Module 2-Mendelian & Non-Mendelian Genetics.pdf

MODULE 2 MENDELIAN GENETICS 1. MENDELIAN GENETICS he determined that discrete units of inheritance...

MODULE 2 MENDELIAN GENETICS 1. MENDELIAN GENETICS he determined that discrete units of inheritance exist and predicted their behavior in the formation of gametes Subsequent investigators, with access to cytological data, were able to relate their own observations of chromosome behavior during meiosis and Mendel’s principles of inheritance. Mendel’s postulates became the basis of Transmission Genetics https://kids.britannica.com/students/article/ Gregor-Mendel/275785/media MODULE 2 MENDELIAN GENETICS 1. MENDELIAN GENETICS What made Mendel succeed where others did not? 1. He had good choice of experimental organism because garden peas are easy to grow and need only 1 season to mature. 2. He used true-breeding (homozygous) plants. He chose varieties that differed in only one trait (monohybrid cross). 3. He was good at keeping accurate records and had good background in statistics and thus was able to analyze the results of his experiments well. https://kids.britannica.com/students/article/ Gregor-Mendel/275785/media MODULE 2 MENDELIAN GENETICS https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/03%3A_Genetics/3.01%3A_Mendel's_Pea_Plants MODULE 2 MENDELIAN GENETICS DEFINITION OF TERMS GENE the physical and functional unit that helps determine the traits passed on from parents to offspring; also called Mendelian factor. a nucleotide sequence in DNA that specifies a polypeptide or RNA. Alterations in a gene’s sequence can give rise to species and individual variation (Russell, 2010) MODULE 2 MENDELIAN GENETICS DEFINITION OF TERMS LOCUS Position on a chromosome where a gene is located. Note that the allele for flower color can be found on the same locus in the homologous chromosomes. MODULE 2 MENDELIAN GENETICS DEFINITION OF TERMS ALLELE OR ALLELOMORPH any 2 or more related genes of a trait DOMINANT: expresses its effect over another allele; masks the recessive allele in the heterozygous organism. An upper case letter is used to represent this in genetic crosses. RECESSIVE: masked in a heterozygous individual by the presence of dominant allele. This is represented by a lower case letter in genetic crosses. MODULE 2 MENDELIAN GENETICS DEFINITION OF TERMS Phenotype: visible appearance Genotype: the genetic constitution Take note that the phenotype purple can genotypically be homozygous (PP) or heterozygous (Pp) because purple is dominant over white. White is always homozygous (pp). MODULE 2 MENDELIAN GENETICS DEFINITION OF TERMS Reciprocal crosses: crossing a male with one trait with a female having the other trait (from a pair of contrasting traits) Selfing: self-pollination; crossing individuals from the same generation Character: observable heritable feature Trait: a variant for a character P: Parental generation; the 1st individuals crossed F1: 1st filial generation offspring; offspring of P F2: 2nd filial generation offspring; offspring of F1 MODULE 2 MENDELIAN GENETICS MENDELIAN LAWS A. Law of Segregation - an organism possesses two alleles encoding a trait and these two alleles separate in equal proportions during gamete formation; each individual has a pair of factors (alleles) for each trait B. Law of Independent Assortment - alleles from one locus segregate into gametes independently of those from another locus. MODULE 2 MENDELIAN GENETICS  MODULE 2 MENDELIAN GENETICS A. Law of Segregation The factors (alleles) segregate/separate during gamete formation. The parent with genotype YY can contribute the gamete Y and the parent with genotype yy contributes the gamete y. Each gamete contains only one factor (allele) from each pair. Since both parents are homozygous, each can only contribute 1 type of gamete. Fertilization gives the offspring two factors for each trait (see F1 generation). https://www.sciencefacts.net/mendels-law-of-segregation.html  MODULE 2 MENDELIAN GENETICS A. Law of Segregation: Monohybrid Cross This is a cross between 2 individuals involving 1 character (e.g., seed color). In the example, parent 1 is true-breeding yellow or homozygous yellow (YY) and parent 2 is true- breeding green or homozygous green (yy) Y from the yellow parent and y from This will result to the genotype Yy of F1 which is heterozygous. Both gametes can be seen as eggs and sperm. F2: 2 phenotypes (yellow, green); 3 genotypes (YY, Yy, yy) Phenotypic ratio: 3 yellow:1 green Genotypic ratio: 1YY:2Yy:1yy https://www.sciencefacts.net/mendels-law-of-segregation.html  MODULE 2 MENDELIAN GENETICS A. Law of Segregation: Monohybrid Cross In crossing F1 with each other, a Punnett Square is used to present the gametes of the parents and the genotypes and phenotypes of the offspring. Punnett Square: a table listing all possible genotypes resulting from a cross. All possible sperm genotypes are lined up on one side. All possible egg genotypes are lined up on the other side. Every possible zygote genotype is placed within the squares. https://www.sciencefacts.net/mendels-law-of-segregation.html MODULE 2 MENDELIAN GENETICS How to make a Punnett Square 1. If the letters representing the trait are not given in the word problem, you have to choose one: UPPERCASE version of the letter for dominant allele and lowercase version of the same letter to represent recessive allele. For clarity, you can start with a legend, specifying what trait is represented by what letter. 2. Draw the Punnett square. 3. Put the female parent on the left and the male parent at the top. (could be interchanged) 4. Segregate alleles into gametes. 5. Perform the cross: bring letters (alleles) “down” and “across” (Or put together 1 letter from parent 1 and another from parent 2.)  MODULE 2 MENDELIAN GENETICS Test Yourself: A garden pea with homozygous wrinkled seed is crossed with another garden pea with homozygous round seed. Just like Mendel’s experiment, F1 are self-pollinated to produce the F2 generation. a. Diagram this cross using letters to represent each trait. b. Make a Punnett Square showing the gametes of each parent and the genotype/s of the offspring up to the F2 generation.  MODULE 2 MENDELIAN GENETICS B. Law of Independent Assortment genes encoding different characters assort independently when gametes are formed based on the random separation of homologous pairs of chromosomes in Anaphase I of Meiosis it takes place when genes encoding two characters are located on different pairs of chromosomes  MODULE 2 MENDELIAN GENETICS B. Law of Independent Assortment: Dihybrid Cross This is a cross between 2 individuals involving 2 characters (e.g., seed shape and seed color). Since there are 2 characters, each gamete will have 2 letters, one from each character. Genotype of Parent 1: RRYY (R and R segregate; Y and Y segregate) Genotype of Parent 2: rryy (r and r segregate; y and y segregate) Since both parents are homozygous, each can only contribute 1 type of gamete; gamete of parent 1 is RY and gamete from parent 2 is ry. To determine the genotype of F1, put together the 2 gametes from the parents to form RrYy. https://microbenotes.com/mendels-law-of-independent-assortment/  MODULE 2 MENDELIAN GENETICS B. Law of Independent Assortment: Dihybrid Cross F2 Generation Phenotypic ratio: Genotypic ratio: https://microbenotes.com/mendels-law-of-independent-assortment/  MODULE 2 MENDELIAN GENETICS B. Law of Independent Assortment: Dihybrid Cross A dihybrid cross can also be solved using the forked-line method or branch diagram. Start by making separate crosses involving 1 character at a time, meaning, cross Aa with Aa and Bb with Bb. Monohybrid cross between Aa x Aa = genotypes under GENE 1 Fraction = proportion of offspring having that genotype The genotypes of the offspring from the cross between Bb and Bb are shown under GENE 2. Each possible genotype produced from the cross between Aa and Aa should combine with each of the genotype produced from the cross between Bb and Bb. Example: AaBb X AaBb Values are then multiplied. The results can be seen under OFFSPRING.  MODULE 2 MENDELIAN GENETICS Test Yourself GENOTYPES LIST OF GAMETES 1. GgMM 2. SSII 3. BbNn 4. aaDd 5. ccff  MODULE 2 MENDELIAN GENETICS Test Yourself GENOTYPES LIST OF POSSIBLE GAMETES 1. GgMM GM, gM 2. SSII SI 3. BbNn BN, Bn, bN, bn 4. aaDd aD, ad 5. ccff cf  MODULE 2 MENDELIAN GENETICS Test Cross: Monohybrid Cross a cross between an individual homozygous for the recessive allele of a particular gene and an individual expressing the dominant gene but whose genotype is unknown. Because it involves only 1 character, it is called a monohybrid test cross. So how do we determine the genotype of the parent which exhibits the dominant trait?  MODULE 2 MENDELIAN GENETICS Test Cross: Monohybrid Cross Determining the genotype of the parent which exhibits the dominant trait SCENARIO 1: If the parent showing the dominant trait is homozygous (PP), then it can only contribute a dominant gamete P. The other parent being homozygous recessive can only contribute p. This means that the offspring of the cross between these 2 individuals can only be Pp, so all are phenotypically purple in terms of flower color. SCENARIO 2: If the parent showing the dominant trait is heterozygous (Pp), then it can contribute 2 types of gametes, P and p. Since the other parent can only contribute p, some of Purple = dominant (P) the offspring is expected to be Pp and some pp, meaning if White = recessive (p) there are offspring with white flowers, it can only happen if both parents contribute the recessive p.  MODULE 2 MENDELIAN GENETICS Test Cross: Dihybrid Cross It is important to remember that it is the dominant parent that has a genotype that is not known while the other parent should express the recessive trait and is homozygous recessive. Based on the figure, if the dominant parent is homozygous, all of the offspring is expected to have long wings and gray body (L-G). Since there are many other phenotypes observed in the offspring, the parent showing the dominant traits can only be heterozygous for each character.  MODULE 2 MENDELIAN GENETICS FORKED-LINE METHOD or BRANCH DIAGRAM In the formation of gametes, segregation can be seen between A and a, B and b and C and c. From each of the alleles of the first character, there are lines that connect to each of the alleles of the next character, and so on. On following the lines, we can identify the gametes (enclosed in the box at the rightmost side). Note that since there are 3 characters, each gamete also has 3 letters.  MODULE 2 MENDELIAN GENETICS FORKED-LINE METHOD or BRANCH DIAGRAM The number of gamete types formed by an individual is determined by the formula 2^n, where n represents the number of heterozygous characters. In this example, all 3 characters are heterozygous, 2^3 = 8 gametes result.  MODULE 2 MENDELIAN GENETICS GROUP ASSESSMENT: ½ crosswise 1. In a cross between an individual which is heterozygous for 2 characters and an individual who is homozygous recessive for both characters, what is the probability of each of the following offspring? a. homozygous recessive b. phenotypically dominant for both characters c. phenotypically recessive for both characters d. heterozygous for both characters Tip: Start by identifying letters to represent the traits involved. MODULE 2 MENDELIAN GENETICS In reality, we are not just made up of up to 2 pairs of genes. We have so many genes at work to make our entire body work. So, let us add more characters!  MODULE 2 MENDELIAN GENETICS Test Cross: Multihybrid Cross If more than 3 characters are involved, there is a shorter way of solving for phenotypic or genotypic ratio or probabilities. What to do: 1. Make separate crosses by character. 2. Compute probabilities from the result of each monohybrid cross. Separate crosses: AA x Aa | Bb x BB | Cc x cc | Dd x Dd | EE x EE Example: AABbCcDdEE x AaBBccDdEE  MODULE 2 MENDELIAN GENETICS Test Cross: Multihybrid Cross EXAMPLE: How much is the probability of having an offspring with the (a) genotype AaBbCcDdEE? Solution: plug in values from the results of the crosses P (AaBbCcDdEE) = (1/2)(1/2)(1/2)(2/4 or ½)(1) = 1/16 or 0.0625 or 6.25% (b) phenotype ABCdE? Solution: determine the phenotypes resulting from the genotypes and plug in their values P (ABCdE) = (1)(1)(1/2)(1/4)(1) Instead of fractions, decimal values can also be used like 0.5 instead of ½, 0.75 instead = 1/8 or 0.125 or 12.5% of ¾, and so on.  MODULE 2 MENDELIAN GENETICS ASSESSMENT: 2. Using the same cross given earlier, how much is the probability of having an offspring with the: a. genotype AABbCCddEE? b. genotype AaBBccDdEe? c. phenotype abcde? d. phenotype ABCDE? e. phenotype AbCdE?  MODULE 2 NON-MENDELIAN GENETICS 2. DOMINANCE RELATIONSHIPS Complete Dominance Whether homozygous or heterozygous, the dominant allele is always expressed while the recessive trait is only expressed when the allele is homozygous recessive.  MODULE 2 NON-MENDELIAN GENETICS 2. DOMINANCE RELATIONSHIPS Complete Dominance In humans, it is easier to look at inheritance if it involves a single gene. In autosomal inheritance, the gene responsible for the phenotype is located on one of the 22 pairs of autosomes (non-sex determining chromosomes). Dominant: conditions that manifest in heterozygotes (individuals with just one copy of the mutant allele) as well as in homozygous dominants. Recessive: conditions only manifest in individuals who have two copies of the mutant allele (are homozygous).  MODULE 2 NON-MENDELIAN GENETICS 2. DOMINANCE RELATIONSHIPS Autosomal Dominant Inheritance One copy of the gene is enough for the individual to be affected. Both males and females are affected with equal frequency. Examples: ✓ Huntington’s disease ✓ achondroplasia (short-limbed dwarfism) ✓ polycystic kidney disease ✓ polydactyly  MODULE 2 NON-MENDELIAN GENETICS 2. DOMINANCE RELATIONSHIPS Autosomal Recessive Inheritance It takes 2 copies of the gene for the trait to be expressed. Both males and females are affected with equal frequency. Examples: ✓ Cystic fibrosis ✓ Tay-Sachs ✓ Hemochromatosis ✓ phenylketonuria (PKU) ✓ deafness/hearing loss  MODULE 2 NON-MENDELIAN GENETICS INCOMPLETE OR PARTIAL DOMINANCE There is a dominant allele and a recessive allele but the dominant one cannot completely mask the effect of the recessive allele such that heterozygotes come out with a blended phenotype. Both parental phenotypes are not seen. Red is dominant over white. Homozygous Instead, there is a new phenotype red-flowered parent crossed with a white- that results from the mixing of the flowered parent (also homozygous) will parental genes produce pink-flowered offspring.  MODULE 2 NON-MENDELIAN GENETICS INCOMPLETE OR PARTIAL DOMINANCE If we use R for red and r for white, then the cross can be presented like this:  MODULE 2 NON-MENDELIAN GENETICS CODOMINANCE both alleles involved are equally dominant the phenotypes of both parent flower plants can be seen simultaneously expressed in the same offspring Codominance in plants  MODULE 2 NON-MENDELIAN GENETICS CODOMINANCE https://byjus.com/rajasthan-board/rbse-solutions-class-12-biology-chapter-35/ https://igcse-biology-2017.blogspot.com/2017/06/321b-understand-meaning-of-term.html https://www.horseproperties.net/blog/things-you-did-not-know-about-the-appaloosa/  MODULE 2 NON-MENDELIAN GENETICS CODOMINANCE In humans, examples of codominance are related to the blood. One is the MN blood group The MN blood group system is under the control of an autosomal locus found on chromosome 4, with two alleles designated LM and LN. Both M and N alleles are dominant. The heterozygote arrangement with M coming first is only based on alphabetical order and has nothing to do with dominance.  MODULE 2 NON-MENDELIAN GENETICS CODOMINANCE Second blood-related example: inheritance of sickle cell gene. While normal hemoglobin is designated as HbA allele, the hemoglobin in sickle celled individuals is designated as HbB allele. Codominance can be seen in terms of hemoglobin in heterozygotes (NS) where both forms of hemoglobin (HbA and HbB) can be found. The codominance here is at the molecular level. Although if you look at the picture, it sure looks like codominance in terms of cell shape.  MODULE 2 NON-MENDELIAN GENETICS OVERDOMINANCE In this type of dominance, the heterozygote has a more extreme phenotype than that of either of his parents. It is also called heterozygote advantage because the heterozygous individuals have a higher fitness than homozygous individuals. In the case of the prev. example, homozygotes (recessive) develop sickle cell anemia, wherein the red blood cells are sickle-shaped, which makes them less efficient at transporting oxygen plus it increases the possibility of these RBCs blocking blood vessels. Sickle cell heterozygotes are usually phenotypically normal and are generally healthy, but are more likely to be oxygen-deprived at high altitudes or during extremely vigorous exercise.  MODULE 2 NON-MENDELIAN GENETICS OVERDOMINANCE Heterozygote advantage can also be apparent in many autosomal recessive genetic diseases/conditions: a. Heterozygotes for cystic fibrosis have resistance against cholera and typhoid fever. b. Heterozygotes for thalassemia are protected against malaria c. Heterozygotes for deafness have thicker epidermis.  MODULE 2 MULTIPLE ALLELISM 3. MULTIPLE ALLELES Multiple allelism occurs when there is a series of 3 or more alternative or allelic forms of a gene, only two of which can exist in any normal, diploid individual. Only a small percentage (1 - 5%) of loci has multiple alleles depending upon the species. But take note that each individual can only have 2 of these alleles, but in numerous alternative forms. One example is the locus for the ABO blood https://rsscience.com/red-blood-cells/ group which has 3 alleles  MODULE 2 MULTIPLE ALLELISM 3. MULTIPLE ALLELES The ABO blood group involves 3 alleles which, in various pair combinations can result to 4 blood types (phenotypes). I = isoagglutinogen (antigen); the alleles A and B are used as superscripts. each phenotype is identifiable by the type of antigen on the surface of red blood cells. ✓ type A = antigen A on the surface of their RBCs ✓ type B = antigen B on the surface of their RBCs ✓ type AB = both antigens ✓ type O = neither A nor B Both type A and B can be homozygous or heterozygous; type O is always homozygous recessive  MODULE 2 MULTIPLE ALLELISM 3. MULTIPLE ALLELES To simplify, we can use just the superscripts to designate the genotypes: a) AA or AO for type A (where AA is homozygous & AO is heterozygous); b) BB or BO for type B (where BB is homozygous & BO is heterozygous); c) AB for type AB; and d) OO for type O. Just be sure to remember that the O allele is recessive while A and B are dominant.  MODULE 2 MULTIPLE ALLELISM 3. MULTIPLE ALLELES Sample Problem 1: Could a child with blood type O be produced from parents with blood types A and B? Steps in solving the problem: a. Identify the letters that should be used to represent each parent. b. Explore all possible combinations involved in the crosses: crossing homozygous with homozygous, homozygous with heterozygous or heterozygous with homozygous and heterozygous and heterozygous.  MODULE 2 MULTIPLE ALLELISM 3. MULTIPLE ALLELES Sample Problem 1: Could a child with blood type O be produced from parents with blood types A and B? Steps in solving the problem: a. Identify the letters that should be used to represent each parent. b. Explore all possible combinations involved in the crosses: crossing homozygous with homozygous, homozygous with heterozygous or heterozygous with homozygous and heterozygous and heterozygous. Solution: Genotypes of parents: AA or AO X BB or BO Make separate crosses following all possible combinations provided in step b above. Answer is YES (only in one of the crosses). Which cross is it?  MODULE 2 LETHAL ALLELES 4. LETHAL ALLELES Lethal genes are alleles that cause an organism to die only when present in homozygous condition, where the gene involved must have been an essential gene. When the allele is fully dominant, the homozygous dominant and heterozygous individuals are killed. Instead of mutations altering the phenotypes of the organisms, a mutant allele could cause death. Completely lethal genes: cause death of the zygote, or later in the embryonic development or even after birth or hatching (individuals cannot attain the age of reproduction). However, in many cases lethal genes become operative at the time the individuals become sexually mature. Subvital, sublethal or semi-lethal genes: lethal genes which handicap but do not destroy their possessor.  MODULE 2 LETHAL ALLELES 4. LETHAL ALLELES: RECESSIVE LETHAL GENES If the mutation is caused by a recessive lethal allele, the homozygote for the allele will have the lethal phenotype. The good thing is that most lethal genes are recessive. In humans: Tay-Sachs disease, sickle cell anemia & cystic fibrosis  MODULE 2 LETHAL ALLELES 4. LETHAL ALLELES: RECESSIVE LETHAL GENES - produced by an allele that is lethal in its homozygous state. - The allele interferes with normal spinal development and in heterozygous cats this results in a short tail or a lack of a tail. - a cross involving 2 Manx cats can produce a homozygous recessive cat which dies before birth. Tailless Manx phenotype  MODULE 2 LETHAL ALLELES 4. LETHAL ALLELES: DOMINANT LETHAL GENES - If the mutation is caused by a dominant lethal allele, the heterozygote for the allele will show the lethal phenotype, the homozygote dominant dies early. - In humans, an autosomal dominant progressive disorder called Huntington’s disease (HD) whose symptoms can develop at any time, but they often first appear when people are in their 30s or 40s. - It affects the brain’s basal ganglia, which are associated with a variety of functions, including voluntary and involuntary motor control, procedural learning relating to routine behaviors or "habits," eye movements, and cognitive, emotional functions.  MODULE 2 LETHAL ALLELES 4. LETHAL ALLELES: CONDITIONAL LETHAL GENES Some lethal alleles exert their effects only under certain environmental conditions, thus the term conditional. They can be expressed due to specific circumstances, such as temperature. An example is the case of temperature-sensitive (ts) lethal genes in Drosophila melanogaster, which can cause death in a developing larva at 30ᵒC Larvae, however, can survive if grown at lower temperatures like 22ᵒC.  MODULE 2 LETHAL ALLELES 4. LETHAL ALLELES: SEMI-LETHAL/SUBLETHAL GENES These genes kill some individuals in a population, but not all of them. Environmental factors and other genes may help prevent the detrimental effects of semi-lethal genes. A good example in humans is hemophilia. Hemophiliacs = low levels of blood-clotting factors. https://www.invitra.com/en/hemophilia-and- pregnancy/  MODULE 2 MODIFIER GENES 5. MODIFIER GENES These genes do not mask the effects of another gene. Instead, they can modify the expression of a second gene. They have a subtle, secondary effect which alters the phenotypes produced by the primary genes. Those mice which possess at least 1 dose of the dominant B have black coat expected of the B gene. However, if only dd is present, the black is diluted and becomes gray. The lighter coat color produced from the bb genes is expressed fully in the presence of at least 1 dose of D gene while it becomes even lighter when only dd genes are present. Gene D (controls color intensity) can modify the expression of gene B (controls coat color).  MODULE 2 MODIFIER GENES 5. MODIFIER GENES When two genotypes produce the same phenotype due to different environments, then each one is called the phenocopy of the other, because they differ genotypically. For example: → In Drosophila melanogaster, the normal (natural or wild) body color is brown and a hereditary variant has yellow color, when the larvae of wild type Drosophila with brown body color are raised on food containing silver salts, they develop into yellow bodied flies. → Thus, these flies are the phenocopies of yellow mutant, but would give rise to wild type brown flies in normal environment.  MODULE 2 MODIFIER GENES 5. MODIFIER GENES When two genotypes produce the same phenotype due to different environments, then each one is called the phenocopy of the other, because they differ genotypically. For example: → In Drosophila melanogaster, the normal (natural or wild) body color is brown and a hereditary variant has yellow color, when the larvae of wild type Drosophila with brown body color are raised on food containing silver salts, they develop into yellow bodied flies. → Thus, these flies are the phenocopies of yellow mutant, but would give rise to wild type brown flies in normal environment.  MODULE 2 MODIFIER GENES 5. MODIFIER GENES In Siamese cats and Himalayan rabbits, the variation of fur color is due to environmental temperature. A similar condition can be seen in Himalayan rabbits. https://petsareyours.blogspot.com/2012/04/himalayan-rabbit.html  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Inter-allelic Genetic Interactions The independent (non-homologous) genes located on the same or on different chromosomes interact with one another for the expression of single phenotypic trait of an organism. The discovery of the inter-allelic genetic interactions has been made after Mendel and although they show the same letter representation for the generations involved, their phenotypes are interpreted differently.  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Non-Epistatic Inter-allelic Genetic Interaction EXAMPLE 1: Eye color in Drosophila melanogaster P (scarlet) (brown) Gametes F1 F2 Genotype Phenotype 9/16 A-B- 9/16 wild-type (brick red) 3/16 A-bb 3/16 scarlet 3/16 aaB- 3/16 brown Color of the eyes in D. melanogaster: (a) brick-red, 1/16 aabb 1/16 white (b) scarlet, (c) brown, and (d) white.  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Non-Epistatic Inter-allelic Genetic Interaction EXAMPLE 1: Eye color in Drosophila melanogaster Only gene A is functional: enzyme A → scarlet pigment Only gene B is functional: enzyme B → brown pigment Both genes A and B are functional: enzymes A and B → brick-red Both genes A and B are not functional: no pigment → white  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Non-Epistatic Inter-allelic Genetic Interaction To summarize, each gene pair affects the same character. There is complete dominance at both gene pairs. New phenotypes (novel phenotypes) result from interaction between dominants, and also from interaction between both homozygous recessives. Two genes interact to produce a comb size and shape, they simply modify a character and are thus also known as supplementary or modifying genes. Note also that the 2 genes involved control only 1 character, the shape of the comb. So even if the F2 phenotypic ratio is the same as the Mendelian ratio, they differ in the number of characters involved.  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Epistasis It occurs when one gene masks or modifies the effect of another gene pair (epistasis) and alters the phenotype. The expected phenotypes could be those that lead to masking (antagonistic effect) while there are those which can be complementary or additive. Recessive Epistasis is when the recessive allele of one gene masks the effects (phenotypic expression) of either allele of the second gene.  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Recessive Epistasis EXAMPLE 1: Fur color in Labrador Retrievers The coat color of is controlled by 2 pairs of genes: 1. Gene E controls melanin production (E = more melanin; e = less melanin); 2. Gene B controls melanin deposition (B = deposit melanin in fur; b = don't deposit in fur).  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Recessive Epistasis EXAMPLE 1: Fur color in Labrador Retrievers The black coat can be produced when there is at least 1 B and 1 E (B-E-) The brown needs at least 1 E but just diluted by the presence of homozygous recessive b (bbEE-). Yellow is produced by the genotype B-ee White or albino is produced by bbee (double recessives). F2 phenotypic ratio: 9 black:3 chocolate brown:4 yellow  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Recessive Epistasis EXAMPLE 1: Fur color in Labrador Retrievers The black coat can be produced when there is at least 1 B and 1 E (B-E-) The brown needs at least 1 E but just diluted by the presence of homozygous recessive b (bbEE-). Yellow is produced by the genotype B-ee White or albino is produced by bbee (double recessives).  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Dominant Epistasis Dominant Epistasis is caused by the dominant allele of one gene, masking the action of either allele of the other gene. EXAMPLE 1: Solid fruit color in summer squash  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Dominant Epistasis EXAMPLE 1: Solid fruit color in summer squash Individuals with at least 1 W are all white, those with at least 1 Y are yellow and the one and only double-recessive is green. Inhibition of production of green pigment by W also means no production of yellow pigment.  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Dominant Epistasis EXAMPLE 1: Solid fruit color in summer squash  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Dominant Epistasis EXAMPLE 1: Solid fruit color in summer squash  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Dominant Epistasis EXAMPLE 2: Color of flowers in foxglove The dominant W controls the flower color in foxglove by allowing pigment deposition only at the throat region. Petals are white. Colored petals are produced when the dominant W gene is absent or non-functional (meaning only recessive forms are present).  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Dominant Epistasis EXAMPLE 2: Color of flowers in Gene D codes for an enzyme that can convert foxglove the colorless precursor to dark red pigment Recessive d codes for an enzyme that can form a light red pigment from the colorless precursor. Whether dark or light red pigment, both will only be found in the throat if the dominant W gene is functional. In the absence of a functional gene W, pigment is deposited on the petals. There is complete dominance at both gene pairs, but one gene, when dominant, is epistatic to the other. In this case, gene W is epistatic to D and d.  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Dominant Suppression Dominant suppression is similar to dominant epistasis but occurs when a dominant allele of one gene completely suppresses the phenotypic expression of alleles of another gene. Example: Feather color in chicken In chickens, for example, feather color requires a dominant allele C. chickens that are homozygous for a recessive allele c have white feathers. the C allele can have its color-producing action suppressed by a dominant suppressor allele, I. the recessive allele i does not exert suppression.  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Complementary Epistasis Complementary epistasis, also called duplicate recessive epistasis, occurs when both gene loci have homozygous recessive alleles and both of them produce identical phenotypes This generates the F2 9:7 phenotypic ratio. This is also known as complementary gene action. CcPp Example: Flower color in sweet pea, Lathyrus odoratus  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Complementary Epistasis Example: Flower color in sweet pea, Lathyrus odoratus There is complete dominance at both gene pairs, but either recessive homozygote is epistatic to the effect of the other gene. That means that whether cc or pp is present, only white flowers are produced.  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Duplicate Dominant Epistasis Duplicate dominant epistasis occurs when the dominant alleles of both gene loci produce the same phenotype without cumulative effect, modifying the F2 phenotypic ratio into 15:1. The two pairs of factors which have identical effect are known as duplicate factors. So, the two genes duplicate one another’s activity such that at least one copy of a dominant allele at either locus will produce the dominant or wild-type phenotype.  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Duplicate Dominant Epistasis Example 1: Flower color in bean Duplicate dominant epistasis illustrating redundancy, thus, only 1 copy of either dominant alleles produce the pigment. Only double homozygous recessives show the mutant phenotype.  MODULE 2 GENE INTERACTIONS 6. GENE INTERACTIONS: Duplicate Dominant Epistasis Example 1: Flower color in bean MODULE 2 GENE INTERACTIONS How do we recognize whether what we are looking at involves gene interactions? Just look at F2 phenotypic ratio. MODULE 2 PLEIOTROPISM 7. PLEIOTROPISM Most genes have their multiple effects and are called pleiotropic genes. The phenomenon of multiple effect (multiple phenotypic expressions) of a single gene is called PLEIOTROPISM.  MODULE 2 PLEIOTROPISM EXAMPLES OF PLEIOTROPISM Example 1: The gene for phenylketonuria (PKU) produces various phenotypic traits in humans, collectively called syndrome. →The affected individuals secrete excessive quantity of amino acid phenylalanine in their urine, cerebrospinal fluid and blood. →short stature, mentally deficient, widely spaced incisors, pigmented patches on skin, excessive sweating, and non- pigmented hairs and eyes.  MODULE 2 PLEIOTROPISM EXAMPLES OF PLEIOTROPISM Example 2: Marfan syndrome: a human malady resulting from an autosomal dominant mutation in the gene encoding the connective tissue protein fibrillin. →Fibrillin is important to the structural integrity of the lens of the eye, to the lining of vessels such as the aorta, and to bones, among other tissues. →lens dislocation, increased risk of aortic aneurysm, and lengthened long bones in limbs. https://www.semanticscholar.org/paper/Marfan-syndrome:-report-of-two-cases- with-review-of-Randhawa-Mishra/3aa06153f83636079a62ea07abb40cc66361eed9  MODULE 2 PLEIOTROPISM EXAMPLES OF PLEIOTROPISM Example 3: Sickle cell anemia: mutation of a beta-globin gene that can cause various organ problems  MODULE 2 Effects of Environmental Factors 8. ENVIRONMENTAL INFLUENCE ON GENE EXPRESSION: EXAMPLES 1. Coat color distribution in Himalayan rabbits and Siamese cats in response to temperature. 2. Temperature-dependent sex determination in freshwater turtle Emys orbicularis embryos: at 28.5°C: a mixed brood of both males and females. at 30°C: all are females at 25°C: only males hatch 3. anthocyanin production in maize in response to temperature: bright red color under bright light and dark violet under dim light https://petsareyours.blogspot.com/2012/04/himala yan-rabbit.html  MODULE 2 Effects of Environmental Factors 8. ENVIRONMENTAL INFLUENCE ON GENE EXPRESSION: EXAMPLES 4. Some chemicals ingested or drugs 5. Deficiency in folic acid in pregnant taken by pregnant women (e.g. women causes birth abnormalities like thalidomide) caused birth deformities spina bifida in babies https://en.wikipedia.org/wiki/Thalidomide_scandal https://www.pinterest.com/pin/218072806931256896/  MODULE 2 Effects of Environmental Factors 8. ENVIRONMENTAL INFLUENCE ON GENE EXPRESSION: EXAMPLES 6. internal environment: Male pattern baldness is influenced by the hormones, testosterone and dihydrotestosterone, but only when levels of the two hormones are https://selfhacked.com/blog/male-pattern-baldness-what-can-you-do-about-it/ high (sex-influenced trait) 7. internal environment: development of mammary glands in women (sex-limited trait) https://www.shutterstock.com/image-illustration/blood-supply-breast-rendering-2239433441 MODULE 2 Penetrance & Expressivity 9. PENETRANCE AND EXPRESSIVITY PENETRANCE – the extent to which a particular gene or set of genes is expressed in the phenotypes of individuals carrying it, measured by the proportion of carriers showing the characteristic phenotype Two types of Penetrance: 1. Complete Penetrance https://pediaa.com/difference-between-penetrance-and-expressivity/ 2. Incomplete Penetrance  MODULE 2 Penetrance & Expressivity 9. PENETRANCE AND EXPRESSIVITY 1. COMPLETE PENETRANCE → most homozygous dominant and recessive genes and many completely dominant genes even in heterozygous conditions give their complete phenotypic expressions. Examples a. In pea: complete penetrance of the alleles RR for red flowers and rr for white flowers b. In guinea pigs, complete penetrance of the dominant allele B for black coat in homozygous and heterozygous condition.  MODULE 2 Penetrance & Expressivity 9. PENETRANCE AND EXPRESSIVITY 2. INCOMPLETE PENETRANCE → Some genes in homozygous as well as in heterozygous conditions fail to provide complete phenotypic expression of them. Examples in humans a. Polydactyly has about 70% penetrance. b. Retinoblastoma has 75% penetrance c. Tendency to develop diabetes mellitus is controlled by certain genes but not all carrying the genes can develop the condition. MODULE 2 Penetrance & Expressivity 9. PENETRANCE AND EXPRESSIVITY EXPRESSIVITY - the degree of effect produced by a penetrant genotype - determined by the proportion of individuals with a given genotype who also possess the associated phenotype https://pediaa.com/difference-between-penetrance-and-expressivity/ MODULE 2 Penetrance & Expressivity 9. PENETRANCE AND EXPRESSIVITY EXPRESSIVITY: Examples a. In man, the polydactylous condition may be penetrant in the left hand (6 fingers) and not in the right (5 fingers); or it may be penetrant in the feet and not in the hands b. In humans, retinoblastoma can affect both eyes while in some, only 1 eye is affected.  MODULE 2 Penetrance & Expressivity 9. PENETRANCE AND EXPRESSIVITY It is noticeable how some resemble the dominant parent while some are like the recessive parent. There are also some which are like All individuals below the arrow are heterozygotes. intermediates. https://www.mun.ca/biology/scarr/Penetrance_vs_Expressivity.html MODULE 2 Penetrance & Expressivity 9. PENETRANCE AND EXPRESSIVITY Expressivity vs. Penetrance - YouTube  MODULE 2 Penetrance & Expressivity 9. PENETRANCE AND EXPRESSIVITY REMEMBER: Both penetrance and expressivity are affected by the environment. The expression of a gene can be influenced by the diet and lifestyle. Expressivity of a completely penetrant gene can be influenced by temperature (like the gene for vestigial wings in Drosophila), with lower temperature producing most effect. Severity of symptoms of an inheritable allergy or the difference in height of identical twins raised in different homes (vary in diet). MODULE 2 Twinning: Concordance & Discordance 10. TWINNING Identical twins are derived from 1 zygote that splits during development. Fraternal twins come from 2 different eggs fertilized separately at the same time. All female dizygotic twins are also referred to as sororal twins. https://www.freepik.com/premium-vector/difference-identical-fraternal-twins-monozygotic-dizygotic-twins_27647867.htm  MODULE 2 Twinning: Concordance & Discordance 10. TWINNING Do monozygotic twins have the same genomes? Differences can actually start after they split from a single zygote or embryo. The more differences can be expected the earlier the splitting occurred. These differences are mostly due to mutations. Only a small percentage of monozygotic twins have identical genomes.  MODULE 2 Twinning: Concordance & Discordance 10. TWINNING Do dizygotic twins have the same genomes? Dizygotic (DZ) twins share 50% genes (just like regular siblings), and if they are raised together, also have the same environment during their early years. But since they are expected to have different sets of friends and careers, environment can then differ greatly.  MODULE 2 Twinning: Concordance & Discordance 10. TWINNING Comparison of monozygotic and dizygotic twins in terms of genes and environment. Little variation → BIG IMPACT! MODULE 2 Twinning: Concordance & Discordance 10. TWINNING: CONCORDANCE & DISCORDANCE Concordance: the Discordance: the degree of probability that a pair of dissimilarity in a pair of twins individuals will both have a with respect to the presence certain characteristic, given or absence of a disease or that one of the pair has the trait. characteristic MODULE 2 Twinning: Concordance & Discordance 10. TWINNING: CONCORDANCE Monozygotic (MZ) twins have more similar genes; dizygotic (DZ) twins share 50% of their genes. Concordance: if both twins carry the trait Discordance: if only 1 twin carries the trait Higher concordance values can be seen in monozygotic twins compared to dizygotic twins. MODULE 2 Twinning: Concordance & Discordance 10. TWINNING MODULE 2 Twinning: Concordance & Discordance 10. TWINNING: DISCORDANCE In Figure A, the bigger between the two has Beckwith-Wiedemann syndrome which is classified as an overgrowth syndrome. - larger than normal (macrosomia); taller than their peers during childhood. They are at an increased risk of developing - usually becomes less apparent cancerous and noncancerous tumors over time especially in the kidneys and liver MODULE 2 Twinning: Concordance & Discordance 10. TWINNING: DISCORDANCE In Figure B, the smaller of the twins has Russell-Silver syndrome - slow growth before and after birth - low birth weight - asymmetric growth of some body parts - affected children are thin and have digestive abnormalities; some Affected children have an increased risk of delayed development, speech and language develop episodes of hypoglycemia problems which affect learning ability. - adults are short MODULE 2 Twinning: Concordance & Discordance 10. TWINNING: DISCORDANCE Discordance in metabolism in monozygotic twins. The one on the left is normal while the one on the right has metabolic problems and has become obese through the years. MODULE 2 Twinning: Concordance & Discordance TWIN STUDIES What identical twins separated at birth teach us about genetics - BBC REEL - YouTube