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

linked and Polygenic linked and Polygenic InheritanceInheritance

Incomplete DominanceIncomplete Dominance• OffspringOffspring have an appearance somewhat in in

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

RWRW

RWRW

RWRW

RWRW

WW

WW

RR RR

All Rr =All Rr = pink pink(heterozygous (heterozygous pink)pink)

FF11 generation generation

Incomplete DominanceIncomplete Dominance• Incomplete dominance occurs when two or more

alleles influence the phenotype• Neither allele is

completely dominant over the other

Incomplete DominanceIncomplete Dominance

CodominanceCodominance• BOTH allelesBOTH alleles are expressedare expressed equallyequally in in heterozygous heterozygous

individualsindividuals

• Neither allele is dominant over the otherNeither allele is dominant over the other– Example:Example: blood typeblood type

• Determined by whether or not you have A or B proteinsDetermined by whether or not you have A or B proteins• IIAA = A protein = A protein• IIBB = B protein = B protein• I = no proteinI = no protein

• Another exampleAnother example

Is the speckled chickenIs the speckled chicken

Codominance

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 Codominance ProblemAnother Codominance Problem

Example:Example: male Type O (ii) x female type AB (IAIB)

IAi IBi

IAi IBi

1/2 = IAi1/2 = IBi

i

IA IB

i

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))

CodominanceCodominance• Answer:Answer:

IAIB

ii

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

IB

IA i

i

Sex-linked TraitsSex-linked Traits• Traits (genes) located on the Traits (genes) located on the

sex chromosomessex chromosomes

• Sex chromosomes are Sex chromosomes are X and YX and Y– XXXX genotype for females genotype for females– XYXY genotype for males genotype for males

• Many Many sex-linked traitssex-linked traits carried carried on on XX chromosome chromosome

Sex – Linked Traits

• Example: Colorblindness– If the mother carries the colorblindness gene

on her X chromosome, her son could get it. – As long as one X chromosome

is ok, a female will not

express the trait XC

X y

X

XcX Xc y

X yX X

Sex-linked TraitsSex-linked Traits

Sex ChromosomesSex Chromosomes

XX chromosome - female Xy chromosome - male

Color-blindness

Example: Example: ColorblindnessColorblindness

Polygenic Inheritance

• Inheritance pattern controlled by two or more genes

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

• Genotypes are 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

a pedigree

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.