Incomplete Dominance and Co-dominance

Incomplete Dominance and Co-dominance SBI3U0 Mr. Tabone

Incomplete and Co-dominance • Not all traits are purely dominant or purely recessive • There is sometimes a blending of traits. This is called incomplete dominance • The heterozygous phenotype is a blending of the two traits • Sometimes both traits can be expressed. This is called co-dominance • The heterozygous phenotype displays BOTH traits

Incomplete Dominance • This occurs when neither allele is dominant over the other • Thus the traits blend • Heterozygous individuals display an intermediate trait • IE: In snapdragons white colour (CW) and red colour (CR) are not dominant over one another • Heterozygous (CRCW) snapdragons turn out to be pink

Snapdragons • Heterozygous plants produce both red and white pigments, resulting in a pink colour IWIW IRIR IRIW

Example • A geneticist notes that crossing a round radish with a long radish produces oval radishes. If two oval radishes are crossed, what would you expect for the genotypes and phenotypes of their offspring? Round (IRIR) ILIL Long (ILIL) IR IR Genotypes IRIL – 100% IRIL IRIL IRIL IRIL Phenotypes Oval – 100%

Co-dominance • In some cases both alleles for a trait are visible in the phenotype • This is called co-dominance • This system of inheritance is seen in chickens. The genes for black (FB) and white (FW) feather colour are co-dominant • Heterozygous birds have a mix of black feathers and white feathers

Chickens • d FBFB FBFW FWFW

Example • In cattle, when a white cow is mated with a red bull, roan coloured offspring result. Upon closer inspection, a roan coloured calf has both red and white hairs. What would be the genotypes and phenotypes of the offspring that result from mating a white cow with a roan bull? HR HR Genotypes White (HWHW) HW Red (HRHR) HW Phenotypes HWHR HWHR HWHR – 100% HWHR HWHR Roan– 100%

Multiple Allelism • Some traits have only two alleles for a certain trait • Seed colour (green vs. yellow) in pea plants • Seed shape (round vs. wrinkled) in pea plants • Some traits have multiple alleles for a certain trait • Blood type (A, B, O) in humans • This is called multiple allelism

Blood Types • All cells have small proteins and carbohydrates bound to the surface of the cells called antigens • In humans, red blood cells have three main types of antigens A, B, and O that determine blood type • There are several other classes of antigens that can determine blood type, but the ABO antigens are the most important for blood transfusions • The type of antigens a person has is determined by their genes

Antibodies • The alleles for A (IA) and B (IB) are co-dominant, while the allele for O (i) is recessive (to both) • The immune system generates antibodies, which are glycoproteins that detect foreign cells • Antibodies stick to specific antigens and signal white blood cells to destroy the foreign material • The antibodies that detect A and B antigens are immunoglobulin glycoproteins. • We call the one that attaches to A-antigens anti-A and the one that attaches to B-antigens anti-B

ABO Blood Types A - antigen O - antigen B - antigen Anti – A antibody Anti – B antibody

Blood Phenotypes IAIA and IAi Type A IBIB and IBi  Type B IAIB  Type AB ii  Type O • If a person is type A their immune system produces antibodies designed to attack B antigens • Vice versa for type B blood • Type O individuals develop both anti A and anti B antibodies

ABO Blood Phenotypes Blood Type – A IAIA IAi Anti – B antibody

ABO Blood Phenotypes Blood Type - B IBIB IBi Anti – A antibody

ABO Blood Types Blood Type - O Blood Type - AB ii IAIB No Antibodies Both Antibodies

Blood Transfusions • These antibodies present a problem with blood transfusions • Placing type B blood cells into an individual with anti-B antigens will trigger the immune system to destroy the donor blood cells • Similar things will occur with type A blood being donated to an individual with anti-A antigens

Blood Transfusions • Which blood can be accepted by a person with blood type; a) A b) B c) AB d) O

Rhesus (Rh) Factor • The Rhesus factor is another blood system based around the Rh– antigen • It is coded for on a separate gene from the ABO antigens • This means it is inherited separately • The phenotypes are termed; • Rh + and Rh – • Positive means the antigen is present on blood cells • Negative means the antigen is not present on blood cells • The Rh+ allele is dominant over the Rh- allele

Hemolytic • If an Rh+ fetus is growing within an Rh- mother there can sometimes be problems • The Rh+ blood cells of the fetus sometimes cross through the placenta into the mother’s blood stream • This causes the mother to develop anti-Rh antibodies • The anti-Rh antibodies can then pass through the placenta and cause the fetus’ blood cells to be broken down • Thus hemolytic disease (hemo = blood, lysis = loosen or destroy)

Multifactorial Inheritance • Some traits have very distinct phenotypes • Seeds are either round or wrinkled. There is no in between • Some traits however have a wide range of phenotypic expression • Height is a good example • This traits are controlled by multiple genes that interact to create a phenotype • These traits are called polygenic

Nature Vs. Nurture • A person’s phenotype is often determined not only by the genes they possess but by their environment • For example, exposure to sunlight can alter the skin tone of certain individuals (tanning) • This applies to both physical and behavioural traits. Everything is susceptible to both biological and environmental influences

Incomplete Dominance and Co-dominance