The Chromosomal Basis of Inheritance

1
The Chromosomal Basis of Inheritance

• Chapter 15
• Biology Campbell Reece

2
Chromosome Theory

• Chromosome Theory of Inheritance
• Mendelian genes have specific loci on chromosomes
• It is the chromosomes that undergo segregation
  and independent assortment
• Thomas Hunt Morgan was the first to associate a
  specific gene with a specific chromosome

3
Linked Genes

• Genes located on the sex chromosome are called
  sex-linked genes
• Ex. Baldness, color-blindness, hemophilia
• Linked genes genes located on the same
  chromosome tend to be inherited together
• This violates Mendels Law of Independent
  Assortment, because it is the chromosomes that
  separate, not the genes

4
Colorblindness Test
5
Sex-Linked Genes

• Tend to be found more often on the X chromosome
  (because it is larger)
• Therefore, men tend to show sex-linked traits
  more because they only have one X
• If the trait is caused by a recessive gene, a
  male only requires the one X to have the trait,
  but a female must have the trait on both Xs

6
Gene Mapping

• Genetic map an ordered list of the genetic loci
  along a particular chromosome
• The farther apart two genes are on a chromosome,
  the more likely it is that crossing over will
  occur between them
• Using the recombination frequencies, you can
  calculate the distance (in map units) between
  genes on a chromosome

7
Gene Mapping
8
Alterations of Chromosome Number

• Nondisjunction the homologous chromosomes fail
  to separate properly during meiosis
• Aneuploidy an abnormal number of chromosomes
• Trisomic have 3 copies of a chromosome
• Monosomic have only 1 copy of a chromosome (the
  other is missing)

9
Alterations of Chromosome Structure

• Deletion a piece of the chromosome is missing
• Duplication an extra copy of a segment of the
  chromosome
• Inversion a reverse segment within a chromosome
• Translocation a piece from one chromosome
  attaches to a nonhomologous chromosome

10
Alterations of Chromosome Structure
11
Chromosomal Disorders

• Down Syndrome (Trisomy 21) caused by an extra
  21st chromosome

12
Chromosomal Disorders

• Klinefelter syndrome an extra X chromosome in a
  male (XXY)
• XYY males that are somewhat taller than normal
• Turner syndrome a missing sex chromosome (XO)
• cri du chat caused by a deletion in chromosome
  5 (causes mental retardation)