What number do you see?

2.

Normal will see “45”

Colorblind will see nothing but spots.

3.

Normal will see “56”

Colorblind will see “56”

4.

Normal will see “6”Colorblind will see

nothing but spots.

5.

Normal will see “8”Colorblind will see

nothing but spots.

6.

Normal will see “5”

Colorblind will see “2”

7.

Normal will see a circle, star and a square

Colorblind will see a circle, star and a square.

8.

Colorblind will see only a yellow square.

Normal vision will see both a yellow square and a ‘faint’ brown circle.

9.

Colorblind will see a yellow circle.

Normal will see a yellow circle and a “faint” brown square.

10.

Normal will see a “faint” brown boat.

Colorblind will see nothing. (only spots)

11.

Normal will see “2”Colorblind will see

nothing but spots.

12.

Normal will see “5”Colorblind will see

nothing but spots.

13.

Normal will see “15”

Colorblind will see either a “13” or “17” or nothing but spots.

14.

Normal will see nothing but spots.

Colorblind will see “5”

15. Can you see a line?

16. Can you see a line?

17.

Normal will see “73”

Colorblind will see nothing but spots.

18.

Normal will see “7”

Colorblind will see nothing but spots.

19.

Normal will see “16”

Colorblind will see nothing but spots.

1910 – Thomas Hunt Morgan

He performed genetic experiments on fruit flies – Why fruit flies?

1. Prolific breeders.

2. New generation every two weeks.

3. Only four pairs of chromosomes.

What did Morgan do?

1. Morgan took a fruit fly with red eyes and mated it to a fly with white eyes.

2. All the F1 had red eyes. (red – dominant)

3. His F2 ratio? He got what he expected…

3 red : 1 white

HOWEVER, all the flies with white eyes were male. He found no female flies with white eyes.

What did Morgan conclude?

1. A fly’s eye color is linked to its gender.

2. The gene for eye color had to be located on the chromosome that determined gender – the X chromosome.

That is why this type of inheritance is referred to as X-linked inheritance. (Sometimes its called a sex-linked trait.)

XX – female; XY – male

What are some other common x-linked conditions?

Androgenetic alopecia

Male pattern baldness is a sex-linked characteristic that is passed from mother to child. A man can more accurately predict his chances of developing male pattern baldness by observing his mother's father than by looking at his own father.

Hemophilia

Duchenne Muscular Dystrophy

How do you use a Punnett Square to solve an X-linked problem?

Since the trait is linked to gender, you must include a X and Y for the father and a X (and possibly a second X) for the mother.

Alleles must be written with the X chromosome only – not the Y

(e.g. XAXA or XAY or XaY or XAXa)

Try this sample question…

• A colorblind man marries a woman with normal vision. They have two daughters and one son. The son is color-blind. One daughter is color-blind and the other has normal vision.

a. Draw a pedigree representing this family. Shade in those symbols representing color-blind individuals. Symbols representing people with normal vision should be left blank. Carriers should be partially colored in or given a dot.

b. What is the genotype of the father?

c. What is the genotype of the mother?

d. What are the genotypes of the children?