Revision: Principles of Inheritance and Variation Zoology HSC Science Class 12 Tamil Nadu Board of Secondary Education

Definitions [11]

Definition: Multiple alleles

Multiple alleles are the three or more alternative forms of the same gene that occupy the same locus on homologous chromosomes and control the same character in a population, though only two alleles occur together in an individual.

Definition: Blood Transfusion

Blood transfusion is the process of introducing blood from a healthy donor into the bloodstream of a patient, typically through a vein, often during surgery or after heavy blood loss.

Definition: Blood Recipient

A Blood Recipient is a person who receives blood during a transfusion, requiring compatibility with the donor's blood group.

Definition: Antigens

Antigens are specific proteins present on the surface of red blood cells that determine an individual’s blood group (e.g., Antigen A or Antigen B).

Definition: Antibodies

Antibodies are proteins present in blood plasma that react against specific antigens not found on the individual's own red blood cells, playing a key role in blood group compatibility.

Definition: Blood Donor

A Blood Donor is a person who voluntarily gives blood to be transfused into another person in need.

Definition: Universal Donor

A person with blood group O is called a universal donor because their blood can be safely transfused to individuals of all major blood groups (A, B, AB, and O).

Definition: Universal Recipient

A person with blood group AB is called a universal recipient because they can receive blood from all major blood groups (A, B, AB, and O) without risk of incompatibility.

Define the following term:

Rh factor

Rhesus factor is a hereditary protein present in red blood cells. If present, the individual is Rh-positive; if absent, they are Rh-negative.

Definition: Sex determination

The biological mechanism by which the sex (male or female) of an individual is established based on genetic or chromosomal factors, is called sex determination.

Definition: Pedigree analysis

The study of inheritance of a particular trait in humans by collecting information from family history and representing it through a family tree using standard symbols is called pedigree analysis.

Key Points

Key Point: Blood Transfusion and Blood Groups

ABO System: Introduced by Karl Landsteiner (1900). Based on the presence/absence of antigen A and B on RBCs, four groups: A, B, AB, O.
Universal Donor & Acceptor: AB = universal acceptor (both antigens, no antibodies); O = universal donor (no antigens, both antibodies).
Rh Factor: Discovered by Landsteiner & Wiener (1940). 80–85% people are Rh⁺; the rest are Rh⁻.
HDN (Erythroblastosis Foetalis): When a Rh⁻ mother carries a Rh⁺ foetus, she produces anti-Rh antibodies (after the first delivery), which attack subsequent Rh⁺ foetuses.
Prevention: Rh⁻ mother is injected with anti-D antibodies during all pregnancies with Rh⁺ foetus to prevent HDN.

Key Points: Chromosomal Disorders or Abnormalities

Chromosomal Disorder	Chromosomal Abnormality	Karyotype	Major Features	Remarks
Down’s Syndrome	Extra copy of chromosome 21 (Trisomy 21)	47, +21	Short stature, small round head, furrowed tongue, partially open mouth, broad palm with single crease, mental and physical retardation	Most common autosomal aneuploidy
Klinefelter’s Syndrome	Extra X chromosome in males	47, XXY	Masculine body with feminine traits (gynecomastia), underdeveloped testes, sterile	Sex-chromosome aneuploidy
Turner’s Syndrome	Absence of one X chromosome	45, XO	Short stature, rudimentary ovaries, absence of secondary sexual characters, sterile female	Only monosomy compatible with life
Aneuploidy	Gain or loss of one or more chromosomes due to non-disjunction	2n ± 1	Severe developmental abnormalities	Includes trisomy and monosomy
Polyploidy	Increase in whole set of chromosomes due to failure of cytokinesis	3n, 4n, etc.	Common in plants; increases size and vigor	Rare and usually lethal in animals