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. C W = white coat C R = 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? C R C W = 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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: I A, I B, i.  I A is the “A” allele, I B 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. X B = not color blind X b = color blind  Females can possess the color-blind allele without being color-blind. X B X b  Males can be color-blind even though they may only have one color-blind allele. X b Y

Sex-Linked Traits – Practice!  The gene for eye color in fruit flies is found on the X chromosome. X R = red eyes X r = 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?