{{

Gene InteractionsGene Interactions

GeneticsGeneticsChapter 4, Part 2Chapter 4, Part 2

4.3 Gene Interaction Modifies Mendelian Ratios

• Genes work together to build the complex structures and organ systems of plants and animals

• The collaboration of multiple genes in the production of a single phenotypic characteristic or group of related characteristics is termed gene interaction

• Geneticists use a variety of symbols for alleles• Dominant alleles:

– an italic uppercase letter (D) or– letters (Wr)– an italic letter or group of letters with the + superscript (Wr+)

• Recessive alleles: – an italic lowercase letter (d) or– an italic letter or group of letters (Wr)

Representing Alleles

S / Sm / Sm+s / sm / Sm

Note that the mutant usually gets the letter name!

Note that the mutant usually gets the letter name!

Representing alleles in Drosophila

• Example: body color– Ebony mutant phenotype is indicated by e– Normal gray (wild-type) is indicated by e+

• e+/e+: gray homozygote (wild type)• e+/e: gray heterozygote (wild type)• e/e: ebony homozygote (mutant)

OR• +/+: gray homozygote (wild type)• +/e: gray heterozygote (wild type)• e/e: ebony homozygote (mutant)

Gene Interaction in Pathways

• Single-gene trait describes an inherited variation of a gene that can produce a mutant phenotype

• However, this is not a complete depiction of genetic reality

• Numerous genes contribute to the normal red eye color of Drosophila, including those responsible for production of eye pigments or transport proteins

Three Genes Involved in Drosophila Eye Color

• The brown gene produces an enzyme in a pathway that synthesizes a bright red vermilion pigment; mutant flies, bb, have brown eyes• Note: gene named after

the mutant, not the gene it encodes for!

• The vermillion genes produces an enzyme in a pathway that synthesizes a brown pigment; mutant flies, vv, have bright red eyes

• The white gene encodes a transporter that carries pigment to the eye; flies that do not produce this protein have white eyes

Three Distinct Types of Genetic Pathways

• Biosynthetic pathways are networks of interacting genes that produce a molecular compound as their endpoint

• Signal transduction pathways receive chemical signals from outside a cell and initiate a response inside the cell

• Developmental pathways direct growth, development and differentiation of body parts and structures

HOW DO GENES CONTROL BIOSYNTHETIC PATHWAYS?

HOW DO WE FIND A GENE THAT AFFECTS A CERTAIN PATHWAY?

Biosynthetic Pathways…

The One-Gene-One Enzyme Hypothesis

• George Beadle and Edward Tatum were among the first to investigate biosynthetic pathways

• They studied growth variants of the fungus, Neurospora crassa

• Their proposal, the one-gene-one enzyme hypothesis came out of their experiments

Red Bread Mold; http://www.biosci.missouri.edu/shiu/

Prototroph: an organism that can synthesize all of its amino acids

Auxotroph: an organism that has lost the ability to synthesize certain substances required for its growth and metabolism

This experiment looked at amino acids, but you could look at other synthetic pathways!

Beadle and Tatum ExperimentCAUSING MUTATION!

The Hypothesis Made a Connection Between Genes, Proteins and Phenotypes

• Each gene produces an enzyme

• Each enzyme has a specific role in a biosynthetic pathway that produces the phenotype

• Each mutant phenotype due to the loss or malfunction of a specific enzyme

Genetic Dissection to Investigate Gene Action

• Biosynthetic pathways consist of sequential steps• Completion of one step generates the substrate for the next

step in the pathway• Completion of every step is necessary to produce the end

product• Genetic dissection is an experimental approach taken to

investigate the steps of biosynthetic pathways

Genetic Dissections: Horowitz’s Experiments on Met- Mutants of Neurospora

• Horowitz’s analysis aimed to:

• Determine the number of intermediate steps in the methionine synthesis pathway

• Determine the order of the steps

• Identify the step affected by each mutation

Genetic Dissection: Results of Horowitz’s Experiments

• Whether or not a mutant strain grows on a medium containing a component of the pathway allows determination of the step at which the mutant is blocked

• Mutation of an enzyme will cause the pathway to become blocked. • If we give an intermediate from before the block, we can’t pass the block and the mutant will not grow.

• If we give an intermediate after the block, the mutant will grow.

• The blocked step is also identified by the substance that accumulates in the auxotroph

• Imagine one mutant:

X Give D, E or F: Will grow!Give A, B or C: Won’t grow!

What Accumulates?

1. Met 1: only on minimal media + methionine, indicating it is the last step of the pathway. Need to add methionine to get past the block

2. Met 2: need minimal + homocysteine , therefore block is at step that produces homocysteine. This result also tell us that homocysteine is the substrate converted to methionine in the biosynthetic pathway.

3. Met 3: grows on minimal, homocysteine & cystathionine. This tells us that Met 3 is blocked at the step that produces cystathionine and that cystathionine precedes homocysteine

4. Met 4 grows with any supplementation of minimal media. This tells us that Met 4 is defective at a step that precedes the production of cysteine.

More Recent Adjustments to the Hypothesis

• Hypothesis confirmed!: Each gene produces an enzyme

• Each enzyme has a specific role in a biosynthetic pathway that produces the phenotype

• Recent Adjustments:

• Some protein producing genes produce transport proteins, structural or regulatory proteins, rather than enzymes

• Some genes produce RNAs rather than proteins

• Some proteins (e.g. -globin) must join with other proteins to acquire a function

So far…So far… AaBb x AaBbAaBb x AaBb

Crossing genotypes leads to a Crossing genotypes leads to a phenotypic ratiophenotypic ratio

BUT, Genes do not act BUT, Genes do not act alone. Now let’s look at alone. Now let’s look at how genes interact to alter how genes interact to alter phenotypic ratios….phenotypic ratios….

Gene InteractionsGene Interactions

Colorless precursor

Colorless intermediate

Purple pigment

Enzyme C Enzyme P

The recessive c allele encodes an inactive

enzyme

The recessive p allele encodes an inactive

enzyme

Epistasis: Gene Interactions

• Epistatic interactions happen when an allele of one gene modifies or prevents the expression of alleles at another gene.

• Epistatic interactions often arise because two (or more) different proteins participate in a common cellular function– For example, an enzymatic pathway

No Interaction (9:3:3:1 Ratio)

• The expected 9:3:3:1 ratio is seen in the absence of epistasis: when the genes do not interact to change the expression of one another

www.integratedbreeding.net

Dihybrid cross, F2 progeny

• Cross involving the brown and vermillion genes in Drosophila

• When pure-breeding brown flies (b/b; v+/v+) are crossed to pure-breeding vermillion flies (b+/b+; v/v), the F1 all have wild type red eyes (b+/b; v+/v)

• When the F1 are interbred (b+/b; v+/v x b+/b; v+/v ), the F2 are:

• 9/16 b+/-; v+/-, wild type, red eyes

• 3/16 b/b; v+/-, brown eyes

• 3/16 b+/-; v/v, vermillion eyes

• 1/16 b/b;v/v, white eyes

• The results show that the genes are not undergoing epistatic interaction with one another

No Interaction (9:3:3:1 Ratio)

Epistatic Interactions

• A minimum of two genes are required for epistatic interactions; these usually participate in the same pathway

• There are six ways epistasis could affect the predicted 9:3:3:1 dihybrid ratio

Gene interaction alters the classic 9:3:3:1 ratio seen in the F2 progeny of the dihybrid cross!Gene interaction alters the classic 9:3:3:1 ratio seen in the F2 progeny of the dihybrid cross!

Epistatic Interactions

Complementary Gene Interaction (9:7 Ratio)• Bateson and Punnett crossed two pure-breeding strains of white flowered sweet peas

• They found all the F1 were purple flowered; the F1 x F1 cross yielded 9/16 purple and 7/16 white flowered progeny

• They recognized that the two genes interact to produce the overall flower color; when genes work in tandem to produce a single gene product, it is called complementary gene interaction

Duplicate Gene Action (15:1 Ratio)

• The genes in a redundant system have duplicate gene action; they encode the same product, or they encode products that have the same effect in a pathway or compensatory pathways

Dominant Gene Interaction (9:6:1 Ratio)

• Plants that have dominant allele(s) for just one of either of the genes will have round fruit and those with only recessive alleles of both genes will have long fruit

• Dominant for either gene (A or B), equals one phenotype. (3+3 = 6)

Recessive epistasis (9:3:4)

• B and b for black and brown melanin (MC1R gene)

• E: controls deposition of pigment in hairs (TRYP1 gene)

• ee is epistatic

• Recessive epistatsis causes yellow coat color

Recessive epistasis (9:3:4)

Brown eumelanin

Eumelanin deposition

Black eumelanin

Not able to deposit!

May or may not be correct in your book!May or may not be correct in your book!

Dominant Epistasis (12:3:1 Ratio)

• In dominant epistasis, a dominant allele at one locus will mask the phenotypic expression of the alleles at a second locus, giving a 12:3:1 ratio

• E.g. in foxglove flowers a dominant allele at one locus restricts the deposition of pigment to a small area of the flower

Dominant Suppression (13:3 Ratio)

• In dominant suppression, a dominant allele at one locus completely suppresses the phenotypic expression of the alleles at a second locus, giving a 13:3 ratio

• In chickens, the C allele is responsible for pigmented feathers and the c allele for white feathers

• The dominant allele of a second gene, I, can suppress the color producing effect of the C allele, leading to white feathers in both C/- and c/c individuals

Dominant I suppresses dominant pigment productionDominant I suppresses dominant pigment production

YOU MUST KNOW FOUNDATION FIGURE 4.21 (p. 130-131!)

4.4 Complementation Analysis Distinguishes Mutations in the Same Gene from Mutations in Different Genes

• When geneticists encounter organisms with the same mutant phenotype, they ask two questions:

1. Do these organisms have mutations in the same or in different genes?

2. How many genes are responsible for the phenotypes observed?

Ex. Two botanists working with petunias both discover a white flower mutation. One works in California and one works in the Netherlands. Are the mutations in the same genes?

Ex. Two botanists working with petunias both discover a white flower mutation. One works in California and one works in the Netherlands. Are the mutations in the same genes?

HOW DO WE KNOW IF THE GENETIC MUTATION IS THE SAME?

We have two Drosophila with the same phenotypic mutation….

Genetic Complementation Analysis

• Genetic heterogeneity is when mutations in different genes can produce the same or very similar mutant phenotypes

• Mating of two organisms with similar mutant phenotypes can lead to wild type offspring, a phenomenon called genetic complementation

• Complementation testing is when two pure breeding organisms with similar mutant phenotypes are mated

• If complementation occurs, wild type offspring are obtained and the mutations are known to affect two different genes

• When the mutations fail to complement, the offspring have the mutant phenotype and the mutations are known to affect the same gene

Example of a complementation test. Two strains of flies are white eyed because of two different autosomal recessive mutations which interrupt different steps in a single pigment-producing metabolic pathway.

A B

Complementation Analysis

• In complementation analysis multiple crosses are performed among numerous pure breeding mutants to try to determine how many different genes contribute to a phenotype

• Mutations that mutually fail to complement one another are called a complementation group• Can’t get back to wild-type!

• Mutation is on the same gene!

• A complementation group in this context refers to a gene

Genetic Complementation?-Vermilion vs. White-Apricot vs. Buff

Genetic Complementation?-Vermilion vs. White-Apricot vs. Buff

+ = cross of pure-breeding mutants yield wild-type (complements)- = cross of pure-breeding mutants yields only mutant progeny+ = cross of pure-breeding mutants yield wild-type (complements)- = cross of pure-breeding mutants yields only mutant progeny

-Mutations that fail to complement each other are on the same gene-Complementation group: consist of one or mutants of a single gene

WHAT ARE THE FUNCTIONAL CONSEQUENCES OF MUTATION?

There are SO many different eye colors for Drosophila!

Functional Consequences of Mutation (See Fig. 4.1)

• A wild type phenotype is produced when an organism has two copies of the wild type allele

• Mutant alleles can be:

• Gain-of-function, in which the gene product acquires a new function or express increased wild type activity

• Loss-of-function, in which there is a significant decrease or complete loss of functional gene product

Loss-of-Function Mutations

Amorphic = no functionAmorphic = no function

Hypomorphic = less functionHypomorphic = less function

Dominant Negative Mutations

• Multimeric proteins, composed of two or more polypeptides that join together to form a functional protein are particularly subject to dominant negative mutations

• These are negative mutations due to their “spoiler” effect on the protein as a whole

Gain-of-Function Mutations

Hypermorphic = more functionHypermorphic = more function

Neomorphic = new functionNeomorphic = new function

Questions?Questions?