Chromosome Theory & Heredity

Gene Linkage Where two or more genes occurs on the same chromosome – Located in close association with one another – Tend to be inherited together – Generally do not follow Mendel's law of independent assortment

Sex-Linked Trait Trait that is determined by alleles carried only on a sex chromosome – i.e. color blindness, hemophilia

Sex Chromosomes XX in females XY in males – In humans the X chromosome is large and the Y chromosome is small – In most organisms the sex of the offspring is determined by the gametes from the male parent – X chromosomes carry info for both sexes – Y chromosomes carries genes related to male development

Sex Chromosomes & Fertilization Gametes – Reproductive cells Egg or sperm – Contain a single copy of each gene Zygote – Sperm and egg unit

Chromosome Information for Humans 46 chromosomes (23 pairs) Autosomes : chromosomes 1-22 Sex Chromosomes: chromosome 23 (determines gender) Normal – Male: 46 XY – Female 46 XX

Sex-Linked Inheritance Nondisjunction: failure of chromosomes to separate properly during one of the stages of meiosis – Produces gametes that contain either an extra copy or no copies of chromosomes Down Syndrome – 3 copies of chromosome 21 Turner Syndrome – sterile female, lack a second sex chromosome (genotype= 45 XO) Klinefelters Syndrome – sterile male, extra sex chromosome (genotype = 47 XXY)

Nondisjunction

Sex-Linked Disorders Carried on chromosome 23 If found on the X, the males will always have it (if they have the allele) Females must have both X chromosomes with the disorder to show it, otherwise they are carriers

Sex-Linked Disorders (cont.) Colorblindness – located on X chromosome – Normal: X B X B or X B Y – Carrier: X B X b – Colorblind: X b X b or X b Y Hemophilia – “bleeders disease” Muscular Dystrophy – gradual wasting of muscles