Comprehensive Guide to Genetics

Introduction to Genetics

Genetics is the branch of biology that focuses on the study of genes, genetic variation, and heredity in living organisms. It explores how traits are passed from parents to offspring and how they are expressed and vary within populations.

Mendel's Laws of Inheritance

Gregor Mendel, through his work on pea plants, laid down the fundamental principles of heredity, now known as Mendel's Laws.

1. The Law of Dominance

This law states that in a cross between two parents with pure, contrasting traits, only one form of the trait, the dominant one, will appear in the next generation (F1). The trait that is not expressed is called the recessive trait.

Example: When a pure-bred tall pea plant (TT) is crossed with a pure-bred dwarf pea plant (tt), all the offspring in the first generation (F1) are tall. The allele for tallness (T) is dominant over the allele for dwarfness (t).

2. The Law of Segregation

This law states that during the formation of gametes (sperm or egg cells), the two alleles for a heritable character separate or segregate from each other, so that each gamete ends up with only one allele for that character.

Example: An F1 generation tall plant (Tt) will produce two types of gametes: half will carry the tall allele (T), and the other half will carry the dwarf allele (t).

3. The Law of Independent Assortment

This law states that the alleles for different traits are sorted and passed on to the offspring independently of one another. The inheritance of one trait does not affect the inheritance of another.

Example: The inheritance of seed shape (round or wrinkled) is independent of the inheritance of seed color (yellow or green).

Genetic Crosses

Monohybrid Cross

A monohybrid cross is a genetic cross between two individuals who have different alleles for a single gene of interest.

Example: A cross between a homozygous tall plant (TT) and a homozygous dwarf plant (tt).
- F1 Generation: All offspring are heterozygous tall (Tt).
- F2 Generation (crossing two F1 individuals):
  - Phenotypic Ratio: 3 Tall : 1 Dwarf (3:1)
  - Genotypic Ratio: 1 Homozygous Tall (TT) : 2 Heterozygous Tall (Tt) : 1 Homozygous Dwarf (tt) (1:2:1)

Dihybrid Cross

A dihybrid cross is a cross between two individuals who differ in two observed traits that are controlled by two distinct genes.

Example: A cross between a plant with round, yellow seeds (RRYY) and a plant with wrinkled, green seeds (rryy).
- F1 Generation: All offspring have round, yellow seeds (RrYy).
- F2 Generation (crossing two F1 individuals):
  - Phenotypic Ratio: 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green (9:3:3:1)

Key Genetic Terminology

Gene: The basic physical and functional unit of heredity. Genes are made up of DNA.
Allele: One of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.
Homozygous: Having two identical alleles for a particular gene (e.g., TT or tt).
Heterozygous: Having two different alleles for a particular gene (e.g., Tt).
Dominant: An allele that is expressed phenotypically, masking the effect of the recessive allele when present.
Recessive: An allele whose phenotypic effect is masked when a dominant allele is present.
Mutation: A permanent alteration in the nucleotide sequence of the DNA of a gene.
Variation: Differences between individuals or groups of organisms of any species, often caused by genetic differences (genotypic variation) or by the effect of environmental factors.
Phenotype: The set of observable physical and biochemical characteristics of an individual, resulting from the interaction of its genotype with the environment. (e.g., tall, dwarf).
Genotype: The genetic makeup of an individual organism (e.g., TT, Tt, tt).

Sex Determination in Humans

Humans have 23 pairs of chromosomes in each cell.

Autosomes: 22 of these pairs are autosomes, which are the same in both males and females.
Sex Chromosomes: The 23rd pair are the sex chromosomes, which determine the sex of an individual.
- Females have two X chromosomes (XX).
- Males have one X chromosome and one Y chromosome (XY).

The sex of a child is determined by the chromosome carried by the sperm. If a sperm with an X chromosome fertilizes the egg, the child will be a girl (XX). If a sperm with a Y chromosome fertilizes the egg, the child will be a boy (XY).

Description: A rare genetic disorder in which the blood does not clot properly due to a lack of sufficient blood-clotting proteins (clotting factors).
Inheritance: It is an X-linked recessive disorder. A son born to a carrier mother has a 50% chance of inheriting the faulty gene and having haemophilia.

2. Colour Blindness

Description: The inability or decreased ability to see color or perceive color differences under normal lighting conditions. The most common form is red-green color blindness.
Inheritance: It is also an X-linked recessive disorder. Like haemophilia, it is much more common in males than in females.

Genetics