The Chromosomal Basis of Inheritance

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The Chromosomal Basis of Inheritance

• Chapter 15

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Objectives

• You will relate Mendelism to chromosomes
• You will understand the basis of sex-linked genes
• You will explore errors and exceptions in
  chromosomal inheritance

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Chromosome Theory of Inheritance

• Developed by Walter S. Sutton in 1902
• States Mendelian genes have a specific loci on
  chromosomes and it is the chromosomes that
  undergo segregation and independent assortment

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Tracing a gene to a specific chromosome

• Thomas Hunt Morgan
• Early 20th Century
• Drosophila melanogaster fruit fly
• Prolific breeders 1 mating 100s of offspring
• Only 4 pairs of chromosomes
• Discovered male fly with white eyes instead of
  red
• Wild type phenotype most common in natural
  populations- red eyes
• Mutant phenotype alternative to wild type- white
  eyes

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Sex Linkage

• Mated white-eyed male with red-eyed female
• All offspring had red eyes suggesting wild type
  is dominant
• F1 bred together 31 ratio in F2
• WHITE EYES ONLY IN MALES
• SEX LINKAGE
• LOCATED ON X CHROMOSOME/ NOT Y
• FEMALE COULD ONLY BE WHITE IF SHE WAS HOMOZYGOUS
  WHICH THE F2 FEMALES COULD NOT BE IN THIS
  EXPERIMENT

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Chromosomal Basis of sex

• XX female
• XY male
• Obviously the female must contribute the X
  chromosome to the offspring
• The male may contribute the X or the Y.

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Sex-linked genes

• In humans sex-linked usually refers to genes on
  the X chromosome. (like Drosophila)
• Therefore fathers pass sex-linked alleles to all
  daughters but none of sons and mothers pass
  sex-linked alleles to both sons and daughters.

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Transmission of sex-linked recessive traits

• A) Father will transmit to daughters but no sons
• B) Carrier mother mating with normal male will
  pass to half of sons (express) and half of
  daughters (carriers)
• C) Carrier mother mating with male with the
  trait will pass on with 50 chance of expression
  to each child. All daughters will be at least
  carriers.

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Sex-linked Disorders in Humans

• More common in males because females must be
  homozygous and males only receive one chromosome
  carrying the gene.
• Color blindness
• Duchenne muscular dystrophy
• Hemophilia

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Duchenne Muscular Dystrophy

• 1/3500 males affected in the US
• Rarely live past 20s
• Progressive weakening of muscles and loss of
  coordination
• Located on X chromosome

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Hemophilia

• Absence of one or more proteins required for
  blood clotting
• Bleed more because of lack of clotting
• Can be treated with injections of the missing
  protein.
• Located on X Chromosome.

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X Inactivation in Female Mammals

• Barr body
• One of the X chromosomes in each cell becomes
  almost completely inactivated and condenses into
  a Barr body.
• The selection of which of the 2 X chromosomes
  forms a Barr body occurs randomly and
  independently of each cell
• Therefore females consist of a mosaic of two
  types of cells

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Seeing mosaicism

• Tortoiseshell cat
• Patches of orange and black hair
• Calico cats have white hair also which is
  determined by another gene.
• Sweat glands in humans
• Heterozygous woman has patches of normal skin and
  patches of skin lacking sweat glands

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X inactivation in tortoiseshell cat
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Errors and Exceptions in Chromosomal Inheritance

• Alterations of Chromosome number
• Aneuploidy
• Polyploidy
• Alterations of Chromosome structure
• Deletion
• Duplication
• Inversion
• Translocation

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Chromosome Number

• Nondisjunction
• Members of a pair of homologous chromosomes do
  not move apart properly
• Offspring an have aneuploidy
• Chromosome is a triplicate (2n 1)
• aka trisomic
• Chromosome is missing (2n 1)
• aka monosomic
• Polyploidy
• having more than two complete chromosome sets

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Chromosome Structure

• Deletion
• Chromosome fragment lacking a centromere is lost
  during cell division thus missing certain genes
• Duplication
• Chromosome fragment could attach as an extra
  segment to a sister chromatid
• Inversion
• Chromosome fragment reattaches to original
  chromosome but with reverse orientation
• Translocation
• Segment goes to nonhomologous chromosome

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Alterations in chromosome structure
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Human Disorders Due to Chromosomal Alterations

• Down syndrome
• Klinefelter syndrome
• Fragile X syndrome

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Down Syndrome

• 1/700 children born in US
• Extra chromosome 21
• Trisomic for chromosome 21
• Total of 47 Chromosomes
• Phenotype
• Characteristic facial features, short stature,
  heart defects, risk of respiratory infection,
  lowered mental capacity
• Generally shorter life span
• Generally sterile

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Down Syndrome
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Klienefelter syndrome

• Having an extra X chromosome in males
• XXY
• 1/2000 live births
• Have male sex organs but are sterile
• Feminine body characteristics
• Normal intelligence
• See book for (282) for other similar conditions

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Fragile X syndrome

• Abnormal X chromosome
• Tip hangs on the rest of the chromosome by a thin
  thread of DNA
• Lower mental capacity
• 1/1,500 males
• 1/2,500 females
• If a male inherits it it must be from mother