$) will be transmitted to all children of an affected mother.
Reason (R): Mitochondrial genomes are present in multiple copies per cell and often exist as a mixture of mutated and wild-type molecules (heteroplasmy), which influences the severity of the phenotype.

Q11. In a testcross to map two linked genes, the F1 produced 150 progeny: 132 parental types and 18 recombinant types. Using $RF=\dfrac{\text{number of recombinants}}{\text{total progeny}}\times 100$, estimate the map distance between the two genes (in cM).

Q12. Two linked genes A and B are $20\ \text{cM}$ apart and are in coupling phase in the F1 with genotype $AB/ab$. The F1 is testcrossed to $ab/ab$. What fraction of progeny will show both dominant phenotypes? (Use the relation $\mathrm{Freq}(AB)=\dfrac{1-r}{2}$ for coupling-phase gametes.)

Q13. In a large randomly mating population in Hardy–Weinberg equilibrium allele frequencies are $p=0.7$ for $A$ and $q=0.3$ for $a$. The heterozygote $Aa$ expresses a trait with penetrance $50\%$ (only half of $Aa$ individuals show the phenotype). In a sample of 1000 individuals, how many are expected to express the trait? (Use genotype frequency $2pq$.)

Q14. In a three-point test cross the recombination frequencies between genes A–B and B–C are $r_{AB}=0.10$ and $r_{BC}=0.15$. Among $N=2000$ progeny, $18$ double crossovers were observed. Calculate interference $I$ using $Expected_{double}=r_{AB}\times r_{BC}\times N$, $COC=\dfrac{Observed_{double}}{Expected_{double}}$, and $I=1-COC$.

Q15. Assertion (A): Incomplete dominance and co-dominance can be distinguished solely by examining the F2 phenotypic ratios.
Reason (R): Both incomplete dominance and co-dominance typically yield an F2 phenotypic ratio of $1:2:1$; the difference lies in the nature of the heterozygote phenotype (intermediate vs both parental traits), not in the ratio.