Genetic Disorders - Inheritance Patterns of Some Genetic Diseases In Humans

Premature Aging (Progeria) seen in boys
Premature Aging (Progeria) seen in boys | Source
Child With Burkitt's Lymphoma
Child With Burkitt's Lymphoma | Source

Analyzing Genetic Diseases

No doubt, you can often blame your first, second or third generation for your genetic disorders! It's a fact that human genetic is often responsible for our traits, behaviors and also many of our inherited health conditions. Because genetic is a true predictor, it is important that we at least have an understanding of the inheritance patterns of some genetic diseases.

This knowledge can help those at risk for certain conditions to seek early prevention interventions. Also, having just the basic understanding of the possible outcome when one or both mates are afflicted with a certain illness can be helpful with family planning decisions.

The rules for analyzing a pedigree according to Mendel's principle, depicts these rules as they apply to human traits. The diagram below is an illustration of three such traits, with each trait showing a different pattern of Mendelian inheritance.

Inheritance of Human Traits Determined By A Single Gene

A. In the diagram above, with a dominant genes on an autosomal chromosome, all affected offspring have at least one affected parent. Half of the offspring will be affected, and half will be normal when one parent in normal and the other is affected.

With recessive genes on autosomal chromosomes, the effected children usually have two unaffected heterzygous parents, and the ratio of offspring will be one normal to two carriers(normal phenotype) to one affected.

B. In a situation where the recessive genes are on x chromosomes, males are affected more often than females. Note that the inheritance is from the mother to son, not father to son. If the father is normal and the mother is a carrier, half the daughters will be normal and half will be carriers, while half the sons will be normal and half will be affected.

Understanding Traits

1. A Dominant Trait

If a child has a rare trait and one parent is affected, the trait is usually dominant.

2. A Recessive Trait

If a child has a rare trait but neither parent is affected, the trait is usually recessive.

3. An Autosomal Trait

If both male and female children are affected about equally, the trait is probably autosomal.

4. X-linked Trait

If all or nearly all the affected people are males, the trait may be X-linked. However, if a son inherits the trait from his father, it cannot be X-linked.

Recessive & X-linked Conditions

Autosomal Recessive Condition

An example of an autosomal recessive condition is Albinism, a genetic condition in which the homozygote's sin cells and hair follicles fail to produce the dark pigment melanin.

X-linked Condition

An X-linked genetic disease is a condition in which the causative allele is on the X chromosome. The most common X-linked recessive genetic disease is Duchenne Muscular Dystrophy(DMD), a degenerative muscle condition that strikes 1 out of every 3500 boys.


Notes - Heterozygous: When the alleles of a pair are considered heterozygous, this simply means one allele is dominant and the other is recessive.

Alleles: Alternative form of genes. All living organisms have two alleles for each trait. While the dominant allele is expressed, the recessive one is masked.

Autosomal Chromosome: Any chromosome with the exception of the sex chromosomes. Humans have 44 autosomal chromosomes.

Conclusion

Please note that the above information is just a basic explanation of the inheritance patterns of some genetic conditions. You are advised to consult a genetic counselor to answer all your questions in regard to parenting decisions and genetic disease interventions.

I.Mcfarlane 2012

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