Beyond Mendel

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Beyond Mendel
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Beyond Mendel

• Some genetic disorders do not follow simple
  Mendelian transmission.
• They do not seem to run in families
• Many of these disorders are caused by changes in
  chromosome number aneuploidy.
• Aneuploidy can result from changes in autosomal
  count or the number of sex chromosomes.
• XO
• XXX
• XXY
• XYY

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Alterations of Chromosomal Number or Structure

• How do alterations in chromosomal number occur?
• Nondisjunction Errors during Meiosis.
• Homologous chromosomes do not separate during
  Meiosis I
• Sister chromatids do not separate during Meiosis
  II.

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Down Syndrome Trisomy 21

• Most zygotes containing chromosomal alterations
  are spontaneously aborted.
• However, some aneuploid conditions are viable.
• Down Syndrome 1 in 700 children will have an
  extra chromosome 21. Trisomy 21

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Karyotype of a Female with Down Syndrome
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Down Syndrome

• Symptoms include
• Syndrome typical facial features
• Short stature
• Heart defects
• Life expectancy 17 yrs
• Mental retardation due to CNS malformations
• Reduced neuron count Low brain weight
• Reduced depth of sulci
• Shorter than normal dendrites
• I.Q. 20 50

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Incidence of D.S. correlated with maternal age

• Before birth, all human female gametes are
  arrested during late prophase I when homologous
  chromosomes are in Tetrads.
• Only a single gamete completes Meiosis during
  each menstrual cycle.
• The older the female, the longer the gametes have
  been sitting in prophase I.

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Tuner Syndrome XO

• Affected people lack a second sex chromosome.
• Sexed as girls at birth.
• Ovaries do not produce estrogens. Must be treated
  during adolescence to induce puberty.
• Other symptoms include
• Short stature
• Syndrome typical facial features
• Sterile
• Heart defects
• Webbing of skin on the neck
• Lack of secondary sex characteristics if not
  treated

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Turner Syndrome

• Overall intelligence is normal
• Specific disabilities in map reading and spatial
  orientation tasks.
• Dreams are deficient in visual imagery and
  resemble those of blind people.

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Turner Syndrome XO
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Klinefelters Syndrome XXY

• Individuals posses an extra X chromosome. 1 in
  600 births
• Presence of TDF allows masculinization sexed as
  males at birth
• Symptoms
• Tall
• Slightly feminized physique
• Poor beard growth
• Breast development
• Testicular atrophy
• Sterile
• Mental retardation

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Superman XYY

• Extra Y chromosome 1 in 850 births
• May be sterile
• Symptoms
• Very tall
• Mental retardation
• Exaggerated male secondary sex characteristics

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XYY

• During the seventies the XYY genotype was
  believed to promote aggressive, antisocial
  behaviors.
• A disproportionably large number of XYY
  individuals were incarcerated.
• Measures of aggression showed no correlation
  between XYY genotype and increased aggression.
  Third variable?
• Less intelligent than average
• Much Taller than average
• Crimes committed were most often non-violent

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XYY

• XYY males less likely to establish good
  relationships with women, less likely to marry.

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XXY and XYY Males
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