Lecture 10: Gene and Environment Interactions PDF

Genetics I Lecture 10: Gene and environment interactions Prepared by: Mohamed A. Abdelrahman Associate Professor School of Biotechnology  10.1. Introduction Much of the early success of genetics can be attributed to the correlation of phenotypes and alleles, as when Men...

Genetics I Lecture 10: Gene and environment interactions Prepared by: Mohamed A. Abdelrahman Associate Professor School of Biotechnology  10.1. Introduction Much of the early success of genetics can be attributed to the correlation of phenotypes and alleles, as when Mendel equated Y with yellow peas and y with green. However, from this logic there arises a natural tendency to view alleles as somehow determining phenotypes. Although this is a useful mental shorthand, we must now examine the relationship between genes and phenotypes more carefully. The fact is that there is no way a gene can do anything alone. (Imagine a gene—a single segment of DNA—alone in a test tube.)  10.1. Introduction For a gene to have any influence on a phenotype it must act in concert with many other genes and with the external and internal environment. So an allele like Y cannot produce yellow color without the participation of many other genes and environmental inputs. In this lecture we examine the ways in which these interactions take place.  10.2. Genes and environment The physical ways in which genes interact with one another and with the environment are summarized by the model in Figure 6- 1.  Some of the interactions shown in the figure are as follows: 1- Transcription of one gene may be turned on or off by other genes called regulatory genes. The regulatory proteins they encode generally bind to a region in front of the regulated gene.  Some of the interactions shown in the figure are as follows: 2- Proteins encoded by one gene may bind to proteins from other genes to form an active complex that performs some function. These complexes, which can be much larger than that shown in the figure, have become known as molecular machines because they have several interacting functional parts just like a machine.  Some of the interactions shown in the figure are as follows: 3- Proteins encoded by one gene may modify the proteins encoded by a second gene in order to activate or deactivate protein function. For example, proteins may be modified through the addition of phosphate groups.  Some of the interactions shown in the figure are as follows: 4- The environment engages with the system in several ways. a. In the case of an enzyme, its activity may depend on the availability of a substrate supplied by the environment. b. Signals from the environment can also set in motion a chain of consecutive gene controlled steps that follow one another like a cascade of falling dominoes. The chain of events initiated by an environmental signal is called signal transduction.  10.2. Genes interaction The 2:1 ratio produced by a cross between two yellow mice results from a lethal allele. Figure 4.1, REF2  10.2. Gene interaction that produces new phenotype Gene interaction, happen when the products of genes at different loci combine to produce new phenotypes that are not predictable from the single-locus effects alone. In pepper color, two separate genes working together (Y&C) for a single trait (pepper color). Both genes are still independent. The genotype of Y gene doesn’t influence C gene.  10.2. Gene interaction that produces new phenotype Gene interaction, happen when the products of genes at different loci combine to produce new phenotypes that are not predictable from the single-locus effects alone. In FETHER COLOR in Budgerigar parakeet, two separate genes working together (B (blue) &Y (Yellow)) for a single trait. Both genes are still independent The genotype of B gene doesn’t influence C gene. Production of feather color in budgerigar parakeets, Pearson, Figure 4.20  10.3. Gene Interaction with Epistasis Epistasis is when the genotype of one gene actually influences the expression of another. The effect of gene interaction is that one gene masks (hides) the effect of another gene at a different locus. Epistasis means stand on top of or control. The gene that does the masking called epistatic gene while the gene whose effect is masked is a. hypostatic gene Epistatic genes may be recessive or dominant in their effect.  10.3.1. Recessive epistasis Recessive epistasis, where the epistatic gene control in the recessive form. Coat color in Labrador retriever is controlled by two genes at two different loci One gene determines which color is made for the hair (black or brown) BB and Bb make black color (dominance) bb makes brown color The other determines how well that color is deposited in hair EE and Ee genotypes allow deposition of the color. ee no deposition. The phenotype And so !? B-E- The ratio Labrador retriever dogs bbE_ !? B_ee bbee  10.3.1. Recessive epistasis Recessive epistasis, where the epistatic gene control in the recessive form. Coat color in Labrador retriever is controlled by two genes at two different loci One gene determines which color is made for the hair (black or brown) BB and Bb make black color (dominance) bb makes brown color The other determines how well that color is deposited in hair EE and Ee genotypes allow deposition of the color. ee no deposition. And so B-E- Black Labrador retriever 9/16 bbE_ brown Labrador retriever 3/16 Labrador retriever dogs B_ee Yellow Labrador retriever 4/16 bbee Yellow Labrador retriever  10.3.2. Dominant epistasis Dominant epistasis, where the epistatic gene control in the dominant form. Dominant epistasis is seen in the interaction of two loci that determine fruit color in summer squash, which is commonly found RATIO in one of three colors: yellow, white, or green. ? Cross a true-breeding white squash plant with a true-breeding green and all the progeny in F1 are white. The if F1 self crossed then the ratio should be !?  10.3.2. Dominant epistasis Dominant epistasis, where the epistatic gene control in the dominant form. Dominant epistasis is seen in the interaction of two loci that determine fruit color in summer squash, which is commonly found W_Y_ & W_yy in one of three colors: yellow, white, or green. wwY_ wwyy Cross a true-breeding white squash plant with a true-breeding green and all the progeny in F1 are white. The if F1 self crossed then the ratio should be !? Gene W is epistatic to Y (W controls Y) If gene W has any dominant alleles then the phenotypes are white squash If the genotype is ww so the color will be expressed  10.3.2. Dominant epistasis Yellow pigment in summer squash is produced in a two-step pathway. REF 1, Figure 4.18 10.3. Gene Interaction with Epistasis  10.3. Gene Interaction with Epistasis A number of all-white cats are crossed and they produce the following types of progeny: 12/16 all-white, 3/16 black, and 1/16 gray. Give the genotypes of the progeny. Which gene is epistatic?  10.3. Gene Interaction with Epistasis A number of all-white cats are crossed and they produce the following types of progeny: 12/16 all-white, 3/16 black, and 1/16 gray. Give the genotypes of the progeny. Which gene is epistatic? The 12 all-white : 3 black : 1 gray ratio is a modification of the 9 : 3 : 3 : 1 ratio produced in a cross between two double heterozygotes: Therefore, the all-white cats have a dominant epistatic allele (W) and are genotype W_ G_ and W_ gg, the black cats lack the epistatic allele (W) and have a dominant allele for black (ww G_), and the gray cats lack the epistatic W and are recessive for gray (ww gg). The allele for all-white (W) is a dominant epistatic gene.  10.3. Gene Interaction with complementation Complementation is the production of a wildtype phenotype when two haploid genomes bearing different recessive mutations are united in the same cell. Complementation test: To carry out a complementation test on recessive mutations, parents that are homozygous for different mutations are crossed, producing offspring that are heterozygous. If the mutations are allelic (occur at the same locus), If the mutations are not allelic (occur at different loci), then the heterozygous offspring have only mutant then the mutations complement each other and the alleles (a b) and exhibit a mutant phenotype: heterozygous offspring have the wild-type phenotype: Example to explain, how does complementation work at the molecular level? The molecular basis The normal blue color of the harebell flower is caused of genetic by a blue pigment called anthocyanin. complementation. Three phenotypically identical white The blue pigment is the end product of a series of mutants—$, £, and ¥— are intercrossed. biochemical conversions of non-pigments. Each step is Mutations in the same catalyzed by a specific enzyme encoded by a specific gene (such as $ and £) cannot complement, gene. because the F1 has one gene with two mutant alleles. The pathway is A homozygous mutation in either of the genes will lead blocked and the to the accumulation of a precursor that will simply flowers are white. When the mutations make the plant white. are in different genes (such as £ and ¥), complementation of The complementation is actually a result of the the wild-type alleles of cooperative interaction of the wild-type alleles of the each gene occurs in the F1 heterozygote. two genes. Pigment is synthesized and the flowers are blue. (What would you predict to be the result of crossing $ and ¥?). Figure 6-16 , REF 2. All rights reserved.  10.3. Gene Interaction with complementation A modified F2 ratio can be useful in supporting a hypothesis of gene complementation. As an example, let’s consider what F2 ratio will result from What are the crossing the dihybrid F1 harebell phenotypes and their ratio!? plants.  10.3. Gene Interaction with complementation The F2 shows both blue and white plants in a ratio of 9 : 7. How can these results be explained? The 9: 7 ratio is clearly a modification of the dihybrid 9:3:3:1 ratio with the 3:3:1 combined to make 7. The cross of the two white lines and subsequent generations can be represented as follows:  10.4. Regulatory genes The example of complementation in harebells involved different steps in a biochemical pathway. Similar results can come from gene regulation. Regulatory gene often functions by producing a protein that binds to a regulatory site upstream of the target gene, facilitating the transcription of the gene by RNA polymerase.  In the absence of the regulatory protein, the target gene would be transcribed at very low levels, inadequate for cellular needs. Let’s cross a pure line r/r defective for the regulatory protein to a pure line a/a defective for the target protein. The cross is r/r ; a+/a+ X r+/r+ ; a/a. The F1 will be r+/r ; a+/a dihybrid will show complementation between the mutant genotypes because both r+ and a+ are present, permitting normal transcription of the wild- type allele. When selfed the F1 dihybrid will also result in a 9 : 7 phenotypic ratio in the F2: Interaction between a regulating gene and its target. The r gene codes for a regulatory protein, and the a gene codes for a structural protein. Both must be normal for a functional (“active”) structural protein to be synthesized. Figure 6-18 REF 2  10.4. Suppressors genes A type of gene interaction that can be detected more easily is suppression. A suppressor is a mutant allele of one gene that reverses the effect of a mutation of another gene, resulting in a wild- type or near wild-type phenotype. Assume that an allele a+ produces the normal phenotype, whereas a recessive mutant allele a results in abnormality. A recessive mutant allele s at another gene suppresses the effect of a, so that the genotype a/a. s/s will have wild-type (a+-like) phenotype. Suppressor alleles sometimes have no effect in the absence of the other mutation; in such a case, the phenotype of a+/a+. s/s would be wild type. In other cases, the suppressor allele produces its own abnormal phenotype.  10.4. Suppressors genes Let’s look at a real-life example from Drosophila. The recessive allele pd will result in purple eye color when unsuppressed. A recessive allele su has no detectable phenotype itself, but suppresses the unlinked recessive allele pd. Hence pd/pd ; su/su is wildtype in appearance and has red eyes. The following analysis illustrates the inheritance pattern. A homozygous purple-eyed fly is crossed to a homozygous red- eyed stock carrying the suppressor.  10.4. Suppressors genes Suppression is sometimes confused with epistasis. However, the key difference is that: a suppressor cancels the expression of a mutant allele and restores the corresponding wild-type phenotype. The modified ratio is an indicator of this type of interaction. Furthermore, often only two phenotypes segregate (as in the preceding examples), not three, as in epistasis.  10.4. Suppressors genes Suppression is sometimes confused with epistasis. However, the key difference is that: a suppressor cancels the expression of a mutant allele and restores the corresponding wild-type phenotype. The modified ratio is an indicator of this type of interaction. Furthermore, often only two phenotypes segregate (as in the preceding examples), not three, as in epistasis.  Resources Pierce, B. A. (2018). Genetics essentials: concepts and connections (p. 488). WH Freeman. Griffiths, A. J. (2005). An introduction to genetic analysis. Macmillan. Chatterjee, A. (2022). Quantitative Genetics. In Genetics Fundamentals Notes (pp. 1029-1076). Singapore: Springer Nature Singapore. Banerjee, S., Bhattacharjee, T., Maurya, P. K., Mukherjee, D., Islam, S. M., Chattopadhyay, A.,... & Hazra, P. (2022). Genetic control of qualitative and quantitative traits in bell pepper crosses involving varied fruit colors and shapes. International Journal of Vegetable Science, 28(5), 477-492.