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

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.

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.

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

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).

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.

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.

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

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.

• Multiple Alleles - More than two alternative forms of a gene in a population occupying the same locus on homologous chromosomes; e.g. ABO blood group in humans.
• Origin - Multiple alleles arise by repeated mutations of the wild-type gene; the wild type is dominant over all other mutant alleles.
• Dominance - Different alleles in a series may show dominant-recessive relationship, co-dominance, or incomplete dominance among themselves.
• Example in Drosophila - Wing size is controlled by multiple alleles; Normal wings (vg⁺) → Nicked (vgⁿⁱ) → Notched (vgⁿº) → Strap (vgˢᵗ) → Vestigial (vg); normal wing is wild type (dominant), vestigial is recessive.

• 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.

• Sex determination: It is the mechanism by which an organism develops into a male or a female based on genetic factors.
• Types of organisms: Organisms may be bisexual (hermaphrodite), having both sex organs, or unisexual (dioecious), like humans, with separate sexes.
• Discovery: Henking (1891) discovered the X-body, later identified as the X chromosome involved in sex determination.
• XX–XY system: Females are XX (homogametic) and males are XY (heterogametic), seen in humans and Drosophila.
• ZW–ZZ system: Females are ZW (heterogametic) and males are ZZ (homogametic), seen in birds and some reptiles.
• Haplodiploidy: In honeybees, unfertilized eggs develop into haploid males and fertilised eggs into diploid females.

• Sex-linked inheritance: It is the inheritance of genes located on sex chromosomes (X and Y) from parents to offspring.
• X-linked genes: These genes are present on the X chromosome and usually do not have corresponding alleles on the Y chromosome.
• Expression in males and females: X-linked recessive traits appear more in males (one X chromosome), while females need two recessive alleles; females with one allele are carriers.
• Examples of X-linked traits: Haemophilia, colour blindness, muscular dystrophy, and night blindness.
• Y-linked genes: These genes are present on the Y chromosome and are passed directly from father to son (e.g. hypertrichosis).

• Pedigree Analysis is the study of inheritance patterns of traits across several generations of a family; the chart representing this is called a family tree (pedigree).
• Since controlled crosses (like in pea plants) cannot be done in humans, family history analysis is used as an alternative to study inheritance.
• Pedigree analysis is a powerful tool in human genetics used to trace the inheritance of a specific trait, abnormality or disease.
• Standard symbols used - Square = normal male, Circle = normal female, Diamond = sex unspecified; filled/shaded shapes = affected individuals.
• A horizontal line between two symbols = mating; a double horizontal line = consanguineous mating (mating between relatives); children are shown below parents in order of birth from left to right.