TG Inter 2nd Year Zoology - Units & Chapters

UNIT V: REPRODUCTION

UNIT VI: GENETICS

UNIT VII: ORGANIC EVOLUTION

UNIT VIII: APPLIED BIOLOGY

Unit VI: Genetics - Important Long Answer Questions

Q1. Explain Morgan's experiment on linkage with Drosophila. What conclusions did he draw?

• Morgan crossed a dihybrid fly (grey body, long wings - BVBV) with a double recessive fly (black body, vestigial wings - bvbv).
• Expected a 1:1:1:1 ratio in testcross offspring (Mendelian expectation).
• Actual result: two parental types (grey-long and black-vestigial) appeared in far greater numbers, and two recombinant types (grey-vestigial and black-long) appeared in very small numbers.

1. The genes for body colour and wing type are located on the same chromosome - this is called linkage.
2. Because they are linked, they tend to be inherited together rather than independently (violation of Mendel's Law of Independent Assortment).
3. The small percentage of recombinant types is due to crossing over during meiosis, where homologous chromosomes exchange segments.
4. The frequency of recombination (crossing over %) is used to calculate the distance between genes on a chromosome (genetic mapping).

Q2. Write about ABO blood groups and their inheritance. Add a note on Erythroblastosis Foetalis.

• I^A and I^B are codominant to each other (both expressed in AB group).
• Both I^A and I^B are dominant over i.

• People with Rh antigen on RBCs are Rh+ (positive); those without are Rh- (negative).
• Rh factor is controlled by a dominant gene (R).

1. An Rh- mother carries an Rh+ foetus (first pregnancy).
2. At delivery, foetal Rh+ RBCs enter the mother's bloodstream.
3. The mother's immune system produces anti-Rh antibodies (sensitization).
4. In the second pregnancy, if the foetus is again Rh+, the mother's anti-Rh antibodies cross the placenta.
5. These antibodies attack and destroy the foetal RBCs, causing haemolysis.
6. The foetus develops anaemia, jaundice, oedema, and in severe cases, death.

Q3. Describe the structure of DNA (Double Helix Model) as proposed by Watson and Crick.

1. Two strands: DNA consists of two polynucleotide chains wound around each other in a right-handed double helix.
2. Nucleotide structure: Each nucleotide has three components:
   • A deoxyribose sugar
   • A phosphate group
   • A nitrogenous base (Adenine, Guanine, Thymine, Cytosine)
3. Backbone: The sugar-phosphate groups form the outer backbone of each strand, connected by 3'-5' phosphodiester bonds.
4. Base pairing (Chargaff's Rule):
   • Adenine (A) pairs with Thymine (T) - 2 hydrogen bonds
   • Guanine (G) pairs with Cytosine (C) - 3 hydrogen bonds
   • The two strands are therefore complementary to each other.
5. Antiparallel: The two strands run in opposite directions - one in 5'→3' direction and the other in 3'→5' direction.
6. Dimensions:
   • The helix has a diameter of 2 nm
   • Each complete turn spans 3.4 nm (34 Å)
   • There are 10 base pairs per turn, each separated by 0.34 nm
7. Major and minor grooves alternate along the helix surface.

Q4. Explain the process of DNA Replication (Semi-conservative replication).

• Replication begins at specific sequences called origins of replication.
• The enzyme helicase unwinds and separates the two strands by breaking hydrogen bonds, forming a replication fork.
• SSB proteins (single-strand binding proteins) stabilize the separated strands.

• Primase synthesizes a short RNA primer complementary to the template strand, providing a free 3'-OH end for DNA polymerase to begin.

• DNA Polymerase III adds new deoxyribonucleotides in the 5'→3' direction only.
• On the leading strand (3'→5' template): synthesis is continuous.
• On the lagging strand (5'→3' template): synthesis is discontinuous, producing Okazaki fragments.

• DNA Polymerase I removes RNA primers and replaces them with DNA nucleotides.

• DNA Ligase joins the Okazaki fragments and seals any nicks, completing the lagging strand.

Q5. Explain sex determination in humans. Add a note on Barr bodies.

• 22 pairs of autosomes (same in both sexes)
• 1 pair of sex chromosomes

• Males are the heterogametic sex (produce two types of gametes: X and Y).
• Females are the homogametic sex (produce one type of gamete: X).

• If an X-bearing sperm fertilizes the egg → XX → Female
• If a Y-bearing sperm fertilizes the egg → XY → Male
• Therefore, the father determines the sex of the child.

• In 1949, Murray Barr discovered darkly staining chromatin bodies in the nuclei of female cat neurons.
• These are called Barr bodies (or sex chromatin).
• A Barr body is an inactivated X chromosome (condensed and transcriptionally silent).
• Lyon's hypothesis: In any cell with more than one X chromosome, all extra X chromosomes are inactivated randomly and permanently - this is called lyonization.
• Number of Barr bodies = (Number of X chromosomes - 1)
  • Normal female (XX): 1 Barr body
  • Normal male (XY): 0 Barr bodies
  • Klinefelter's (XXY): 1 Barr body
  • Turner's (X0): 0 Barr bodies

Q6. What is DNA Fingerprinting? Explain the technique and its applications.

• Human DNA contains short, repetitive sequences called VNTRs (Variable Number of Tandem Repeats) or microsatellites/STRs.
• The number of repeats varies between individuals, making the pattern unique (except in identical twins).

1. DNA Extraction: DNA is extracted from the sample (blood, saliva, hair, semen, etc.).
2. Restriction Digestion: DNA is cut into fragments using restriction endonucleases.
3. Gel Electrophoresis: Fragments are separated by size on an agarose gel - smaller fragments move faster.
4. Southern Blotting: DNA fragments are transferred from gel to a nitrocellulose membrane.
5. Hybridization: A radioactive probe (complementary to VNTR sequences) is added. It binds to specific fragments.
6. Autoradiography: An X-ray film is exposed to the membrane. Dark bands appear where the probe has bound, creating the "fingerprint" pattern.

1. Forensic science - identifying criminals from crime scene samples
2. Paternity/maternity disputes - confirming biological parentage
3. Immigration disputes - verifying family relationships
4. Identifying disaster victims
5. Studying genetic diversity and population genetics
6. Identifying missing persons

Exam Tip: Questions 1, 2, 4, and 5 are the most frequently repeated in TSBIE exams. Make sure you can draw the DNA double helix diagram and the ABO blood group inheritance table for full marks.