Incomplete & Codominance

Reviewing

All photos from: http://www.naturalselectionreptiles.com/Genetics.html

Incomplete Dominance• (a.k.a. partial dominance or blending) 

• occurs when both alleles contribute to a trait that is not like either parent. 

• this condition is only seen in the heterozygous individual 

– ex.. Snap Dragons or Four O’Clocks or Morning Glory or Primrose (Flowers)

– Curly (homozygous) or straight (homozygous), but if you are heterozygous you have wavy hair.

Example: Snap Dragons

 CR = red  CW =  white  Parents are both true breeding red and white.Parents: Red  X  White

CRCR X CWCW

Genotype: All CRCW

Phenotype: All Pink

CR CR

CW CRCW CRCW

CW CRCW CRCW

http://www.usask.ca/biology/genetics/extensions/snapdragon.jpg

F1 x F1 Cross

CRCW X CRCW

Remember: CR = red  CW =  white 

Genotypic Ratio:1 CRCR : 2 CRCW : 1 CWCW

Phenotypic ratio:1 Red: 2 Pink: 1 White

CR CW

CR CRCR CRCW

CW CRCW CWCW

***NOTE:  Because there is no dominance in the heterozygote the phenotypic ratio is identical to the genotypic ratio**** 

http://www.visionlearning.com/library/modules/mid129/Image/VLObject-3228-050216120240.jpg

• http://courses.bio.psu.edu/fall2005/biol110/tutorials/tutorial5_files/figure_14_9.gif

http://library.thinkquest.org/20465/media/flowers.gif

How the coloring works . . .

• CRCR = have enough pigment to be red.

• CWCW = flowers don’t have any pigment.= no color

• CRCW = only have enough pigment to be pink.

Codominance• the two dominant genes are expressed at the

same time 

• both traits appear in the heterozygous individual at the same time

– ex.. Roan (Red or Blue) coat color in horses and cattle 

– Chestnut and White color in horses are both dominant traits; the heterozygous individual is a palomino (golden).

Red Roan Example: http://www.naturalselectionreptiles.com/Genetics/exCodominance.jpg

Blue Roan Example: http://www.virginiacowboy.com/sitebuilder/images/DSC_0040-803x536.jpg

Red Roan Example:http://www.brokenlfarm.com/images/sale/kbrowdypetejr/102_8424.JPG

Coat Color in Horses• C= Color gene• CRCR  = Red • CWCW = White • CRCW = Red Roan If you cross a true breeding Red horse with a true breeding white

horse, what ratios will you get?

Genotypic Ratio:All CRCW

Phenotypic Ratio:All Red Roan

CR CR

CW CR CW CR CW

CW CR CW CR CW

Example: MN Blood Group

• MN blood group – is characterized by a certain type of glycoprotein on the surface of the RBCs. 

• There are two forms: M and N• Designate is L• LMLM = M• LNLN = N• LMLN  = MN type 

MN Blood Group• LMLM = M• LNLN = N• LMLN  = MN type 

What types of offspring would be produced from a cross of a homozygous M type with an MN type?Parents: LMLM X LMLN

Genotypic Ratio:1 LMLM : 1 LMLN

Phenotypic Ratio:1 M blood type: 1 MN blood type

LM LM

LM LMLM LMLM

LN LMLN LMLN

Multiple Alleles• More than two alleles for a gene

• O (ii) is recessive to IA and IB.• Genotypes IAIA and IAi result in Type A phenotype.• Genotypes IBIB and IBi result in Type B phenotype.• Genotype IAIB results in Type AB phenotype (codominant).• Genotype ii results in Type O phenotype.

• Surface proteins are like nametags and antibodies clump up foreign blood

• Useful in blood transfusion and organ transplant

Resource Sites• Incomplete & Codominance Explained:

– http://www.hobart.k12.in.us/jkousen/Biology/inccodom.htm• Codominance - Incomplete Dominance (Brightstorm Video)

– http://www.brightstorm.com/science/biology/mendelian-genetics/codominance-incomplete-dominance/

• Practice: Drag and Drop Pedigrees– http://www.zerobio.com/drag_gr11/pedigree/pedigree4.htm

• More Practice from KSU:– http://www.ksu.edu/biology/pob/genetics/incom.htm