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Section 4: Genetic and Molecular Biology Chapter 11 (Notes) Sexual reproduction and meiosis  Meiosis • Meiosis: type of nuclear division which results in half as many chromosomes as the parent cell (Reduction division). ◊ Leads to formation of eggs and sperm (gametogenesis) • Fertilization: process when sperm and egg unite sperm and egg ◊ Leads to offspring that are genetically distinct from each other and from their parents ◊ Each cell produced by meiosis receives a different gene combination • Genes are located on chromosomes ◊ Mistakes during meiosis result in cells containing the wrong number of chromosomes  Chromosomes and autosomes • Karyotype: number and types pf chromosomes present in an organism (basically a picture of someone’s chromosomes) • Sex chromosomes: The 23 chromosome which determine the sex of the individual. (1 pair) ◊ All other chromosomes are autosomes (22 pairs) • Humans have 46 chromosomes in every cell except their gametes • Chromosomes carry genes ◊ Gene: a section of DNA that influences one or more hereditary traits in an individual • Different types of genes are called alleles EXAMPE: Eye shape (gene) Narrow or round eyes (alleles)  Homologous chromosomes • Homologous chromosomes: Chromosomes of the same type (also called homologs) ◊ Homologs carry the same genes in the same locations but each one may contain different alleles.  Concept of ploidy • Ploidy: number of each type of chromosome present (n, 2n, 3n, etc.) ◊ Haploid (n): Organism whose cells contain just one of each type of chromosomes (ex: gametes) ◊ Diploid (2n): organisms whose cells contain two versions of each type of chromosome (one paternal chromosome and one maternal chromosome) ◊ Polypoid (3n,4n, etc..): Organisms with three or more versions of each type of chromosomes