Question

A number of human-mouse somatic cell hybrid closes were examined for the presence or human chromosomes and the expression of specific human genes. Part of the results are summarized in the table below. Assign each gene to the chromosome on which it is located.

	Hybrid Cell Lines
	5	12	23	30	61	68
Genes Expressed
HBB (defects causes Sickle Cell Anemia)	+	-	-	-	+	+
CFTR (defects causes Cystic Fibrosis)	-	-	+	+	+	+
BRCA1 (defects causes Breast/Ovarian Cancer)	-	-	-	-	-	-
HEXA (defects causes Tay-Sachs Disease)	+	+	-	+	+	-
LRRK2 (defects causes Parkinson's Disease)	-	+	-	+	-	+
Chromosomes (Present or Absent)
16	+	-	+	-	+	-
17	+	-	-	-	+	+
18	+	+	-	+	+	-
19	+	-	+	+	-	+
20	-	-	+	+	+	+
21	-	+	-	+	-	+
22	+	+	+	+	+	+

Answer 1

HBB is located in chromosome 17
CFTR is located in chromosome 20
BRCA1 is located in chromosome 22
HEXA is located in chromosome 18
LRRK2 is located in chromosome 21

Each gene is expressed only when the chromosome is present, this way we can correlate the location of each gene. For example: gene HBB is expressed exclusively in cell lines 5, 61 and 68; and only the chromosome 17 is reported to be present on cell lines 5, 61 and 68; therefore we can conclude that gene HBB is located on chromosome 17.

However, if we see the results, we can see that gene BRCA1 is located in chromosome 22 in spite of the fact that chromosome is 22 is present in ALL cell lines and BRCA1 is not expressed in any of the cell lines; therefore, how did we get this result? The answer is related to the disease caused by each mutations in each gene. For BRCA1, a gene in charge of repairing damaged DNA and suppressing tumor formation, this protein coded by BRCA1, has to be absent in order to lead to tumor formation. In contrast to all the other diseases, where the proteins coded by those genes, are truncated, modified and/or decreased in quantity, but they are still produced.