Genetics — Set 2

Correct Answer: A. Law of Independent Assortment

• **Law of Independent Assortment** = Mendel's second law states that allele pairs for different traits on different chromosomes separate independently of each other during gamete formation, producing new combinations of traits in offspring. • **Basis of dihybrid cross** — This law explains the 9:3:3:1 phenotypic ratio observed when two independently assorting traits are crossed in the F2 generation. • Independent assortment applies only to genes located on different chromosomes or far apart on the same chromosome — linked genes are an exception. • 💡 Option B (Law of Segregation) is wrong because segregation deals with how the two alleles of a single gene separate during gamete formation; Option C (Law of Purity of Gametes) is wrong because it is essentially the same as segregation — describing that each gamete carries only one allele for a trait; Option D (Law of Dominance) is wrong because dominance describes which allele is expressed when two different alleles are present together.

Correct Answer: B. Sutton and Boveri

• **Sutton and Boveri** = Walter Sutton (1902) and Theodor Boveri (1902) independently proposed that chromosomes carry the units of heredity — they noticed that chromosome behaviour during meiosis precisely parallels Mendel's laws of segregation and independent assortment. • **Sutton studied grasshoppers, Boveri studied sea urchins** — Both arrived at the same conclusion by observing meiosis in different organisms, which gave the theory strong cross-species support. • This chromosomal theory became the foundation for linking genes to physical structures inside cells. • 💡 Option A (Watson and Crick) is wrong because Watson and Crick determined the double-helix structure of DNA in 1953 — a different discovery; Option C (Mendel and Morgan) is wrong because Mendel described inheritance laws without knowing about chromosomes, and Morgan later provided genetic linkage evidence; Option D (Bateson and Punnett) is wrong because they developed the Punnett square and studied gene linkage, not the chromosomal theory.

Correct Answer: D. Color blindness

• **Color blindness** = Colour blindness is a sex-linked recessive disorder caused by a defective gene on the X chromosome; males (XY) have only one X, so a single defective copy causes the condition, while females need two defective copies to be affected. • **Carrier females** — A woman with one defective allele (X^b X) is a carrier and appears normal but can pass the condition to her sons. • About 8% of males and only 0.5% of females are colour-blind, reflecting this X-linked pattern. • 💡 Option A (Cystic fibrosis) is wrong because it is an autosomal recessive disorder carried on chromosome 7, affecting males and females equally; Option B (Sickle cell anaemia) is wrong because it is also autosomal recessive — located on chromosome 11, not on sex chromosomes; Option C (Down syndrome) is wrong because it results from trisomy 21 (an extra chromosome 21), which is a chromosomal number abnormality, not a sex-linked gene disorder.

Correct Answer: C. Plasmids

• **Plasmids** = Plasmids are small, circular, double-stranded DNA molecules that exist separately from the bacterial chromosome and replicate autonomously — they often carry genes conferring antibiotic resistance or other survival advantages. • **Vectors in genetic engineering** — Scientists use plasmids as vectors to insert foreign genes into bacteria, enabling mass production of useful proteins like insulin. • Plasmids can also be transferred between bacteria via conjugation, spreading antibiotic resistance rapidly. • 💡 Option A (Introns) is wrong because introns are non-coding sequences within a gene that are spliced out of pre-mRNA — they are part of the chromosome, not separate circular molecules; Option B (Histones) is wrong because histones are proteins around which DNA wraps to form chromatin — they are not DNA molecules themselves; Option D (Nucleosomes) is wrong because nucleosomes are the bead-like units of chromatin (DNA + histone octamer), not independent small circular DNA.

Correct Answer: B. Polyploid

• **Polyploid** = A polyploid organism has more than two complete sets of chromosomes (3n, 4n, 6n etc.) — this condition is very common in flowering plants and is a major driver of plant speciation. • **Agricultural importance** — Many crops like wheat (hexaploid, 6n = 42) and strawberry (octoploid, 8n = 56) are polyploids, which often results in larger fruits, greater yield, and improved hardiness. • Polyploidy is rare in animals; in plants it can arise naturally from errors in meiosis or from hybridisation between species. • 💡 Option A (Aneuploid) is wrong because aneuploidy means having an abnormal number of individual chromosomes (e.g., trisomy 21 = 47) — not complete extra sets; Option C (Diploid) is wrong because diploid simply means having the normal two sets of chromosomes (2n); Option D (Haploid) is wrong because haploid means having only one set of chromosomes (n), as found in gametes.

Correct Answer: A. tRNA

• **tRNA (Transfer RNA)** = tRNA is the adaptor molecule that physically carries specific amino acids to the ribosome and matches each codon on the mRNA with the correct amino acid via its anticodon loop. • **Each tRNA is amino acid-specific** — There are at least 20 types of tRNA (one per amino acid), and each is charged by a specific aminoacyl-tRNA synthetase enzyme. • This charging reaction uses energy from ATP, ensuring fidelity in protein synthesis. • 💡 Option B (DNA) is wrong because DNA remains in the nucleus as the permanent template — it does not travel to ribosomes; Option C (mRNA) is wrong because mRNA carries the coded instructions from DNA to the ribosome but does not itself deliver amino acids; Option D (rRNA) is wrong because ribosomal RNA forms the structural and catalytic core of the ribosome — it does not transport amino acids.

Correct Answer: D. Genes

• **Genes** = A gene is a specific segment of DNA that encodes the instructions for producing a functional protein or RNA molecule — genes are the fundamental units of heredity passed from parent to offspring. • **Human genome has ~20,000–25,000 genes** — These are distributed across 46 chromosomes, but genes account for only about 1–2% of total DNA; the rest is regulatory or non-coding. • Genes were called 'factors' by Mendel before the discovery of DNA; W. Johannsen coined the term 'gene' in 1909. • 💡 Option A (Nucleotides) is wrong because nucleotides are the individual building-block units (sugar + phosphate + base) of DNA and RNA — they are the alphabet, not the word; Option B (Proteins) is wrong because proteins are the products encoded by genes, not the hereditary units themselves; Option C (Chromosomes) is wrong because chromosomes are the thread-like structures that contain thousands of genes — a chromosome is the storage structure, not the unit of inheritance.

Correct Answer: D. Genotype

• **Genotype** = The genotype is the complete genetic constitution of an organism — represented by letter pairs like AA, Aa, or aa — specifying which alleles an individual carries for any given trait. • **Hidden blueprint** — Two organisms with the same phenotype (e.g., tall plants) can have different genotypes (TT or Tt), which is why genotype reveals what can be inherited by offspring. • Genotype is determined by methods such as test crosses, DNA sequencing, or molecular markers. • 💡 Option A (Karyotype) is wrong because karyotype is a photograph of an individual's chromosomes arranged by size and shape — it shows structure, not allele identity; Option B (Archetype) is wrong because archetype is a concept from literature and psychology meaning an original model — it has no meaning in genetics; Option C (Phenotype) is wrong because phenotype describes the observable traits that result from the genotype's interaction with the environment.

Correct Answer: A. Pleiotropy

• **Pleiotropy** = Pleiotropy occurs when a single gene influences two or more seemingly unrelated phenotypic traits — one gene, many effects — because the protein it encodes plays a role in multiple biological pathways. • **Sickle cell anaemia example** — The single HBB gene mutation causes abnormal haemoglobin, which pleiotropically affects red blood cells, oxygen delivery, joints, kidneys, and the spleen all at once. • Marfan syndrome is another classic example, where one fibrillin gene mutation affects the heart, eyes, and skeletal system. • 💡 Option B (Epistasis) is wrong because epistasis is when one gene masks or suppresses the expression of a different gene at another locus; Option C (Codominance) is wrong because codominance means both alleles of a gene are fully expressed simultaneously in the heterozygote (e.g., AB blood type); Option D (Polygenic inheritance) is wrong because it describes multiple genes collectively controlling one trait (e.g., skin colour) — the opposite situation to pleiotropy.

Correct Answer: B. Adenine

• **Adenine** = Adenine is a purine — a nitrogenous base with a fused double-ring structure (a pyrimidine ring fused to an imidazole ring) — and is found in both DNA and RNA. • **Purines vs Pyrimidines** — Purines (Adenine, Guanine) have two rings; Pyrimidines (Cytosine, Thymine, Uracil) have a single ring — purines always pair with pyrimidines in nucleic acids. • Adenine also forms part of ATP (adenosine triphosphate), the cell's primary energy currency. • 💡 Option A (Cytosine) is wrong because Cytosine is a pyrimidine with a single-ring structure; Option C (Thymine) is wrong because Thymine is also a pyrimidine — it is DNA-specific and has a single ring; Option D (Uracil) is wrong because Uracil is an RNA-specific pyrimidine with a single ring that replaces Thymine.