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