1. Sex-Linked Traits

2. It’s all about the SEX (gender)!
Sex-linked genes are located on the 23rd chromosome (sex chromosome).
Sex chromosomes are X and Y.
XY = Boy
XX = Girl
Sex-linked genes are shown as SUPERSCRIPTS on the X chromosome.

3. Sex-Linked Traits Sex-linked traits can be Dominant or Recessive.
A = dominant a = recessive
What would be the genotypes of a male and female that have a Sex-linked Dominant trait and do not express (have) the trait?
Expresses Trait: Male - XA Y Female - XA XA or XA Xa
No Expression: Male - Xa Y Female - Xa Xa

4. Sex-linked Traits Most Sex-linked traits are Recessive!
What would be the genotypes of a male and female that have a Sex-linked Recessive trait and do not express the trait?
Expresses Trait: Male - Xa Y Female - Xa Xa
No Expression: Male - XA Y Female - XA XA or XA Xa (Carrier)
Most Sex-linked traits are Recessive!

5. Sex-Linked Recessive Traits
More males are affected than females.
An affected son can have parents who have the normal phenotype. (XAY, dad) x (XAXa, mom)
For a daughter to have the trait, her father must also have it. Her mother must have it or be a carrier (XaY, dad) (XaXa or XAXa, mom)

6. Sex-Linked Recessive Traits
The trait often skips a generation from the grandfather to the grandson.
If a woman has the trait (XaXa), ALL of her sons will be affected.
There are no male carriers because they can only contribute one X chromosome.

7. Example #1: Color-Blindness
XB = not colorblind, Xb = colorblind
Cross XBY male with a XBXb female
XB Y XB Xb

8. Result
XBXB
XBY
XBXb
XbY
What does this mean?!

9. Example #2: Hemophilia
Cross a normal carrier woman with a normal male.
XHXh with a XHY
What are the possible genotypes (4) and phenotypes (3) of their children?

10. XHXH XHY XHXh XhY Results! XH Y XH Xh Genotypes: XHXH, XHXh, XHY, XhY
Phenotypes: Normal female, normal male, affected male
XHXH
XHY
XHXh
XhY

11. Examples of Sex-Linked Recessive Disorders:
Red/Green Colorblindness – Difficulty perceiving differences between colors (red or green, blue or yellow).
Hemophilia – Absence of one or more proteins necessary for normal blood clotting.
Cataracts – opacity in the lens that can lead to blindness
Night blindness – (Nyctalopia) rods do not work so that can not see in the dark
Glaucoma – pressure in the eye that can lead to optic nerve damage and blindness
Duchenne Muscular Dystrophy – progressive weakness and degeneration of skeletal muscles that control movement due to absence of dystrophin (protein that maintains muscle integrity). Mainly in boys, onset 3-5 yrs, by 12 years can’t walk, and later needs respirator.

12. To See or Not To See

13. Test yourself www.enchroma.com
Click on ‘Color blindness test’ on the top.
Hit ‘Start Test’

14. Color Blind: Recessive SL Trait

15. Pedigrees:
Diagram that shows the inheritance of traits through several generations
Typical Pedigree Information:
Lines are drawn to show relation
Circle means female
Square means male
No color/nothing filled in = normal
Filled in = affected, has the
Carrier for the trait:
Half filled in
With a / through the shape
Dot that doesn’t fill in the circle

16. Color-Blind Pedigrees
Draw a pedigree showing a cross between a man with normal vision and a woman who carriers the color-blind trait.
Color-blind is a recessive trait.
Genotypes of Parents:
Male = XR Y Female = XR Xr

17. Does yours look like this?
Pedigree of color-blind trait.
Genotypes of Parents:
Male = XR Y Female = XR Xr
XRY
XRXr
XrY
XRXR

18. Can a female be color-blind?
Think about it.
If so, how? Why?

19. YES—she can! If dad (XrY) is color-blind and mom (XRXr) is a carrier =
50% chance XrXr
She’s color-blind.

20. Hemophilia It is a rare blood disorder where the blood does not clot.
A hemophiliac will bleed “freely”, while a normal person will eventually stop bleeding because a scab forms.
Hemophiliacs require medical intervention to stop the flow of blood.
Usually they are given clotting factors, which help them form scabs.
Hemophilia is inherited.
About 1 out of 5,000 males are born with hemophilia each year.

21. Pedigree Royale: Hemophilia