Genetics

Question 1

Normal number chromosomes?

Answer 1

In humans, the normal diploid number of chromosomes is 46, consisting of 22 pairs of autosomal chromosomes (numbered 1–22 in decreasing size) and one pair of sex chromosomes (XX in females and XY in males).

Question 2

Trisomy’s besides 21?

Answer 2

In addition to trisomy 21, only two other autosomal trisomies, 13 and 18, occur with any frequency in livebirths. Incidence rates for trisomies 13 and 18 in livebirths are 1/20,000 and 1/10,000, respectively. Unlike trisomy 21 that is associated with near-normal life expectancy, both trisomies 13 and 18 are associated with death in infancy, typically occurring during the first year of life.

Three sex chromosome trisomies—the 47,XXX, 47,XXY (Klinefelter’s syndrome), and 47,XYY conditions—are quite common, with each occurring in about 1/2000 newborns.