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Chapter 12
Patterns of Heredity
& Human Genetics
Section 12.1
Mendelian Inheritance
of Human Traits
NCSCOS: 3.03
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.
 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
Label the following symbols from a
pedigree:
Carrier
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
Constructing and Reading a pedigree
I.
1
2
II.
1
2
3
4
5
III.
1
2
3
4
5
*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.
1
2
3
4
5
6
7
III.
1
2
3
4
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?
5
 What
does a half shaded circle or square
represent?

A carrier
 Define

a carrier:
A heterozygous individual
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.
Facts about X-linked Disorders
*carried 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
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
Simple Recessive Heredity
 Most
genetic disorders are caused by
recessive alleles. This means the disorder
is inherited when both parents have a
recessive allele.
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.
Pedigree for Cystic Fibrosis
 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
 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 TaySachs disease usually die by age 4, from
recurring infection.
Pedigree for Tay-Sachs
Simple Dominant Heredity
 Dominant
disorders are inherited as
Mendel’s rule of dominance predicted:
Only one dominant allele has to be
inherited from either parent.
Common Dominant Traits &
Disorders
 Simple




Dominant Traits
1. cleft chin
2. widow’s peak hairline
3. unattached earlobes
4. almond shaped eyes
Disorders: Huntington’s 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 allele…would you want to
know?
Is it Dominant or Recessive…
I.
1
2
3
4
II.
1
2
4
3
5
III.
1
2
3
Dominant, only one parent has the disorder.
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Is it Dominant or Recessive…
I.
1
2
3
4
II.
1
2
4
3
5
6
III.
1
2
3
Recessive, neither parent has the disorder. Both are heterozygous.
Section 12.2 When
Heredity Follows Different
Rules
NCSCOS: 3.03
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)
 Red
flower color (RR) is dominant
 White
 Pink
flower color (rr) is recessive
colored flowers (Rr)
 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
 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
 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
 Y-linked
disorders: only passed on from
father to son
 Examples:
excessive hair growth of the
ears & male infertility
 Polygenic
inheritance: when a trait is
controlled by many genes
 Examples:
blood type
height, eye color, skin color, &
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
chromosomes: 23rd pair in humans
that determine a person’s sex
 Sex
 Example:
Down’s Syndrome (trisomy 21)
8 Environmental Factors That Can
Also Influence Gene Expresssion

1. temperature

5. infectious agents

2. light

6. hormones

3. nutrition

7. structural
differences

4. chemicals

8. age
12.3 Complex Inheritance
in Humans
Are your earlobes attached or
unattached?
Unattached
Attached
Can you roll your tongue?
Can roll
Cannot roll
 Do
you have dimples?
 Are you right-handed or left-handed?
 Do you have Hitchhiker’s 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)
Clasp your hands together…
 Which
thumb is on top, left or right?
Is your hairline straight, or does it come to
a point in the middle of your forehead
(aka widow’s peak)?
Straight
Widow’s peak
(12.3) Complex Inheritance in
Humans
 Skin
color, eye color, height =
polygenic inheritance
 Hemophilia, red-green
colorblindness, male patterned
baldness = sex-linked traits
Complex Inheritance in Humans
Cell Anemia – an example of
codominance.
 Sickle



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
Sickle Cell Anemia
Complex Inheritance in
Humans





Blood Typing –
multiple alleles
One gene – I, with
multiple alleles
A B
 I , I , 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