Human Genetics and Disorders

Certain traits such as baldness in human biings, which are actually determined autosomal genes but their expression being influenced by sex hormones are called sex influenced traits.

Both Assertion (A) and Reason (R) statements are correct, and R is correct explanation of A. In chromosomal aberration there is an alteration in the structural organisation of chromosome.  Aberrations are of two types, Intrachromosomal aberration :  when the alteration is within single chromosome. Interchromosomal aberration : when alteration involves the intervention of two or more chromosomes. Cri-du-chat syndrome in humans is caused by deletion of a part of short arm of chromosome 5. It involves alteration in only one chromosome; hence it is an intrachromosomal aberration.

Albinism and Phenylketonuria are examples of autosomal recessive hereditary disorders. Albinism is caused due to inability to produce melanin pigment characterized by absence of normal colouration of skin. Phenylketonuria is a metabolic disorder which arises due to deficiency of an enzyme phenylalanine hydroxylase,  which converts phenylalanine to tyrosine.

Achondroplasia occurs as a sporadic mutation in approximately 85% of cases (associated with advanced paternal age) or may be inherited in an autosomal dominant genetic disorder that is a common cause of dwarfism.

The cause of Tay–Sachs disease is a genetic defect that is passed from parent to child. This genetic defect is located in the HEXA gene, which is found on chromosome 15. The HEXA gene makes part of an enzyme called beta-hexosaminidase A, which plays a critical role in the nervous system.

In humans, the inherited causes account for 50% of all cases of severe sensorineural hearing impairment and 80% are due to single-gene autosomal recessive disorders.

Thomas Bourgeron discovered the first monogenic mutations involved in autism.

Loss-of-function mutations are the result of gene product having less or no function. When the allele has a complete loss of function (null allele) it is often called an amorphic mutation. Phenotypes associated with such mutations are most often recessive. It is not a point mutation.

Most amino acids are encoded by several different codons. For example, if the third base in the TCT codon for serine is changed to any one of the other three bases, serine will still be encoded. Such mutations are said to be silent because they cause no change in their product and cannot be detected without sequencing the gene (or its mRNA).

Phenylketonuria (PKU) is an autosomal recessive metabolic genetic disorder characterized by an error in the genetic code for the hepatic enzyme phenylalanine hydroxylase (PAH), rendering it nonfunctional.This enzyme is necessary to metabolize the amino acid phenylalanine (Phe) to the amino acid tyrosine.

A single gene disorder is the result of a single mutated gene. There are estimated to be over 4000 human diseases caused by single gene defects. Single gene disorders can be passed on to subsequent generations in several ways.

Lethal mutations are mutations that lead to the death of the organisms which carry the mutations.

Huntington's disease, is a neuro-degenerative genetic disorder that affects muscles coordination and leads to cognitive decline and dementia. It typically becomes noticeable in middle age.

Marfan syndrome is a genetic disorder of the connective tissue. People with Marfan's tend to be unusually tall, with long limbs and long, thin fingers.

2