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Epistasis

Mendel’s studies in pea plants implied that the sum of an individual’s phenotype was controlled by genes (or as he called them, unit factors), such that every characteristic was distinctly and completely controlled by a single gene. In fact, single observable characteristics are almost always under the influence of multiple genes (each with two or more alleles) acting in unison. For example, at least eight genes contribute to eye color in humans.

Eye color in humans is determined by multiple genes. Use the Eye Color Calculator to predict the eye color of children from parental eye color.

In some cases, several genes can contribute to aspects of a common phenotype without their gene products ever directly interacting. In the case of organ development, for instance, genes may be expressed sequentially, with each gene adding to the complexity and specificity of the organ. Genes may function in complementary or synergistic fashions, such that two or more genes need to be expressed simultaneously to affect a phenotype. Genes may also oppose each other, with one gene modifying the expression of another.

In epistasis    , the interaction between genes is antagonistic, such that one gene masks or interferes with the expression of another. “Epistasis” is a word composed of Greek roots that mean “standing upon.” The alleles that are being masked or silenced are said to be hypostatic to the epistatic alleles that are doing the masking. Often the biochemical basis of epistasis is a gene pathway in which the expression of one gene is dependent on the function of a gene that precedes or follows it in the pathway.

An example of epistasis is pigmentation in mice. The wild-type coat color, agouti ( AA ), is dominant to solid-colored fur ( aa ). However, a separate gene ( C ) is necessary for pigment production. A mouse with a recessive c allele at this locus is unable to produce pigment and is albino regardless of the allele present at locus A ( [link] ). Therefore, the genotypes AAcc , Aacc , and aacc all produce the same albino phenotype. A cross between heterozygotes for both genes ( AaCc x AaCc ) would generate offspring with a phenotypic ratio of 9 agouti:3 solid color:4 albino ( [link] ). In this case, the C gene is epistatic to the A gene.

A cross between two agouti mice with the heterozygous genotype AaCc is shown. Each mouse produces four different kinds of gametes (AC, aC, Ac, and ac). A 4 × 4 Punnett square is used to determine the genotypic ratio of the offspring. The phenotypic ratio is 9/16 agouti, 3/16 black, and 4/16 white.
In mice, the mottled agouti coat color ( A ) is dominant to a solid coloration, such as black or gray. A gene at a separate locus ( C ) is responsible for pigment production. The recessive c allele does not produce pigment, and a mouse with the homozygous recessive cc genotype is albino regardless of the allele present at the A locus. Thus, the C gene is epistatic to the A gene.

Epistasis can also occur when a dominant allele masks expression at a separate gene. Fruit color in summer squash is expressed in this way. Homozygous recessive expression of the W gene ( ww ) coupled with homozygous dominant or heterozygous expression of the Y gene ( YY or Yy ) generates yellow fruit, and the wwyy genotype produces green fruit. However, if a dominant copy of the W gene is present in the homozygous or heterozygous form, the summer squash will produce white fruit regardless of the Y alleles. A cross between white heterozygotes for both genes ( WwYy × WwYy ) would produce offspring with a phenotypic ratio of 12 white:3 yellow:1 green.

Questions & Answers

hi I'm asking a question about HIV infection, can HIV infection transmitted from Mother to unbron child? please help me I So confused.
Khushboy Reply
what is the difference between primary and secondary active transport in detail? I didn't understand the steps in the textbook specifically
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you are a doctor?
Mohammed
what is the meaning of connective tissue?
Mohammed Reply
what are the characteristics of living things
Owolo Reply
hi
Mohammed
what's the meaning of connective tissue?
Mohammed
Reproduction, adaptation, interaction, movement, growth, respiration, made of cells, responsive to environment (homeostasis), metabolic action (consumption of food converted into energy)
garret
state two most important factors that favour exponential growth of population of a gazelle in a pack
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Ben
what are the two types of electron microscope
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Sama
Enzymes are biological catalyst which alter any reaction and protein in nature
Nkoue Reply
Thanks
Gaudi
Your welcome sir
Nkoue
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Royd
What is translation and transcription
Nkoue
Transcription is making RNA from DNA. Translation is going from RNA to proteins.
Eric
full meaning of RNA and DNA
Charity
Nkoue what homeostits means?
Mohammed
Mohammed khalfan, In biology, homeostasis is the state of steady internal, physical, and chemical conditions maintained by living systems.[1] This dynamic state of equilibrium is the condition of optimal functioning for the organism and includes many variables, such as body temperature and fluid bal
Khushboy
what is the definition of enzymes
Royd Reply
enzymes are biological catalyst that speed up chemical reaction.
Alale
What are enzymes?
Gaudi
Enzymes are made of proteins and lower the energy of activation. In other words, they bring things together which helps to lower the amount of energy for a reaction to go forward.
Eric
they are catalyses that speeds up chemical reaction.... e.g they break down the food we consume.
Azeez
These are catalyst that speeds up the chemical reaction.
Micheal
What is connective tissue?
Mohammed
what homeostits means ?
Mohammed
what is a spirogyra
Talabi Reply
Spirogyra is a filamentous chlorophyte green algae of the order Zygnematales. It is named for the helical or spiral arrangement of the chloroplasts. That is characteristic of the genus. It is commonly found in freshwater habitats. And there are more than 400 species of Spirogyra in the world.
Eric
what is the mean of biology
Bello Reply
what is cell
Bello
A cell is the smallest living unit.
Eric
Hi I'm new in this group can someone please help with the list features shared by plants and charopytes that are not shared with most other eukaryotes
Wendy
iz a basic units of a living thing?
simongc
what is eutrophication
Chinaza Reply
hi.. I'm asking a question about HIV infection.... Can HIV infection transmitted from Mother to unbron child?
Khushboy
Eutrophication is an enrichment of water by nutrient salts that causes structural changes to the ecosystem such as: increased production of algae and aquatic plants, depletion of fish species, general deterioration of water quality and other effects that reduce and preclude use.
Khushboy
Show well labeled diagram of female reproductive organs
Lanlege Reply
phenotype is your big head
Amagiya Reply
The phenotype is the physical appearance or things you can see. Or the traits expressed by ones DNA.
Eric
phenotypes are appearance that can be seen and touched
Chidera
example of phenotype
Charity
what is a dichotomous key
Moses Reply
explain the role of
Moses
Dichotomous key : Is the key that is use to classify or group an organism base on their common features
Alale
an amoeba is what kind of cellular organism?
Mercy Reply
It is a protizoa with bilayer membrane bound organelles. Therefore it is eukaryotic.
Eric
what is amoeba
Muhammad
amoeba is a unicellular organisms. Therefore it is made u of only one call.
Alale
what is phenotype
Muhammad
amoeba is a unicellular organism with one cell
Chinaza

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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