Deletion origin of cri du chat syndrome of known and unknown function. The abnormal phenotype is thus caused by haploinsufficiency of one or more of these 17 genes. Sequence analysis also reveals the origin of this deletion because the normal sequence is bounded by repeated copies of a gene called PMS, which happens to encode a DNA-repair protein. As we have seen, repeated sequences can act as substrates for unequal crossing over. A crossover between flanking copies of PMS on opposite ends of the 17-gene segment leads to a duplication (not found) and a Wil- liams syndrome deletion, as shown in Figure 17-23. Most human deletions, such as those that we have just consid- ered, arise spontaneously in the gonads of a normal parent of an affected person; thus, no signs of the deletions are usually found in the chromosomes of the parents. Less commonly, deletion-bearing individuals appear among the offspring of an individual having an undetected balanced rearrangement of chromosomes. For example, cri du chat syndrome can result from a parent heterozygous for a reciprocal translocation, because (as we will see) segregation pro- duces deletions. Deletions may also result from recombination within a heterozygote having a pericentric inversion (an inversion spanning the centromere) on one chromosome. Both mechanisms will be detailed later in the chapter. Animals and plants show differences in the survival of gametes or offspring that bear deletions. A male animal with a deletion in one chromosome produces sperm carrying one or the other of the two chromosomes in approximately equal numbers. These sperm 15.3 Deletion 15.2 15.1 15.1 P 1 14 1 14 13 13 12 12 11 11.1 11.2 11.2 12 12 13 13 14 14 15 15 -21- -21- 22 22 2 23 23 -31- -31- 32 32 3 33 3 33 34 34 FIGURE 17-22 Cri du chat syndrome is caused by the loss of the tip of the short arm of one of the homologs of chromosome 5. 35 35 Normal 5 Deleted 5

The technique of linking biological information to genome sequences is termed genome annotation. Gene annotation is the method of identifying gene locations and coding sections. It helps us understand what these genes are doing in the body through establ…