Chapter 12: Patterns of Heredity & Human Genetics
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Transcript Chapter 12: Patterns of Heredity & Human Genetics
Chapter 12:
Patterns of Heredity &
Human Genetics
Section 1: Mendelian
Inheritance of Human Traits
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?
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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
Tay-Sachs 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.
2.
3.
4.
cleft chin
widow’s peak hairline
unattached earlobes
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.
Chapter 12
Section 2: When Heredity
Rules are Different
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 flower color (rr) is recessive
Pink 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: height, eye color, skin color, &
blood type
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
Sex chromosomes: 23rd pair in humans
that determine a person’s 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