A karyotype represents a picture of all chromosomes that a cell contain, arranged from the biggest to the smallest. Chromosomes are being photographed during mitosis when the genetic material is the most densely packed. Then, the scientists cut off chromosomes from this picture and arrange them according to their size and centromere position into homolog pairs.

Recessive traits will appear only if both parents pass the recessive allele to their offspring. However, among sex chromosomes, X is dominant over the Y chromosome. Males have genotype XY, where X is always inherited from a mother and Y from a father. A recessive trait, that is determined by a sex-linked gene, will appear in males if it is inherited from their mother. In females (genotype XX), the recessive trait will appear if the recessive allele is inherited from both of the parents. The females that have one dominant and one recessive allele for a sex-linked gene are known as the carriers of the recessive trait. There will be 50 percent chance for this recessive trait to appear in their male children.

A pedigree shows us the appearance of the certain traits among the family members. Based on a pedigree, we can conclude whether the allele is the dominant or recessive, autosomal or sex-linked.

Colorblindness is a recessive trait that is determined by a sex-linked gene. The three genes that are responsible for the color vision are found on the X chromosome.
Since the X is dominant over the Y chromosome, colorblindness will appear in males if it is inherited from their mother. In females, the recessive trait will appear if the recessive allele is inherited from both of the parents.
If a mother carries a recessive allele for colorblindness (red X chromosome), while the father does not, there is a 25 percent chance for that trait to appear in their offspring (genotype {color{Red}X}{color{Blue}Y}), while females (genotype {color{Blue}X}{color{Red}X}) are the carriers whose male children may be colorblind.

begin{center}
begin{tabular}{ c c c}
& {color{Blue}X} & {color{Red}X}\
{color{Blue}X} & {color{Blue}XX} & {color{Blue}X}{color{Red}X}\
{color{Blue}Y} & {color{Blue}X}{color{Blue}Y} & {color{Red}X}{color{Blue}Y}
end{tabular}
end{center}

In the ABO blood type inheritance, we have three different alleles – A, B, and O, but we have four possible phenotypes – A (AA, AO), B (BB, BO), AB (AB), and O (OO) type of blood.
A woman with blood type O must have a genotype OO, and a man with blood type AB has only one possibility for a genotype – AB.

begin{center}
begin{tabular}{ c c c}
& {color{Blue}O} & {color{Blue}O}\
{color{Blue}A} & AO & AO\
{color{Blue}B} & BO & BO\
end{tabular}
end{center}

We can conclude that their offspring have 50 percent chance for the blood type A (genotype AO) and a 50 percent for a blood type B (genotype BO).

The white forelock is an autosomal dominant trait. The only person that is labeled on the pedigree that is a potential homozygous dominant for this trait is a granddaughter that has both parents with this phenotype. Since the grandfather has children with and without a white forelock, we can conclude that he must have a heterozygous genotype (Aa). This means that his son is also heterozygous for this trait (Aa). The son’s wife also has a white forelock. Since one of their daughters has, while the other one doesn’t have this trait, we can conclude that the wife also has heterozygous genotype (Aa). Their daughter that is inherited a dominant allele may be heterozygous (Aa), but there is a chance that both of her parents passed on their dominant alleles, which would make her a homozygous dominant. If her genotype is AA, she would pass the dominant allele for this trait to all of her children, which will inherit a white forelock.

Tay-Sachs disease is a progressive genetic disorder that affects the nervous system. Since the parents are healthy, we can conclude that this disease isn’t determined by a dominant allele, because the parent who would have a dominant allele must himself have the disease. Therefore, this disease is autosomal recessive, which means that both of the parents are heterozygous and the carriers of the recessive allele. Their children will be affected by this illness only if they inherit recessive allele from both of their parents, which would make them homozygous recessive.