Section 12.1
Mendelian Inheritance of Human Traits
NCSCOS: 3.03

 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.
II.
1 2
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, …)
6

I.
II.
Reading the pedigree…
1 2
III.
1
1 2 3 4 5 6 7
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

Colorblind sees: yellow square
Colorblindness Tests
Normal color: yellow square & faint brown circle
Colorblind sees: the number 17
Normal Color sees: the number 15
Test Name: Ishihara Test

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

 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.

 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.

 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

 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?

I.
Is it Dominant or Recessive…
1 2 3 4
II.
1 2 3
4 5
6
III.
1 2 3
Dominant, only one parent has the disorder.

Is it Dominant or Recessive…
I.
1 2 3 4
II.
1 2 3
4 5
6
III.
1 2 3
Recessive, neither parent has the disorder. Both are heterozygous.

NCSCOS: 3.03

 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

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

 Sex chromosomes: 23 rd 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
 3. nutrition
 4. chemicals
 6. hormones
 7. structural differences
 8. age

Are your earlobes attached or unattached?
Attached
Unattached

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)

 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

 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

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