Sex-Linked Inheritance

1. Sex-Linked Inheritance SBI3UI – Unit 5: Lesson 5

2. Big Ideas in this Lesson: • It has been estimated that the human X chromosome carries between 100 and 200 different genes. This means that there are numerous sex-linked traits, including red-green colour blindness, hemophilia, hereditary myopia, and night blindness. • These traits affect men more often than women. Why?

3. Terminology in this Lesson: • X-linked recessive inheritance • hemophilia • carrier • colour blindness • Duchenne Muscular Dystrophy

4. Leading Question: • HOW are X linked traits different from autosomal traits?

5. A. Sex Determination • The human X chromosome (found in BOTH males& females) is quite large and it contains many genes • The Y chromosome (found only in males) is primarily involved in determining “maleness”. • Note: If only one X chromosome is present (recall Turner Syndrome?), then the offspring develops as a female phenotype, not male.

6. Autosomal ≠ X linked Inheritance • Any traits controlled by the X-chromosome are calledX-linked genes or traits. • This form of linkage gives results contrary to Mendel’s Law of Independent Assortment: a gene on the X chromosome in the male has no matching allele on the Y chromosome! • Therefore, any gene present on the X chromosome, whether dominant or recessive, is expressed in maleoffspring (and females with Turner Syndrome)

7. Consider sex determination • We know that there are diseases related to the number of sex chromosomes… we considered these in the last unit… but for a moment, let’s pretend we are all perfect. (We are, aren’t we?)

8. Punnett Square - Sex Determination: • Try this Punnet square for sex determination • P: XX (female) crossed with XY (male) • Note: capital letters for both ‘X’ and ‘Y’!

9. Yours should look like this… • Female: XX X X Phenotypic Ratios: 50% female 50%male X Y • Male: XY

10. B. Colour Blindness:

11. Colour blindness can be X-linked: • Humans see colour because we have protein structures on the retina called cones, which respond to specific light frequencies. • Humans have 3 “colours” of cones: red, green and blue. We see a continuous rainbow because of the way that these cones are stimulated together. • Side note: After dark falls…humans see in black and white because of different protein structures called rods, which are also found on the retina.

13. Types of colour blindness: Blue-Yellow Red-Green The genes coding for red cones and green cones occur only on the X chromosome. • The genes coding for blue (and yellow) colours occur only on an autosomal chromosome.

14. Yellow-blue colour blindness is rare. • Males and females are equally affected at about 0.01% of the population. • This is because this yellow-blue colour blindness is an autosomal recessive disease, carried on chromosome 7. • You’d need 2 recessive alleles for this phenotype to be expressed.

15. Red-green colour blindness is more common. • Either set of cones can be affected with the same phenotypic result. • Colour blindness can also be the result of other disease processes…diabetes, vitamin A deficiency, shaken baby syndrome, macular degeneration, etc. • There are many, many more descriptive categories …but in the general case, 7 to 10% of males are red-green colour blind. • Males only have one X chromosome - and so the information coded there is always expressed!

16. Females? • Colour blindness is quite rare in females… • about 1 in 100 000 females is affected. • WHY? Females have two X chromosomes (as compared to 1 in males) One is maternal, the other paternal. Therefore females have a much better probability of normal vision.

17. What could colour blindness look like?

18. Colorblind individuals see a number 17. Color normal individuals see a number 15. Colorblind individuals see a yellow circle. Color normal individuals see a yellow circle and a faint brown square.

19. An Ishihara test image as seen by subjects with normal color vision and by those with a variety of color deficiencies Thank you to wiki!

20. Colour blindness as an X-linked trait… • Females (mammals) have two X chromosomes in normal individuals. In each cell, only one of the X chromosomes is activated. In some cells, this may be the maternal X and in other cells it may be the paternal X. • Because females have two X chromosomes, they have a better chance of normal vision.

21. Males and colourblindness • Normal males have one X chromosome and one Y chromosome. • The X chromosome is always inherited from the female parent in males. • Consequently the probability of a male being red-green colour blind is about 1 in 10. (The rate of colour blindness varies dependent on which relatively segregated population of humans we look at.)

22. Gametes involved in RG colour blindness: • How do we write the alleles for the genotypes? • X- NORMAL DOMINANT ALLELE Xr - RECESSIVE ALLELE FOR RG COLOUR BLINDNESSY - LACKS THE GENE FOR THIS TRAIT (therefore designated just ‘Y’)

23. A female can either be colour blind herself, OR she can be a carrier for colour blindness. What genotype does a colour blindness carrier have?

24. A female can either be colour blind herself, OR she can be a carrier for colour blindness. What genotype does a colour blindness carrier have? A female carrier has the genotype XXr

25. Can a male be a carrier of an X linked disease?

26. Can a male be a carrier of an X linked disease? A male “carrier” could be XXrY Klinefelter’s Syndrome!

27. Let’s look at a family tree! What are the genotypes?

28. Father: Red-green colourblind Goetze:red-green colour blind Mother: Normal Father: Normal Father: Colour blind Mother Normal Mother: Normal Goetze’s Brother: Normal Colour vision!

29. Goetze:red-green colour blindXrXr Father: Red-green colourblindXrY Mother: Normal vision X? X? Father: Normal XY Father: Colour blind XrY Mother normal XX Mother: Normal vision X?X? Goetze’s Brother Normal Colour vision! XY

30. Mother: Normal vision X Xr Father: Normal XY Father: Colour blind XrY Mother normal XX Mother: Normal vision XXr Goetze’s Brother Normal Colour vision! XY Goetze:red-green colour blindXrXr Father: Red-green colourblindXrY

31. C. Hemophilia

32. Hemophilia is… • A group of blood clotting disorders • In an affected individual, blood fails to clot properly and the individual can hemorrhage to death • Modern solutions are usually to supply the missing clotting factor intravenously • Carriers for hemophilia have blood clotting times that are slightly slower than normal

33. Back to Queen Victoria and her brood… • The following picture is a pedigree. • Pedigrees are a graphical representation of the phenotype of a large number of people in a family tree. • Pedigrees can be used to track patterns of diseases- to locate carriers, or to establish if a disease is heritable or the result of chance.

35. Gametes involved in hemophilia: • XH - NORMAL DOMINANT ALLELE (can be designated with just an ‘X’) • Xh - RECESSIVE ALLELE FOR HEMOPHILIA • Y - LACKS THE GENE FOR THIS TRAIT (therefore designated just ‘Y’)

36. Phenotypes are: normal, carrier or affected (hemophiliac) Males Females

37. Phenotypes are: normal, carrier or affected (hemophiliac) Males Females

38. Phenotypes are: normal, carrier or affected (hemophiliac) Males Females

39. Sample Cross: Suppose we cross an affected male (hemophiliac) with a female who is not a carrier. What will the phenotypes of their daughters be?

40. Sample Cross:Cross an affected male (hemophiliac) with a normal female who is not a carrier for the disease. What will the phenotypes of their daughters be? Cross: hemophiliac male x normal female XhYxXHXH XHXH Since all daughters of this cross have the genotype XHXh therefore all daughters are carriers for the disease. They will have slightly longer than normal clotting times as a result. Xh Y

41. D. Assignment • Read and take notes from p. 219-225 in your textbook. Clarify the ways in which X linked traits differ from autosomal traits! • Complete the practice questions on your handout • Complete questions # 3, 4 on p. 225. • Create a vocabulary page for the terminology in this lesson and the assignment.