Extensions to Mendel Unit 4 Genetics Color Blindness

Extensions to Mendel Unit 4 Genetics

Color Blindness • X-Linked Recessive Condition X Xc X XX XXc Y XY Xc. Y

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 You Color Blind?

Everyone can see these shapes

People who are colorblind cannot see the circle.

Example: Find the Red Crayon

Key

Questions 1) 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 Xc. X-Normal vision Xc. Xc-colorblind 4) Can a male inherit color blindness from dad? No, a male only inherits the Y chromosome from dad

Exceptions to Mendelian ratios: 1 Incomplete or Codominance - Two or more alleles exist, but none is dominant to the other/s 2 Multiple alleles for a single gene 3 Epistasis - interactions b/t more than one gene 4 Sex-linkage - locus of a gene is on a sex chromosome 5 Sex influenced or limited expression is influenced or limited by gender (hormones…)

Incomplete Or Codominance • 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 F 1 generation RED + WHITE

Incomplete Or Codominance � In the F 2 generation a 1: 2: 1 ratio results of red to pink to white • F 2 results shown below C RC R C W F 2 Generation 1: 2: 1 C RC W CR CW C RC R C RC W C W C W

v pathway Epistasis 1 2 A B C produces a red pigment, C, in flowers and that A is a colorless precursor and B is a yellow intermediate A v If X 1 B 2 C the gene for enzyme 1 was knocked out, the flower would be colorless

Epistasis (cont. ) A v If 1 B X 2 C the gene for enzyme 2 was knocked out, the flowers would be yellow

Epistasis v If both genes were knocked out, the flowers would be colorless A X 1 B X 2 C

1 F 1 n 2 F 2 n X 1 F 1 n 2 F 2 n Confusing 1 F 2 F 1 F 2 n 1 n 2 F 1 n 2 n F 1 F 2 F 2 F 1 F 1 F 2 F 2 n 1 F 1 n 2 F 2 F 1 F 1 n 2 F 2 n F F 1 1 2 1 F 2 n 1 F 1 F 2 n 2 n 1 F 1 n 2 F 2 n 1 F 1 n 2 n 2 n 1 n 2 F 1 F 1 n 2 F 2 n 1 n 1 n 2 F 2 F 1 n 1 n 2 F 2 n F 1 n 2 F 2 n 1 F 1 n 2 n 2 n 1 n 1 n 2 F 2 n 1 n 1 n 2 n 2 n n n 1 1 2

1 F 1 n 2 F 2 n X 1 F 1 n 2 F 2 n Mendelian- 9: 3: 3: 1 1 F 2 F 1 F 2 n 1 n 2 F 1 n 2 n F 1 F 2 F 2 F 1 F 1 F 2 F 2 n 1 F 1 n 2 F 2 F 1 F 1 n 2 F 2 n F F 1 1 2 1 F 2 n 1 F 1 F 2 n 2 n 1 F 1 n 2 F 2 n 1 F 1 n 2 n 2 n 1 n 2 F 1 F 1 n 2 F 2 n 1 n 1 n 2 F 2 F 1 n 1 n 2 F 2 n F 1 n 2 F 2 n 1 F 1 n 2 n 2 n 1 n 1 n 2 F 2 n 1 n 1 n 2 n 2 n n n 1 1 2

1 F 1 n 2 F 2 n X 1 F 1 n 2 F 2 n Epistatic 1 F 2 F 1 F 2 n 1 n 2 F 1 n 2 n F 1 F 2 F 2 F 1 F 1 F 2 F 2 n 1 F 1 n 2 F 2 F 1 F 1 n 2 F 2 n F F 1 1 2 1 F 2 n 1 F 1 F 2 n 2 n 1 F 1 n 2 F 2 n 1 F 1 n 2 n 2 n 1 n 2 F 1 F 1 n 2 F 2 n 1 n 1 n 2 F 2 F 1 n 1 n 2 F 2 n F 1 n 2 F 2 n 1 F 1 n 2 n 2 n 1 n 1 n 2 F 2 n 1 n 1 n 2 n 2 n n n 1 1 2

A 9: 4: 3 Ratio v 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 v Other possible phenotypic ratios for a dihybrid cross involving epistasis are below: v – 9: 7 = 9: (3+3+1) – 12: 3: 1 = (9+3): 3: 1 – 12: 4 =(9+3): (3+1) v – 10: 3: 3 = (9+1): 3: 3 – 10: 6 = (9+1): (3+3) – 13: 3 = (9+1+3): 3 A ratio made up of some combination of 9: 3: 3: 1 is generally a good hint that epistasis is at work