Human skin colour is an example of polygenic (quantitative) inheritance, controlled by three pairs of genes:

A/a
B/b
C/c

Each dominant allele (A, B, C) contributes one unit of pigment, while each recessive allele (a, b, c) contributes no pigment.

The amount of melanin produced depends on the total number of dominant alleles.

Transcription is the process by which the genetic information present in one strand of DNA (the template strand) is copied into an RNA molecule by the enzyme RNA polymerase.

It is the first step of gene expression, producing messenger RNA (mRNA), transfer RNA (tRNA), or ribosomal RNA (rRNA), depending on the gene.

This question is based on the Lac Operon model proposed by Jacob and Monod, explaining gene regulation in E. coli.

Correct Matching

Gene: Encodes (Function)
i: Repressor protein (Binds the operator and blocks transcription in the absence of lactose)
z: β-Galactosidase (Breaks lactose into glucose and galactose)
y: Permease (Facilitates the entry of lactose into the bacterial cell)
a: Transacetylase (Transfers acetyl groups; its exact physiological role is less significant in lactose metabolism)

Aneuploidy is a chromosomal abnormality in which an individual has one or more extra or missing chromosomes due to nondisjunction during meiosis.

Among the given disorders:
(A) Haemophilia
A sex-linked recessive genetic disorder caused by mutation in genes responsible for blood clotting factors.
It is a single-gene disorder, not due to aneuploidy.
(B) Down's Syndrome
Caused by Trisomy 21 (an extra copy of chromosome 21).
It is a classic example of aneuploidy.
(C) Thalassemia
An autosomal recessive single-gene disorder affecting haemoglobin synthesis.
It is not caused by a change in chromosome number.
(D) Turner's Syndrome
Caused by monosomy X (45, X).
Individuals have only one X chromosome instead of two.
It is an example of aneuploidy.