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

Symbols of a pedigree:***need to know***

Carrier –

heterozygous individual

Constructing and Reading a pedigree

***an inverted “v” means the married couple had twins*** (know)

****a horizontal line between a male and female with a

strike means the persons are divorced.**** (Need to

know)

Constructing and Reading a pedigree

***Roman Numerals (I, II, III) refers to the generations.***

*Arabic numbers refers to individuals. (1, 2, 3, 4, 5, …)

321 4 5 6

1

1

2

2

3 4 5

I.

III.

II.

Reading the pedigree…

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?

321 4 5

61

1

2

2

3 4 5

I.

III.

II.

7

Types of Pedigrees

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.

•*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

•Facts about X-linked Disorders

*carried on the X-chromosome

Colorblindness Pedigree

Colorblindness Tests

Test Name: Ishihara Test

Colorblind sees:

yellow square

Normal color:

yellow square &

faint brown circle

Colorblind sees:

the number 17

Normal Color sees:

the number 15

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.

– blind, deaf, and unable to swallow. Muscles begin to atrophy and paralysis sets in and other neurological symptoms

– Death usually by age 4

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. unattached earlobes– 3. 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…

Dominant, only one parent has the disorder.

3

21 4

56

1

1

2

2 3

4

3

I.

III.

II.

Is it Dominant or Recessive…

Recessive, neither parent has the disorder. Both

are heterozygous.

3

21 4

56

1

1

2

2 3

4

3

I.

III.

II.

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

• 2. light

• 3. nutrition

• 4. chemicals

• 5. infectious agents

• 6. hormones

• 7. structural differences

• 8. age

*** know examples***