Cameo

In Sickness and in Health by Barry Chess

Part IV—Sex-Linked Inheritance

“Alright,” Olga began, “so factor VIII deficiency is sex-linked because it only affects men. Does it require the presence of testosterone or something like that?”

“No, but there are many traits that do depend on the presence or absence of sex hormones. We call them sex-influenced traits. Sex-linked traits get their name from the fact that the genes that cause them are carried on the X chromosome, which is one of the chromosomes responsible for determining what sex a person will become. Let’s take a look at how factor VIII deficiency runs in both your families.”

Questions

  1. What are the characteristics of X-linked recessive inheritance?
  2. Why does a son never inherit his father’s defective X chromosome?
  3. What is required for a woman to display a sex-linked recessive trait?
  4. Return to the pedigree drawn earlier for Greg and Olga; mark those persons who are carriers of the factor VIII deficiency gene.
  5. What is the chance that Olga carries the gene for factor VIII deficiency? Calculate the probability that she will pass it to her offspring. Will male children be affected in a different way than female children?
  6. What is the chance that Greg carries the factor VIII gene? Can he pass the gene on to his sons? His daughters? How will each be affected?

References

Human Genetics for First Year Students: X-linked Recessive Inheritance
http://www.uic.edu/classes/bms/bms655/lesson7.html
X-linked Inheritance: Red-Green Color Blindness, Hemophilia A
http://www.musckids.com/health_library/genetics/xlink.htm

Go to Part V—“Population Genetics”

Originally published at http://www.sciencecases.org/sickness_and_health/sickness_and_health4.asp

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