Genetics — Set 3

Correct Answer: B. Watson and Crick

• **Watson and Crick** = In 1953, James Watson and Francis Crick used X-ray crystallography data — crucially Photo 51 taken by Rosalind Franklin — to deduce the double-helix structure of DNA and published their model in Nature. • **1962 Nobel Prize** — Watson, Crick, and Maurice Wilkins shared the Nobel Prize in Physiology or Medicine in 1962; Rosalind Franklin had died in 1958 and could not be awarded posthumously. • Their model instantly revealed how DNA could replicate (complementary base pairing) and store genetic information. • 💡 Option A (Morgan and Bridges) is wrong because they studied gene linkage and sex-linked inheritance in Drosophila melanogaster, not DNA structure; Option C (Mendel and Darwin) is wrong because Mendel and Darwin lived in the 19th century — long before DNA structure was understood; Option D (Hershey and Chase) is wrong because their 1952 blender experiment proved that DNA — not protein — is the genetic material, which was a separate discovery.

Correct Answer: C. Test cross

• **Test cross** = A test cross is performed by crossing an individual with a dominant phenotype (but unknown genotype) with a homozygous recessive individual (tt) — if any offspring show the recessive trait, the dominant parent must be heterozygous (Tt). • **Introduced by Mendel** — This elegant technique was first used by Gregor Mendel to verify his theoretical ratios and remains a standard tool in genetics today. • If all offspring show the dominant phenotype, the parent is homozygous dominant (TT); if half show recessive, the parent is heterozygous (Tt). • 💡 Option A (Dihybrid cross) is wrong because a dihybrid cross involves two different traits simultaneously (e.g., AaBb × AaBb) and is not specifically designed to reveal an unknown genotype; Option B (Monohybrid cross) is wrong because it crosses two individuals for one trait without requiring one parent to be homozygous recessive — so it does not definitively reveal the genotype; Option D (Back cross) is wrong because a back cross means crossing an F1 offspring back to one of its original parent lines — it is not the same as testing against a homozygous recessive.

Correct Answer: C. Snapdragon flower color

• **Snapdragon flower colour** = In Antirrhinum (snapdragon), crossing red-flowered (RR) and white-flowered (rr) parents produces pink-flowered (Rr) offspring — neither allele is fully dominant, so the phenotype is an intermediate blend. • **Blending, not mixing** — In F2, the ratio is 1 red : 2 pink : 1 white (1:2:1) because the original phenotypes re-appear, proving alleles did not fuse permanently. • This distinguishes incomplete dominance from blending inheritance — the alleles remain distinct and can segregate again. • 💡 Option A (Attached earlobes) is wrong because earlobe shape follows simple complete dominance (free earlobes are dominant over attached); Option B (ABO blood groups) is wrong because the ABO system is an example of codominance (A and B alleles are both fully expressed in type AB) and multiple alleles — not incomplete dominance; Option D (Human height) is wrong because height is polygenic — controlled by many genes — not a case of incomplete dominance between two alleles.

Correct Answer: C. Crossing over

• **Crossing over** = During prophase I of meiosis, non-sister chromatids of homologous chromosomes physically exchange segments at points called chiasmata — this shuffles allele combinations and generates genetic diversity in gametes. • **Why it matters** — Crossing over breaks apart linked genes and creates recombinant chromosomes, which is why siblings sharing the same parents are genetically different from each other. • The frequency of crossing over between two genes is used to calculate genetic distance in centimorgans (cM) for chromosome mapping. • 💡 Option A (Segregation) is wrong because segregation is the separation of homologous chromosomes to opposite poles during meiosis — it does not involve physical exchange of segments; Option B (Inversion) is wrong because inversion is a chromosomal mutation where a segment is flipped 180°, not an exchange between non-sister chromatids; Option D (Mutation) is wrong because mutation is a random change in the DNA sequence — crossing over is a normal, regulated meiotic process.

Correct Answer: D. 23

• **23 chromosomes** = Human gametes (sperm and egg) are haploid (n = 23) — they carry one chromosome from each homologous pair plus one sex chromosome — so that fertilisation restores the diploid number (2n = 46) in the zygote. • **Meiosis halves the count** — Meiosis I and II together reduce a diploid germ cell (46) into four haploid gametes (23 each), ensuring chromosome number remains constant across generations. • A sperm contains either 22 autosomes + X or 22 autosomes + Y; an egg always contains 22 autosomes + X. • 💡 Option A (22) is wrong because 22 is the number of autosomes in a gamete — the sex chromosome (either X or Y) is the 23rd; Option B (44) is wrong because 44 is the autosome count in a somatic cell, far more than a gamete; Option C (46) is wrong because 46 is the total chromosome number of a diploid somatic cell, not a haploid gamete.

Correct Answer: A. Introns

• **Introns** = Introns ('intervening sequences') are non-coding regions within a gene that are transcribed into pre-mRNA but are then cut out by spliceosomes during RNA splicing — the remaining exons are joined to form the mature mRNA. • **Exons express, introns intervene** — A useful memory trick: Exons are EXpressed; Introns are INTervening (removed). • Alternative splicing of introns allows one gene to produce multiple different proteins, greatly expanding the proteome. • 💡 Option B (Exons) is wrong because exons are the coding sequences that are retained after splicing and are translated into protein; Option C (Operons) is wrong because an operon is a cluster of prokaryotic genes (with a shared promoter) regulated together — it is a gene regulation unit, not a spliced-out sequence; Option D (Codons) is wrong because codons are three-nucleotide sequences on mRNA that specify individual amino acids during translation.

Correct Answer: C. Point mutation

• **Point mutation** = A point mutation is the substitution of a single nucleotide for another in the DNA sequence — it may be silent (no change in amino acid), missense (different amino acid), or nonsense (creates a stop codon). • **Sickle cell anaemia classic example** — A single A→T point mutation in the HBB gene changes the sixth amino acid of haemoglobin from glutamic acid to valine, causing the disease. • Point mutations are the smallest category of mutation, yet can have profound effects depending on the codon changed. • 💡 Option A (Duplication) is wrong because duplication is a chromosomal mutation where a segment of DNA is copied twice — it involves addition of extra genetic material, not a single base swap; Option B (Deletion) is wrong because deletion removes one or more nucleotides entirely — if it removes a number not divisible by three, it causes a frameshift; Option D (Frameshift mutation) is wrong because frameshift mutations result from insertions or deletions that shift the reading frame, altering every codon downstream — far more disruptive than a point mutation.

Correct Answer: A. Father

• **Father** = Human males have XY sex chromosomes and produce two types of sperm — X-bearing and Y-bearing; females have XX and always contribute an X egg. A Y-bearing sperm fertilising an egg produces a male (XY); an X-bearing sperm produces a female (XX). • **Mother's contribution is always X** — Since all eggs carry only one X chromosome, the mother cannot determine sex — only the type of sperm from the father decides the outcome. • This was first conclusively established by T. H. Morgan's work on sex-linked traits in Drosophila. • 💡 Option B (Environment) is wrong because in humans, sex is genetically determined at fertilisation — environmental factors like temperature do not alter chromosomal sex (unlike in some reptiles); Option C (Both parents) is wrong because the mother always contributes X — only the father's sperm varies between X and Y, so he is the sole determinant; Option D (Mother) is wrong for the same reason — she can only pass on X, contributing no variability in sex determination.

Correct Answer: A. 3:1

• **3:1 phenotypic ratio** = When two heterozygous F1 plants (Tt × Tt) are crossed, the F2 offspring are 1 TT : 2 Tt : 1 tt in genotype — because TT and Tt both show the dominant trait, the phenotypic ratio is 3 dominant : 1 recessive. • **Genotypic ratio is 1:2:1** — It is crucial to distinguish: the genotype ratio in F2 is 1:2:1, but since two genotypes (TT and Tt) give the same phenotype, the phenotype ratio simplifies to 3:1. • Mendel first demonstrated this with tall vs. dwarf pea plants, observing exactly 787 tall and 277 dwarf in F2 — almost precisely 2.84:1. • 💡 Option B (1:1) is wrong because a 1:1 phenotypic ratio results from a test cross (Tt × tt), not an F2 monohybrid cross; Option C (9:3:3:1) is wrong because that is the F2 phenotypic ratio for a dihybrid cross involving two independent traits; Option D (1:2:1) is wrong because 1:2:1 is the genotypic ratio (TT:Tt:tt) of the F2, not the phenotypic ratio.

Correct Answer: C. ABO system

• **ABO system** = The ABO blood group system demonstrates codominance — individuals with genotype I^A I^B express both A and B antigens on their red blood cells simultaneously, producing blood type AB where neither allele masks the other. • **Multiple alleles add to complexity** — The ABO gene has three alleles (I^A, I^B, and i); I^A and I^B are codominant to each other, while both are dominant over i (the O allele). • AB individuals are universal recipients; O individuals are universal donors — clinically important in blood transfusions. • 💡 Option A (Rh factor) is wrong because the Rh system (Rh+ vs. Rh−) follows simple dominance — Rh+ is dominant over Rh−, with no codominance; Option B (Kell system) is wrong because it also follows a dominant/recessive pattern and is not a classic textbook example of codominance; Option D (MN system) is also codominant in reality, but it is far less commonly cited in Indian exam syllabuses than ABO — the ABO system remains the standard answer.