Human Genetics

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Human Genetics
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Difficulties in Studying Human Heredity

• Controlled experiments are not possible
• Small numbers of offspring per generation
• Long periods between generations

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Sex Determination

• Sex chromosomes - 1 pair, last
• XX female
• XY male
• Autosomes - body chromosomes - first 22 pairs
• same in both male and female

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• XX 50
• XY 50

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• You always have a 50 chance of having a girl and
  a 50 chance of having a boy!
• Who determines the sex of the offspring?

Daddy!!...uh oh, what did King Henry VIII do????
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Sex Determination
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Multiple Alleles

• Three or more alleles for a trait in a human
  population
• ex. Blood types IA, IB, i (type O)
• Karl Landsteiner discovered antigens which are
  responsible for different blood types

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Multiple Alleles - Blood Types

• IA - contains antigen A
• IB - contains antigen B

• IAIB - contains antigens for both A B
• i - contains neither A nor B

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Blood Groups
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• Universal donor O
• Universal recipient AB
• Dominant Trait in Blood Groups

• Rh antigen named after Rhesus Monkey
• people who have the Rh antigen Rh (positive)
• people who do not have it Rh- (negative)
• Rh-, Rh person positive

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Non-disjunctions

• Chromosomes fail to separate during meiosis
• Individuals have too many or too few chromosomes

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Non-disjunction AnimationMonosomy
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Sex-linked Traits

• Traits controlled by recessive genes located on
  sex chromosomes (normally associated with the X
  chromosome)

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• Females
• XR XR normal
• XR Xr carrier
• Xr Xr disease
• Males
• XR Y normal
• Xr Y disease

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Sex-limited Traits

• Controlled by genes located in the autosomes
• Only expressed in the presence of sex hormones
• Only expressed in one gender
• First seen at puberty (hormones produced in large
  enough quantities)

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Examples

• Male birds colorful plumage
• Beard growth in males
• Milk production in females

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Sex-influenced Traits

• Traits that are caused by a gene whose expression
  differs in males and females
• also located on autosomes
• expressed in the presence of male or female
  hormones

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Example

• Male pattern
• baldness
• BB normal
• Bb males bald, females are not
• bb males and females bald

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Hereditary Disorders
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Techniques for detecting genetic disorders

• Amniocentesis - technique used that removes and
  studies amniotic fluid during pregnancy
• only identifies chromosomal disorders

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• Karyotyping - an enlarged photo of the chromosome
  pairs (map) to identify any abnormalities in the
  chromosomes - done during metaphase

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• Pedigree charts and analysis
• chart that shows how a trait and the genes that
  control it are inherited within a family
• identifies the presence or absence of particular
  traits in members of each generation

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• Males
• Females
• Generations Roman numerals
• Individuals Numbered sequentially
• Trait expressed Filled in
• Non carriers Empty
• Carrier (not ill) Half filled
• individual who carries a recessive
• allele that is not expressed

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I.
1
2
I.
II.
1
2
6
7
3
4
5
II.
7
6
3
4
5
III.
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9
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III.
8
9
10
IV.
IV.
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12
11
12
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Autosomal Recessive Disorders
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• Albinism
• 1/17,000 are affected
• lacking the pigment melanin
• linked to brain
• and eye
• development

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• Tay-Sachs
• incidence rate of infantile Tay-Sachs
• Ashkenazi Jewish groups, French Canadian Cajun
  1 in 30 are carriers
• other groups 1 in 300 are carriers

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• accumulation of lipids on brain
• brain malfunction death by
• age 5

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• Cystic Fibrosis
• 1/31 is carrier in U.S.
• mucus in lungs, pancreas, digestive tract and
  other organs
• most common lethal genetic disease in U.S.

• heterozygote is resistant to typhoid fever

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• PKU (phenylketonuria)
• 1/15,000 in U.S.
• inability to breakdown phenylalanine

• infant screening at birth

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mental retardation
special diet to prevent retardation
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Autosomal Dominant Disorders
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Woody Guthrie

• Huntingtons
• incidence 1/10,000 in U.S.
• nervous system degenerates

Thirteen on House
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• Neuron with inclusions (stained orange)
• Inclusions are part of progression

Nancy Wexler - high incidence in Lake Maracaibo
region of Venezuela Research in these villages
led to location of gene markers
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• late onset
• may not show symptoms until past repro-ductive age

Areas affected by degeneration
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Sex-linked Disorders
Usually associated with X Chromosome
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• Duchenne Muscular Dystrophy
• 1 boy in every 3,000
• progressive weakening of muscles and loss of
  coordination
• more common in males

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• Red-green Color Blindness - individuals cannot
  distinguish
• between
• the colors
• red green

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• mild disorder
• more common in males

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• Hemophilia - blood clotting enzyme is
• absent

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• excessive bleeding
• high frequency in European royal families Royal
  disease
• possible mutation in one of Queen Victorias
  parents
• (Tsarevich Alexis, the son of Tsar Nicholas II of
  Russia )

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Non-Disjunctions

• Result in too many or too few chromosomes

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• Down Syndrome
• trisomy 21
• developmental and mental retardation
• much variation in degree
• possibly related to maternal age

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• Incidence
• 1 per 800 births, average
• maternal age below 20 1/1600 births
• maternal age above 35 1/370 births
• 75-80 of DS born to younger women due to
  increased numbers of births in younger women

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• Klinefelter Syndrome
• incidence 1/700
• trisomy of sex chromosomes, XXY

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• abnormal testicular development
• sterility
• normal intelligence, though increased risk of
  learning disabilities behavior problems
• Increases with each X
• about 1/3 have some increased breast tissue
  development
• generally normal life span

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Klinefelters gynecomastia male breast reduction
surgery
Before
After
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• Turner Syndrome
• incidence 1/2500 births
• monosomy of sex chromosomes, XO
• female phenotype
• secondary sex characteristics do not develop

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• Webbed neck
• Short in stature (48)
• Cardiovascular problems
• Infertile/sterile
• Life expectancy reduced by 10 yrs

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• only known viable monosomy in humans
• about 98 are miscarried
• accounts for as many as 15 of all spontaneous
  abortions/miscarriages
• Some Turners are caused by circular X chromosome
  or may only be missing X in some of cells
  (mosaicism) fewer symptoms

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