Human late prophase karyotypes have about 2000 visible G bands. The human genome
ID: 41627 • Letter: H
Question
Human late prophase karyotypes have about 2000 visible G bands. The human genome contains approximately 22,000 genes. Consider the region 4q32.3 through the end of the long arm of chromosome 4 that is identified on the late prophase chromosome in the following figure, and assume the entire region is deleted.
Approximately how many genes will be lost as a result of the deletion? (Hint: Consider both how many bands are lost as a result of the deletion and the gene density per band.)
Express your answer using two significant figures.
Explanation / Answer
Gene density per band refers to number of genes present in a single G band obtained on karyotyping.
Gene density per band = Total number of genes present in the genome / Number of G bands
Gene density per band = 22,000 / 2000 = 11 genes per G band
In 4q32.3
4 - chromosome number
q Long arm of chromosome
3 Region around the centromere
2 Position of band
.3 Position of sub-band
Generally heterochromatic region will be rich in adenine and thymine residues and contain lesser number of active genes. They stain darkly on G banding. But euchromatic region rich in guanine and cytosine residues which contain majority of active genes will stain lightly on G banding.
When a specific region of chromosome is deleted. Majority of active genes present in lightly stained bands will be lost whereas very few or limited number of genes might be lost in darkly stained regions. Hence we consider only number of lightly stained bands to approximate the number of active genes deleted.
When the entire region of 4q32.3 is deleted that means region 4 in q arm of chromosome 4 in human is deleted.
Number of darkly stained bands present in region 3 is 7.
Number of lightly stained bands present in region 3 is 7.
Hence on deletion of region 3, 14 bands are lost.
Number of genes deleted = Number of lightly stained G bands deleted x Gene density per band
= 7 x 11 = 77 genes
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