Right before the start of the meiotic division, genetic material is being doubled. Nondisjunction can occur during meiosis I when chromosomes are unable to accurately divide, or during meiosis II, when sister chromatids don’t separate correctly. At the end of a meiotic division, there will be four daughter cells, some of which may be haploid and others aneuploid. When a haploid gamete merges its genetic material with aneuploid gamete that will result in aneuploidy. The zygote will contain one or three sets of chromosomes.

Genotype changes are caused by nucleotide sequence modifications of the DNA molecule. If they lead to changes in codons as well in amino acids, they could have a major effect on the function of the final protein, especially if the active site of a protein is modified. In this way, altered protein can cause a chained reaction which may result in phenotype changes.

Klinefelter syndrome is a genetic disorder that occurs because of nondisjunction during meiosis among sex chromosomes. Males with this condition have an extra X chromosome, so their genotype is 47, XXY. They can have more than one extra chromosome, but such cases are extremely rare. Usually, the symptoms and signs appear during puberty and they include reduction of secondary sexual characteristics, gynecomastia, and infertility. Therapy is a hormonal substitution.

Mutations in the level of genes are called point mutations and they include deletion, insertion, and substitution. In deletion and insertion, a whole sequence of nucleotides from the mutation point is displaced because amino acids that build a protein are determined by codons, which are made by three nucleotides in a row. In substitution, only one amino acid can be changed. These processes sometimes alter an organism’s phenotype.
An example for point mutation that alters the phenotype of a person is the sickle cell disease. The cause of this genetic disorder is substitution of thymine for adenine in one strand of the DNA molecule, which changes codon of the mRNA for a beta-globin from GAG (glutamic acid) to GUG (valine). This change in amino acid alters the structure of hemoglobin molecule making it less soluble which lead to the formation of sickle-shaped erythrocytes.

The sickle cell disease is a genetic disorder that is caused by substitution of thymine for adenine in one strand of the DNA molecule, which changes codon of the mRNA for a beta-globin from GAG (glutamic acid) to GUG (valine). This change in amino acid alters the structure of hemoglobin molecule making it less soluble which lead to the formation of sickle-shaped erythrocytes. However, it is discovered that persons who have one copy of the sickle cell allele are less prone to malaria because this allele provides them protection from the parasite.