THE MECHANICS

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CHAPTER 3

• THE MECHANICS
• OF HEREDITY
• (Gene-Context Interaction)

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Human life begins with union of mothers ovum
(egg cell) and fathers sperm cell.

• Once every 28 days an egg cell is released
• All womans ova present at birth (maybe a
  million) only 400 will mature and be released
  between puberty and menopause
• Males produce billions of sperm per month
  (200-300 million every 3-4 days) from puberty
  till death
• Ovum is largest cell in body sperm one of
  smallest

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TWO TYPES OF CELLS

• Body cells (somatic) - contain 23 pair of
  rod-like structures chromosomes
• Sex cells (ova sperm) - contain 23 (versus 23
  pair) chromosomes each
• Of the 23 chromosomes in each sperm or ovum, one
  (labeled the sex chromosome) determines the sex
  of the offspring
• Father produces 2 types of sperm X and Y
• X sperm are larger and stronger Y sperm are
  smaller and weaker but faster.
• XX-female XY-male

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Each chromosome contains 1,000s of genes (units
of heredity)

• Skin pigmentation
• Eye color
• Some personality traits (shyness, activity level)
• To some extent, intelligence
• All of these more are influenced/determined
  genetically!

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GENES

• Dominant genes
• Recessive genes
• Ex- The gene for brown eyes are dominant over the
  gene for blue eyes.

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MORE GENE INFORMATION

• GENOTYPE
• Your actual genetic makeup
• All genes inherited from parents

• PHENOTYPE
• What you see
• Manifested characteristics

Genotype is hidden inferred from phenotype
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GENES AND DISEASES

• Genes are sometimes responsible for certain
  diseases defects
• In most cases these disorders linked with
  recessive rather than dominant genes

If a person has a dominant gene for a disease it
will be manifested in the carrier. An individual
can be carrier of a single recessive gene for an
abnormality not know it (it will not show up in
offspring unless both parents are carriers pass
it on.
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GENETIC DISEASES

• Huntingtons Chorea
• Fatal disease associated with a dominant gene
• Doesnt manifest itself until one is 30-40 (maybe
  50) years of age

• Sickle-Cell Anemia
• Linked to recessive gene
• Carried primarily by African-Americans
• 10 carry the gene

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GENETIC DISEASES (contd)

• PKU
• Liver enzyme missing that prevents certain foods
  from breaking down properly
• Undetected can lead to MR brain seizures
• Can be detected in newborns and avoided by
  special diet

• Tay-Sacks
• Enzyme disorder where the brain is unable to
  break down certain fatty substances
• 1st symptoms occur at 6 months
• Child dies before age 3-5
• Can be detected but not prevented

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CHROMOSOMAL DISORDERS

• DOWNS SYNDROME (Trisomy 21)
• Occurs in 1 of every 680 births
• Extra chromosome on chromosome 21 (3 instead of
  normal pair of 2)
• Risk increases with parents age (especially
  mother)
• Mothers 15-24 1 in 1420 births
• Mothers over 45 1 in 30 births

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CHROMOSOMAL DISORDERS associated with the Sex
Chromosome

• Turner Syndrome
• 1-2500 females missing an X sex chromosome (XO)
• Most are spontaneously aborted (90)
• Survivors underdeveloped secondary sex
  characteristics
• Treatable with estrogen
• Females are sterile

• Klinefelters Syndrome
• Presence of extra X in males (XXY)
• 1-1000
• Small, underdeveloped testicles
• Some breast development
• High-pitched voice
• Little facial hair
• Many infertile
• Treatable with testosterone

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CHROMOSOMAL DISORDERS associated with the Sex
Chromosome (contd)

• XYY Syndrome (Super males)
• Taller
• Lower than average intelligence
• Tend to be more aggressive (?)

• Fragile X Syndrome
• The X sex chromosome is abnormally compressed or
  broken
• More common in males
• Results in mental retardation
• no symptoms until puberty
• www.fraxa.org