Genetics and Molecular Inheritance — Practice

Question 1

The phenotypic ratio of the F₂ generation in a Mendelian monohybrid cross is:
(a) 3:1 (b) 1:2:1 (c) 9:3:3:1 (d) 1:1

Accepted Answer

(a) 3:1.

The F₂ phenotypic ratio is 3 dominant : 1 recessive. The underlying genotypic ratio is 1:2:1 (1 homozygous dominant : 2 heterozygous : 1 homozygous recessive).

Question 2

The F₂ phenotypic ratio of a dihybrid cross is:
(a) 9:3:3:1 (b) 3:1 (c) 1:2:1 (d) 1:1:1:1

Accepted Answer

(a) 9:3:3:1.

A dihybrid F₂ shows 9 both-dominant : 3 : 3 : 1 both-recessive, reflecting independent assortment of two gene pairs on different chromosomes.

Question 3

A dihybrid plant of genotype RrYy produces how many types of gametes?
(a) four (b) two (c) one (d) eight

Accepted Answer

(a) four.

RrYy produces four gamete types — RY, Ry, rY, ry — in equal proportion, because the two gene pairs assort independently. This is why a dihybrid test cross gives 1:1:1:1.

Question 4

In Mirabilis jalapa, red (RR) × white (rr) gives pink (Rr) flowers. This is an example of:
(a) incomplete dominance (b) codominance (c) epistasis (d) complete dominance

Accepted Answer

(a) incomplete dominance.

The heterozygote is intermediate (pink) because neither allele fully dominates. The F₂ ratio is 1 red : 2 pink : 1 white, where phenotype equals genotype (1:2:1).

Question 5

The AB blood group in humans is an example of:
(a) codominance (b) incomplete dominance (c) pleiotropy (d) epistasis

Accepted Answer

(a) codominance.

Alleles I^A and I^B are codominant, so an I^A I^B individual expresses both A and B antigens fully (blood group AB). ABO also illustrates multiple alleles (I^A, I^B, i).

Question 6

In humans, the sex of the child is determined by:
(a) the father's sperm (b) the mother's egg (c) the number of autosomes (d) temperature

Accepted Answer

(a) the father's sperm.

The mother (XX) always gives an X; the father (XY) gives either X (→ girl) or Y (→ boy). So the sperm's sex chromosome determines the child's sex.

Question 7

Haemophilia and colour blindness in humans are inherited as:
(a) X-linked recessive traits (b) autosomal dominant traits (c) Y-linked traits (d) mitochondrial traits

Accepted Answer

(a) X-linked recessive traits.

Both genes lie on the X chromosome and are recessive, so they appear far more often in males, who have only one X and thus express a single defective allele.

Question 8

Down's syndrome is caused by:
(a) trisomy of chromosome 21 (b) monosomy of the X chromosome (c) an extra X in a male (d) deletion of chromosome 5

Accepted Answer

(a) trisomy of chromosome 21.

An extra copy of chromosome 21 (47 chromosomes) causes Down's syndrome. Klinefelter's is XXY (47); Turner's is XO (45).

Question 9

In a DNA sample, adenine is 20% of the bases. The percentage of cytosine is:
(a) 30% (b) 20% (c) 40% (d) 60%

Accepted Answer

(a) 30%.

By Chargaff's rule A = T = 20%, together 40%. The remaining 60% is split equally between G and C, so C = 30% (and G = 30%).

Question 10

The number of hydrogen bonds between guanine and cytosine in DNA is:
(a) three (b) two (c) one (d) four

Accepted Answer

(a) three.

G pairs with C via three hydrogen bonds, while A pairs with T via two. Higher G–C content therefore makes DNA more thermally stable.

Question 11

The Meselson–Stahl experiment established that DNA replication is:
(a) semiconservative (b) conservative (c) dispersive (d) bidirectional only

Accepted Answer

(a) semiconservative.

Using ¹⁵N labelling in E. coli, Meselson and Stahl showed each daughter DNA molecule keeps one parental strand and one newly made strand — semiconservative replication.

Question 12

The short DNA segments synthesised discontinuously on the lagging strand are called:
(a) Okazaki fragments (b) exons (c) introns (d) primers

Accepted Answer

(a) Okazaki fragments.

Because DNA polymerase synthesises only 5′→3′, the lagging strand is built as short Okazaki fragments that DNA ligase later joins into a continuous strand.

Question 13

In RNA, the base that replaces thymine of DNA is:
(a) uracil (b) cytosine (c) guanine (d) adenine

Accepted Answer

(a) uracil.

RNA contains uracil (U) in place of thymine (T); U pairs with adenine. The mRNA sequence otherwise matches the coding strand of DNA.

Question 14

The genetic code is said to be degenerate because:
(a) most amino acids are coded by more than one codon (b) one codon codes many amino acids (c) it differs in every species (d) it is read two bases at a time

Accepted Answer

(a) most amino acids are coded by more than one codon.

There are 64 codons but only 20 amino acids (61 coding + 3 stop), so most amino acids have several codons — the code's degeneracy. It is also universal, non-overlapping and comma-less.

Question 15

In the lac operon of E. coli, the addition of lactose switches transcription on because lactose:
(a) binds the repressor and inactivates it (b) binds the operator (c) activates RNA polymerase directly (d) removes the promoter

Accepted Answer

(a) binds the repressor and inactivates it.

Lactose (the inducer) binds the repressor protein and changes its shape so it can no longer bind the operator. RNA polymerase then transcribes the structural genes — the operon is switched on (inducible).

Practice — Genetics and Molecular Inheritance

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