The Punnett square below represents the couple's possible offspring, with the mother having genotype  and the father having genotype .

Since the disorder is X-linked, we know that any sons will necessarily inherit an affected allele from the mother. Any daughters will inherit an chromosome from each parent; by necessity, any daughters will be heterozygous carriers. The probability of any daughters being phenotypically normal is 100%, and the probability of any sons being colorblind is also 100%.

The question states that the couple had two sons and one daughter, and asks the probability that one son is colorblind, one son is normal, and one daughter is colorblind. These probabilities are 100%, 0%, and 100%, respectively.

There is a 0% chance that this combination of children is possible.

The first generation shows us a father with the disease and a mother without the disease. They produce three children, none of whom have the disease. Knowing that they do not have the disease allows us to eliminate dominant from consideration. In order for the third generation to be affected, the mother from the second generation must be a carrier. In the third generation, we see that the carrier mother has a male child with the disease with a father who does not have the disease. The male child will inherit the Y chromosome from his father, but must receive an X chromosome from the mother. He inherits the disease on this X chromosome.

Were the disease autosomal recessive, the father of the third generation child would need to be affected in order for him to inherit the trait. The disease must be X-linked recessive.

We know that red eyes are the dominant allele, which means white eyes are the recessive allele. Both sexes of offspring present the recessive allele. It is especially important to note that the daughters can express the recessive allele. This means that they must have inherited one recessive allele from each parent, while the sons must have inherited the recessive allele from the mother (they inherit the Y-chromosome from the father).

White-eye daughters: 

White-eye sons: 

Each parent must have at least one recessive, white-eye allele. Since the father has only one X-chromosome, this chromosome must carry the white eye allele. We know that they father must have white eyes.

Father: 

Since one parent has white eyes and the other has red eyes, we know the mother must have red eyes. She also carries the recessive allele, meaning that she is heterozygous.

Mother: 

From this cross, we are able to get the percentages reported in the question. 50% of daughters will have red eyes and 50% will have white. The same percentages will be seen for the sons.

In order to solve the problem, trace the colorblind allele all the way to the grandparent's alleles. It is possible that the daughter is a carrier, but this is not a guarantee. Males can not be carriers, as they only have one X chromosome; they either have the allele or they do not. Finally, the maternal grandmother may be colorblind, but this is not guaranteed.

All that we know is that at least one of the maternal grandmother's alleles codes for colorblindness. The maternal grandmother passed this allele to the mother. The color blind son inherits his Y chromosome from his father, and his only X chromosome from his mother; thus, the mother must be a carrier to pass down a color blind X chromosome to the son.

If cell division occurs incorrectly, and the resulting cell contains an abnormal number of chromosomes, that cell is considered to be aneuploid. Polyploidy is the presence of an entire extra copy of the genome in a cell. Euploid (true ploid) refers to a cell with the correct number of chromosomes.

Linked genes are found on the same chromosome, and do not always separate according to independent assortment. Alleles are different versions of a gene. Codominance is a mode of inheritance in which both alleles, if present, are fully expressed.

Although X-linked dominant traits tend to be rare, it is certainly possible that the boy would inherit it from an affected mother. A father only gives a Y chromosome to his sons. This means that the boy could not inherit the X-linked dominant trait from his father.

Since the cell has a total of 23 chromosomes it must be a gamete. Recall that the diploid number for humas is 2n=46. The presence of a Y chromosome means that it corresponds to a male gamete so it must be a sperm cell.  

Autosomes are all the chromosomes that do not determine an individual's sex. X and Y chromosomes both determine sex, and are sex chromosomes. The other 22 pairs of chromosomes are autosomes. 

The union of two gametes is called fertilization, which results in a zygote. The zygote is a diploid cell that will grow into the entire organism.