Comparative Genomics: Chromosome Number

Comparative Genomics: Chromosome Number
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  • 0:01 Chromosome Numbers
  • 0:51 Chromosome Pairs
  • 2:20 Chromosomes in Other Organisms
  • 3:42 Species Hybrids
  • 4:36 Lesson Summary
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Lesson Transcript
Instructor: Meredith Mikell
The genetic blueprint of an organism is contained in its chromosomes. Different species have different numbers of chromosomes. In this lesson, we will examine the concept of chromosome numbers as it applies to genomics and follow up with a brief quiz.

Chromosome Numbers

Your DNA contains the code that determines your physical characteristics, behavior, and more. It's the reason why you look like your genetic relatives but not so much your non-related peers. DNA is made up of nucleotides, individual molecules that form a very long double-helix chain. The chain is then wound up into little packages called chromosomes, bundles of DNA containing specific genes for specific traits.

Humans have a total of 46 chromosomes, arranged in 23 pairs. But not all organisms have this many; some have more chromosomes, some have fewer. This karyotype is a map of a set of human chromosomes and can be useful in identifying differences in chromosome number between species.

Chromosome Pairs

Each pair of chromosomes contains several specific types of genes on them, a sequence of nucleotides that code for a given trait or characteristic. So one chromosome pair might have the genes for eye color and skin pigmentation, while a different pair has the genes for right or left handedness, freckles, and lactose tolerance.

Chromosomes are paired because an individual receives one set from each parent. For example, chromosome pair number 15 codes for the same genes, but each chromosome might have different alleles, or variations of the same gene. This means if mom has blue eyes and dad has brown eyes, then their child's pair of chromosomes number 15, which has the genes for eye color, will have one brown allele and one blue allele.

Of the 23 pairs of human chromosomes, one pair is the sex chromosomes, which are either two X chromosomes in the case of females or one X and one Y chromosome in the case of males. The other 22 pairs are autosomal chromosomes, meaning they are not sex chromosomes, but instead code for most of the organism's other characteristics.

Each chromosome actually has anywhere from a couple hundred to a couple thousand genes on it! The Human Genome Project is ongoing research to try to map all 30,000 or more human genes by identifying every known nucleotide sequence. That's a lot of information to code for!

Chromosomes in Other Organisms

Other organisms have varying numbers of chromosomes, ranging anywhere from four to over 200. A fruit fly has eight; a guinea pig has 64; a mallard duck has 80. More chromosomes does not necessarily mean more complex organisms. The onion, for example, has about five times more chromosomes than humans have! Yet, it definitely is not five times more complex.

Chromosomes sometimes contain several copies of the same gene sequence, have long strands of non-coding sequences, or are polyploid, meaning they have multiple sets of each chromosomes, not just two! Plants are notorious for having evolved some amazing genetic tricks like polyploidy. But when we look at organisms more closely related to ourselves, say other great apes, we still see some differences in chromosome number.

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