Chromosomes are bundles of coiled DNA and are found in every cell
that has a nucleus. In humans, there are 46 chromosomes in every nucleus of every
body cell. In fact, every nucleus in the body has an identical set of
chromosomes. In total, there are 23 pairs (or kinds) of chromosomes in a human
nucleus. Mothers and fathers contribute a chromosome to make each pair. A human
nucleus has roughly 20,000 genes. This equals 40,000 alleles.
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If you look to the left, you will see one
pair of homologous chromosomes. Let’s pretend that the first chromosome of
this pair came from the critter’s dad and the second came from its mother. As
you can see, a gene is always comprised of 2 factors, or “alleles”. In this
example, the “E” allele is on the first chromosome and the “e” allele is
located on the second chromosome. Together, “E” and “e” make up a “gene”. Everything
is made up of DNA. |
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If you look to the right, you will see a
different pair of homologous chromosomes. As you can see, this pair of
chromosomes hosts 3 different genes. The first 2 genes are heterozygous and
the last gene is homozygous dominant. As usual, a gene is always comprised of
2 factors, or “alleles”. The first
chromosome of this pair came from the critter’s mom and the second came from
its father. Three chromosome pairs are shown below.
Lines on the chromosomes represent different alleles. Two alleles make up a
gene.
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This table
shows that different critters have different chromosome numbers.
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Peas
= 14 |
House
mouse = 40 |
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Cricket
= 22 |
Boa
Constrictor = 36 |