Human Allelic Disorders

The first Mendelian trait in humans was described in 1905 (brachydactly) by Dr. Farabee (no relation to your author). Now more than 3500 human genetic traits are known.
Albinism, the lack of pigmentation in skin, hair, and eyes, is also a Mendelian human trait. Homozygous recessive (aa) individuals make no pigments, and so have face, hair, and eyes that are white to yellow. For heterozygous parents with normal pigmentation (Aa), two different types of gametes may be produced: A or a. From such a cross 1/4 of the children could be albinos. The brown pigment melanin cannot be made by albinos. Several mutations may cause albinism: 1) the lack of one or another enzyme along the melanin-producing pathway; or 2) the inability of the enzyme to enter the pigment cells and convert the amino acid tyrosine into melanin.
Phenylketonuria (PKU) is recessively inherited disorder whose sufferers lack the ability to synthesize an enzyme to convert the amino acid phenylalanine into tyrosine Individuals homozygous recessive for this allele have a buildup of phenylalanine and abnormal breakdown products in the urine and blood. The breakdown products can be harmful to developing nervous systems and lead to mental retardation. 1 in 15,000 infants suffers from this problem. PKU homozygotes are now routinely tested for in most states. If you look closely at a product containing Nutra-sweet artificial sweetener, you will see a warning to PKU sufferers since phenylalanine is one of the amino acids in the sweetener. PKU sufferers are placed on a diet low in phenylalanine, enough for metabolic needs but not enough to cause the buildup of harmful intermediates.
Tay-Sachs Disease is an autosomal recessive resulting in degeneration of the nervous system. Symptoms manifest after birth. Children homozygous recessive for this allele rarely survive past five years of age. Sufferers lack the ability to make the enzyme N-acetyl-hexosaminidase, which breaks down the GM2 ganglioside lipid. This lipid accumulates in lysosomes in brain cells, eventually killing the brain cells. Although rare in the general population (1 in 300,000 births), it was (until recently) higher (1 in 3600 births) among Jews of eastern central European descent. One in 28 American Jews is thought to be a carrier, since 90% of the American Jewish population emigrated from those areas in Europe. Most Tay-Sachs babies born in the US are born to non-Jewish parents, who did not undergo testing programs that most US Jewish prospective parents had.
Sickle-cell anemia is an autosomal recessive we have discussed in other sections. Nine-percent of US blacks are heterozygous, while 0.2% are homozygous recessive. The recessive allele causes a single amino acid substitution in the beta chains of hemoglobin. When oxygen concentration is low, sickling of cells occurs. Heterozygotes make enough "good beta-chain hemoglobin" that they do not suffer as long as oxygen concentrations remain high, such as at sea-level.