Different Forms of Genetic Expression

Genetic expression is how a gene is “shown” (phenotype)

We have been using a model known as complete dominance.

Complete Dominance The form of genetic expression discovered

by Gregor Mendel. Only 2 alleles for each gene. One is dominant – shown with a capital

letter. One is recessive – shown with a lower case

letter. If one dominant allele is present, that is the

only trait that will be expressed!

Incomplete Dominance Dominant alleles combine to form a new trait. Each allele is represented with its own capital letter.

R = red W = white

A red plant would have the genotype _______. A white plant would have the genotype _______. The heterozygous plant, RW, has two dominant

alleles, it would form a new phenotype which mixes the two. RW = _________

Incomplete Dominance – Practice!

The two alleles for an aliens skin color are black and white, and is genetically expressed through incomplete dominance.

Show a Punnett square for a mix between a gray and white alien.

Co-dominance

Dominant alleles are both expressed but do not mix. (Huh?)In a particular breed of horse, there are two alleles for coat color.

CW = white coat CR = red coat Note the use of superscripts. The base

“C” is for coat or color, and the superscripts represent red and white, the allele varieties.

Co-dominance continued…

What is the genotype of a red horse?_______ What is the genotype of a white horse?_______ What happens in the heterozygous condition?

CRCW = roan, this is a horse that has both red hairs and white hairs.

Co-dominance – Human Example!

People can have four different blood types.

______, _____, _____, or _____. Blood type AB is a result of an

individual having alleles for both the A and B antigen and they are both present on cells. (Co-dominant)

IAIB

Multiple Alleles

Multiple alleles are combinations of different kinds of genetics due to more than two forms of alleles present.

A good example of this is human blood type.

There are 3 alleles: IA , IB, i. IA is the “A” allele, IB is the “B” allele,

and i = the “O” allele.

Multiple Alleles continued…

What genotypes are possible for individuals with blood type… A – B – AB – O –

Multiple Alleles – Practice!

A female with blood type AB marries a man with blood type B. The mans mother was blood type O.

What is the chance that they have a child with blood type A?

Sex-Linked Traits

Remember that there are two types of sex chromosomes. X Y Males have the genotype XY. Females have the genotype XX.

Genes that are found on the sex chromosomes are called sex-linked.

Sex-Linked Traits continued… There is a gene for color-blindness found on

the X chromosome. XB = not color blind Xb = color blind

Females can possess the color-blind allele without being color-blind. XBXb

Males can be color-blind even though they may only have one color-blind allele. XbY

Sex-Linked Traits – Practice! The gene for eye color in fruit flies is

found on the X chromosome. XR = red eyes Xr = white eyes

What is the genotype of… a red-eyed male – a white-eyed female – a white-eyed male – a red-eyed female -

Sex-Linked Traits – More practice!

Cross a white-eyed male fruit fly with a pure red-eyed female.

What is the chance that a white-eyed male fruit fly will be born?