Genetics Exam 3
What is auxotroph or auxotrophic?
"mutant"
What is prototroph or protrophic?
"wild-type"
Why is the inheritance of Tay-Sachs disease characterized as complete dominance, yet expression of the gene that causes the illness (HEXA) has incomplete dominance?
*Complete dominance*: It acts as a Mendelian trait HH and Hh --> healthy hh --> Tay Sachs disease *Incomplete dominance*: HH --> normal level of hexosaminidase Hh --> half the normal level of hexosaminidase hh --> no hexosaminidase - Tay-Sachs disease depends on how you measure it (levels of hexosaminidase or healthy/sick)
What is the phenotypic ratio is expected when there is *no interaction* among two genes, each of which controls different aspects of the same phenotypic trait?
- 9:3:3:1
What is epistasis?
- A group of specific patterns of gene interaction in which an allele of one gene modifies or prevents that expression of alleles of another gene. - A minimum of two genes are required - For a dihybrid cross, a 9:3:3:1 ratio of four phenotypes in F2 is predicted, but epistasis results in fewer than four phenotypes.
What is a temperature-sensitive allele?
- A mutation evident only at or above a certain temperature due to an abnormality of the protein product that affects its stability. - Example: Allele c^h
What is pleiotropy?
- A single gene mutation that affects multiple and seemingly unconnected properties of an organism. - Ex. when the same gene is related to respiratory problems and sterility
What is duplicate gene action?
- Allows dominant alleles of either duplicate gene to produce the wild-type allele. - Only organisms with homozygous mutations of both genes have a mutant phenotype.
What F2 ratios are expected when there is duplicate gene action?
- Bean flower color - PpRr (purple) x PpRr (purple) - 9/16 P_R_ purple - 3/16 P_rr purple - 3/16 ppR_ purple - 1/16 pprr white RATIO: 15:1 (Just need dominant allele from either gene to produce wild-type phenotype; mutant phenotype only shows up in homozygous recessive)
What is the C-gene system for mammalian coat color?
- C (color) gene that is responsible for coat color in mammals such as cats, rabbits, and mice. - This gene has dozens of alleles but we will focus on four alleles that form an allelic series.
What are dominant negative mutations?
- Certain loss-of-function mutations produce dominant mutant phenotypes through alterations in the function of a multimetric protein of which the mutant polypeptide forms a part. - Multimetric proteins, composed of two or more polypeptides that join together to form a functional protein are particularly subject to this as a consequence of some change that prevents the polypeptides from interacting normally to produce a functional protein.
What F2 ratios are expected when there is dominant suppression?
- Chicken feather color - CcIi (white) x CcIi (white) - 9/16 C_I_ white - 3/16 C_ii pigement - 3/16 ccI_ white - 1/16 ccii white - RATIO: 13:3 (Big C suppresses Big I)
What is the ABO blood type?
- Chromosome 9 - Ex. of allelic series --> 3 alleles: A=B>O - A and B are antigens (cellular products on the surface of red blood cells) - It is an example of Codominance - A and B antigens are carbohydrate group (sugar) bound to lipid molecules on the surface of RBCs. - Specificity of A & B antigens based on terminal sugar of carbohydrate group.
What is an example of indirect evidence for gene-enviroment interactions?
- Consider an allele in an organism as the *gene* - Environment is the rest of the genome, plus RNA, proteins, etc. that interact with the gene (ex. why is BRCA1 80% penetrance instead of 100%)
How does haploinsufficiency related to oncogenes?
- Example: C-myc - Genotype --> outcome - ++ --> normal amount of cyclin - +C-mycmutant --> excessive amount of cyclin - C-mycmutantC-mycmutant --> excessive amount of cyclin - One good allele is *not* enough to get the job done - MANY WILDTYPE ALLELES OF ONCOGENES ARE HAPLOINSUFFICENT
How does haplosufficency relate to tumor-suppressor genes?
- Example: Retinoblastoma gene & pRB - Genotype --> outcome - ++ --> blocks progression - +RBmutant --> blocks progression - RBmutantRBmutant --> doesn't block progression - One good allele is enough to get the job done
Why are mutations of the COL1A1 gene often dominant negative?
- Explains autosomal dominant disorders - Type 1 collagen is a trimetric protein (3 polypeptides) - 2 of the peptides are produced by COL1A1 - 1 of the peptides are produced by COL1A2 - Most cases of osteogenesis imperfect are from 1 mutated COL1A1 gene
What phenotypic ratio is observed in the F2 of a dihybrid cross involving two genes with recessive epistasis?
- Labrador retriever coat color - ee blocks B and b - BbEe x BbEe Genotypes: - 9/16 B_E_ - 3/16 bbE_ - 3/16 B_ee - 1/16 bbee sooo, Phenotypes: - 9/16 B_E_ --> Black - 3/16 bbE_ --> Chocolate - 4/16 B_ee or bbee --> Yellow RATIO: 9:3:4
What is the one gene-one enzyme hypothesis as originally tested by Beadle and Tatum?
- More complicated with modern knowledge of the various coding products of genes - Protein-coding genes produce enzymes, transport proteins, structural proteins, etc. - RNA genes produce RNAs (tRNA and rRNA) - **each gene produces an enzyme, and each enzyme has a specific functional role in a biosynthetic pathway - Their hypothesis proposed that each mutant phenotype was attributable to the loss or defective function of a specific enzyme.
What is dominant gene interaction?
- Occurs between genes that each contribute to a phenotype, producing one phenotype if dominant alleles are present at each gene (A-B-), a second phenotype if recessive alleles are homozygous for either gene (A-bb or aaB-), and a third phenotype if recessive homozygosity occurs at both genes (aabb). - Dominant allelic actions produces both proteins --> disk-shaped fruit - If only one of the proteins is produced --> spherical fruit - Neither protein is produced --> long fruit
What is complementary gene interaction?
- Occurs when genes must act in tandem to produce a phenotype. - The wild-type action from both genes is required to produce the wild-type phenotype. - Mutation of one or both genes produces a mutant phenotype.
What is recessive epistasis?
- Occurs when recessive alleles at one gene mask or reduce the expression of alleles at the interacting locus.
What is the H substance?
- On chromosome 19 --> FUT1 gene - Complete dominance - Present on the surfaces of all red blood cells - A and B can modify the H antigen, but O cannot. - FUT1 gene converts the H substance into either the A allele and B allele from I^A and I^B
What F2 ratios are expected when there is dominant gene interaction?
- Squash shape - AaBb (disk) x AaBb (disk) - 9/16 A_B_ --> disk - 3/16 A_bb -->sphere - 3/16 aaB_ --> sphere - 1/16 aabb --> long RATIO: 9:6:1
What phenotypic ratio is observed in the F2 of a dihybrid cross involving two genes with dominant epistasis?
- Summer Squash - W>w and Y>y - W_ --> no pigment - Y_ --> yellow - yy --> green - WwYy x WwYy Genotypes: - 9/16 W_Y_ white - 3/16 W_yy white - 3/16 wwY_ yellow - 1/16 wwyy green soo... Phenotypes: - 12/16 White - 3/16 yellow - 1/16 green RATIO: 12:3:1
What F2 ratios are expected when there is complementary gene interaction?
- Sweet pea flower color - CcPp purple x CcPp purple - 9/16 C_P_ purple - 3/16 C_pp white - 3/16 ccP_ white - 1/16 ccpp white RATIO: 9:7 (Wild type alleles from both genes needed to produce pigment)
What is genetic dissection?
- Tease apart the role of various genes along a pathway - The use of mutations and recombinants in genetic analyses to identify and assemble the genetic components of a biological property or process. - Separately tests the ability of a mutant to execute each step of a biosynthetic pathways and assembles the steps of a pathway by determining the point at which the pathway is blocked in each mutant.
How does the C-gene system for mammalian coat color work?
- The C-gene alleles form an allelic series that is revealed by phenotypes of offspring of various mating. - Allele C is dominant to all other alleles of the gene - Allele c^ch produces the chinchilla phenotype (hypomorphic and generates reduced coat color as a results of reduced level of activity of gene product) - Allele c^h produces the Himalayan phenotype - Allele c produces a protein product with no enzymatic activity so it is albino.
What is the Bombay phenotype?
- The H antigen is a precursor to the A and B antigens. - For instance, the B allele must be present to produce the B enzyme that modifies the H antigen to become the B antigen. It is the same for the A allele. - However, if only recessive alleles for the H antigen are inherited (hh), as in the case above, the H antigen will not be produced. - Subsequently, the A and B antigens also will not be produced. - The result is an O phenotype by default since a lack of A and B antigens is the O type
What is genetic complementation?
- The ability of two mutants with the same mutant phenotype to produce progeny with the wild-type phenotype is called this and it indicates that more than one gene is involved in determining the phenotype. - Any given pair of mutants that complement one another by producing wild-type progeny are mutations of different genes
How can the mutant allele causing yellow coat color in mice be both dominant and recessive?
- The mutant allele as dominant or as recessive depending on the particular phenotype we are looking at. - *Dominance:* Agouti (AA) x Yellow (AA^y) --> 1:1 ratio which indicates dominance of the mutant alleles over wild type; dominance is due to GOF of the yellow pigment by the mutant allele. - *Recessive:* Yellow (AA^y) x Yellow (AA^y) --> 2:1 ratio which indicates the homozygous lethality of the mutant allele. Lethality only affects homozygotes and the mutant allele is recessive to the wild type. Due to LOF of Raly gene.
What is an example of direct evidence for gene-enviromnent interactions?
- The phenotype of an individual is a product of its *genotype* and interactions of the genotype with external environment
What is the C gene?
- produces the enzyme tyrosinase, which is active in the first two steps of a multistep biochemical pathway that synthesizes the pigment melanin. - In the initial melanin pathway steps, tyrosinase is responsible for the breakdown (catabolism) of the amino acid tyrosine.
How has the hypothesis changes as new information about genes and the products they code for has been gathered? (First example)
1.) Some protein-producing genes do not produce enzymes, but produce transport proteins, structural proteins, regulatory proteins, or tiger non enzyme proteins
How has the hypothesis changes as new information about genes and the products they code for has been gathered? (Second example)
2.) Some genes produce RNAs rather than proteins
How has the hypothesis changes as new information about genes and the products they code for has been gathered? (Third example)
3.) Some proteins (eg. B-globin) must join with other proteins to acquire a function.
What is dominant epistasis?
A dominant allele of one gene masks or reduces the expression of alleles of a second gene.
What are sex-limited traits?
A gene or trait expressed exclusively in one sex
What are sex-influenced traits?
A gene, usually autosomal, whose expression differs between males and females of a species
What is an allelic series?
A group of alleles of a gene that display a hierarchy of dominance relationship among them.
What is complete dominance?
A kind of dominance where the dominant allele completely masks the effects of the recessive allele in a heterozygous condition.
Type of LOF mutation: Null mutations (aka amorphic mutations)
A loss-of-function mutation that results in a *complete loss* of gene function in comparsion to the wild-type gene product; the mutant alleles produce no functional gene product and are often lethal in a homozygous genotype.
Type of LOF mutation: Leaky mutations (aka hypomorphic mutations)
A mutation resulting in *partial loss* of gene functions; it implies that a small percentage of normal functional capability is retained by the mutant allele, but at a lower level than is found for the wild-type allele.
What is a lethal allele?
An allele that results in the premature death of the organisms that carry it. Lethality most often affects homozygous organisms.
Type of GOF mutation: Neomorphic ("new form") mutations
Aquire novel gene activities not found in the wild-type and are usually dominant. The gene products are functional, but have structures that differ from the wild-type gene product.
What is the allelic series at this gene for the C-gene system for mammalian coat color?
Dominance relationship --> C > c^ch > c^h > c
What are haplosufficiency?
Dominant wild type alleles that are used in this example are identified as this since one (haplo) copy is sufficient to produce the wild-type phenotype in the heterozygous genotype.
What is penetrance?
Expression of the phenotype corresponding to a particular genotype
What is partial dominance?
Generally considered to be a type of incomplete dominance, with the heterozygote resembling one homozygote more than the other.
Is the wild-type allele of Tay-Sachs disease haplosufficient or haploinsufficient?
Haplosufficient
What are gain-of-functions mutations?
Identifies alleles that are required a new function or have their expression altered in a way that gives them substantially more activity than the wild-type allele.
What are gene-environment interactions?
Interactions taking place between particular genes and specific environmental factors
What is the function of the Raly gene?
Involved in embryonic development
What is the function of the Agouti gene?
Involved in production of phaeomelanin
What are catabolic pathways?
Large molecules are broken down into smaller molecules; Release energy
What is the MN blood group?
Look on sheet
What is the Rh blood group?
Look on sheet
O blood type is what? AB blood type is what?
O --> Universal donor because no antigens AB --> Universal recipient because no antibodies
What is dominant suppression?
Occurs when the dominant allele of one gene suppresses the expression of a dominant allele of a second gene.
Type of GOF mutation: Hypermorphic ("greater than wild-type form") mutations
Produce more gene activity per allele than the wild type and are usually dominant
What is haploinsufficiency?
Recessive wild type alleles that are used in this example are identified because a single copy is not sufficient to produce the wild-type phenotype in the heterozygous genotype.
What is a gene interaction?
Referring to genes that interact with one another due to their participation in the production of a particular product or trait.
What are anabolic pathways?
Requires input of energy; Final product is outcome of multiple sequential steps along a pathway.
What are loss-of-function mutations?
Results in a significant decrease or in the complete loss of the functional activity of a gene product
What is codominance?
The equal and detectable expression of both alleles in a heterozygous organism.
What is complete penetrance?
The observation that the phenotype for a trait is always produced when the corresponding genotype(s) are present
What is incomplete dominance ("partial dominance") ?
The observation that the phenotype occurring in heterozygous organisms is intermediate between the phenotypes of homozygous organisms, but more similar to one homozygous phenotype than to the other.
What is incomplete penetrance?
The occurrence of individual organisms that have a particular genotype or allele, but not the corresponding phenotype; if its less than 100% ex. Breast Cancer (80%)
How do pathways work in terms of interpreting different types of gene interactions?
gene 1 --> protein --gene 2--> converted protein --gene 3--> Final product
In genetic complementation analysis, the number of complementation groups =
the numbers of genes