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← Sex Chromosomes vs Autosomes

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Showing Revision 1 created 09/30/2013 by Cogi-Admin.

  1. Now it's no secret, the Y chromosome is so
  2. much smaller than the X chromosome. And there's an amazingly
  3. cool story behind all of this. Please go read about
  4. it. It's amazing. It's been going on for a long
  5. time. The Y chromosome has been getting smaller and
  6. smaller for the last oh, I don't know 100 million
  7. years or so. But the result has had dramatic implications
  8. for genes that are expressed from the X chromosome and
  9. they don't exist on the Y chromosome. Think about it
  10. for a second. Let's say we're interested in this gene
  11. that causes red green colorblindness and it's on the X
  12. chromosome. Well, females have two X chromosomes. So anytime we're
  13. looking at their inheritance patterns. It's pretty much the same
  14. thing and straightforward as we've ever looked at for the
  15. autosomes, because females have two copies. They have two alleles,
  16. they can be homozygous they can be heterozygous, they can be
  17. plus, plus, plus minus, or minus, minus. And for
  18. recessive traits, when they're homozygous recessive, then they're going to show
  19. the trait, let's say, for red green color blindness. But
  20. for the sake of it, let's say that the female
  21. is heterozygous, dominant recessive. So they're not actually red-green
  22. colorblind, but they carry an allele for it. Males don't
  23. have that luxury. It's almost like the females have a
  24. backup copy, functioning copy of the gene, so they aren't
  25. red-green colorblind. But males sure they have one X
  26. chromosome. The Y chromosome is missing the piece of information
  27. that would correlate to the same region on the X
  28. chromosome. They don't have this gene. So if the male
  29. has the dominant copy grade, he's going to see color
  30. vision just fine. But if he inherits even just one
  31. allele, one recessive allele for red-green colorblindness, that's all it
  32. takes. He doesn't have anything to counteract the effect, and
  33. so he's going to be red-green colorblind. So in our
  34. pedigree up at the top here, we have an affected
  35. great-grandfather. And if he is XY, we know that
  36. he has an affected allele. If he has any daughter,
  37. he must pass that allele on, because both daughters
  38. are XX. They get one X from mom and one
  39. X from dad. And the X that came from dad
  40. had to have been the one with the recessive alleles.
  41. So both of these daughters are carriers. They have to be,
  42. because we know they're unaffected. See they're not shaded, that means
  43. they're not red green colorblind. It must have gotten the recessive
  44. allele from their father. And then if we look at this individual's
  45. children here, these two. She had an unaffected daughter, we have
  46. no idea if she passed on the recessive allele or not
  47. because it's masked. But in the case of her son, we
  48. see very clearly, he must have the recessive allele because he's affected.
  49. So, although the trait looks like it skipped a
  50. generation, right? It looks like it went from grandfather
  51. to grandson. The allele didn't skip anybody, the allele
  52. went right from grandfather to mother to son. You
  53. can see it, you can track it in the pedigree. So the lesson from all of this is
  54. that, although phenotypes appear to skip generation sometimes, the
  55. alleles never skip. And in the case of sex-linked traits,
  56. when they're linked to either the X or the Y chromosome,
  57. because there're some traits linked only to the Y chromosome. The
  58. patterns can be a little bit more tricky to discern because
  59. the males don't have another copy to complement their X chromosome.