GENE 314 Lab 3: Gene Interactions and Epistasis

Monohybrid cross

Aa x Aa

Results in phenotypic ratio of 3:1

Dihybrid cross

AaBb x AaBb

Results in a phenotypic ratio of 9:3:3:1

Test cross

back cross to the homozygous recessive

Monohybrid

One segregating gene

Dihybrid

Two segregating genes

Monohybrid test cross

Aa x aa

Results in a phenotypic ratio of 1:1

Dihybrid test cross

AaBb x aabb

Results in a phenotypic ratio of 1:1:1:1

Epistasis

Form of gene interaction in which one gene masks the phenotypic expression of a different, non-allelic gene

What is the result of epistasis?

The alteration of the characteristic ratios we expect from the dihybrid cross types above

What used to be a common method to track transmission?

Examine pedigrees of large families or inbred populations

What is the scientific name for “baker’s yeast?”

Saccharomyces cervisiae

What are the characteristics of S. cervisiae?

• Unicellular

• Eukaryotic (fungus)

• Haploid number of 16 chromosomes

• Aerobic or anaerobic

• Requires C, N, and biotin for survival

• Has both a haploid and diploid life cycle

What do haploid S. cervisiae cells excrete?

Pheromones that are either type “a” or type “alpha”

What happens when type “a” and type “alpha” pheromones are mixed on a nutrient rich agar plate?

The cells fuse to form a diploid zygote

When does sporulation/meiosis occur?

When diploid yeast are stressed by nutritional deficiency

What holds together spores/gametes in S. cervisiae?

Ascus

List the steps of S. cervisiae life cycle

1. Diploid (a/alpha)

2. Mitosis of diploid cells

3. Starvation leads to meiosis of diploid cells & ascus formation

4. 4 haploid ascospores

5. Haploid cells are separated from ruptured ascus and grown separately

6. Haploid cells of opposite type (a/alpha) mate to form diploid

What is AIR a precursor to?

Adenine, an essential building block for DNA

What genes are involved in the adenine pathway?

ade2 and ade4

What does a non-functioning ade2 gene lead to?

The buildup of a red-colored intermediate (instead of colorless)

Which 2 pathways determine maize kernel color?

1. Determination of the endosperm color

2. Determination of the aleurone color

What is the aleurone?

Outer coating of cells on the endosperm that is either colors (purple or red) or colorless

When can the color of endosperm be observed?

Only when the aleurone is colorless

What is the pericarp?

The surrounding layer of the maize kernel

Describe the locus for endosperm color

Has 2 alleles, Y and y

Yellow = YY, Yy

White = yy (and the absence of carotenoid pigments)

Describe the determination of aleurone color

4 different loci, each with 2 alleles

Functional enzymes are dominant, and non-functional enzymes are recessive, with one exception: C¹

What is the C¹ allele in aleurone color?

C¹ is a dominant, yet non-functional allele that results in a colorless phenotype

When is the aleurone colorless?

When 3 of the 4 loci (C, R, or A) have non-functional enzymes, resulting in the lack of anthocyanin production

When does a red aleurone occur?

Only from the loss of function at the P locus

When does a purple aleurone occur?

Only when all enzymes are functional

CC or Cc, RR or Rr, AA or Aa, PP or Pp

Functional enzyme

cc, rr, aa, pp

Non-functional enzyme

C¹C or C¹c

Dominant non-functional blockage pathway

How are statistics used in science?

As a quantitative measure of the soundness of a hypothesis

What is the Chi-Square formula?

x² = Σ ((obs - exp)² / exp)

What does a 95% confidence interval mean?

5% of the time, our hypothesis will be rejected

When do we reject the hypothesis?

If the calculated value is greater than the critical value (0.05)

When do we fail to reject the hypothesis?

If the calculated value is less than the critical value (0.05)

What are the 4 strains of yeast in exercise 1?

1. Wild type

2. Δade2

3. Δade4

4. Δade2Δade4

What is a YPD plate?

Yellowish-brown

What is a minimal plate?

White

How much of the designated strain should be spotted in its corresponding quadrant on each plate?

10 uL

Data for how many ears of corn have been provided to us?

6 (A-F)

Each kernel on an ear of corn is the result of a ______ fertilization, but all the kernels on that ear have _____ parents.

Separate; the same

How to determine degrees of freedom?

Number of classes minus 1

How do you know if there is segregation of P alleles?

If there are both purple and red kernels present