2.1.1 - Genes, alleles and genomes

Gene

A section of DNA with a functional purpose; the basic unit of heredity; codes for proteins and traits and is passed to offspring through sexual reproduction.

Allele

An alternative form of a gene; individuals have two copies of each gene (one from each parent); alleles can be dominant or recessive.

Genome

The complete set of genetic material in a cell; the organism's full set of genetic instructions (the genetic jigsaw).

Locus

The specific position on a chromosome where a gene or allele is located.

Homologous chromosomes

Chromosomes that carry the same genes at the same loci; one inherited from each parent.

Autosomes

Non-sex chromosomes; humans have 22 pairs of autosomes.

Sex chromosomes

Chromosomes that determine biological sex (in humans, X and Y).

Karyotype

A visual representation of an organism's chromosomes used to identify abnormalities.

Meiosis

The cell division process that produces haploid gametes from diploid cells and introduces genetic diversity via crossing over and independent assortment.

Crossing over

Exchange of chromatids between homologous chromosomes during meiosis, creating new allele combinations.

Independent assortment

Random orientation of chromosome pairs during meiosis I, contributing to genetic variation.

Diploid

A cell with two complete sets of chromosomes (2n); typical of somatic (body) cells.

Haploid

A cell with a single set of chromosomes (n); typical of gametes (sperm and egg).

Chromosome

A DNA-containing structure that carries genetic information; tightly packed during cell division.

Chromatin

DNA wrapped around proteins; forms chromosomes during division.

Nucleotide

The monomer of DNA consisting of a sugar (deoxyribose), a phosphate group, and a nitrogenous base.

DNA

Deoxyribonucleic acid; a double-stranded helix built from nucleotide monomers.

Base pair

A pair of nucleotides on opposite DNA strands held by hydrogen bonds; A pairs with T, C pairs with G.

5' to 3' ends

Directionality of DNA strands; 5' end has a phosphate group and 3' end has a sugar.

Complementary base pairing

Bases pair specifically (A with T, C with G) to form the two DNA strands.

Genome size (human)

The human genome comprises about 3 billion base pairs, roughly 21,000 genes, organized into 46 chromosomes.

Human Genome Project

The project completed in 2003 that sequenced the human genome; revealed about 3 billion base pairs and ~21,000 genes across 46 chromosomes; aimed to identify gene function and applications.

Somatic cells

Body cells (not reproductive); in humans, contain 46 chromosomes (22 autosomes + 2 sex chromosomes).

Gamete

A haploid reproductive cell (sperm or egg) produced by meiosis; fusion of gametes forms a diploid zygote.