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What is epistasis?
The phenomena where multiple genes effect one trait (phenotype).
Depending on which form of a gene in epistasis that an organism has, what changes
in the expression of other genes, resulting in a modified phenotype.
What is double recessive epistasis? (duplicate recessive)
Either homozygous recessive genotypes result in the same pehno type.
Most often how does epistasis effect a trait?
It interferes with the expression of a gene product from another one.
What is the rare but alternative form of epistasis?
Different loci can work in complement with each other.
Finish the statement:
Epistasis is an interaction dealing with …
phenotype.
True or False:
The segregation of genotypes does not change due to epistasis?
True; it is the law of independent assortment that is changed.
What are the five specific epistasis ratios?
Recessive Epistasis
Double Recessive Epistasis
Dominant Epistasis
Double Dominant Epistasis
Dominant Recessive Epistasis
WHat is recessive epistasis?
A genetic interaction where one recessive allele masks the expression of another non-allelic gene, leading to a specific phenotypic ratio.
What are the two ratios that denote a dihybrid recessive epistasis?
9:3:4 and 3:1
What is are the two tell tale signs of a double recessive epistasis situation?
The F1 generation shows the dominant phenotype and the ratio of the F2 generation is about 9:7
How does double recessive epistasis work?
Say you have a protein to begin with known as a precursor proteins. The first gene makes an intermediate protein and the second gene makes a product protein from the intermediate protein. Both the intermediate protein and final protein are needed for expression of the dominant phenotype, therefore if either the intermediate protein or final protein do not function, the dominant phenotype is not expressed.
What is domiant epistasis?
One allele for a dominant trait hides the phenotype of the other gene.
How does a dominant epistasis situation work?
There is a Dominant allele that can be considered “super dominant” meaning that the other gene in the trait “defers” to the super dominant allele. If the super dominant allele is not present in the genotypes then the deferential allele can be expressed, however if neither the super dominant allele of one gene and the deferential dominant allele of the other genes are present then the recessive traits can show.
What is the tell tale sign of a Dominant epistasis situation?
12:3:1 F2 ratio
What is double dominant epistasis?
Both genes are dominant and the protein products result ion the same trait.
What is the underlying mechanism of double dominant epistasis?
Say you start with a precursor protein. Both gene A and gene B make a protein that results in the same trait, if both are expressed the dominant phenotype is expressed, if one of the two dominant alleles is expressed the dominant phenotype is expressed. Only if both proteins are inactive by a double recessive genotype is the recessive phenotype expressed.
What is the tell-tale sign of the double dominant epistasis situation?
15;1 F2 phenotypic ratio
What is dominant recessive epistasis?
Sometimes, a Dominant allele in one locus and a recessive alleles in another locus can produce the same phenotype.
What is the molecular explanation of a dominant recessive epistasis situation?
Say you begin with a precursor protein, the dominant alleles of one gene codes for a specific phenotype and is expressed via the activation of a protein. Another gene that is in its recessive state, meaning that it has deactivated the protein associated with the gene also codes for that same phenotype. This means that if either the dominant allele of the first gene is present or if the recessive allele of the second gene is present, the dominant phenotype is expressed. The only way to express the recessive phenotype is for the recessive/ inactivated version of the protein of the first gene is expressed at the same time as the dominant/ activated version of the protein of the second gene.
What is the tell tale sign of domiant recessive epistasis?
13;3 F2 ratio
In the dominant recessive epistasis situations, what is considered the epistatic gene?
Typically the gene that is homozygous recessive with the same phenotype is a suppressor protein/ epistatic protein.
What are the Tips and Tricks for Epistasis?
Look at the F2 phenotypic ratios
Genes involved - Ratios of 3:1, 1:2:1, or 2:1
Genes involved - can be a variety of ratios based entirely on the cross
Determine how many genes and how many traits are involved in the question
Understand you epistatic ratios but don’t be constrained by them
Understand a dihybrid heterozygote cross!!!
What is considered the epistatic gene in Double Domiant Epistasis?
Either gene, determined by which domiant gene is present, masking the other recessive traits.
What is considered the epistatic gene in Dominant Epistasis?
The super dominant gene is the epistatic gene because all other forms defer to it. If the super dominant is not present then the deferential dominant allele would be epistatic
What is the epistatic gene in double recessive epistasis?
Which ever gene is acting as the suppressor gene at the moment.
i.e which ever gene is in its recessive state.
What is the epistatic gene in recessive epistasis?
The gene that determines genotype in its recessive state masking all other genotypes.
How is biological sex, determined>
In early development, by the 6 or 7th week of gestation, biological sex is determined by the presence of sex chromosomes, specifically the Y chromosome in males which triggers the development of male characteristics.
What was Aristotle’s theory of sex determination?
He believed that males had an internal fire or heat and women hd an internal cold or ice. These competing forces would come together and which ever force over took the other was the resulting offspring’s sex.
What is sexual dimorphism?
The secondary characteristics that help to define and differentiate males from females.
i.e. facial hair, sex organs, body size, plumage coloration.
Who was the first to come up with the idea of sex chromosomes being the determining factor is sex determination?
Clarence McClung
What was th element discovered by Hermann Henking that was used by Clarence McClung to determine Sex Chromosomes?
The X element with was present in all cells adn was present in ½ the produced zygotes.
What is an allosome?
The sex chromosomes, a chromosome that differs from an ordinary autosome in form, size and behaviors. Determines the sex of an individual organism.
What is an autosome?
A non-sex chromosome, the appears in pairs whose members are homologous but differ from other pairs of chromsomes.
The XX chromosomes of a female human are considered what?
Homogametic (homozygous)
The XY chromosomes of a male human are considered what?
Heterogametic (hemizygous)
Why are XY chromosomes hemizygous and not heterozygous?
Because the chromsomes, X & Y, are completely different
Can a eukaryote handle an extra allosome?
Yes, but an autosome not so much.
In a fruit fly what are the sex chromosomes pairs and what determines sex?
The sex chromosomes in a fruit fly are X and Y, with the ratio of X chromosomes to sets of autosomes determining the sex. The male only has one X.
In birds what are the sex chromosome pairs and what determines sex?
In birds, the sex chromosomes are Z and W, with the ratio of Z chromosomes to W chromosomes determining the sex. The heterogametic organism is female.
What determines sex on teh y chromosome?
The SRY gene which is the Sexual Determining Region Y gene, initiates male sex development at around the 6-7 week mark of gestation.
Why do organisms have a PAR ( pseudoautosomal region) on their sex chromosomes?
allows for homologous recombination between sex chromosomes during meiosis, ensuring proper segregation and stability of the sex chromosomes.
What are differential regions on sex chromosomes?
Regions that contain specific genes that have no counterparts on the other Sex Chromosome
In flies what is a sex-linked trait?
eye color
How do we write a sex linked trait genotype?
AX aX
When the parental organism that is female is homozygous dominant for the sex linked trait what is the resulting F1 generation phenotype and Genotype?
The resulting F1 generation phenotype will display the dominant trait, while the genotype will be represented as AXaX or AXY.
When the parental organism that is female is homozygous dominant for the sex linked trait what is the resulting F2 generation phenotype and Genotype?
The resulting F2 generation phenotype will show a mix of dominant and recessive traits, with a genotype ratio of 1 AXAX : 1 AXaX : 1 AXY : 1 aXY. Meaning only male offspring are affected by the recessive trait.
If the reciprocal cross is done for a sex linked trait, where the homozygous recessive genotype is given to the female parent. What is the genotype and phenotype of the F1 generation?
1 AXaX : 1 aXY making all males recessive for the trait.
If the reciprocal cross is done for a sex linked trait, where the homozygous recessive genotype is given to the female parent. What is the genotype and phenotype of the F2 generation?
Both sexes will have a 1:1 expression of the domiant and recessive phenotypes.
Fill in the Blank:
If the _________ cross yields the same results as the original cross, then the genes is ________ sex ______.
reciprocal; not; linked
When looking at the reciprocal cross what should you look for?
Different reciprocal cross ratios in different sexes
Differences between the reciprocal crosses that follow a specific sex.
What is linkage?
The Physical connection that exist between geneswhen their loci are on the same chromosome.
What are sex linked traits?
Traits whose loci are located on the Sex Chromosomes so that their transmission from one generation to the next is affected by which Sex Chromosomes an individual acquires.
Who is more likely to be effected by an X-linked recessive trait?
X-linked recessive traits appear Much more commonly in males than females. This is because the males Always receive their chromosome from their mother and will always express ANY X-linked traits their mother has.
i.e. color blindness.
There are traits that are linked to the Y chromosome, what is a holandric trait?
Traits passed directly from FATHER to SON on the Y chromosome. There are very few y-linked traits.
i.e. Retinitus Pigmentosa
What is a sex limitd trait?
A trait generally autosomeal that is expressed in only one gender.
i.e xanthine oxidoreductase → only in females
Interferon regulartory factor 4 (IRF4) → only in males
What are sex influenced traits?
Autosomal traits that differ in the wya that the two genders express the gene.
i.e. pattern baldness
WHat are the TIPS & TRICKS of Sex-linked traits?
Look for a specific trait segregating with one gender only.
Look for key words or information given in the problem
“a trait located on an autosome that only appears in one gender”
“a reciprocal cross was performed that gave a different outcome related to gender.”
Note 
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