Q.(a) Mention the symptoms of Phenylketonuria and Down's Syndrome. OR

(a) PKU and Down's syndrome are two distinct genetic disorders -- one a metabolic enzyme defect, the other a chromosomal trisomy -- each with a characteristic set of physical/developmental symptoms. (b) The Meselson-Stahl density-gradient experiment provided the definitive proof that DNA replicates semiconservatively.

(a) Phenylketonuria (PKU) is an inborn error of metabolism, inherited as an autosomal recessive trait, caused by a deficiency of the liver enzyme phenylalanine hydroxylase, which normally converts the amino acid phenylalanine into tyrosine. In its absence, phenylalanine and its abnormal breakdown products (such as phenylpyruvic acid) accumulate in the blood and are excreted in the urine; if undetected and untreated (typically through a special low-phenylalanine diet from early infancy), this accumulation causes severe mental retardation, along with reduced pigmentation of hair and skin (since the tyrosine pathway also feeds into melanin production), and other neurological problems. Down's syndrome, by contrast, is a chromosomal disorder caused by the presence of an extra copy of chromosome 21 (trisomy 21, giving 47 chromosomes instead of the usual 46), usually arising from non-disjunction during meiosis. Its characteristic symptoms include short stature, a small, round head, a furrowed and often protruding tongue with a partially open mouth, a single transverse (simian) crease across the broad, short palms, low muscle tone (hypotonia), slanting eyes with a prominent epicanthic fold, and mild to moderate mental retardation, along with an increased incidence of congenital heart defects. …