Genetics — Set 4

Correct Answer: A. 47

• **47** = Down Syndrome is caused by Trisomy 21 — an extra copy of chromosome 21 is present due to non-disjunction during meiosis. • **Three copies of chromosome 21** — instead of the usual two, giving a total of 47 chromosomes. • This is the most common chromosomal disorder in live births and causes intellectual disability and distinct physical features. • 💡 Option B (45) is wrong because 45 chromosomes characterises Turner Syndrome (45,X), not Down Syndrome; Option C (48) is wrong because 48 chromosomes occur in rarer aneuploidies like XXYY syndrome; Option D (46) is wrong because 46 is the normal diploid count in a healthy human.

Correct Answer: B. AB+

• **AB+** = AB+ individuals have A, B, and Rh antigens on their red blood cells and carry no anti-A, anti-B, or anti-Rh antibodies in their plasma. • **Can receive from all ABO and Rh groups** — because they will not mount an immune reaction against any of the four common blood types. • The AB+ group is the rarest (about 3% of population) yet medically the most versatile as a recipient. • 💡 Option A (O-) is wrong because O- is the Universal Donor, not recipient — O- individuals have antibodies against A, B, and Rh; Option C (O+) is wrong because O+ people cannot accept Rh-negative blood safely and carry anti-A and anti-B antibodies; Option D (AB-) is wrong because AB- individuals cannot accept Rh+ blood, making them a restricted recipient.

Correct Answer: B. Wilhelm Johannsen

• **Wilhelm Johannsen** = In 1909 he coined the word 'gene' to replace Mendel's vague term 'element' for the hereditary unit. • **Also coined 'genotype' and 'phenotype'** — two foundational terms that separated the genetic blueprint from its physical expression. • Johannsen was a Danish botanist and his precise vocabulary helped standardise the language of genetics worldwide. • 💡 Option A (Gregor Mendel) is wrong because Mendel used the word 'element', not 'gene', in his 1865 pea-plant experiments; Option C (Hugo de Vries) is wrong because de Vries proposed the Mutation Theory, not the naming of the gene; Option D (Thomas Hunt Morgan) is wrong because Morgan is credited with discovering sex-linked inheritance and gene linkage through his Drosophila work.

Correct Answer: B. Hydrogen bonds

• **Hydrogen bonds** = Hydrogen bonds form between complementary nitrogenous bases — A pairs with T (2 H-bonds) and G pairs with C (3 H-bonds). • **Weak individually, strong collectively** — thousands of H-bonds along the helix provide stability while still allowing the strands to unzip during replication and transcription. • The base-pairing specificity (Chargaff's rules) is maintained entirely by these hydrogen bonds. • 💡 Option A (Ionic bonds) is wrong because ionic bonds involve charged ions and are not responsible for base pairing in DNA; Option C (Peptide bonds) is wrong because peptide bonds join amino acids in proteins, not nucleotides in DNA; Option D (Covalent bonds) is wrong because covalent bonds do exist in DNA — but they form the sugar-phosphate backbone of each individual strand, not the inter-strand connection.

Correct Answer: A. Separate from each other

• **Separate from each other** = Mendel's First Law states that the two alleles of a gene separate during meiosis so that each gamete carries only one allele. • **Each offspring receives one allele from each parent** — the combination at fertilisation determines the offspring's genotype. • This law explains why a heterozygous (Tt) parent can produce both T and t gametes in a 1:1 ratio. • 💡 Option B (Are always dominant) is wrong because alleles are neither always dominant nor recessive — dominance is a relationship between two alleles, not a property of alleles during segregation; Option C (Fuse together) is wrong because alleles segregate apart during meiosis and do not fuse — fusion would eliminate allele diversity; Option D (Multiply in number) is wrong because allele number does not increase during gamete formation; the diploid pair simply separates into two haploid alleles.

Correct Answer: A. Fruit fly

• **Fruit fly (Drosophila melanogaster)** = Morgan chose Drosophila because it has a 14-day life cycle, only 4 pairs of chromosomes, and produces hundreds of offspring per mating. • **Discovered gene linkage and sex-linked inheritance** — his work on white eye and other visible mutations provided the first chromosomal maps. • Morgan won the Nobel Prize in Physiology or Medicine in 1933 for his work establishing that genes are carried on chromosomes. • 💡 Option B (Mouse) is wrong because mice have long gestation periods and fewer offspring, making them impractical for rapid genetic studies of the era; Option C (Pea plant) is wrong because pea plants were used by Gregor Mendel — not Morgan — and they deal with single-gene traits, not linkage; Option D (Bacteria) is wrong because bacterial genetics was pioneered later by scientists like Griffith, Avery, and Lederberg, not by Morgan.

Correct Answer: B. A functional unit of a gene

• **A functional unit of a gene** = A cistron is the smallest segment of DNA that codes for a single polypeptide chain, defined by the cis-trans (complementation) test. • **Equivalent to a gene in molecular terms** — the term was introduced by Seymour Benzer to define the functional genetic unit more precisely than older definitions. • Cistron, recon, and muton are three levels of genetic fine-structure identified by Benzer's rII phage experiments. • 💡 Option A (A part of a ribosome) is wrong because ribosomes are made of rRNA and proteins — they are the site of translation but have no cistron component; Option C (A non-coding DNA) is wrong because non-coding sequences are called introns or junk DNA — a cistron is by definition a coding unit; Option D (A type of cell) is wrong because a cistron is a molecular genetic unit, not a cellular structure of any kind.

Correct Answer: C. Translation

• **Translation** = Translation is the process by which ribosomes read the codons in mRNA and link the corresponding amino acids into a polypeptide chain. • **Requires tRNA and rRNA** — tRNA brings the correct amino acid to each codon, while rRNA forms the catalytic core of the ribosome. • Translation is the final step of the central dogma: DNA → RNA → Protein. • 💡 Option A (Replication) is wrong because replication is the copying of DNA into another DNA molecule — it produces no protein; Option B (Splicing) is wrong because splicing removes introns from pre-mRNA to produce mature mRNA — it is an RNA processing step, not protein synthesis; Option D (Transcription) is wrong because transcription converts DNA into mRNA — it precedes translation and does not itself produce protein.

Correct Answer: A. UGA

• **UGA** = UGA is one of the three stop codons (UAA, UAG, UGA) that signal the ribosome to terminate translation and release the finished polypeptide. • **No tRNA matches stop codons** — instead, release factors bind the ribosome and trigger polypeptide release. • UGA is also called 'Opal' codon; UAA is 'Ochre', and UAG is 'Amber' — traditional names used in molecular biology. • 💡 Option B (AUG) is wrong because AUG is the Start codon that codes for the amino acid Methionine and initiates translation — it is the opposite of a stop codon; Option C (AAA) is wrong because AAA codes for the amino acid Lysine and is a sense codon, not a termination signal; Option D (GUA) is wrong because GUA codes for the amino acid Valine and participates in protein building rather than ending it.

Correct Answer: B. Is expressed in the heterozygote

• **Is expressed in the heterozygote** = A dominant allele masks the effect of the recessive allele when both are present together (heterozygous condition Aa). • **Only one dominant copy needed** — in Mendel's pea plants, one allele for round seeds (R) was enough to produce round seeds even alongside the wrinkled allele (r). • Dominance is about expression, not health: dominant traits can be normal (tongue rolling) or disease-causing (Huntington's disease). • 💡 Option A (Always causes disease) is wrong because many dominant traits — like free earlobes or dark hair — are completely healthy phenotypes; Option C (Is only found in males) is wrong because dominance applies equally to both sexes; sex-limited expression is a different concept entirely; Option D (Is never inherited) is wrong because dominant traits are the most visibly inherited — they appear in every generation that carries even one copy of the allele.