In some cells, the X from the father becomes inactive, and in other cells, the X from the mother becomes inactive.

X inactivation occurs early in the life of the fetus. However, in females, most of the genes on one of the two X chromosomes are turned off through a process called X inactivation (except in the eggs in the ovaries). Normally, in the nonsex chromosomes, the genes on both of the pairs of chromosomes are capable of being fully expressed. Although the protection against malaria can help a carrier survive, sickle cell disease (in a person who has two copies of the gene) causes symptoms and complications that may shorten life span. Malaria causes fever, chills, sweating, a general feeling of illness (malaise), and sometimes. However, when a person inherits only one copy of the sickle cell gene (called a carrier), the person develops some protection against malaria Malaria Malaria is infection of red blood cells with one of five species of the protozoa Plasmodium. For example, in the case of the sickle cell gene, when a person inherits two copies of the abnormal gene, the person will develop sickle cell disease Sickle Cell Disease Sickle cell disease is an inherited genetic abnormality of hemoglobin (the oxygen-carrying protein found in red blood cells) characterized by sickle (crescent)-shaped red blood cells and chronic. In rare cases, a mutation introduces a change that is advantageous. This deficiency allows the amino acid phenylalanine (absorbed from the diet) to accumulate in the body, ultimately causing severe intellectual disability. read more, a mutation results in the deficiency or absence of the enzyme phenylalanine hydroxylase. For example, in phenylketonuria Phenylketonuria (PKU) Phenylketonuria is a disorder of amino acid metabolism that occurs in infants born without the ability to normally break down an amino acid called phenylalanine. An absent or nonfunctioning protein is often harmful or fatal. If the protein has a different amino acid sequence, it may function differently or not at all. Depending on its size and location, the mutation may have no apparent effect or it may alter the amino acid sequence in a protein or decrease the amount of protein produced.

For the two strands of the double helix to fit neatly, a pyrimidine must always be paired with a purine.Mutations may involve small or large segments of DNA.

If these pairings were ever to occur, the structure of DNA would be like a staircase made with stairs of different widths.

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