b. Alternative forms of a gene occurring at a single locus are called alleles (or,
rarely, allelomorphs); for example, the Kell blood group system includes two alleles, K
(Kell) and k (Cellano). Any cell, including the fertilized egg or zygote, in which the
alleles at a paired locus are identical is called a homozygote. Using the example given
above, two homozygous allelic pairs are possible, KK and kk. The remaining
combination, Kk, contains non-identical or heterozygous alleles.
c. The term phenotype refers to the expressed characteristics or traits of an
individual which are due to genetic factors (for example, the appearance of the organism--such
as eye color). The term genotype, on the other hand, refers to the specific genetic
makeup of an organism. To be homozygous, an organism must have like genes at
corresponding loci on a specific pair of homologous chromosomes. To be
heterozygous, the organism must have dissimilar genes at the corresponding loci.
a. Blood group antigens fall into three general chemical classes as shown in
Table 2-1. Glycoprotein and glycolipid antigens owe their specificity to the carbohydrate
portion of the molecule, while specificity resides in the protein moiety of lipoproteins.
Chemistry of cellular antigens.
b. According to the central dogma, information coded in the nucleotide bases of
DNA is transcribed to messenger ribonucleic acid (mRNA), and subsequently, through
numerous, events translation into structural proteins is accomplished. There is no place
for the synthesis of carbohydrate in this scheme. It is interesting to note however, that
the carbohydrate specificities of so many blood group antigens are inherited precisely in
accord with the traditional rules of genetics.