9. Y-LINKED GENES AND DISORDERS

Question 1

1. What is the SRY Gene?

Answer 1

• this is a Y Chromosome Gene
• it codes for the development of the testes

Question 2

2. What does an absence of the SRY gene in females result in?

Answer 2

• it results in the development of gonads in the ovaries

Question 3

3. What do rare abnormalities in the SRY gene result in?

Answer 3

• they result in the development of XY females
• they result in the development of XX males
• EG: Swyer Syndrome

Question 4

4. What is Swyer Syndrome?

Answer 4

• this is an XY Gonadal Dysgenesis
• this means that the Gonads do not develop normally

Question 5

5. What causes Swyer Syndrome?

Answer 5

• it is caused by mutations in the SRY gene
• the SRY gene is often inactivated
• this mutation gives rise to XY females
• they are karyotypically male
• they have extremely underdeveloped testes

Question 6

6. What is the SRY gene essential for?

Answer 6

• Maleness

Question 7

7. What happens when there is an inactivation of the SRY gene on the Y Chromosome?

Answer 7

• this means that XY individuals who are normally male
  will now have female characteristics
• they are karyotypically male
• they have phenotypical female traits

Question 8

8. What is XX Male Syndrome?

Answer 8

• this is the translocation of part of the Y chromosome to
  the X chromosome
• the translocated part contains the SRY gene
• the X chromosome now carries the SRY gene
• this gene is normally a male gene

Question 9

9. What are the effects of having XX Male Syndrome?

Answer 9

• the females will have male characteristics
• even though they are karyotypically female

Question 10

10. What is one societal issue caused by the presence of the SRY gene in females?

Answer 10

• there are Gender Verification problems in the Olympics
• athletes with the SRY gene were not permitted to
  participate as females
• this rule was omitted in 1992

EXAMPLE:
- Caster Semenya
- She has Swyer Syndrome
- she is a female with an XY karyotype
- 46 XY male with female characteristics
- this is due to SRY inactivation

Question 11

11. What 2 Chromosomal Alterations result in SRY Gene Disorder (XX Male Syndrome)?

Answer 11

1. Translocation
2. Genetic Recombination

Question 12

12. What is Translocation?

Answer 12

• this is the exchange of chromosomal fragments
  between non-homologous chromosomes
• this is an abnormal process
• this can happen during Spermatogenesis

Question 13

13. What is Genetic Recombination?

Answer 13

• this is the exchange of chromosomal fragments
  between homologous chromosomes
• this is a normal process
• this happens during Prophase of Meiosis I
• it is also known as Crossing Over

Question 14

14. What is Colour Blindness?

Answer 14

• this is the inability or decreased ability to perceive
  colour differences

Question 15

15. What is Duchenne Muscular Dystrophy?

Answer 15

• this is a disorder that affects 1 out of 3 500 males in
  the US
• it is the progressive weakening of the muscles
• it is the progressive loss of co-ordination
• affected individuals rarely pass the age of 20

Question 16

16. What is Haemophilia?

Answer 16

• this is a disease in which progressive bleeding is
  prolonged in the affected individual
• this follows after an individual has sustained an injury

Question 17

17. What causes Haemophilia?

Answer 17

• there is a deficiency in Factor VIII (8)
• this factor is responsible for blood clotting

Question 18

18. What is X Inactivation in Female Mammals?

Answer 18

• 1 of the 2 X-Chromosomes in each Somatic cell will be
  randomly inactivated
• this happens during Embryonic Development

Question 19

19. What happens to the inactivated X Chromosome?

Answer 19

• the X chromosome will condense into a Barr Body
• this Barr Body then lies inside a nuclear envelope

Question 20

20. What happens to the Barr Body X-Chromosomes in the ovaries?

Answer 20

• the Barr Body chromosomes are reactivated
• they are reactivated in the cells that give rise to eggs
• this means that every female gamete has an Active X
  chromosome

Question 21

21. What happens if a female is Heterozygous for a particular gene located on the Active X Chromosome?

Answer 21

• she will be a mosaic for that character

Question 22

22. Provide a definition for Mosaicism.

Answer 22

DEFINITION 1:
- some somatic cells will express the phenotype of the
one X-linked gene
- the other cells will express the phenotype of the other
gene on the X chromosome

DEFINITION 2:
- Mosaicism occurs when a person has two or more
genetically different sets of cells in their body

DEFINITION 3:
- Genetic mosaicism is the presence of DNA alterations
in only some of the body’s cells
- A person with mosaicism has a mixture of normal and
mosaic DNA in the same type of cells

Question 23

23. Provide 2 examples of Mosaicism in humans?

Answer 23

1. Hypohidrotic Ectodermal Dysplasia
2. Heterochromia

Question 24

24. What is Hypohidrotic Ectodermal Dysplasia?

Answer 24

• this is an X-linked mutation
• it prevents the development of sweat glands

Question 25

25. What can be said about a woman who is Heterozygous for Hypohidrotic Ectodermal Dysplasia?

Answer 25

• she has patches of normal skin and patches of skin
  lacking sweat glands

Question 26

26. What is Heterochromia?

Answer 26

• Heterochromia is the presence of different coloured
  eyes in the same person
• Heterochromia in humans appears either as a
  hereditary trait

NB:
- the X-Activation exists as an evolutionary measure to
ensure that females do not have a genetic advantage

Question 27

27. What is Gene Linkage?

Answer 27

• genes that are located near each other on the same
  chromosome tend to be inherited together
• these are called Linked Genes

Question 28

28. Thomas Hunt Morgan did an experiment in which he crossed flies that differed in traits of body colour and wing size.
    What were his results?

Answer 28

• Thomas discovered that body colour and wing size are
  usually inherited together in specific combinations
  from the Parental Phenotypes
• this is because the genes for these characters are on
  the same chromosome

Question 29

29. Thomas Hunt Morgan did an experiment in which he crossed flies that differed in traits of body colour and wing size.
    What else did Thomas Hunt Morgan notice?

Answer 29

• there were also Non-Parental phenotypes that were
  produced
• this is due to Genetic Recombination
• this result in offspring with a combination of traits that
  differ from either parent

THOMAS HUNT MORGAN DISCOVERED THAT:
- genes can be linked
- the linkage in this experiment was incomplete
- this resulted in the production of recombinant
phenotypes

Question 30

30. What can be said about Mendel’s Law of Independent Assortment and Gene Linkage?

Answer 30

• Mendel’s Law of Independent Assortment does not
  apply to linked genes
• this is because these linked genes are inherited as a
  package
• these kinds of genes do not assort independently
  because they are on the same chromosome

Question 31

31. What are the ideal results of a Complete Linkage?

Answer 31

IT SHOULD RESULT IN 2 PHENOTYPES:
- 50% grey normal wings
- 50% black reduced wings

Question 32

32. What do the Genetic Findings of Mendel and Morgan relate to?

Answer 32

• they relate to the Chromosomal Basis of
  recombination

Question 33

33. What did Thomas Hunt Morgan propose with regards to Genetic Recombination?

Answer 33

• Genetic Recombination can sometimes break the
  physical connection between genes on the same
  chromosome
• the genes are unlinked and seperated

Question 34

34. Provide a definition for Genetic Recombination?

Answer 34

• this is the crossing over of non-sister chromatids
• these chromatids are if homologous chromosomes
• this all happens during Meiosis I

Question 35

35. What is the mathematical calculation to work out the Recombination Frequency?

Answer 35