Principles of Inheritance and Variation - Mutation and Genetic Disorders

Mutation is a phenomenon that results in the alteration of DNA sequences, leading to changes in the genotype and phenotype. It can be caused by mutagens like UV radiation ($254 \text{ nm}$), chemicals, or errors during replication.

Point Mutation: A change in a single base pair of DNA. A classic example is Sickle-cell anemia, where the codon $GAG$ (coding for Glutamic acid) is mutated to $GUG$ (coding for Valine) at the sixth position of the $\beta$-globin chain.

Frame-shift Mutation: Deletion or insertion of one or two bases changes the reading frame from the point of mutation onwards.

Mendelian Disorders: These are caused by alteration or mutation in a single gene and follow Mendelian inheritance patterns. Examples include Haemophilia (sex-linked recessive), Sickle-cell anemia (autosomal recessive), and Phenylketonuria ($PKU$).

Chromosomal Disorders: Caused by the absence, excess, or abnormal arrangement of chromosomes. Aneuploidy (gain or loss of chromosomes) occurs due to failure of chromatid segregation during cell division.

Down's Syndrome: Caused by the presence of an additional copy of chromosome number $21$ (Trisomy of $21$). Features include a short stature, furrowed tongue, and partially open mouth.

Klinefelter's Syndrome: Caused by the presence of an additional copy of X-chromosome resulting in a karyotype of $47, XXY$. These individuals are sterile males with feminine development (Gynaecomastia).

Turner's Syndrome: Caused due to the absence of one of the X chromosomes, resulting in a karyotype of $45$ with $X0$. Affected females are sterile as ovaries are rudimentary.