Codominance and Incomplete Dominance



Codominance and Incomplete Dominance

Definitions

Incomplete Dominance:

e.g. Snapdragon Flowers: RR(red) x rr(white) = Rr (pink)

Codominance:

e.g. Coat Colour in Horses: CRCR x CWCW = CRCW (roan – a coat with red and white hairs)

1. Identify each of the examples below as incomplete dominance (ID) or codominance (C) by circling your choice.

Then, using the above examples, practice writing out genotypes for the phenotypes listed in each set. The "medium" trait is always heterozygous.

a) Birds can be blue, white, or white with blue-tipped feathers.

ID/C

Genotypes: blue _____, white ______, white with blue tips ______

b) Flowers can be white, pink, or red.

ID/C

Genotypes: white _____, pink ______, red ______

c) A Hoo can have curly hair, spiked hair, or a mix of both curly and spiked.

ID/C

Genotypes: curly _____, spiked ______, curly and spiked ______

d) A Sneech can be tall, medium, or short.

ID/C

Genotypes: tall _____, medium ______, short ______

e) A Bleexo can be spotted, black, or white.

ID/C

Genotypes: spotted _____, black ______, white ______

3. In Smileys, eye shape can be starred, circular, or a circle with a star.

a) Is this an example of incomplete dominance or codominance?

b) Write the genotypes for the pictured phenotypes below.

[pic]

_________ _________ _________

4. A star-eyed and a circle eyed Smiley are crossed. Using a punnett square, determine the phenotypes and genotypes of their offspring.

5. A circle-star eyed and a circle eyed Smiley are crossed. Using a punnett square, determine the phenotypic and genotypic ratios of their offspring.

6. Two circle-star eyed Smileys are crossed. Using a punnett square, determine the phenotypic and genotypic ratios of their offspring.

Multiple Allele Traits

|Eye color is more complex than two genes  |

In humans, three genes involved in eye color are known. They explain typical patterns of inheritance of brown, green, and blue eye colors. However, they don't explain everything. Grey eye color, hazel eye color, and multiple shades of blue, brown, green, and grey are not explained. The molecular basis of these genes is not known. What proteins they produce and how these proteins produce eye color is not known. Eye color at birth is often blue, and later turns to a darker color. Why eye color can change over time is not known. An additional gene for green is also postulated, and there are reports of blue-eyed parents producing brown-eyed children (which the three known genes can't easily explain [mutations, modifier genes that suppress brown, and additional brown genes are all potential explanations]).

The known Human Eye color genes are: EYCL1 (also called gey), the Green/blue eye color gene, located on chromosome 19 (though there is also evidence that another gene with similar activity exists but is not on chromosome 19). EYCL2 (also called bey1), the central brown eye color gene, possibly located on chromosome 15. EYCL3 (also called bey2), the Brown/blue eye color gene located on chromosome 15. A second gene for green has also been postulated. Other eye colors including grey and hazel are not yet explained. We do not yet know what these genes make, or how they produce eye colors. The two gene model (EYCL1 and EYCL3) used above explains only a portion of human eye color inheritance. Both additional eye color genes and modifier genes are almost certainly involved.

Codominance, Multiple Alleles and Blood Types

Read the following information and fill in the remaining spaces in the table.

A number of human traits are the result of more than two types of alleles. Such traits are said to have multiple alleles for that trait.

Blood type is an example of a common multiple allele trait. There are three different alleles for blood type, (A, B, & O). A is dominant to O. B is also dominant to O. A and B are codominant.

Type O Blood: Universal Donor as it contains no A or B antigens, so the receivers' blood will not clot when given O blood.

Type AB Blood: Universal Receiver as it contains no Anti-A or Anti-B antibodies in its plasma. It can receive all blood types.

Antigen: Protein on the surface of the blood cell. (Allele A makes A antigen. Allele B makes B antigen. Allele O makes no antigens.)

Antibody: Protein in blood plasma that reacts with specific antigens that enter the blood (usually something that isn't supposed to be there!). (Ex.: Anti-A is an antibody that recognizes A-antigen, binds to it, then causes clumping together or clotting of similar A-antigens.)

Try this: Fill in the remaining spaces in the chart below.

|Blood Type Chart |

|Parent 1 Phenotype |

|Parent 1 Genotype |

|Parent 2 Phenotype |

|Parent 2 Genotype |

|Child |

|Phenotype |

|Child Genotype |

| |

|Type O |

|i i |

|Type O |

| |

| |

| |

| |

|Type A |

|IA IA |

|Type A |

| |

| |

| |

| |

|Type B |

|IB IB |

|Type B |

| |

| |

| |

| |

|Type A |

| |

|Type O |

| |

| |

| |

| |

|Type B |

| |

|Type O |

| |

| |

| |

| |

|Type A |

| |

|Type B |

| |

| |

| |

| |

Distribution and Characteristics of Human Blood Factors

|Blood |Distribution in USA |Antigen on Red |Antibody in Serum|Will Clot with |Can Receive |Can Give to: |

|Type |(%) |Blood Cell |Plasma |Blood From These |From | |

| | | | |Donors | | |

|O |48 |None |Anti-A, Anti-B |A, B, AB |O |All |

|A |42 |A |Anti-B |B, AB |A & O |A & AB |

|B |7 |B |Anti-A |A, AB |B & O |B & AB |

|AB |2 |A & B |None |None |All |AB |

 

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