Incomplete Dominance, Co-Dominance, and Sex-linked ... · CO-DOMINANCE –BLOOD TYPES Human blood...

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NON-MENDELIAN GENETICS

Incomplete Dominance, Co-Dominance, and Sex-linked dominance

INCOMPLETE DOMINANCE

Two alleles – dominant and recessive

Genotypes are the same as simple Mendeliangenetics: AA, Aa, aa

Some phenotypes are different than simple Mendelian genetics

When an organism is homozygous dominant, they look dominant

When an organism is homozygous recessive, they look recessive

When an organism is heterozygous, they are a blend of dominant and recessive

Example – flower color

Red petals are dominant to white petals

A flower with red petals is crossed with a flower that

has white petals

The offspring will have pink petals

If two heterozygous flowers are crossed, they

will give a 1:2:1 genotypic ratio AND a 1:2:1

phenotypic ratio

Genotypic

Phenotypic

If two heterozygous flowers are crossed, they

will give a 1:2:1 genotypic ratio AND a 1:2:1

phenotypic ratio

Genotypic

Phenotypic

2 pink

1 white

CO-DOMINANCE

The alleles are both dominant; neither is

recessive

Both alleles are FULLY expressed in the

offspring

Co-dominant alleles are written as capital

letters with superscripts

CO-DOMINANCE

Example: In chickens, the alleles for feather

color are co-dominant (letter for the feather

color trait is F). One allele is for white feathers,

the other is for black feathers

CO-DOMINANCE

Black feathers = FB

White feathers = FW

CO-DOMINANCE

Black feathers = FB

White feathers = FW

Genotype Combinations

CO-DOMINANCE

Black feathers = FB

White feathers = FW

CO-DOMINANCE

Black feathers = FB

White feathers = FW

FBFB = a chicken with only black feathers

FWFW = a chicken with only white feathers

FBFW = A chicken with BOTH white AND black feathers

CO-DOMINANCE

CO-DOMINANCE – BLOOD TYPES

Human blood types are an example of co-dominant traits with multiple alleles (3)

Type O is recessive

Types A and B are codominant with each other (neither dominates over the other)

They are both fully dominant to type O

The dominant alleles are written with the letter I and a superscript. The recessive allele is a lowercase i

Type A –

Type B –

Type AB –

Type O -

Type A – IAIA or IAi

Type B – IBIB or IBi

Type AB – IAIB

Type O - ii

Example cross

One parent is heterozygous for type A blood

The second parent is heterozygous for type B blood

Example cross

One parent is heterozygous for type A blood

The second parent is heterozygous for type B blood

What is the genotypic

ratio?

What is the phenotypic

ratio?

25% IAIB

25% IAi

25% IBi

25% ii

ratio?

25% IAIB

25% IAi

25% IBi

25% ii

ratio?

25% AB 25% A 25% B 25% O

BLOOD TYPE EXAMPLES

A child is born to a mother who is homozygous

for type B blood and a father who is

heterozygous for type A blood. What is the

probability the child will have type A blood?

Type B blood?

A child is born to a mother who has type AB

blood and a father who has type O blood. What

are the possible blood types for the child?

SEX-LINKED DOMINANCE

SEX-LINKED

Traits that are found on the X chromosome are

known as “sex-linked” since they are controlled

by one of the sex chromosomes

Since females have two X chromosomes, they

are more likely to be heterozygous carriers

(having one recessive X and one normal X)

SEX-LINKED

Since males only have one X chromosome

(from their mother), they are more affected

than females by recessive sex-linked traits

They either have the recessive trait or they don’t

(they cannot be heterozygous)

Examples: color-blindness and hemophilia

COLOR BLINDESS

SEX-LINKED

Example: A child is born to a mother who is a

carrier for hemophilia, and a father who does

not have the disease. Hemophilia is a recessive

sex-linked disease. What are the chances that

the child will be:

A boy with hemophilia?

A boy without hemophilia?

A girl with hemophilia?

A girl without hemophilia?

SEX-LINKED

PEDIGREE

Used to trace the inheritance of a trait through

several generations

Uses symbols to illustrate inheritance

PEDIGREES

PEDIGREE TYPES

Autosomal Dominant

Affected offspring cannot have non-affected parents

Examples

Huntington’s Disease

Achondoplasia

(dwarfism)

PEDIGREES

Autosomal Recessive

Non-affected parents can have affected offspring

(parents are carriers)

If both parents are affected,

they MUST have affected

offspring

Examples

Cystic Fibrosis

Albinism

Tay-Sachs Disease

PEDIGREES

X-Linked Dominant

Affected fathers must have affected daughters

Examples: Fragile X Syndrome (causes autism)

PEDIGREES

X-Linked Recessive

High incidence of affected males (since they have only one X

chromosome)

Examples

Hemophilia

Red/Green Color-blindness

PEDIGREES

PRACTICE

Identify this pedigree as autosomal recessive,

autosomal dominant, X-linked dominant, or X-

linked recessive

PRACTICE

linked recessive

PRACTICE

linked recessive

Incomplete Dominance, Co-Dominance, and Sex-linked ... · CO-DOMINANCE –BLOOD TYPES Human blood...

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