Chromosomes and Genes

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DNA Interactive

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.

Description: 1 gene on 2 chromosomes

 

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.

 

 

 

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.

Description: 3 genes on 2 chromosomes

This table shows that different critters have different chromosome numbers.

Peas = 14

House mouse = 40

Cricket = 22

Boa Constrictor = 36

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