Mendelian Inheritance in Humans

Chapter 3

Law of Segregation

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

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

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 3 16

1 4 1 4 1 4

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 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