Download 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?
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
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