Biology — Reproduction, Genetics, Evolution, Biotechnology & Ecology — Practice

Question 1

What is double fertilisation? Why is it unique to angiosperms?

Accepted Answer

Double fertilisation is the fusion of TWO male gametes with TWO female structures in the embryo sac. (1) First sperm + egg cell → ZYGOTE (2n) → embryo. (2) Second sperm + TWO POLAR NUCLEI → Primary Endosperm Nucleus (PEN, 3n) → endosperm (nourishes embryo). This is unique to angiosperms. In gymnosperms, there is only SINGLE fertilisation — one sperm fuses with the egg. The triploid endosperm (3n) formed in angiosperms is nutritive tissue that feeds the developing embryo.

Question 2

In guinea pigs, rough coat (R) is dominant over smooth (r) and black colour (B) is dominant over white (b). Cross a rough black guinea pig (RrBb) with a smooth white guinea pig (rrbb). What is the expected ratio of phenotypes in offspring?

Accepted Answer

Cross: RrBb × rrbb (test cross). Using dihybrid test cross: gametes of RrBb = RB, Rb, rB, rb (equal probability). Gametes of rrbb = rb only. Offspring: (1) RrBb (rough black): 1, (2) Rrbb (rough white): 1, (3) rrBb (smooth black): 1, (4) rrbb (smooth white): 1. Phenotypic ratio = 1 rough black : 1 rough white : 1 smooth black : 1 smooth white = 1:1:1:1. This is a classic DIHYBRID TEST CROSS.

Question 3

Describe the Polymerase Chain Reaction (PCR). State ONE advantage and TWO applications.

Accepted Answer

PCR (Polymerase Chain Reaction) is an in vitro technique to AMPLIFY a specific DNA sequence exponentially. STEPS (repeated 25-30 cycles): (1) DENATURATION (~94°C): Heat separates the two DNA strands. (2) ANNEALING (~55°C): Short synthetic oligonucleotide PRIMERS anneal to complementary sequences flanking the target region on each strand. (3) EXTENSION (~72°C): Thermostable Taq polymerase (from Thermus aquaticus) extends the primers, synthesising new complementary strands. Each cycle DOUBLES the target DNA. After 30 cycles: ~1 billion copies from a single molecule. ADVANTAGE: Requires extremely SMALL amounts of starting DNA — even a single cell or degraded ancient DNA is sufficient. APPLICATIONS: (1) FORENSICS — DNA fingerprinting of crime scene samples. (2) DIAGNOSIS — detection of HIV, TB, COVID-19, and other pathogens. (3) Amplifying ancient DNA (paleogenomics). (4) Prenatal diagnosis of genetic disorders.

Practice — Biology — Reproduction, Genetics, Evolution, Biotechnology & Ecology

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