21 W

21.1 Talking Glossary: X-chromosome (1.25 min)

Length: 1.25 min

https://www.genome.gov/genetics-glossary/X-Chromosome

Abstract: The X chromosome is one of two sex chromosomes. Humans and most mammals have two sex chromosomes, the X and Y. Some individuals have two X chromosomes in their cells, while others have X and Y chromosomes in their cells. Egg cells all contain an X chromosome, while sperm cells contain an X or a Y chromosome. This arrangement means that during fertilization, it is the XY parent that determines the whether the offspring will be XY or XX.

Image: https://www.genome.gov/sites/default/files/tg/en/illustration/x_chromosome.jpg

Audio: https://www.genome.gov/sites/default/files/tg/en/narration/x-chromosome.mp3

Transcript “So this, because I’m a female, is truly one of my favorite chromosomes. As you know, females have two X chromosomes. They’re quite large in comparison to the male chromosomes. They are carried by the egg, and so consequently you pass on–if you have an egg–you can only pass on an X chromosome to your offspring. What’s also quite interesting is the number of genes that are found on the X chromosome. It is predicted that there are approximately 155 million base pairs, which translates to about 900 to 1,400 genes on the X chromosome. Meaning that it carries about five percent of the total DNA in the entire cell. Which is quite in contrast to the Y chromosome, which is considerably smaller. Again, if you look at the X chromosomes in the genes that it carries, often times you’ll see that sex-linked disorders are carried on the X chromosome. Which is why, as I stated before, they’re more predominant in male, because there’s not a protective mechanism against having a mutation on one of those genes because we don’t have the normal copy of that gene on the X chromosome.”

Carla Easter, Ph.D.

Image: https://www.genome.gov/sites/default/files/genome-old/images/content/easter_carla.jpg

Biography

Carla Easter, Ph.D., is chief of the Education and Community Involvement Branch at the National Human Genome Research Institute (NHGRI). She played a major role in the development of the NHGRI/Smithsonian exhibition Genome: Unlocking Life’s Code, and its accompanying website, and serves as a liaison to the K-12 and university community as a speaker on genomic science and career preparation and pathways. Dr. Easter also serves as an adjunct faculty member at the University of the District of Columbia Department of Biology, Chemistry and Physics.

From 2003-2006, Dr. Easter was director of outreach for Washington University School of Medicine’s Genome Sequencing Center. Before assuming her role as outreach director, Dr. Easter was a research associate in the Department of Education at Washington University (2001-2003) where she explored the notions of science among secondary students. She served as pre-college coordinator for the NASA Summer High School Apprenticeship Research Plus Program and project associate for the Quality Education for Minorities Network. From 1997-2000, Dr. Easter conducted post-doctoral research at Washington University School of Medicine on the virulence factors associated with Streptococcus pyogenes.

Dr. Easter earned her bachelor’s degree in microbiology from the University of California, Los Angeles and her doctoral in biology with an emphasis on molecular genetics from the University of California, San Diego.

Video: https://youtu.be/S1-Y8cO-sMc Dr. Easter is a laboratory scientist who now specializes in science communication and outreach. The video below is a discussion between Dr. Easter and a highschool student sponsored by the Children’s Science Center Lab

The YouTube video below contains an audio-only interview with Dr. Easter https://youtu.be/X_CX7FWSfoM

21.2 Talking Glossary: X-inactivation (“Lyonization”; 1.45 min))

https://www.genome.gov/genetics-glossary/Lyonization (Links to an external site.)

Abstract: “Lyonization is commonly known as X-inactivation. In mammals, males receive one copy of the X chromosome while females receive two copies. To prevent female cells from having twice as many gene products from the X chromosomes as males, one copy of the X chromosome in each female cell is inactivated. In placental mammals, the choice of which X chromosome is inactivated is random, whereas in marsupials it is always the paternal copy that is inactivated.”

Audio

Transcript: “Lyonization is named after Mary Lyon, who was a geneticist who first figured out that in females who have two copies of the X chromosome, that one copy of each gene is turned off permanently in one chromosome or another. So that females, who have two copies of the X chromosome, and males, who have one copy of the X chromosome, are both human, and they can both operate fairly normally. So this process of turning off one copy of one gene or another on the X chromosome is called lyonization, and it happens from a series of essentially irreversible chemical modifications to one copy of the gene. The fascinating thing about this is that in so-called X-linked diseases, if the female inherits a gene responsible for an X-linked disease, and has one copy that’s abnormal and one copy that’s normal, the abnormal gene is almost always the one that’s turned off. And the normal gene is almost always allowed to stay on. In X-linked diseases in males, of course, X-linked diseases manifest because they only have one X chromosome, and so those mutated genes have to show up. But the fascinating part about it, which we really don’t understand, is how it is that the body knows if the female inherits one copy of a gene that is abnormal and would otherwise cause an X-linked disease if the normal copy was lyonized and turned off. That doesn’t happen. The abnormal copy gets turned off, leaving the normal copy to function, and saving the female from having the disease.”

Christopher P. Austin, M.D.

History:

English geneticist Mary Lyons (https://en.wikipedia.org/wiki/Mary_F._Lyon) first characterized X-inactivation. You can read a short article about her life here (https://www.nature.com/articles/518036a).