The gene is the basic physical unit of inheritance. Genes are inherited from parents to children and contain the information needed to specify traits. The human genome provides the complete set of genetic information in a person. Genes are arranged, one after another, on structures called chromosomes.

A chromosome contains a single, long DNA molecule, only a portion of which corresponds to a single gene. Each chromosome contains hundreds to thousands of genes. Every normal human cell, except for sperm and egg cells, has 23 pairs of chromosomes for a total of 46 chromosomes.

The sex chromosomes are one of the 23 pairs of chromosomes. Normally, a person has two sex chromosomes, and each is either an X or a Y chromosome. Normal female have two X chromosomes (XX), and normal male have one X and one Y chromosome (XY).

Single Gene Disorder

Also called Mendelian or monogenic disorders. Single-gene disorders in recognisable patterns are inherited and are classified according to how they are transmitted from one generation to the next.

Autosomal Dominant- Only one abnormal allele (an allele is one of a number of alternative forms of the same gene or same genetic locus. Abnormal alleles are forms of gene that arise by mutation and are found at the same place on a chromosome) of a gene is needed to express an autosomal dominant trait. The risk of occurrence for each child of affected parents is 50%. Examples of this type of disorder are Marfan syndrome, Neurofibromatosis type 1, Neurofibromatosis type 2, Huntington’s disease, Von Willebrand disease and Tuberous sclerosis.

Autosomal Recessive- Two copies of an abnormal allele are needed to express an autosomal recessive trait.

Chances of Inheriting a trait:

The statistical expectation for each child born to parents both of whom carry the gene (but do not have signs of disease) is as follows:

  • 25% chance that the child is born with two normal genes (normal)
  • 25% chance that the child is born with two abnormal genes (at risk for the disease)
  • 50% chance that the child is born with one normal and one abnormal gene (carries the disease but is not affected by the disease)

X-Linked Dominant- Traits are carried on the X chromosome. Usually, males are more severely affected, and some disorders are often lethal in males. The chance of passing on X-linked dominant disorder differs between men and women. An affected male never transmit the trait to his sons while all daughters are a carrier. An affected female has 50% chance of having an affected embryo with each pregnancy.

X-Linked Recessive- Is a condition that is also caused by mutations in genes on the X chromosome. Males are more frequently affected than females. An affected female has 50% chance of having sons who are affected and 50% chance of having daughters are a carrier. Examples are hemophilia A, Lesch-Nyhan syndrome, and Duchenne muscular dystrophy.

Y Linked- This disorder can only be passed from men to their sons as females can never be affected because they do not possess Y sex chromosome.

Mitochondrial- Also known as maternal inheritance. It is a type of inheritance where he traits of the offspring are maternal origin, due to the expression of extra-nuclear DNA present in the egg during fertilisation. Example -Leber’s hereditary optic neuropathy.

Abnormalities due to several factors.

One or more genes plus exposure to certain other factors, including in the environment (called multifactorial inheritance) can cause some disorders. Example, common medical problems such as heart disease, diabetes, obesity, asthma, cleft palate, and cancers do not have a single genetic cause. They are associated with the effects of multiple genes in combination with lifestyle and environmental factors. Thus making it difficult to determine a person’s risk of inheriting or passing on these disorders.

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