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The key points covered in this video include:
1. Introduction to Epistasis
2. Recessive Antagonistic Epistasis
3. Dominant Antagonistic Epistasis
4. Complementary Epistasis
Introduction to Epistasis
Sometimes different genes on different homologous chromosomes can interact with each other. Epistasis is the interaction between two non-linked genes which causes one gene to mask the expression of the other in the phenotype. Epistatic genes can work antagonistically (against each other) or in a complementary fashion. When a gene suppresses another gene, the gene doing the suppressing is called the epistatic gene. The gene which is being suppressed is called the hypostatic gene.
Recessive Antagonistic Epistasis
Antagonistic epistasis can be either recessive or dominant. Recessive epistasis occurs when the presence of two copies of the recessive allele at the first locus prevents the expression of another allele at a second locus. An example of recessive epistasis is the control of flower colour in the Salvia plant. In the Salvia plant there are two genes, A/a and B/b which are located on two separate chromosomes. In the hypostatic gene, the dominant B allele encodes purple flowers and the recessive B allele encodes pink flowers. In the epistatic gene, the dominant A allele allows the hypostatic gene to be expressed whereas the recessive a allele suppresses the expression of the hypostatic gene. Therefore if the flower has two copies of the recessive a allele neither the dominant B or recessive B allele will be expressed in the phenotype. So if the genotype of the flower is AaBB or AaBb the phenotype will be purple, if it is Aabb it will be pink but if it is aaBB or aabb or aaBb it will be white.
Dominant Antagonistic Epistasis
In dominant antagonistic epistasis, the expression of the dominant allele of the epistatic gene prevents the expression of the hypostatic gene. An example of dominant antagonistic epistasis is in the inheritance of feather colour in chickens. In chickens the hypostatic gene, C/c encodes feather colour, whereas the epistatic gene, I/i encodes the formation of colour. In the hypostatic gene the dominant C allele encodes coloured feathers whereas the c recessive allele encodes white feathers. In the epistatic gene the dominant I allele prevents the formation of colour whereas the recessive allele allows colour to form. Therefore if the dominant I allele is expressed no colour will be formed so the chicken feathers will be white, regardless of whether the dominant C or recessive c allele is expressed. However, if a chicken is homozygous recessive (ii) for the epistatic gene and has the dominant C allele, they will be coloured.
Complementary Epistasis
In complementary epistasis, the two genes work together with each other - for example, they may encode two enzymes that work in succession. In mice the CC or Cc genotype in the epistatic gene produces coloured fur whereas the cc genotype stops the formation of the pigment. The second hypostatic gene, A/a determines the distribution of the pigment if it is made. The dominant A allele encodes for a type of brown coloured hair called agouti hair - each agouti hair has a black and yellow band. The recessive allele encodes for a black colour where each hair only has a black band and no yellow band. Therefore if the mouse is homozygous cc then the mice will be white because no pigment will be produced. If the mouse genotype is CcAA, CCAA or CcAa it will have agouti fur but if it is Ccaa it will have black fur. If two mice with the heterozygous CcAa genotype are crossed, a phenotypic ratio of 9:4:3 (agouti: white:black) is produced.
Summary
Epistasis is the interaction between two non-linked genes in which one gene suppresses the expression of the other in the phenotype
Antagonistic epistasis is when the genes interact to work against each other
Recessive epistasis is when the recessive allele of the epistatic gene encodes the suppression of the other gene
Dominant epistasis is when the dominant allele of the epistatic gene encodes the suppression of the other gene
Complementary epistasis is when the genes interact to work together
