Genetics Lesson 1: Introduction to DNA

Genetics - Lesson 1

• Genetics is the branch of biology that studies genes, variation, and heredity.
• A gene is the basic unit of heredity, a sequence of nucleotides in DNA or RNA.
• Variation refers to diversity and differences among individuals or populations.
• Heredity is the genetic heritage passed down from biological parents.

Understanding DNA, Chromosomes, and Genes

• DNA (deoxyribonucleic acid) is a molecule composed of two polynucleotide chains forming a double helix.
• DNA carries genetic instructions for the development, functioning, growth, and reproduction of all known organisms and many viruses.
• Chromosomes are thread-like structures inside the nucleus of animal and plant cells, composed of protein and a single DNA molecule.

DNA Structure

• DNA is made of nucleotides, each containing a phosphate group, a pentose sugar (deoxyribose), and a nitrogenous base.
• Nitrogenous bases include Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).
• These bases pair specifically (A with T, and C with G).

Types of Bases

• Adenine, Thymine, Cytosine, and Guanine are the four bases in DNA.
• The sequence of these bases codes for traits.

Father of Genetics

• Gregor Mendel conducted experiments with pea plants to understand inheritance patterns.
• His work formed the foundation for modern genetics.

Important Terms

• Allele: One of a pair of genes at a specific location on a chromosome, controlling a characteristic (e.g., blood type, color blindness).
• Dominant Trait: The trait that appears or is visibly expressed in an organism.
• Recessive Trait: The trait that is masked and does not show itself in the organism.
• Genotype: The combination of alleles an organism inherits (genetic code).
• Phenotype: The physical appearance of traits in an organism.
• Homozygous Dominant: Both alleles are dominant (e.g., AA).
• Heterozygous: One dominant allele and one recessive allele (e.g., Aa).
• Homozygous Recessive: Both alleles are recessive (e.g., aa).

Traits of Pea Plants

• Mendel studied seven traits in pea plants, each with two forms (e.g., round/wrinkled seeds, tall/short plants).
• He identified pure-breeding plants that consistently showed one form of a trait after generations of self-pollination.

Mendel's Experiments

• Mendel crossed different pea plant varieties.
• His experiments showed that traits are inherited as discrete units (genes).
• He found a 3:1 ratio of dominant to recessive traits in the second generation (F2).

Punnett Square

• A Punnett square is a tool to predict the probabilities of offspring traits.

Mendel's Laws of Heredity

• Law of Segregation: Parental genes separate randomly into sex cells; each offspring inherits one allele from each parent.
• Law of Independent Assortment: Genes for different traits are sorted separately, so inheritance of one trait doesn't depend on others.
• Law of Dominance: Organisms with alternate forms of a gene express the dominant form.

Other Inheritance Patterns

• Incomplete Dominance: Neither allele fully dominates, resulting in a new phenotype (e.g., a blend of traits in flower color).
• Codominance: Both alleles are expressed, resulting in both phenotypes being visible (e.g., spotted coat color in animals).