(a) What is polygenic inheritance? Explain with the help of a suitable example.

(b) How are pleiotropy and Mendelian pattern of inheritance different from polygenic pattern of inheritance?

OR

(a) Name the stage in the cell cycle where DNA replication occurs.

(b) Explain the mechanism of DNA replication. Highlight the role of enzymes in the process.

(c) Why is DNA replication said to be semiconservative?

a) • A type of inheritance in which the traits are controlled by three or more genes is known as polygenic inheritance.

• It is a type of inheritance that is dependent upon the collective or cumulative effect of the alleles.

• Greater the number of dominant alleles, more distinct is the trait expression.

• Greater the number of recessive alleles, less distinct and more diluted is the trait expression.

• If the number of dominant and recessive alleles is equal then the effect is intermediate.

• Example: human skin colour and human heights are examples.

b) In case of pleiotropy, a single gene controls multiple phenotypic expression whereas in case of Mendelian pattern of inheritance, a single gene is responsible for controlling a single phenotypic expression.

OR

a) It is during the S phase or the synthetic phase of Interphase that DNA replication occurs.

b) • The replication begins in definite regions which are called the origin of replication.

• Replication occurs within a small opening of the DNA referred to as Y shaped replication fork (uncoiling of DNA is by some enzymes eg Helicase and topoisomerase.)

• Polymerisation of the nucleotides are catalysed by DNA dependent DNA polymerase in 5' -3' directions.

• Deoxyribonucleotides act as substrates and also provide energy for the process.

• The new strands formed on 3 '-5' template is continuous.

• New strands formed on 5-'3' template is discontinuous.

• The discontinuously formed fragments are joined by enzyme DNA ligase.

c) During DNA replication in the two newly synthesised daughter DNA one strand is parental (conserved) and the other is newly synthesised.