The U.S. Constitutionâs 10th Amendment is arguably the most important of all the amendments in the brilliant document that helped shape the United States.
Not even identical twins have the same DNA--although theirs would be far closer than anyone else's. DNA changes over time. Odds of two siblings having the same DNA who did not split from the same zygote are vanishingly small.
Genes are segments of DNA arranged along a chromosome. A single chromosome can have hundreds or even thousands of genes. Most sexually reproducing organisms, like people and pigeons, have two copies of each chromosome, called homologous chromosomes. Humans have 23 chromosome pairs, or 46 chromosomes in all. Homologous chromosomes have the same genes arranged in the same order, but they have slightly different DNA sequences. Different versions of the same gene are called alleles (uh-LEELZ); homologous chromosomes often contain different alleles.