Embryo Selection - Brutes or Angels: Human Possibility in the Age of Biotechnology - page 85

Table 3.1. Continued

Disease Inheritance Symptoms; treatment Diagnostic tests

Fragile X syndrome Dynamic mutation,

autosomal, dominant

Cognitive deicits, elongated face, large ears,
poor muscle tone, social anxiety; no cure,
behavioral therapy relieves some symptoms

PCR

a An autosome is a non-sex chromosome. Humans have twenty-two autosomes and two sex chromosomes, X and Y.
b Recessive refers to a mutation that must be present in both pairs of matched chromosomes in order to manifest itself. Thus, persons with only a
single copy of a mutant, disease-causing gene are carriers of the disease condition but do not actually have the disease.
c A dynamic mutation changes (worsens) from generation to generation. Examples are trinucleotide repeats, sequences of three bases in chro-
mosomal DNA that are repeated contiguously over and over. Huntington disease is caused by the repetition within human chromosome 4 of the
trinucleotide CAG. The number of repetitions increases within each generation of carriers, and at a threshold level of repeats the disease mani-
fests itself.
d Dominant refers to a mutation that only needs to be present in one member of a pair of chromosomes in order to manifest itself. Thus, persons
with only a single copy of the gene variant that causes Huntington disease will have the disease.
e Karyotype refers to the number and appearance of chromosomes in a mitotic cell from a particular species. For example, the normal human
karyotype is forty-six chromosomes, twenty-two pairs of autosomes (non-sex chromosomes) and one pair of sex chromosomes (XX in females
and XY in males). One member of each pair of chromosomes comes from each parent. A missing chromosome (monosomy) or an extra chromo-
some (trisomy) causes syndromes like Turner (XO), Klinefelter (XXY), and Down (trisomy 21), which are diagnosed by a karyotype analysis.
f X-linked speciies a gene located on the X chromosome; most X-linked, recessive genetic diseases affect only males since they have only a single
X chromosome. For females to be affected, both of their X chromosomes must carry the mutated, disease-causing gene.