Incomplete Dominance,Incomplete Dominance,Codominance, Sex-Codominance, Sex-

linked and Polygenic linked and Polygenic InheritanceInheritance

Mendel’s Dominance• Mendel’s rule of dominance was

complete dominance– Homozygous dominant organisms have

the dominant phenotype– Heterozygous organisms have the

dominant phenotype– Example: Both PP and Pp plants have

the dominant PURPLE phenotype (P=purple and p=white flowers)

Incomplete DominanceIncomplete Dominance• All alleles are dominant for the trait but different.All alleles are dominant for the trait but different.• OffspringOffspring have an appearance somewhat in betweenin between

the phenotypes phenotypes of the two parents – “Mixed”– Blended

RWRW

RWRW

RWRW

RWRW

WW

WW

RR RR

All All RWRW = = pink pink(heterozygous (heterozygous pink)pink)

RR = pure red RR = pure red flowerflowerWW = pure whiteWW = pure white

FF11 generation generation

Incomplete DominanceIncomplete Dominance• Incomplete dominance occurs when two or more dominant alleles are present.• Neither allele is completely dominant over the other.• The offspring produced are a blending of both alleles.

• P = RR (red) x WW (white) • F1’s = 100% RW (pink)

Incomplete DominanceIncomplete Dominance• F2 Generation

RW X RW (pink) (pink)

RW

RW

WW RW

RW RR

Offspring

25% - WW, white50% - RW, pink25% - RR, red

CodominanceCodominance

• All alleles are dominant for the trait butAll alleles are dominant for the trait but

have a different phenotype.have a different phenotype.

• BOTH allelesBOTH alleles are expressedare expressed equallyequally inin heterozygous individuals.heterozygous individuals.

• Neither allele is dominant over the other.Neither allele is dominant over the other.

Codominance example:

RR = pure red bullWW = pure white cow

All offspring have the genotype = RW

Phenotype = All offspring are roan cows. Roan = a mix of red & white hairs.

RR

WW

RW

Codominance example #2

White hen Black rooster

WW BB

BW

X

Checkered chicken

W W

B

B

BW BW

BW BW

F1’sGenotypes = 100% BW

Phenotypes = 100% checkered

Multiple Alleles•When there are 4 or more possible phenotypes for a particular trait, then more than 2 alleles for that trait must exist in the population.

•There may be multiple alleles within the population, but individuals have only two of those alleles.

•Why? Because you have only two biological parents. 

•Half of your genes (alleles) from Pa, & the other half from Ma, so you end up with two alleles for every trait in our phenotype.

ALLELE IA IB i

CODES FOR Type "A" Blood Type "B" Blood Type "O" Blood

•Human blood type is an example of multiple alleles.

•Blood exists as four possible phenotypes: A, B, AB, & O. •There are 3 alleles for the blood gene BUT remember, you only have 2 of them 1 from Ma & 1 from Pa.

* The alleles are as follows:  

Note: *The allele for "O" (i) is recessive. *The alleles for "A" & "B“ are dominant.

BLOOD TYPES

With three alleles we have a higher number of possible combinations in creating a genotype.  

BLOOD TYPES cont’d

* There are 6 different genotypes & 4 different phenotypes for blood type.

• "A" & "B" blood can be either homozygous (IAIA or IBIB) or heterozygous (IAi or IBi), with one recessive allele for "O“.• "O" blood is homozygous recessive (ii). • What's the deal with "AB" blood?

AB blood is codominant!

The "A" trait & the "B" trait appear together in the phenotype.

Multiple Allele & Multiple Allele & Codominance ProblemCodominance Problem• Example:homozygous male Type B (IBIB) x

heterozygous female Type A (IAi)

IAIB IAIB

IBi IBi

1/2 = IAIB

1/2 = IBi

IA

IB IB

i

Another Multiple Allele & Another Multiple Allele & Codominance ProblemCodominance Problem

Example:Example: A woman with type O blood and a man with type AB blood are expecting a child. What are the child’s possible blood types?

IAi IBi

IAi IBi

50% = IAi, type A.50% = IBi, type B.

i

IA IB

i

Multiple Alleles & Multiple Alleles & CodominanceCodominance

• QuestionQuestion::If a boy has a blood type O and his sister If a boy has a blood type O and his sister has blood type AB, has blood type AB, what are the genotypes what are the genotypes and phenotypes of their parents?and phenotypes of their parents?

boy - boy - type O (ii) type O (ii) X girl - X girl - type AB (Itype AB (IAAIIBB))

Hint: You have to work this one backwards. Hint: You have to work this one backwards. Fill in what you know to find the parents.Fill in what you know to find the parents.

Multiple Alleles & Multiple Alleles & CodominanceCodominance

•Answer:Answer:

IAIB

ii

Parents must be:Parents must be:genotypesgenotypes = IAi and IBiphenotypesphenotypes = A and B

IB

IA i

i

girl

boy

Sex-Linked Traits• These are recessive traits located on the sex chromosomes, #23.

•Females genotype is XX.

•Male genotype is XY.

•Many of these traits are found on the X chromosome.

•Sex chromosomes are non-matching Sex chromosomes are non-matching chromosomes in males & females.chromosomes in males & females.

•WhateverWhatever is on the is on the XX chromosome, the chromosome, the male will get! There is nothing on the male will get! There is nothing on the

Y Y chromosome to cover it up!chromosome to cover it up!

•For females to get these recessive For females to get these recessive traits, the trait must be present on both traits, the trait must be present on both

X X chromosomes.chromosomes.

•When a female has the trait on one When a female has the trait on one XX chromosome, she is a carrier and can chromosome, she is a carrier and can pass the trait on to her son.pass the trait on to her son.

Sex-Linked Traits

Sex – Linked Traits

• Example: Colorblindness– A mother carries the

colorblindness gene on her X chromosome and has a 25% chance of passing on the disorder to her son.

– X Y = colorblind male.– X X = carrier female.

X

X X

Y

XcX X X

X Y

c

X Yc

c

c

Sex-linked TraitsSex-linked TraitsExample 2: Example 2: ColorblindnessColorblindness

A colorblind father and a carrier A colorblind father and a carrier mother are having child. Will mother are having child. Will their child be colorblind?their child be colorblind?

Probability:Probability:25% colorblind female, X25% colorblind female, X XX25% colorblind male, X Y25% colorblind male, X Y25% normal female, XX25% normal female, XX25% normal male, XY25% normal male, XY*Mom gives her bad X to her *Mom gives her bad X to her son.son.*Dad gives his bad X to his *Dad gives his bad X to his daughter.daughter.

X

Y

X X

X X X X

X Y X Y

c

c c c

c

c c

c c

Polygenic Inheritance

• Inheritance pattern controlled by two or more genes

• Genes can be on the same chromosome or on different chromosomes

• Genotypes written as AA, Aa, or aa except you will have three or more genes each with a dominant and recessive trait

• Skin color is influenced by the additive effects of melanin by three to six genes. Eye color is also a result of melanin added from multiple genes.

Polygenic Inheritance of Skin ColorGametes ABC ABc AbC Abc aBC aBc abC abc

ABC AABBCC AABBCcAABbCc

AaBBCC

ABc

AbC

Abc AAbbcc

aBC aaBBCc

aBc

abC

abc aaBbcc

The table shows typical ranges of skin color combinations. Other polygenic influenced conditions include Spina bifida and Cleft palate.