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Chromosomes in cells

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You have 2 copies of every gene (one from each parent), so you must have 2 copies of every chromosome. The pair of the mom one and the dad one are called homologous chromosomes.

We can figure out where the genes are, on each chromosome.

Here is a rough map of some genes, found on homologous chromosomes.

 

Different forms of life have different numbers of chromosomes.

number of chromosomes in a species

Image from What’s a Genome, courtesy of http://www.GenomeNews Network.org/J. Craig Venter Institute. http://www.expeditions.udel.edu/extreme08/genomics/

Each chromosome contains many genes, so the total number of genes is huge.

Number of genes in a chromosome for each species

Image from What’s a Genome, courtesy of http://www.GenomeNews Network.org/J. Craig Venter Institute. http://www.expeditions.udel.edu/extreme08/genomics/

How many chromosomes are in a cell

Each organism has a characteristic number of chromosomes in a cell.

Humans have 46 chromosomes in almost every cell.

A karyotype allows us to extract them from a nucleus, and photograph them.

Then one can cut-and-paste each of the chromosome images, and line them up, to clearly show how we have 2 of each. Half are from one’s mother, and half from one’s father.

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