Chapter 12 Patterns of Heredity

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Chapter 12Patterns of Heredity Human Genetics
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Section 12.1

• Mendelian Inheritance
• of Human Traits
• NCSCOS 3.03

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Making a Pedigree

• When genetic inheritance is represented by a
  picture, this is called a pedigree.
• Pedigrees are used by geneticists to map
  inheritance from generation to generation.

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• It is a diagram made of symbols that identify
  three things
• 1. Male or female
• 2. Individuals affected by the trait being
  studied
• 3. Family relationships

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Label the following symbols from a pedigree
Carrier
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Constructing and Reading a pedigree
a horizontal line between a male and female with
a strike means the persons are divorced.

• an inverted v means the married couple had
  twins

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Constructing and Reading a pedigree
I.
1
2
II.
1
2
3
4
5
III.
3
2
1
4
5
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Roman Numerals (I, II, III) refers to the
generations. Arabic numbers refers to
individuals. (1, 2, 3, 4, 5, )
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Reading the pedigree
I.
1
2
II.
6
1
2
3
4
5
7
III.
3
2
1
4
5
How many generations are there? How many children
did II-1 have? II-7? How are III-5 and III-2
related? Who is III-2 in reference to I-2?
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• What does a half shaded circle or square
  represent?
• A carrier
• Define a carrier
• A heterozygous individual

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Types of Pedigrees
Step One

• Is the pedigree autosomal or X-linked. Pedigrees
  can be
• a.) autosomal
• There is a 50/50 ratio between men and women of
  affected individuals.
• b.) X- linked
• Most of the males in the pedigree are affected.

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Facts about X-linked Disorderscarried on the
X-chromosome
X-linked are carried by females, but not
expressed in females. X-linked are expressed
most often in MALES. In males, to express an
X-linked disorder, he only needs to have one
gene. (XY - heterozygous) In females, to
express an X-linked disorder, she needs TWO
alleles to show the disorder. (XX homozygous
recessive) Ex Colorblindness, hemophilia,
baldness
Colorblindness Pedigree
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Colorblindness Tests
Normal color yellow square faint brown circle
Colorblind sees yellow square
Colorblind sees the number 17
Normal Color sees the number 15

• Test Name Ishihara Test

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Simple Recessive Heredity

• Most genetic disorders are caused by recessive
  alleles. This means the disorder is inherited
  when both parents have a recessive allele.

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Common Recessive Disorders

• Cystic Fibrosis (CF)
• A defective protein in the plasma membrane of
  cells causes thick mucus to build up in the lungs
  and digestive system.
• Mostly found among white Americans.

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Pedigree for Cystic Fibrosis
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• Tay-Sachs Disease
• The absence of an enzyme causes lipids to
  accumulate in the tissues and nerve cells of the
  brain.
• Mostly found in people of Jewish descent

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• The child becomes blind, deaf, and unable to
  swallow. Muscles begin to atrophy and paralysis
  sets in. Other neurological symptoms include
  dementia, seizures, and an increased startle
  reflex to noise.
• Even with the best care, children with Tay-Sachs
  disease usually die by age 4, from recurring
  infection.

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Pedigree for Tay-Sachs
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Simple Dominant Heredity

• Dominant disorders are inherited as Mendels rule
  of dominance predicted Only one dominant allele
  has to be inherited from either parent.

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Common Dominant Traits Disorders

• Simple Dominant Traits
• 1. cleft chin
• 2. widows peak hairline
• 3. unattached earlobes
• 4. almond shaped eyes

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Disorders Huntingtons Disease

• A lethal genetic disorder that causes certain
  areas of the brain to break down.
• Does not occur until 30-50 years of age so this
  is why it can be passed along.
• There is a genetic test that can test the
  presence of the allelewould you want to know?

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Is it Dominant or Recessive
I.
2
1
4
3
II.
6
5
4
3
1
2
III.
1
2
3
Dominant, only one parent has the disorder.
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Is it Dominant or Recessive
I.
2
1
4
3
II.
6
5
4
3
1
2
III.
1
2
3
Recessive, neither parent has the disorder. Both
are heterozygous.
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Section 12.2 When Heredity Follows Different Rules

• NCSCOS 3.03

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Complex Patterns of Heredity

• Most traits are not simply dominant or recessive
• Incomplete dominance when the phenotype of the
  heterozygous individual is in between those of
  the two homozygotes (homozygous dominant
  homozygous recessive)

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• Red flower color (RR) is dominant
• White flower color (rr) is recessive
• Pink colored flowers (Rr)

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• Codominace when the alleles of both homozygotes
  (BB or WW) are expressed equally in the
  heterozygous individual
• If a black chicken (BB) is crossed with a white
  chicken (WW), all offspring will be checkered
• Example sickle-cell anemia

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• Sex-linked traits when traits are controlled by
  genes located on sex chromosomes
• X-linked disorders generally passed on from
  mother to son
• The genetic abnormality is found on the X
  chromosome
• Females are XX, males are XY

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• If a female has a normal X, it would be dominant
  over the defective X
• In males, it will not be masked by a
  corresponding dominant allele because they have a
  Y chromosome
• Ex hemophilia Lesch-Nyhan syndrome

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• Y-linked disorders only passed on from father
  to son
• Examples excessive hair growth of the ears
  male infertility

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• Polygenic inheritance when a trait is
  controlled by many genes
• Examples height, eye color, skin color, blood
  type

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Changes in Chromosomal Numbers

• Humans have 23 pairs of chromosomes (46 total)
  more or less disorder
• Autosomes a non-sex chromosome
• Known as chromosomes 1-22

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• Sex chromosomes 23rd pair in humans that
  determine a persons sex
• Example Downs Syndrome (trisomy 21)

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8 Environmental Factors That Can Also Influence
Gene Expresssion

• 1. temperature
• 2. light
• 3. nutrition
• 4. chemicals

• 5. infectious agents
• 6. hormones
• 7. structural differences
• 8. age

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12.3 Complex Inheritance in Humans
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Are your earlobes attached or unattached?
Unattached
Attached
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Can you roll your tongue?
Cannot roll
Can roll
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• Do you have dimples?
• Are you right-handed or left-handed?
• Do you have Hitchhikers thumb?
• Do you have naturally curly or straight hair?
  (consider curly if not straight, ex. wavy)
• Do you have a cleft in your chin?
• Do you have allergies? (grass, mold, foods, etc)

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Clasp your hands together

• Which thumb is on top, left or right?

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Is your hairline straight, or does it come to a
point in the middle of your forehead (aka widows
peak)?
Straight
Widows peak
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(12.3) Complex Inheritance in Humans

• Skin color, eye color, height polygenic
  inheritance
• Hemophilia, red-green colorblindness, male
  patterned baldness sex-linked traits

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Complex Inheritance in Humans

• Sickle Cell Anemia an example of codominance.
• Homozygous normal normal red blood cells (RBC)
• Homozygous for sickle cell RBC have sickled
  shape causes poor blood flow, pain, clots, etc.
• Heterozygous produce normal and sickled RBC
  lead a normal life

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Sickle Cell Anemia
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Complex Inheritance in Humans

• Blood Typing multiple alleles
• One gene I, with multiple alleles
• IA, IB, i
• IA and IB are codominant over i
• Of the three, each person carries two leads to
  multiple blood types
• Fig 12.17 page 325

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