Human Genetics

1
Human Genetics

• Chapter 14

2
Human Chromosomes

• How many kinds of chromosomes do humans have?
• 23
• How many chromosomes do humans have?
• 46 (23 from each parent)
• What are the sex chromosomes?
• The two chromosomes that determine your sex (X
  Y females XX males XY) The X is much
  larger (X has 1200 genes Y has about 140).

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Human Chromosomes

• What are autosomes (autosomal chromosomes)?
• The other 22 chromosomes that are not sex
  chromosomes. (they are named chromosomes 1, 2, 3
  . . . 22)
• The sex chromosomes are the 23rd pair.

4
Human Chromosomes

• What is a karyotype?
• An arrangement of pictures of chromosomes taken
  during metaphase. This picture is called a
  metaphase spread.
• 2. Chromosomes are arranged based on size,
  centromere location, and banding patterns. (The
  banding was the inspiration for bar codes.)
• 3. Karyotypes are used to diagnose genetic
  disorders based on chromosomal number and for sex
  determination.

5
Human Chromosomes
Metaphase Spread
Karyotype of a normal human male. 46,XY
6
Human Pedigree

• What is a pedigree?
• It is a chart that traces a Mendelian trait
  through a family.

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• Examples of Autosomal Dominant Traits
• Dwarfism
• Progeria
• Polydactyly and Syndactyly

• Examples of Autosomal Recessive Traits
• Albinism
• Cystic Fibrosis
• Phenylketoneuria (PKU)

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Human Blood Types

• ABO blood types are traits inherited by multiple
  alleles that show both codominance complete
  dominance.
• Rh blood group determines the () or () part of
  the blood type. () is completely dominant over
  the ().

9
Human Autosomal Disorders
10
Cystic Fibrosis

1. Caused by a recessive allele on chromosome 7 that
   affects a chloride ion channel in the plasma/cell
   membrane. The CFTR protein folds improperly.
2. Causes the production of a thick mucus that
   interferes with digestion breathing.
3. Allele is found in populations originating from
   Northern Europe. The heterozygous people during
   the Middle Ages had an evolutionary advantage
   against typhoid fever.

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Sickle Cell Disease

1. Found in people of Sub-Saharan African descent.
2. The allele is the result of a point mutation that
   changes just one amino acid in hemoglobin.
3. The affected hemoglobin causes the red blood
   cells to be misshapen when O2 levels drop.
4. The allele shows codominance (sickle beta-globin
   gene along with the normal beta-globin gene).
   Homozygous recessive people are most affected,
   heterozygous are partially affected, and
   homozygous dominant are not affected.
5. The allele is relatively common because the
   heterozygous people have resistance to malaria.

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Sickle Cell Disease
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Human Chromosomes

1. Each diploid cell has 6 billion base pairs.
2. Only 2 actually codes for proteins.
3. The average gene is 3000 base pairs the longest
   is 2 million base pairs.

PS-1, PS-2, APOE, APP are all genes involved in
Alzheimers Disease
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Sex-Linked Traits

• These are traits whose genes are found on one of
  the sex chromosomes.
• Most of these genes are found on the X chromosome
  because it is larger.
• X-linked traits have a tendency to show up more
  in males because they only have one X chromosome.

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Sex-Linked Traits

• Red/Green Colorblindness
• X-linked recessive allele
• Cant see the colors red and green very well.
• Most common in males.

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Sex-Linked Traits
XH
Xh

• Hemophilia
• X-linked recessive
• One of the proteins that is involved in the
  clotting process is defective.
• Once known as the royal disease because it was
  common in the royal families of Europe.

XHXH
XHXh
XH
XHY
XhY
Y
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Sex-Linked Traits

• Duchenne Muscular Dystrophy
• X-linked recessive
• Causes progressive weakening loss of skeletal
  muscle.
• Affects 1 out of 3000 American males

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X-Chromosome Inactivation

1. In females, one of the X chromosomes is randomly
   inactivated.
2. This process is called Lyonization after Mary
   Lyon, the scientist who discovered it.
3. The inactive X chromosome forms a dense region in
   the nucleus called a Barr body.
4. This phenomenon can cause interesting traits like
   the calico color pattern in cats. The genes for
   the black and orange color are on the X
   chromosome.

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Nondisjunction

1. Sometimes during meiosis, the homologous
   chromosomes fail to separate.
2. This causes the gametes to have more or less than
   23 chromosomes.
3. Down Syndrome is caused by nondisjunction of
   chromosome 21.
4. One gamete had two copies of chromosome 21. The
   resulting child has 3 copies of chromosome 21
   (trisomy 21)

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Nondisjunction

• Nondisjunction can also occur with the sex
  chromosomes.
• Turners Syndrome (monosomy X)
• - 45,X (1 in 2500 females)
• - Mentally normal, typically short, webbed
  neck, sterile, underdeveloped sex organs 2o sex
  characteristics, heart problems.
• B. Klinefelters Syndrome (47, XXY)
• - 1 in 1000 males
• - Some cognitive problems, tall slim, some
  female 2o sex characteristics, sterile.