Unit: Mendelian Genetics

For example:

◦Some alleles are neither dominant nor recessive.

◦In most organisms, the majority of genes have more than two alleles.

◦Many important traits are controlled by more than one gene.

Incomplete dominance - acondition in which offspringdisplay an intermediate traitbetween two parents.

Example: The F1 generation

produced by a cross between many red-flowered (RR) and white-flowered (WW) plants consists of pink-colored flowers (RW)

Codominance- a phenomenon in whichtwo dominant alleles are expressed at thesame time.

Example #1 Speckled Chickens: In certain varieties of chicken, the allele for black feathers (B) is codominant with the allele for white feathers (W).

◦Heterozygous chickens have a color described as “erminette,” (BW) speckled with black and white feathers.

If two erminettechickens werecrossed, what is the probability that: 

a. They would have a black chick?

b. They would have a white chick?

In horses and cattle, red coloration (R) and white coloration (W) are codominant. Individuals with each allele are roan (RW)

Take two minutes to summarize whatyou have learned by stating thesimilarities & differences that existbetween traits that exhibit codominanceand traits that exhibit incompletedominance.

Multiple alleles- a term used todescribe a genewith more than twoalleles.

Example: ABO bloodgroups in humans.

Alleles: IA IB

and i

• A person only has two of these three alleles though! There are 4 possible blood types.

Dominance IA and IB are bothdominant over i, butneither is dominant over the other (A=B) 

Type A blood Type B blood IA IA IB IB IA i IB i

Type AB blood Type O blood IA IB ii

Answer: If the wrong blood type is used in a transfusion, antibodies in the recipients blood couldattack the new blood received and make the person very ill.

Why are they called that?

Since there are no antigens on thesurface of Type O their blood cells, there is

norisk of the antibodies in the recipients bloodattacking the new blood received from atransfusion.

Why are they called that?

Since people with AB blood have noantibodies for A or B, they can safely receiveany type of blood in a transfusion.

Polygenic Traits: Traits that are controlled bytwo or more genes.

Examples: human skin color, eye color, height

◦The genes that code for the trait may be scattered along the same chromosome, or located on different chromosomes.

◦Polygenic traits often show a wide range of phenotypes.

Take two minutes to summarize what you havelearned regarding the similarities & differences

that exist between traits that have multiplealleles and traits that are referred to as

polygenic.

◦The characteristics of any organism are not determined solely by the genes that organism inherits.

◦Genes provide a plan for development, but how that plan unfolds also depends on the environment.

◦The phenotype of an organism is only partly determined by its genotype.

◦Western white butterflies that hatch in the summer have different color patterns on their wings than those hatching in the spring.

◦Scientific studies show that butterflies hatching in springtime had greater levels of pigment in their wings than those hatching in the summer.

◦In other words, the environment in which the butterflies develop influences the expression of their genes for wing coloration.

◦In order to fly effectively, the body temperature of the Western white butterfly needs to be 28–40°C.

◦Since it is cooler in the spring, more pigmentation allows a butterfly to reach the warm body temperature faster.

◦Similarly, in the hot summer months, less pigmentation prevents the butterflies from overheating.

Take two minutes to summarize what you have

learned regarding the role the environment has

in gene expression.