Fundamentals of Genetics: DNA, Chromosomes, and Inheritance

Fundamentals of Genetics

In the nucleus of every cell, there are a number of long threads called chromosomes. Each chromosome is made up of a large number of genes coding for the formation of different proteins, which ultimately determine our characteristics or traits.

Defining Key Genetic Components

DNA (Deoxyribonucleic Acid)

A molecule that carries most of the genetic instructions used in the development of living organisms. It serves as the blueprint for all living things.

Chromosome

A very long DNA molecule that carries a large amount of genetic information.

Gene

A section of a DNA molecule that codes for a specific protein or characteristic (e.g., hair color). A part of a DNA molecule coding for one protein is called a gene.

Trait

A notable feature or quality in a person.

DNA Structure and Gene Count

• Genes are made of DNA.
• The four nitrogenous bases (letters) of DNA are A, T, C, and G.
• Humans possess approximately 100,000 genes.
• Genes contain the instructions for building proteins.

Cell Division and Chromosome Number

Mitosis

A nuclear division giving rise to genetically identical cells in which the chromosome number is maintained by the exact duplication of chromosomes.

Meiosis

A reduction division in which the chromosome number is halved from diploid to haploid.

Human Inheritance

Humans have 23 pairs of chromosomes (46 total) in every cell. They exist in pairs because you inherit half of them from your mother and half from your father.

Gametes and Heredity

Egg cells and sperm cells are called gametes. Each gamete contains half of the parent's 46 chromosomes (one of each pair).

Heredity is the passing of traits from parent to child. Each parent contributes one set of chromosomes. This set is passed on randomly, ensuring each child receives a unique combination.

Human sex chromosomes are labeled X (typically female) and Y (typically male).

Alleles and Gene Expression

Since we have two of each chromosome, we have two copies of each gene; these copies are called alleles. You receive one allele from each parent.

Allele Combinations

• You can be homozygous, meaning both alleles are the same.
• You can be heterozygous, meaning each allele is different.

Dominance and Recessiveness

Some genes are dominant and some are recessive:

• A dominant gene is always expressed if present.
• A recessive gene only shows its characteristic if there are two copies present (homozygous recessive).

Phenotype vs. Genotype

Phenotype

A description of the characteristics that are physically shown (e.g., brown hair).

Genotype

A description of the exact combination of genes you possess (e.g., BB).