MENDELIAN GENETICS (MODIFICATION OF MENDEL RATIO)

CHAPTER 1. (part two)

I. Epistasis

Two or more separate genes (not separate alleles at one genetic locus) interact to control a phenotypic character.

If one gene locus prevents the expression of a second gene, the first locus is epistatic to the second, and the second is hypostatic to the first.

Example 1: H/h and AB pathway

Precursor H substance H Substance

A antigen

B antigen

A allele

B allele

HH or Hh

hh

Epistasis example 2: Coat color in mice

Wt coat color is agouti - A (dominant to black); Nonagouti (black) coat color - aPigmentation expression - B (dominant to albino); No pigmentation (albino) - b

If individual is bb, then is albino regardless of allele at a locus - due to gene interaction.

Agouti Markings

A- hair made with bands of black pigment and yellow pigment.

Aa hair all black.

AABB (agouti) x aabb (albino) AaBb (all agouti)

AaBb x AaBb A-B- agouti 9/16 9/16A-bb albino 3/16 4/16aaB- black 3/16 3/16aabb albino 1/16

Genotype Phenotype F2 ratio Final phenotypic ratio

Coat color example of epistasis, cont.

Due to gene interaction, we see a 9:3:4 F2 ratio. The b locus is epistatic to the a locus.

Example of dominant epistasis, a 12:3:1 ratio

Inheritance of fruit color in summer squash: two loci together control color and a dominant allele at one locus can mask the expression of the alleles at the second locus.

A--- white

aaB- yellowaabb green

A is dominant to a, and the a locus is epistatic to the b locus.

Therefore, if AaBb is crossed to AaBb, the F2 is as follows:

A-B- white 9/16A-bb white 3/16aaB- yellow 3/16aabb green 1/16

12/16 white3/16 yellow1/16 green

II. Novel phenotypes and F2 ratios due to gene interaction

F2 = A-B- disc 9/16 9/16 discA-bb sphere 3/16 6/16 sphereaaB- sphere 3/16aabb long 1/16 1/16 long

Summer squash fruit shape inheritance:

AABB - disc shape x aabb - long shapeF1 = AaBb - disc shape

Summary of modified F2 ratios - was Mendel just wrong?

No, none of these cases has violated the principles of segregation and independent assortment - just added complexity.

Practice Problem

In a plant, a tall variety was crossed with a dwarf variety. All F1 plants were tall. When two F1 plants were interbred, 9/16 of the F2 were tall and 7/16 were dwarf.

Explain the inheritance of height by a) indicating the number of gene pairs involved and b) by designating which genotypes yield tall and which yield dwarf.

1. When studying a single character, a ratio expressed in 16 parts suggests that two gene pairs are “interacting” during the expression of the phenotype.

2. A 9:7 ratio implies a dihybrid condition with epistasis.

A-B- 9/16A-bb 3/16aaB- 3/16aabb 1/16

3. From any dihybrid cross of double heterozygotes, we see the following genotypic ratio:

4. In our problem, we see a 9:7 phenotypic ratio. If the 3:3:1 groups above were lumped together, a 9:7 ratio emerges.

Assign tall to any plant with both A-B- and dwarf to any plant that is homozygous recessive for either or both the recessive alleles.

talldwarfdwarfdwarf

III. Complementation Analysis Reveals Whether 2 Mutations are at the Same Locus

The mutations do complement each other, so they are in different complementation groups.

Mutations do not complement, soin same complememtation group.

IV. X Linkage

Affects males and females differently

Example: red-green color blindness

Other examples in humans: Hemophilia Duchenne’s muscular

distrophy

“3” or “8”??

X-linkage means genes on X chromosome

Sex chromosomes are “unlike” in many species; all others are autosomes.

In humans and Drosophila, males contain XY and females contain XX.

Y chromosome contains region of pairing homology with X: pseudoautosomal region.

Y contains few genes; X contains many.

Color Blindness Pedigree

Males are hemizygousfor this gene locus; only

females can be carriers.

Hemophilia in the European Royal Families

• Disorder is lethal or debilitating prior to reproductive maturation.• X-linked recessive: heterozygous females are carriers with 50% chance of passing trait to sons only.• Arose from spontaneous mutation in Queen Victoria? • Hemizygous males were affected in Russian, German, Spanish royal families, trait not passed on in British royal family.

V. Is phenotype always due to genotype? Penetrance vs. Expressivity

If 70% of the individuals show the mutant phenotype, then the penetrance is 70%.

Expressivity reflects the range of expression of the mutant genotype.

Variable penetrance or expressivity?

Temperature can affect coat coloration.

Siamese cat fur in the extremities is darker due to cooler temperatures. The enzyme making darker pigment doesn’t work well at the higher temperatures in the rest of the body.

Temperature effects on phenotype

Temperature also affects primrose flower color and fur of Himalayan rabbits.

Influence of chemicals on phenotype

PKU: phenotype is mental retardation due to metabolic disorder; severity is affected by diet and whether phenylalanine is restricted in the diet.

Phenocopies: non-hereditary phenotypic modification that mimics a phenotype caused by a known gene mutation.

example: deafness can be caused in a developing baby if a mother is infected with rubella during the first 12 weeks of pregnancy; also caused by homozygous, recessive mutation