Dihybrid crosses a study of inheritance patterns for organisms differing in two traits… more realistic

Dihybrid cross  Monohybrid: heterozygous (having 2 different alleles for one gene pair).  Ex: Aa  So what is a dihybrid????  Di: meaning two  Hybrid: means heterozyous  instead of one trait we are looking at two traits at the same time  AaBb represents the genotype of a dihybrid

Remember Mendels laws:  Law of segregation.  Only one allele for each trait can be passed at a time. Only pass on one ‘A’ AND one ‘B’ and one…  Law of independent assortment.  How each type of letter (allele) sorts or is passed on to the next generation is independent  i.e. Which “A” is passed on doesn’t affect which “B” is passed on

What combinations of alleles are possible?  If an organism has a genotype of AaBb  What are all the different combinations of A’s and B’s?  AB  Ab  aB  ab  These combinations represent the possible gametes (sperm or egg) that could form during meiosis.

An example of a dihybrid cross  Key:  E = Two Eyes, e = one eye  T = Triangular Head, t = pentagonal head  So, lets have two dihybrid parents; therefore their genotypes are:  EeTt x EeTt : What do they look like?  Phenotype= two eyes and triangular heads.  Phenotype: physical characteristics of an organism: what you “see”.  What types of combinations are possible?  ET, Et, eT, et. Is this all?

Punnett squares

Incomplete Dominance  Incomplete Dominance = neither allele is dominant. Heterozygous individuals show an intermediate (in between) phenotype.  Example: petal color in carnations RR = red RW = pink WW = white

Codominance  Codominance = both alleles contribute to the phenotype. Heterozygous individuals show both traits separately (not blended like in incomplete dominance).  Example: coat color in cattle  RR = red  WW = white  RW = roan

Multiple Alleles  Multiple Alleles = genes that have more than two alleles.  Example: blood type in humans