COMPLEX PATTERNS OF INHERITANCE

Patterns of inheritance that do NOT follow Mendel’s law.

Review Mendel’s Simple Genetics

• There are 2 alleles for each trait.• An allele is either Dominant

• Which is represented by upper case, D.• The phenotype is visible w/just one allele.• So the phenotype is visible with either a

heterozygous or a homozygous genotype.

• Or Recessive• Which is represented by lower case, d.• The phenotype is visible ONLY w/2 recessive

alleles: that’s the homozygous recessive genotype.

Review Mendel’s Simple Genetics

• Mendel’s Law of Segregation: these alleles will separate from each other during the formation of gametes.

• Mendel’s law of Independent Assortment: theses alleles will be assorted into the newly formed gametes independently of each other.

Incomplete Dominance

• The phenotype of the heterozygote is in between those of the two homozygotes.

– It is an intermediate, and a third phenotype appears when the heterozygote is present

– In heterozygotes, neither allele is completely dominant.

Example-Snapdragons

• RR = Red• R’R’ = White• RR’ = Pink

R R

R’

R’

CODOMINANCE

• The phenotypes of both alleles are equally expressed in the heterozygote.

– Example: Horses

• HRHR = Red Horse

• HWHW = White Horse

• HRHW = Roan Horse

HR HR

HW

HW

Red

Horse

White

Horse

Roan

Horse

CODOMINANCE

Multiple Alleles

• Traits controlled by more than 2 alleles (more than 2 phenotypes)

• A diploid individual can only have 2 alleles of a trait.

– Example: Blood type = 3 different alleles:

IA, IB, i

Blood TypeBlood Type That Can Be

Received

Type of Antigens Present

Type of Antibodies Produced

A ( IA ) A and O A antigens B antibodies

B ( IB ) B and O B antigens A antibodies

O ( i )Universal

DonorOnly O No antigens

A and B antibodies

AB ( IAIB ) Universal Recipient

All typesA and B antigens

No antibodies

Multiple Alleles

HR HR

HR

HR

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

•Phenotypes usually show a continuous range or variability.

- Example: Height = A, B, and C all contribute to height.

Polygenic Traits

A a C c

B b

•Tallest = AABBCC

•Shortest = aabbcc

•In between = AbBbCc

•Polygenic traits include: skin color, body weight, eye color, hair color, and height.

Polygenic traits