Provide detailed information about Sickle-cell anemia and Phenylketonuria $(PKU)$.

(N/A) Sickle-cell anemia: This is an autosome-linked recessive trait that can be transmitted from parents to offspring when both partners are carriers for the gene (or heterozygous).
The disease is controlled by a pair of alleles,$Hb^A$ and $Hb^S$. Out of the three possible genotypes,only individuals homozygous for $Hb^S$ $(Hb^SHb^S)$ show the disease symptoms.
Heterozygous $(Hb^AHb^S)$ individuals appear unaffected but are carriers of the disease. There is a $50\%$ probability of transmission of the mutant gene to the progeny.
The defect is caused by the substitution of Glutamic acid $(Glu)$ by Valine $(Val)$ at the sixth position of the $\beta$-globin chain of the hemoglobin molecule.
This substitution of amino acid in the globin protein results due to the single base pair substitution at the sixth codon of the $\beta$-globin gene from $GAG$ to $GUG$.
Under low oxygen tension,the mutant hemoglobin molecule undergoes polymerization,causing the shape of the red blood cell to change from biconcave disc to an elongated sickle-like structure.
Phenylketonuria $(PKU)$: This is an inborn error of metabolism that is also inherited as an autosomal recessive trait.
The affected individual lacks an enzyme that converts the amino acid Phenylalanine into Tyrosine. As a result,Phenylalanine accumulates and is converted into Phenylpyruvic acid and other derivatives.
Accumulation of these in the brain results in mental retardation. These are also excreted through urine because of poor absorption by the kidney.