Extensions to Mendel

Unit 4

Genetics

Color Blindness• X-Linked Recessive Condition

X Xc

X XX XXc

Y XY XcY

Physiology of Color Blindness• Defective Cone Cells which detect color

• Shift the wavelengths of light so the signal sent to the brain is the wrong color.

• L, M and S wavelengths (Long, Medium and Short)

• Mixture of abnormalities with cones (missing…)Most Common- M Cone

The eye

• Rods and Cones are on the cells of the retina

Are YouColor Blind?

Everyone can see these shapes

People who are colorblind cannot see the circle.

Example: Find the Red Crayon

Key

Questions1) Can a female inherit colorblindness from dad?

Yes - nearly 100%

2) Does this mean that she will be colorblind? No - need to factor in the other X chromosome from

mom

3) Why?

It’s a recessive XcX-Normal vision XcXc-colorblind

4) Can a male inherit color blindness from dad?

No, a male only inherits the Y chromosome from dad

1 Incomplete or Codominance - Two or more alleles exist, but none is dominant to the other/s

2 Multiple alleles for a single gene3 Epistasis - interactions b/t more than one

gene4 Sex-linkage - locus of a gene is on a sex

chromosome5 Sex influenced or limited expression is

influenced or limited by gender (hormones…)

Exceptions to Mendelian ratios:

• Incomplete or codominance - Two or more alleles exist, but none are dominant to the other

• Incomplete dominance results in blending of the parental traits

• Example: In some flowers red crossed with white results in pink F1 generation

Incomplete Or Codominance

RED +

• F2 results shown below

Incomplete Or Codominance

CRCWCRCR CWCW

CRCR

CRCW

CRCW

CWCW

CR CW

CR

CW

F2 Generation

In the F2 generation a 1:2:1 ratio results of red to pink to white

1:2:

Epistasis

B CA 21

If the gene for enzyme 1 was knocked out, the flower would be colorless

B CA 21

X

pathway produces a red pigment, C, in flowers and that A is a colorless precursor and B is a yellow intermediate

If the gene for enzyme 2 was knocked out, the flowers would be yellow

Epistasis (cont.)

B CA 21 X

Epistasis

B CA 21X X

If both genes were knocked out, the flowers would be colorless

1F1n2F2n X 1F1n2F2n

Confusing

1F2n

1n2F

1n2n

1F2F

1n2n1n2F1F2n1F2F

1F1n2F2n1F1n2F2F1F1F2F2n

1F1n2n2n1F1n2F2n1F1F2n2n1F1F2F2n

1n1n2F2n1n1n2F2F1F1n2F2n1F1n2F2F

1n1n2n2n1n1n2F2n1F1n2n2n1F1n2F2n

1F1F2F2F

1F2n

1n2F

1n2n

1F2F 1F1n2F2n1F1n2F2F1F1F2F2n

1F1n2n2n1F1n2F2n1F1F2n2n1F1F2F2n

1n1n2F2F1F1n2F2n1F1n2F2F

1n1n2n2n1n1n2F2n1F1n2n2n1F1n2F2n

1F1F2F2F

1F1n2F2n X 1F1n2F2n

Mendelian- 9:3:3:1

1F2n

1n2F

1n2n

1F2F

1n2n1n2F1F2n1F2F

1F1n2F2n1F1n2F2F1F1F2F2n

1F1n2n2n1F1n2F2n1F1F2n2n1F1F2F2n

1n1n2F2n1n1n2F2F1F1n2F2n1F1n2F2F

1n1n2n2n1n1n2F2n1F1n2n2n1F1n2F2n

1F1F2F2F

1F1n2F2n X 1F1n2F2n Epistatic

1F2n

1n2F

1n2n

1F2F

1n2n1n2F1F2n1F2F

1F1n2F2n1F1n2F2F1F1F2F2n

1F1n2n2n1F1n2F2n1F1F2n2n1F1F2F2n

1n1n2F2n1n1n2F2F1F1n2F2n1F1n2F2F

1n1n2n2n1n1n2F2n1F1n2n2n1F1n2F2n

1F1F2F2F

– 10:3:3 = (9+1):3:3– 10:6 = (9+1):(3+3)– 13:3 = (9+1+3):3

A 9:4:3 Ratio

– 9:7 = 9:(3+3+1)– 12:3:1 = (9+3):3:1– 12:4 =(9+3):(3+1)

A ratio made up of some combination of 9:3:3:1 is generally a good hint that epistasis is at work

A biochemical pathway like this one = 9:4:3 ratio as long as there are two alleles each of which

behaves in a simple dominant/recessive way The 9:4:3 ratio is really a 9:(3+1):3 ratio Other possible phenotypic ratios for a dihybrid

cross involving epistasis are below: