Principles of Inheritance and Variation, on the exam screen.

Incomplete dominance is shown by flower colour in Antirrhinum, where the heterozygote is intermediate; co-dominance is illustrated by ABO blood groups, where alleles $I^A$ and $I^B$ are both expressed; pleiotropy is exemplified by phenylketonuria, where one gene affects several traits; and polygenic inheritance controls human skin colour. This gives P-II, Q-IV, R-III, S-I.

Honeybees use a haplodiploid system (Statement V). A fertilised egg (diploid) becomes a female, queen or worker (Statement I); an unfertilised egg develops parthenogenetically into a haploid male (Statement II); the haploid male therefore has half the chromosome number of the diploid female (Statement III). Statement IV is false because haploid males cannot undergo normal reductional meiosis and instead form sperm by a modified mitosis, so the correct set is I, II, III and V.

Sickle-cell anaemia is an autosomal recessive disorder caused by a point mutation (GAG to GUG) that substitutes glutamic acid with valine at the sixth position of the beta-globin chain. Thalassemia results from reduced or absent globin-chain synthesis, phenylketonuria from a defect in phenylalanine metabolism, and haemophilia is an X-linked clotting disorder.

A heterozygous type-A parent is $I^A i$ and a heterozygous type-B parent is $I^B i$. Their cross gives offspring $I^A I^B$ (AB), $I^A i$ (A), $I^B i$ (B) and $ii$ (O) in a 1:1:1:1 ratio. Only the $ii$ genotype is blood group O, giving a probability of 1 in 4, i.e. 25%.

Why the other options are wrong:

- (2) Since both parents carry the recessive i allele, an O child is possible.
- (3) 75% would be the chance of a non-O child, not an O child.
- (4) 50% does not match the 1:1:1:1 offspring ratio for this cross.

In sickle cell anaemia the sixth codon of the $\beta$-globin gene changes from GAG to GUG, so glutamic acid is replaced by valine. This substitution makes haemoglobin polymerize at low oxygen tension and distorts the red blood cell (RBC) into a sickle shape, so the mutant codon is GUG.

Grasshoppers follow the XO type of sex determination: males are XO (a single X and no partner sex chromosome) while females are XX. Therefore the individual with 23 chromosomes lacks the second sex chromosome and is male, while the one with 24 chromosomes is female. Hence males and females respectively.

In honeybees the males (drones) are haploid, developing from unfertilised eggs by parthenogenesis, so their gametes must be formed by mitosis rather than meiosis. Male earthworms, frogs and grasshoppers are diploid and produce gametes through meiosis. Hence male honeybees.

Both statements have been swapped. Down's syndrome is an AUTOSOMAL trisomy - an extra copy of chromosome 21, giving 47 chromosomes - and has nothing to do with the X chromosome. Turner's syndrome is the opposite kind of error: a MISSING X, giving the 45,X karyotype in a phenotypic female with rudimentary ovaries. So the descriptions belong to each other's conditions, and neither statement as written is correct. NCERT Class 12, Principles of Inheritance and Variation.

Blood group O is the homozygous recessive genotype $ii$, so EACH parent must be able to contribute an $i$ allele. A father of genotype $I^A I^B$ carries no $i$ allele at all and can only pass on $I^A$ or $I^B$, so option (4) cannot produce an O child. Every other option pairs two heterozygous carriers, each of whom can donate $i$. NCERT Class 12, Principles of Inheritance and Variation (multiple allelism).

Crossing true-breeding tall and dwarf pea plants gave an $\mathrm{F}_{1}$ generation that was uniformly tall, with no intermediates and no dwarfs - one of Mendel's central observations. The reason states exactly why: the $\mathrm{F}_{1}$ plants are heterozygous $Tt$, and because the tall allele is dominant it masks the recessive dwarf allele in the phenotype. The dwarf allele reappears in the $\mathrm{F}_{2}$ generation, confirming it was never lost. NCERT Class 12, Principles of Inheritance and Variation.

Thomas Hunt Morgan provided experimental verification of the chromosomal theory of inheritance through his work with Drosophila melanogaster. Chapter: Principles of Inheritance and Variation.

Mendel selected 14 true breeding pea plant varieties as pairs, each differing in one character with contrasting traits. Chapter: Principles of Inheritance and Variation.

- Haemophilia is X-linked recessive. - Phenylketonuria is autosomal recessive. - Sickle cell anaemia is an autosomal recessive trait linked to chromosome 11. - Thalassemia is autosomal recessive, not X-linked. Chapter: Principles of Inheritance and Variation.

The gene 'I' has three alleles: $I^A$, $I^B$, and i. A person will have only two of these alleles. When $I^A$ and $I^B$ are present together, they express different types of sugars, not the same type. The allele i does not produce any sugar. Chapter: Principles of Inheritance and Variation.