ABSTRACT
Figure 6.1 Principles of PGD for single-gene disorders by two-step sequential analysis of first and second polar bodies (PB1 and PB2) shown for thalassemia mutations. The large circle represents the genotype of an oocyte obtained from a heterozygous carrier of a mutant gene prior to PB1 extrusion (metaphase I, MI). The other seven large circles show oocytes resulting from the first (metaphase II, MII) and second meiotic divisions. The seven medium-sized circles show extruded PB1s, and the five smallest circles show PB2s. The upper portion of the figure shows extrusion of a homozygous normal PB1, resulting in an affected oocyte. This is confirmed by a mutant PB2 (shaded small circle). The left portion presents the opposite sequence of events. The crossover situation (evidenced by heterozygous PB1) is shown in the middle portion, resulting in either an affected (C1) (evidenced by normal PB2 extrusion) or normal (C2) oocyte (evidenced by abnormal PB2 extrusion). N, normal allele; T, affected (thalassemia) allele; C1 and C2, possible outcomes from heterozygous MII oocyte after the second meiotic division.
