Because males only have one X chromosome, while females have two, they are more likely to be affected by a problematic X chromosome. Females can mask recessive X-linked alleles as carriers; males will express all alleles on their singular X chromosome.

Males only pass on a Y chromosome to their sons, so it is impossible for them to pass an X-linked trait to a son. Furthermore, Y chromosomes are virtually free of contributing to inheritance-linked diseases.

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.

Haploid cells have 23 chromosomes each (22 autosomes and 1 sex chromosome) so when they fuse the resulting cell will have 46 chromosomes (44 autosomes and 2 sex chromosomes). This idea is demonstrated during fertilization, where two haploid cells (gametes) fuse to form the zygote, which is diploid.

A haploid cell has only one set of chromosomes as compared to the two sets normally found in diploid cells. In a haploid cell there are 22 autosomes and 1 sex chromosome for a total of 23 chromosomes.

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.

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.

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.

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.

X-linked disorders are inherited when a parent passes on his or her X-chromosome. Since females have two X-chromosomes, they are less likely to exhibit symptoms of a recessive disorder than males, who have only one. Females are capable of carrying a recessive X-linked trait without expressing it, while males are not. A male must inherit his Y-chromosome from the father and an X-chromosome from the mother, while a female must inherit X-chromosomes from both parents.

If a genotypically healthy mother and a colorblind father have a son, then this child must inherit an X-chromosome from the mother and a Y-chromosome from the father. The mother's chromosome are both genotypically normal, and do not possess the colorblind allele. This means that the son cannot possibly inherit a colorblind allele if the mother is genotypically normal.

All other presented answer represent scenarios that are possible.

If an autosomal trait skips a generation, it must be recessive; however, if an autosomal trait does not skip a generation, it can be either recessive or dominant.

These concepts can be easily seen when outlined via a pedigree analysis. A dominant trait cannot skip a generation; any presence of the allele will lead to expression, thus if the trait is not expressed in a given generation, it cannot be passed down (cannot skip). A recessive allele can be masked by carriers and reappear in a later generation.