Mendelian Inheritance in Humans

Chapter 3

Gregor Mendel 1822-1884

Father of Genetics

P generation: pollen from stamens transferred to carpels

F1 generation

Cross-pollination Experiment Mendel’s 1st Law:

Law of Segregation

Allele pairs separate during gamete formation

Appearance:

P Generation

Genetic makeup:

Gametes

F1 Generation

Appearance: Genetic makeup:

Gametes:

F2 Generation

Purple flowers Pp

P p 1 2 1 2

P p

F1 sperm

F1 eggs PP Pp

Pp pp

P

p

3 : 1

Purple flowers

PP

White flowers

pp

P p

Punnett Square

Monohybrid Cross

Phenotype

Purple

Purple 3

Purple

Genotype

PP (homozygous)

Pp (heterozygous)

Pp (heterozygous)

pp (homozygous) 1

2

1

Ratio 1:2:1

White

Ratio 3:1

1

Segregation of alleles into eggs

Rr

Sperm

R r

R

r R R

R

r

Eggs

r R

r

r

Segregation of alleles into sperm

Rr

1 2

1 2

1 4

1 4

1 4

1 4

1 2

1 2

Probability Math Segregation of Albinism

Human Disorders

Autosomal Recessive:

Albinism

Cystic fibrosis

Sickle Cell Anemia

Tay Sachs Disease

Inherited Deafness

Autosomal Dominant:

Achondroplasia

Familial hypercholesterolemia

Huntington’s Disease

Marfan syndrome

P Generation

F1 Generation

YYRR

Gametes YR yr

yyrr

YyRr Hypothesis of dependent assortment

Hypothesis of independent assortment

Sperm Eggs

YR

Yr

yr YR

YR

yr

Eggs

YYRR YyRr

YyRr yyrr yR

yr Phenotypic ratio 3:1

F2 Generation (predicted offspring)

YYRR YYRr YyRR YyRr

YYRr YYrr YyRr Yyrr

YyRR YyRr yyRR yyRr

YyRr Yyrr yyRr yyrr

Phenotypic ratio 9:3:3:1

YR Yr yR yr

Sperm

1 2 1 4

1 4

1 4

1 4

1 4 3 4

1 2

1 2

1 2

1 4

9 16 3 16 3 16 1 16

1 4 1 4 1 4

Mendel’s 2nd Law: Law of Independent Assortment

Meiosis I

Metaphase II

Gametes

AaDd AaDd

AD Ad aD ad

AD

Ad

aD

ad

AADD Pigment Hearing

aadd Albino Deaf

AADd Pigment Hearing

AaDD Pigment Hearing

AaDd Pigment Hearing

AADd Pigment Hearing

AAdd Pigment

Deaf

AaDd Pigment Hearing

Aadd Pigment

Deaf

AaDD Pigment Hearing

AaDd Pigment Hearing

aadd Albino Hearing

aaDd Albino Hearing

AaDd Pigment Hearing

Aadd Pigment

Deaf

aaDd Albino Hearing

Dihybrid Cross

Dihybrid Cross Eye Color Incomplete Dominance

!  The expression of a phenotype that is intermediate to those of the parents •  R1R1 (red) x R2R2 (white) → R1R2 (pink)

Example: Sickle Cell Anemia in Humans

Multiple Alleles, Codominant Alleles Genes Can Interact to Produce Phenotypes

!  Epistasis •  A form of gene interaction in which one gene

masks or prevents expression of another gene •  Example: Bombay gene (h) can block expression

of blood types A and B

Sperm

BC bC Bc bc

BbCc BBCc BbCC BBCC

BbCC bbCC BbCc bbCc

Bbcc BBcc BbCc BBCc

BbCc bbCc Bbcc bbcc

BC

bC

Bc

bc

BbCc BbCc

1 4

1 4

1 4

1 4

1 4

1 4

1 4

1 4

9 16

3 16

4 16

Epistasis

Mendelian Genetics Applications

Mr. and Mrs. Waverly both have curly hair. Five of their children have curly hair. One child has straight hair, just likely Grandma Waverly. a. Which trait appears to be dominant? b. Which trait seems to be recessive? c. What are the probable genotypes of the entire Waverly family mentioned above? d. What is the probability of the Waverly's NEXT child having straight hair?

Mendelian Genetics Applications

A black guinea pig crossed with an albino guinea pig produces 12 black offspring. When the same albino guinea pig is crossed with a second black one, 7 blacks and 6 albinos are obtained.

What is the best explanation for this genetic situation? Write genotypes and gametes for the parents and offspring.

I

II

1 2 3

1 2

P

Pedigreesgree

Circles = Females Squares = Males Dark Color = Affected Individuals P = Proband

I

II 1 2 3

Male

Female

Mating Mating between relatives (consanguineous)

Parents and children. Roman numerals symbolize generations. Arabic numbers symbolize birth order within generation (boy, girl, boy)

Monozygotic twins

or

or

or

or

P P

d. 1910 d. 1932

Unaffected individual

Affected individual

Proband; first case in family that was identified

Known heterozygotes

Carrier of X-linked recessive trait

Indicates date of death

Pedigrees, cont.

Dizygotic twins

Offspring of unknown sex

Aborted or stillborn offspring

Deceased offspring

Questionable whether individual has trait

Asymptomatic/ presymptomatic

Infertility

or

? ?

Pedigrees, cont.

Pedigree Applications

The pedigree below traces the inheritance of alkaptonuria, an autosomal biochemical disorder. Affected individuals, indicated here by the colored circles and squares, are unable to break down a substance called alkapton, which colors the urine and stains body tissues. Does alkaptonuria appear to be caused by a dominant allele or by a recessive allele? Fill in the genotypes of the individuals whose genotypes can be deduced. What genotypes are possible or each of the other individuals? (Use A or a for the gene in question)

Alcaptonuria

George Arlene

Sandra Tom Sam Wilma Ann Mike

Dan Alan Tina Carla

Carl