1. Polygenic Inheritance Discontinuous variation
The language – Lots
Sex chromosomes
Polygenic Inheritance
Carriers
Continuous variation
Normal distribution
Sex-linked
Haemophilia
Superscripts
Allele
Discontinuous variation
Autosomes
Additive
Homozygous

2. The stuff you Need to know!
Sex-linked inheritance and the effects of the presence of genes on the X-chromosome and not on the Y-chromosome.
Polygenic inheritance leading to characteristics

3. Normal Body Cells
In the nucleus of every body cell there are 46 chromosomes
22 homologous pair (AUTOSOMES) and one pair of sex chromosomes

4. Female and Male Sex Chromosomes
In the female, the sex chromosomes make up a fully homologous pair, the X chromosomes.
In the male, the sex chromosomes make up a pair consisting of an X and a much smaller Y, which is homologous to only part of the X chromosome.

5. Sex-linked genes.
The X and Y chromosomes behave as a homologous pair at meiosis.
However, the X chromosome differs from the Y chromosome in that the larger X carries many genes not present on the smaller Y. These genes are said to be sex-linked.

6. Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance
At fertilisation
When an X chromosome meets a Y chromosome at fertilisation, each sex-linked gene on the X chromosome becomes expressed in the phenotype of the human male produced.
This is because his Y chromosome does not possess alleles of any of these sex-linked genes and cannot offer dominance to them.
Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

7. Symbols
In crosses and family trees involving sex-linked gees, the sex chromosomes are represented by the symbols X and Y and the alleles of the sex-linked gene by appropriate superscripts.
Sex-linked genes

8. X-linked recessive disorder - Red Green Colour Blindness
Inability to distinguish between red and green
A red green colour blind person does not see the number 29 on the right
In humans normal vision (C) is completely dominant to red-green colour blindness (c)

9. Genetics of Colour Blindness
Normal vision C
Red-green colour blindness c
These are the alleles are sex-linked because...
Heterozygous females are called carriers (Cc)
Although they are unaffected themselves there is a 1 in 2 chance (50%) chance that they will pass the allele on to each of the offspring.

10. Five possible genotypes for normal and red-green colour blindness
Phenotype
XCXC
Female with normal colour vision
XCXc
Female (carrier) with normal colour vision.
XcXc
Female with colour blindness (very rare e.g. 0.5%)
XCY
Male with normal colour vision
XcY
Male with colour blindness more common (8%)
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Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

11. Work out the genotypes of the following family tree

12. Answers Carrier mother XCXc Nomal father XCY Normal daughter XCXC
Carrier daughter XCXc
Normal son XCY
Colour-blind son XcY

13. Why is colour blindness more common in males?
Red green colour blindness is rare in females since 2 recessive alleles must be inherited.
It is more common in males where only one is needed.

14. Colour blindness problem set

15. Puzzle 1

16. Puzzle 1 - Answer

17. Puzzle 2

18. Puzzle 2 - Answer

19. Puzzle 3

20. Answer: Puzzle 3

21. Puzzle 4

22. Puzzle 4 - Answer

23. Puzzle 5

24. Puzzle 5 - Answer

25. Puzzle 6

26. Answer puzzle 6
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27. Puzzle 7

28. Puzzle 7 - Answer
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29. Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance
Puzzle 8
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Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

30. Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance
Puzzle 8 - Answer
04/06/2018
Mrs Smith: Ch12 Sex Linkage and Polygenic Inheritance

31. Puzzle 9

32. Puzzle 9 - answer

33. Puzzle 10

34. Puzzle 10 - Answer

35. Puzzle 11

36. Puzzle 11 Answer

37. Haemophilia: mutated form of factor VIII in platelets
Haemophiliacs cannot make the blood clotting protein Factor VIII.
This is a problem with blood clotting. So, if a tissue is damaged and blood vessels are broken, bleeding continues for longer than normal.
Some bleeding is obvious such as when the skin is cut or broken. Others are less easy to spot like bleeding into or around the joints.

38. X-linked recessive disorder -Haemophilia
It caused by a recessive allele carried on the X (e.g. The gene is located on the non-homologous region of the x-chromosome) but not the Y chromosome.
The haemophiliac allele (Xh)is recessive to the normal allele (XH).
Hence is sex-linked.

39. More common in males than females
Haemophilia is more common in men than women.
Fequency in britian is 1:5000
Males inherit the allele from their mother and develop the disease.
Since (until recently) the prognosis for survival was poor and haemophiliac males did not survive to pass on the allele to their daughters (its on the X-chromosome). Therefore females with haemophilia where rare.

40. There is now treatment -FYI
‘Clotting factor concentrates’ revolutionised haemophilia care allowing patients to travel, have jobs, and live full and independent lives.
Transfusion with whole blood and plasma.

41. When the father is normal and the mother is an unaffected carrier

42. Family Tree of Haemophilia

43. Puzzle 1

44. Puzzle 1- Answer

45. Muscular Dystrophy
Duchenne Muscular Dystrophy is the most common form of this disease.
Sufferers are severely disabled from an early age.
The normally die without passing allele onto the next generation.
Afects 1:3000 male infants.
Skeletal muscles loose their normal structure and fibrous tissue develops in their place.
Caused by a recessive allele carried on the X chromosome and is sex-linked.

46. Family Tree of Muscular Dystrophy
In this family the allele survives from female carrier to female carrier.

47. Y-linked Inheritance
Y-linked traits are controlled by alleles on the Y chromosome
Another word for Y-linked traits is holandric traits as they are “wholly male”.

48. Y-linked Inheritance
Do the words homozygous or heterozygous apply to Y-linked traits?
Are females affected by Y-linked traits?

49. What is this?

50. Y-linked Inheritance
An example of a Y-linked trait is HAIRY EARS!

51. Y-linked Inheritance
All of the sons of an affected male will display this Y-linked trait.

52. A final example of Sex-linked Traits
Eye color in fruit flies ( Drosophilia melanogaster )
Eye color is controlled by the X chromosome.
Red eyes are dominant to white.
White eyes are most common in males.
Females only display white eyes if they are homozygous recessive for the trait.

53. Fruit Flies Continued What type of X-linked inheritance is this?
Try your new knowledge out on the handout you are about to receive!
X-linked recessive Inheritance because the males show the trait more often that the females. Also, the females must have two X chromosomes, both of which carry the recessive alleles for white eye color in order to have white eyes!

54. Let’s Try These
Practice Problems