DNA & GENETICS

DNA (Deoxyribonucleic Acid)

The hereditary material that stores genetic instructions used in the growth, development, functioning, and reproduction of all living organisms.

Nucleotide

The repeating subunit of DNA composed of a deoxyribose sugar, a phosphate group, and a nitrogenous base.

Shape of DNA

Double helix, resembling a twisted ladder.

DNA Handrails

Alternating sugar and phosphate groups.

DNA Rungs

Pairs of nitrogenous bases held together by hydrogen bonds.

Chromatin

Uncoiled DNA and protein found in non-dividing cells.

Chromosome

Condensed chromatin visible during cell division.

Nitrogenous Bases

Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).

Chargaff's Rule

Adenine pairs with Thymine; Cytosine pairs with Guanine. The amount of A = T and C = G.

Complementary Base Pairing Example

If one strand reads A T G C C A T, the complementary strand is T A C G G T A.

Rosalind Franklin

Used X-ray crystallography to image DNA's structure.

Watson and Crick

Built the 3D model of DNA using Franklin's data.

Heredity

The passing of traits from parents to offspring.

Gregor Mendel

Known as the Father of Modern Genetics.

Gene

A segment of DNA that codes for a specific trait.

Allele

A variant form of a gene.

Genotype

The genetic makeup of an organism (e.g., Bb).

Phenotype

The physical expression of a genotype (e.g., brown eyes).

Dominant Allele

Masks the presence of a recessive allele (represented by a capital letter).

Recessive Allele

Expressed only when both alleles are recessive (represented by a lowercase letter).

Homozygous Dominant

Having two identical dominant alleles (BB).

Heterozygous

Having one dominant and one recessive allele (Bb).

Homozygous Recessive

Having two identical recessive alleles (bb).

Cystic Fibrosis (CF)

An autosomal recessive disorder caused by a defective CFTR gene, leading to thick mucus buildup in the lungs and digestive tract.

Punnett Square

A tool used to predict the possible genotypes and phenotypes of offspring.

Autosomal Trait

A gene located on a non-sex chromosome.

X-linked Trait

A gene located on the X chromosome.

Carrier (X-linked)

A heterozygous female who does not express the trait but can pass it on to offspring.

Incomplete Dominance

Both alleles blend to form an intermediate phenotype (e.g., red × white = pink flowers).

Codominance

Both alleles are fully expressed (e.g., AB blood type).

Polygenic Trait

Controlled by multiple genes (e.g., height, skin color).

Multifactorial Trait

Influenced by both genes and the environment (e.g., depression, heart disease).

X-linked Expression in Males

Males (XY) express an X-linked trait if their single X chromosome carries it.

X-linked Expression in Females

Females (XX) must inherit two copies of an X-linked recessive gene to express the trait.

Percentage of DNA Identical Among Humans

Approximately 99.9%.

Example of X-linked Recessive Trait

Hemophilia.

Codominant Alleles in Blood Type

A and B alleles are codominant.

Chargaff's Rule Importance

Ensures complementary base pairing, maintaining DNA's uniform structure.

Difference Between Chromosome and Chromatin

Chromatin is uncoiled DNA in non-dividing cells; a chromosome is condensed DNA visible during division.

Why Males Express More X-linked Traits

Males have only one X chromosome, so a single recessive allele results in expression.

Codominance in Blood Type

Both A and B alleles are expressed equally, resulting in AB blood.

Punnett Square Example

A heterozygous (Bb) × homozygous recessive (bb) cross produces 50% Bb (dominant phenotype) and 50% bb (recessive phenotype).

Multifactorial Trait Example

Clinical depression—shaped by genetic predisposition and environmental stressors.

Incomplete Dominance Example

Red and white flower cross producing pink offspring.

Codominance Example

AB blood type, where both alleles are expressed.

Complete Dominance Example

A dominant allele completely masks a recessive one (e.g., brown eyes over blue).