Question

Suppose two independently assorting genes are involved in the pathway that determines fruit color in squash. these genes interact with each other to produce the squash colors seen in the grocery store. at the first locus, the w allele codes for a dominant white phenotype, whereas the w allele codes for a colored squash. at the second locus, the allele y codes for a dominant yellow phenotype, and the allele y codes for a recessive green phenotype. the phenotypes from the first locus will always mask the phenotype produced by the second locus if the dominant allele (w) is present at the first locus. this masking pattern is known as dominant epistasis.

Answer 1

Epistasis is a phenomenon in which more than one gene loci is responsible for a particular phenotype but the contribution of these loci is not additive in nature. This implies that the expression of one gene locus is masked or modified by the presence of another gene locus.

Further Explanation:

In Mendel’s experiment of a dihybrid cross (pea plant), he examined two different traits that were controlled by two different genes simultaneously. For example, Mendel performed crossing of two pea plants out of which one was homozygous for yellow seed color and round seed shape (RRYY) while the other was homozygous for green seed color and wrinkled seed shape (rryy). The outcome of such crossing was a heterozygous pea plant with round seed shape and yellow seed color (RrYy). This F1 progeny was then self-crossed (RrYy x RrYy) which resulted in an F2 generation containing phenotypes in the ratio 9:3:3:1.

In such a dihybrid cross, a single phenotype is a result of the independent effect of each gene locus. Therefore, alleles Y and y had only effect on seed color while allele R and r could only affect seed shape and both allelic pairs had no influence on each other’s phenotype. As a result, two different characteristics were encoded by two different genes.

In certain cases where both the loci affect one particular characteristic, the phenotypic ratio is different from that observed in Mendel’s dihybrid cross. This is because products of two genes are affecting the same phenotype and their effects are not additive. Such genes are known as epistatic genes and the relationship is called epistasis.

Epistasis can occur in the following situations:

• When the effect of an allele at a locus is masked by the other allele at a different locus.

• When different loci interact with each other to form new phenotypes.

• When the presence of one allele at a locus modifies the expression of another allele at other loci.

Epistasis can be of two types:

• Dominant epistasis: When the expression of all the other alleles is masked or suppressed by one dominant allele, it is called dominant epistasis. For example, squash is available in three colors. The yellow color is determined by gene A and is expressed when present as AA or Aa which is dominant over green squash (aa). Another gene B is also responsible for white squash color. The dominant allele B is epistatic to gene A and masks its expression when both the genes are present at the same locus.

• Recessive epistasis: When a homozygous recessive allele inhibits the expression of the other dominant gene, it is termed as recessive epistasis. For example, in Labrador retrievers, B gene locus is responsible for melanin production. The allele B produces black color and is dominant over allele b which produces a brown color. Another locus E gene regulates the melanin deposition in hairs. Therefore, atleast one functional allele E is necessary for the production of brown or black colored hair. The presence of homozygous allele ‘e’ is unable to deposit melanin. As a result, regardless of the presence of any genotype at locus B, the recessive allele ‘ee’ suppresses the expression of B gene locus.

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Answer Details:

Grade: College Biology

Chapter: Epistasis

Subject: Biology

Keywords:

Epistasis, Mendel’s experiment, dihybrid cross, gene, allele, phenotype, genotype, dominant epistasis, recessive epistasis.

Answer 2

The arrangement of the practical relationships between genes is called gene interaction. One such relationship is epistasis, which is the connection of non-allelic genes where the impact of one gene is covered by another gene to result either in the concealment of the impact or the two of them consolidate to create another characteristic (character).

Further explanation

Law of Independent assortment

Law of Independent Assortment depicts how various genes autonomously separate from each other when regenerative cells create. Independent assortment and their relating traits were first seen by Gregor Mendel in 1865 during his investigations of hereditary qualities in pea plants. Mendel was performing di hybrid crosses, which are crosses between creatures that vary concerning two traits. He found that the blends of attributes in the posterity of his crosses did not generally coordinate the mixes of characteristics in parental living beings. From his information, he detailed the Principle of Independent Assortment.

Dominance relation

It is the phenomenon wherein one allele of a gene on a chromosome mask the impact of an alternate allele of a similar gene on the other duplicate of the chromosome. The primary variant is named dominant and the subsequent recessive.

Types of dominant relations

It can be of following types.

1. Complete dominance
2. Incomplete dominance
3. Codominance
4. Over dominance

Epistasis

It is the impact of one gene is dependent on the nearest one or more 'modifier genes', i.e. the hereditary foundation. This term implies that the phenotypic effect of one gene is covered by another gene.

Answer details

Subject: Biology

Level: College

Keywords

• Law of Independent assortment
• Dominance relations
• Types of dominant relations
• Epistasis

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