First of all, there are usually 22 pairs of somatic chromosomes and one pair of sex chromosomes (for 46 total chromosomes). You can write the formula for the karyotype of an individual by stating the total number of chromosomes in the followed by the sex chromosomes, then (if there is an abnormal number of chromosomes) what is extra or missing. So, the formula for a normal male is "46, XY" and for a normal female "46, XX".

1. Write a formula for:

(a) a male with Down syndrome (due to the presence of an extra chromosome 21)

47, XY, +21

(b) a female with Turner syndrome (only one X chromosome)

45, XO

(c) a female with cri du chat syndrome (deletion of a portion of p arm of chromosome 5)

46, XX, -5p

(d) a male with Klinefelter syndrome (two X chromosomes and one Y chromosome)

47, XXY

(e) a female with a Philadelphia chromosome (tanslocation between chromosomes 9 and 22). People suffering from a form of leukemia (chronic myeloid leukemia) frequently harbor cells containing the Philadelphia chromosome.

46, XX, t(9, 22)

2. Abnormalities in chromosome number are detrimental and usually lethal. Speculate as to why the impact of abnormal numbers in chromosome 21 and in sex chromosomes are relatively mild.

Abnormal numbers of sex chromosomes are usually not lethal because (1) only one X chromosome is active in any given cell, so if there is only one X, that one will be active in each cell, and if there is an extra X (as in Klinefelter's), most of the genes on it will be inactivated, i.e. it will become a Barr body, and (2) Y chromosomes don't have very many genes in them.

I am not sure why there are some diseases that involve having an abnormal number of autosomes (trisomy 21, 18, or 13), but that are not lethal. I would speculate that these chromosomes have fewer genes than other chromosomes and/or have genes in which the amount of protein that they make is not critical to life - i.e. the genes can make more protein than normal (because there are 3 copies of the gene instead of 2), and the extra amount doesn't affect the functioning of the cell too much.