Fundamentals of Heredity and Genetic Principles

Core Concepts in Classical Genetics

Genetics:

The branch of biology responsible for the transmission of hereditary traits from one generation to the next.

Gene:

A fragment of DNA information that determines a character. It produces a protein responsible for determining a character, such as eye color or hair color.

Alleles:

The different variants of a gene.

Dominant Inheritance:

Occurs when one gene has greater strength to manifest itself than another gene.

Dominant Gene:

The allele that has more power to manifest itself (expressed even in heterozygotes).

Recessive Gene:

The allele that has less force to become apparent (expressed only in homozygotes).

Intermediate Inheritance:

The pattern in which both genes have the same force to manifest themselves, resulting in a blended phenotype (e.g., incomplete dominance).

Homozygous Organism (Pure):

An organism in which the two alleles that determine a character are identical (e.g., AA or aa).

Heterozygous Organism (Hybrid):

An organism in which the two alleles that determine a character are not equal (e.g., Aa).

Genotype:

The set of genes an organism possesses, whether those genes are expressed or not (represented by letters).

Phenotype:

The set of observable characteristics resulting from the interaction of the genotype and the environment (what is physically expressed).

Gregor Mendel and the Laws of Inheritance

Mendel: Known as the Father of Genetics. He conducted experiments crossing pea plants to understand how traits are inherited.

Mendelian Inheritance Principles

1. Law of Uniformity of Hybrids (F1 Generation)

   When crossing two pure organisms (homozygous) for a given character, all offspring in the first filial generation (F1) are heterozygous.

   Example Cross: GG x gg = Gg (All offspring are heterozygous).

2. Law of Segregation (Separation of Alleles)

   If two organisms from the F1 generation are crossed, phenotypes not seen in the F1 generation (the recessive trait) can reappear in the F2 generation. This is because the pair of alleles separates during gamete formation.

3. Law of Independent Assortment

   This principle states that the alleles for different characters are transmitted independently of one another, provided the genes are located on different chromosomes.

Advanced Genetic Concepts

Test Cross (Retrocreuament)

A test cross involves crossing an organism of unknown genotype (displaying the dominant phenotype) with a homozygous recessive organism.

• If any offspring show the recessive phenotype, the unknown parent is heterozygous.
• If all offspring show the dominant phenotype, the unknown parent is likely homozygous dominant.

Sex-Linked Inheritance

Humans have 46 chromosomes. 44 are autosomes (the same in both sexes). The remaining two are the sex chromosomes:

• Women: XX
• Men: XY

The X chromosome is significantly larger than the Y chromosome.

Recessive X-Linked Disorders

Examples include Haemophilia (which impairs blood coagulation) and Daltonism (color blindness).

Women (XX):

• XX: Normal
• X'X: Normal, but carrier (X' represents the affected allele).
• X'X': Will have the disease.

Men (XY):

• XY: Normal
• X'Y: Will have the illness (since they only have one X chromosome).

Dominant X-Linked Disorders (Rickets)

Women (XX):

• XX: Normal
• X'X: Affected (Rachitic)
• X'X': Affected (Rachitic)

Men (XY):

• XY: Normal
• X'Y: Affected (Rachitic)

Human Blood Groups (ABO System)

Agglutinogen (Antigen):

A protein found on the surface of red blood cells.

Donor:

Refers to the presence of specific Agglutinogens (Antigens) on the red blood cells.

Recipient (Receiver):

Refers to the presence of specific Agglutinins (Antibodies) in the plasma.