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: XBXB or XBY– Carrier: XBXb

– Colorblind: XbXb or XbY• Hemophilia – “bleeders disease”• Muscular Dystrophy – gradual wasting of

muscles