According to the law of independent assortment, there are 2ⁿ combinations where chromosomes can assort into different gametes. So where n is the haploid number, you get 2²³=8,388,608. There are 8,388,608 possible combinations of chromosomes when assorting into gametes.

All three contribute to giving rise to genetic variation. Independent assortment allows for the chromosomes to assort in millions of random of combinations during fertilization. Crossing over between chromosomes produces recombinant chromosomes, or the combination of chromosomal DNA from two parents into one chromosome. Random fertilization allows aids with variation because it means any sperm can fertilize any egg. It is sometimes easy to overlook, but humans do not mate randomly. Lots of energy is put into choosing an optimal mate with whom to reproduce. 

The child is blood type AB, meaning that the child has both the "A" antigen and the "B" antigen on his or her red blood cells. The child is able to express the products of both genes simultaneously. The A antigen was inherited from mom, and the B antigen was inherited from dad. The "A" and "B" alleles are codominant because they can both be expressed in the same person at the same time if the person inherits both alleles, as is the case in this example.

The easiest way to solve this problem is to draw a punnet square. The genotypes of the parents are "AO" and "AB". The potential genotypes of their children are "AA", "AO", "BA", and "BO". Only genotype "BO" will produce type B blood. "BO" is one out of four results of this punnet square, so the probability of this outcome is .

Dominant alleles are referred to with capital letters, so let's call the dominant blue-petal allele B. Recessive alleles are referred to using lower case letters, so we will call the recessive white-petal allele b.

A heterozygous organism has one dominant and one recessive allele, so the heterozygous flower has one B allele and one b allele. Its genotype is Bb. Because B is dominant to b, its phenotype (the trait produced by its genotype) is blue petals.

A homozygous organism has two of the same allele. The homozygous flower will either have two BB alleles or two bb alleles. The question states that the flower with white petals is homozygous recessive, so its genotype is bb and its phenotype is white petals. The only genotype that produces a white phenotype is bb, because you need two recessive alleles in order to express the recessive trait.

When you cross the two flowers, each parent donates one of its two alleles for petal color to the offspring. Accounting for every possible combination of alleles from each parent, there are four possible outcomes from a cross between Bb and bb: Bb, Bb, bb, and bb. (It may also help to draw a punnet square to visualize the four possible combinations). As you can see, these outcomes lead to two possible genotypes: Bb and bb.  The Bb genotype produces flowers with blue petals, and the bb genotype leads to flowers with white petals. Because two of the four possible outcomes are genotype bb, two of the four possible outcomes are for flowers with white petals. Two out of four is equal to , so  is the correct answer.