Single Gene Disorder in Human

 

Single Gene Disorder in Human

Single gene disorders are caused by DNA changes in one particular gene, and often have predictable inheritance patterns. Over 10000 human disorders are caused by a change, known as a mutation in a single gene. These are known as single gene disorders. The mutated version of the gene responsible for the disorder is known as mutant, or disease, allele. Single gene disorders can be divided into different categories like dominant, recessive and X- linked.

Single Gene Disorder

Gene mutation in autosomes:

i.                    Alkaptonuria: it is caused by disorder in single gene. The symptom may include blackening of urine on exposure to O2 and darkening of cartilages.

ii.                   Albinism: it is caused by lack of pigment melanin in skin, hair and iris of eye. It is caused by absence of enzyme tyrosinase which produces melanin. It is seen when both the alleles of gene are recessive.

iii.                Sickle cell anemia: it is autosomal recessive disease so it is transmitted from parents to offspring when both parents are carrier (heterozygote) for the gene. The disease is controlled by single pair of alleles Hb^A  and Hb^s .

Three genes types are possible.

a.     Hb^A  Hb^A    (Normal)

b.     Hb^A  Hb^s     (Carrier)

c.      Hb^s Hb^s       (Disease)

This disease is caused when glutamic acid is replaced by valine.

Thalassemia: it is autosomal recessive disease. it is caused by defect in synthesis of globin polypeptide in RBC resulting in severe anemia. 

X-linked inheritance

X-linked inheritance means that the gene causing the trait or the disorder is located on the X chromosome. Females have two x chromosomes; male have one x and y. Genes on the x chromosome can be recessive or dominant. X-linked recessive genes are expressed in females only if there are two copies of the gene (one on each X chromosome). However, for males, there needs to be only one copy of an X-linked recessive gene in order for the trait or disorder to be expressed. For example, a woman can carry a recessive gene on one of the X chromosomes unknowingly, and pass it on to a son, who will express the trait.

    There is a 50 percent chance that daughters carry the gene and can pass it to the next generation. There is a 50 percent chance that a daughter will not carry the gene, and therefore, cannot pass it on. There is a 50 percent chance that sons do not have the gene and will be healthy. However, there is a 50 percent chance that a son will have inherited the gene and will express the trait or disorder.

Hemophilia and Genetics

Hemophilia is an inherited genetic condition, meaning it is passed down through families. It is caused by a defect in the gene that determines how the body makes VIII, IX, or XI. These genes are located on the X chromosomes, making hemophilia an X-linked recessive disease. Each person inherits two sex chromosomes from their parents. Females have two X chromosomes. Male have one X and one Y chromosomes.

Father does not			XY			XX		Mother is a carrier of the
have hemophilia								hemophilia gene
		xy	xy		xx	xx
		50% chance sons will			50% chance daughter wil be carrier
		have hemophilia XY			of the hemophilia gene XX
	Key	does not have hemophilia			caarrier of the hemophilia gene			Has hemophilia
		Fig.	Hemophilia and its inheritance chances

Male inherit an X chromosome from their mother and a Y chromosome from their father. Females’ recessive an X chromosome form each parent. Because the genetic defect that causes hemophilia is located on the X chromosome, fathers cannot pass the disease to their sons. This also means that if a male gets the X chromosome with the altered gen has a 50 percent chance of passing that gene to her children, male or female.

            A female who has the altered gene on one of her X chromosome is typically called a “carrier.” This means she may pass the disease to her children but she does not have the disease herself. This is because she has sufficient clotting factors from her normal X chromosome to avoid serious bleeding issues. However, females who are carriers often have an increased risk of bleeding

 Color Blindness

A normal X chromosome is shown as (X) while  a color blind carrying X chromosome is shown in bold (X).

    The color blind “gene” is carried on one of the X chromosomes. Since men have only one X chromosome, if his X chromosome carries the color blind ‘gene’ (X). he will be color blind (XY).  A woman can have either.

    01. two normal X chromosomes, so that she will not be color blind or be a carrier (XX),

    02. one normal X and one color blind carrying X chromosome, in which case she will be a carrier (XX), or rarely.

    03. she will inherit a color blind X from her father and a color blind X from her mother and be color blind herself (XX). She will pass on color blindness to all of her sons if this is the case.