Mendelian Inheritance

Introduction

Genetics is the way the genes are formed and the mechanism through which they transfer from parents to offsprings. Although many have attempted to explain the mechanisms behind the genetics, John Gregor Mendel has made the most remarkable achievement by explaining the science behind the transmission of inherited traits. Along with Mendel Dagfinn Aarskog, Jon Aase, John Abelson, Susan L. Ackerman, Jerry, Bruce, Watson, Crick, Nirenberg, Khorana, and many other scientists contributed to genetics and heredity. Mendel is one of the popular geneticists and he proposed the laws of inheritance for the first time, though, earlier some attempts were made by the others but Mendel came up with a better explanation about genetics. Many genetic illnesses were identified by basing his theories. According to him,  genetically abnormal genes could pass from both the parents resulting in wrong genetic composition in the children. The genetic abnormalities passed from the parents might be dominant or recessive. The dominant once will take a phenotypical expression whereas the recessive may not be phenotypically visible outside. As per Mendel's explanation, when the appropriate number of copies are transferred from the parents, an abnormal trait may be easily identified. Some of the well-known genetic variations are explained as under.

 

1. Autosomal dominant inheritance

It is a condition in which the genetic traits are inherited in the presence of only one dominant gene. Nearly 50% of the Mendelian conditions ( out of 5000) are autosomal dominant. A specific phenotype appears in every generation, with an affected individual has at least, one affected parent. In an autosomal dominant trait, a child of an affected parent has a 50% chance of inheriting the abnormal gene, and, conversely, the child can be phenotypically normal. Men and women are equally likely to pass the phenotype to their children irrespective of their gender.  Copies of abnormal dominant genes (homozygosity) are generally lethal. Common examples of autosomal dominant disorders are achondroplasia, Huntington disease, and neurofibromatosis (4). The symptomatology and it`s severity do vary from individual to individual depending upon associated illnesses- comorbidities, and the general health status of the affected. Some of the common autosomal dominant inheritance disorders are Marfan syndrome, Ehlers-Danlos syndrome, Neurofibromatosis and Achondroplasia.

 

Autosomal dominant inheritance-credit to US National Library of Medicine

 

 

2. Autosomal recessive inheritance

It is presented with a mutation in the genes of both the parents, however, only one of the parents may be dominant and the other one is recessive. In a recessive condition, chances of being affected are less likely than dominant. However, in this scenario, the child may assume a carrier status without being affected by itself. In such a case, both the parents of an affected individual will carry an autosomal recessive gene equally (one copy of the mutated gene). Recessive inheritance does not show signs and symptoms of the condition and it is not mandatory that an autosomal recessive disease has to show it`s symptoms in every generation. In addition, the traits may not be seen outside of the siblings of an affected member of the family. The risk of occurrence of the disease in each sibling of the affected individual is only 25%. An unaffected sibling of an affected individual has 2/3  chances of being a carrier (heterozygote). Both the male and female offspring are equally affected by autosomal recessive illness. Some of the recessive disorders are  Tay-Sachs disease, cystic fibrosis, sickle cell disease, phenylketonuria, and maple syrup urine disease.

 

Autosomal recessive inheritance-credit to US National Library of Medicine

 

3. Sex-linked dominant and recessive trait

It is an autosomal trait as a result of XX or XY defect ( sex chromosomes). The dominant forms require only a single gene on the X chromosome to express the abnormality. The recessive forms require only one gene on the X chromosome of males and 2 genes on the X chromosomes of females. The  X-linked recessive conditions are much more common in a male child than the female. Phenotypically, the daughters of affected men (the recessive form) have a 50% chance of passing the abnormal gene to their sons but an affected man never result in an affected son. Heterozygous women (carriers of the recessive form) are generally unaffected. In the rare X-linked dominant conditions, affected men have no affected sons and no normal daughters. Both male and female offspring of female carriers have a 50% chance of inheriting the phenotype. Not all women who inherit either a single dominant gene or a pair of recessive genes will phenotypically express their genotype, as explained by the Lyon hypothesis (see discussion below).

 

 

4. X-linked dominant traits

X-linked dominant inheritance occurs as a result of mutations in the genes of the X chromosome. A female chromosome has XX- allosomes, out of the 2XX, anyone X need to be affected to cause this condition. On the contrary, the male has XY, in this case, X must be affected but if the  Y also gets affected then it cannot be called X-linked. In most of cases, males experience more severe symptoms of the disorder than females. One of the unique characteristics of X-linked inheritance is that fathers cannot pass X-linked traits to their sons but it is only possible to transmit them to daughters.

 

X-Linked dominant trait-credit to US National Library of Medicine


 

5. X-linked recessive trait

This occurs due to the mutations in genes found on the X chromosome. In the case of males, one of the X is enough to be mutagenic to cause defect but in the case of females who have two X chromosomes, a mutation must exist in both copies of the gene to cause the disorder. As it is highly unlikely that females will have two defective copies of this gene, males are affected by X-linked recessive disorders frequently than females. Some of the common conditions are haemophilia and Fabry disease.

 

X-Linked recessive trait-credit to US National Library of Medicine

 

 

6.Y-linked inheritance

Y-linked inheritance occurs when the defective genes on the Y chromosome are transferred to the son. Y-linked inheritance is also called holandric inheritance. Y-linked transmission is possible between the father and son.

 

X-Linked inheritance -credit to US National Library of Medicine

 

 

 

 

Read more

1.        

Mendel`s experiments

2.

Mendelians laws                                            

3.

Chromosomal theory of inheritance              

4.     

mechanism of evolution

 

 

Questions

  1. Describe the Mendel`s contribution to genetics in brief.

  2. Explain the autosomal dominant inheritance.

  3. What is the reason behind X-linked recessive trait?

  4. What is the difference between dominant and a recessive trait?

 



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