Genetics — Set 1

Correct Answer: C. DNA

• **DNA (Deoxyribonucleic Acid)** = DNA carries the complete set of genetic instructions needed for an organism to develop, function, and reproduce — making it the universal hereditary molecule in nearly all life forms. • **Double-stranded stability** — Its double-helix structure with complementary base pairing provides the chemical stability required for long-term storage of genetic information. • Viruses are the only exception where RNA can serve as the primary genetic material. • 💡 Option A (RNA) is wrong because RNA is a single-stranded, less stable molecule used for temporary messages, not permanent storage; Option B (Carbohydrates) is wrong because they are energy sources and structural materials, not information carriers; Option D (Proteins) is wrong because they are the products encoded by DNA, not the coding molecule itself.

Correct Answer: B. The physical appearance

• **The physical appearance** = Phenotype encompasses all observable traits of an organism — including colour, height, disease susceptibility, and behaviour — that result from the interaction of genes and environment. • **Environment can alter phenotype** — Identical twins share the same genotype but can have different phenotypes due to lifestyle, nutrition, or exposure to toxins. • The word comes from the Greek 'phainein' meaning 'to show', reflecting what is visible or measurable. • 💡 Option A (The number of chromosomes) is wrong because chromosome count is part of the karyotype, not phenotype; Option C (The genetic makeup) is wrong because that defines genotype — the hidden blueprint; Option D (The sequence of amino acids) is wrong because that describes a protein's primary structure, not an observable organismal trait.

Correct Answer: C. Thymine

• **Thymine** = Adenine (a purine) always pairs with Thymine (a pyrimidine) via two hydrogen bonds in the DNA double helix — this is called complementary base pairing. • **Chargaff's Rule** — In any DNA sample the amount of Adenine always equals Thymine (A = T) and Guanine always equals Cytosine (G = C). • This specific pairing is what allows DNA to be accurately copied during replication. • 💡 Option A (Cytosine) is wrong because Cytosine pairs with Guanine via three hydrogen bonds; Option B (Guanine) is wrong because Guanine is paired with Cytosine, not Adenine; Option D (Uracil) is wrong because Uracil is found in RNA, not DNA — it pairs with Adenine only in RNA.

Correct Answer: C. Gregor Mendel

• **Gregor Mendel** = Gregor Mendel (1822–1884), an Austrian monk, conducted meticulous experiments on pea plants and formulated the three fundamental laws of inheritance — segregation, independent assortment, and dominance. • **Rediscovered in 1900** — Mendel's work was largely ignored during his lifetime and was only recognised as the foundation of genetics after being independently rediscovered by three scientists in 1900. • His quantitative approach of counting thousands of offspring made his findings statistically robust and groundbreaking. • 💡 Option A (James Watson) is wrong because Watson co-discovered the double-helix structure of DNA in 1953; Option B (Thomas Hunt Morgan) is wrong because Morgan studied gene linkage in fruit flies, earning the 1933 Nobel Prize; Option D (Charles Darwin) is wrong because Darwin proposed the theory of evolution by natural selection, not the laws of inheritance.

Correct Answer: D. Locus

• **Locus** = A locus (plural: loci) is the precise, fixed location on a chromosome where a specific gene or genetic marker resides — like a permanent address in the genome. • **Mapping uses loci** — Genetic linkage maps are constructed by determining the relative distance between loci on the same chromosome. • Two or more alternative forms of a gene found at the same locus are called alleles. • 💡 Option A (Centromere) is wrong because the centromere is the constricted region that holds sister chromatids together during cell division; Option B (Genotype) is wrong because genotype refers to the full genetic constitution of an organism, not a chromosomal address; Option C (Allele) is wrong because an allele is one of several versions of a gene at a given locus, not the location itself.

Correct Answer: C. TT (variant)

• **TT (homozygous dominant)** = An organism is homozygous when both alleles at a given locus are identical — TT has two identical dominant alleles, making it homozygous dominant. • **Guaranteed phenotype** — A homozygous dominant individual always passes the dominant allele to every offspring, so its trait is completely predictable. • Homozygous recessive (tt) also exists; homozygous simply means both alleles match. • 💡 Option A (Tt) is wrong because Tt carries two different alleles (one dominant, one recessive), making it heterozygous; Option B (Ab) is wrong because it mixes upper and lower case different letters — also heterozygous; Option D (XY) is wrong because it represents the male sex chromosome pair, not a homozygous autosomal condition.

Correct Answer: D. Mutation

• **Mutation** = A mutation is a sudden, heritable change in the nucleotide sequence of DNA — it can arise from replication errors, chemical mutagens, UV radiation, or ionising radiation. • **Neutral, harmful, or beneficial** — Most mutations are neutral, some are harmful (causing disease), and a rare few are beneficial, providing the raw material for evolution by natural selection. • Mutations in somatic cells affect only the individual; mutations in germ cells can be passed to offspring. • 💡 Option A (Transcription) is wrong because transcription is the process of copying DNA into mRNA — a normal cellular step, not a change in sequence; Option B (Translation) is wrong because translation reads mRNA to build proteins — it does not alter the DNA; Option C (Linkage) is wrong because linkage refers to genes on the same chromosome tending to be inherited together, not a DNA change.

Correct Answer: A. Uracil

• **Uracil** = Uracil is unique to RNA — it replaces Thymine and pairs with Adenine using two hydrogen bonds, just as Thymine does in DNA. • **Why the difference?** — DNA uses Thymine (which has a methyl group) for greater stability and error-resistance; RNA uses the simpler, cheaper-to-make Uracil because it is a temporary molecule. • This structural distinction between RNA and DNA is a key exam concept tested repeatedly. • 💡 Option B (Guanine) is wrong because Guanine is present in both DNA and RNA; Option C (Thymine) is wrong because Thymine is the DNA-specific base that Uracil replaces in RNA; Option D (Cytosine) is wrong because Cytosine is also present in both DNA and RNA, pairing with Guanine in each.

Correct Answer: B. Transcription

• **Transcription** = Transcription is the first step of gene expression, in which RNA polymerase reads the DNA template strand and synthesises a complementary mRNA molecule. • **Occurs in the nucleus** — In eukaryotes, transcription happens in the nucleus, and the resulting pre-mRNA is processed (capping, polyadenylation, splicing) before leaving for the cytoplasm. • The mRNA then carries the genetic code to ribosomes where it is read during translation. • 💡 Option A (Replication) is wrong because replication duplicates the entire DNA molecule, not just one segment into RNA; Option C (Translation) is wrong because translation converts the mRNA sequence into a protein — it is the step after transcription; Option D (Mutation) is wrong because mutation is a permanent alteration in the DNA sequence, not a copying process.

Correct Answer: C. 44

• **44 autosomes** = A normal human somatic (body) cell contains 46 chromosomes in total — 44 autosomes arranged in 22 homologous pairs, plus 2 sex chromosomes (XX in females, XY in males). • **Autosomes carry non-sex traits** — These 22 pairs determine characteristics like eye colour, height, and blood group but do not determine biological sex. • Autosomes are numbered 1–22 from largest to smallest based on a standardised karyotype. • 💡 Option A (22) is wrong because 22 is the haploid count of autosomes in a gamete, not a somatic cell; Option B (23) is wrong because 23 is the total chromosome count in a gamete (22 autosomes + 1 sex chromosome); Option D (46) is wrong because 46 is the total chromosome count — autosomes alone number only 44.