Although they come from the same family, they are different species. Here is a cute article on the subject from National Geographic. What's the Difference Between Rabbits and Hares? read more
Two chromosomes fused end to end to form what we now call human chromosome 2. The region of the fusion, in the middle of chromosome 2, still carries the characteristic sequences that are usually only found at the ends of chromosomes, which is how we know that this kind of fusion must have taken place. read more
A normal human has 23 pairs of chromosomes, i.e. 46 chromosomes. People with Down Syndrome have an extra copy of the 21st chromosome so they have 47 chromosomes. read more
This number, along with the visual appearance of the chromosome, is known as the karyotype, and can be found by looking at the chromosomes through a microscope. Attention is paid to their length, the position of the centromeres, banding pattern, any differences between the sex chromosomes, and any other physical characteristics. read more