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CHAPTER 4 GENETIC INHERITANCE 4.2 DEVIATIONS FROM THE MENDELIAN INHERITANCE

CHAPTER 4 GENETIC INHERITANCE 4.2 DEVIATIONS FROM THE MENDELIAN INHERITANCE. Epistasis. DEVIATIONS FROM THE MENDELIAN INHERITANCE. What is Epistasis ?. Epistasis come from the Greek – “ epi ” means “upon” “ histani ” means “to place” So it means to place upon or to stand upon.

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CHAPTER 4 GENETIC INHERITANCE 4.2 DEVIATIONS FROM THE MENDELIAN INHERITANCE

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  1. CHAPTER 4 GENETIC INHERITANCE 4.2 DEVIATIONS FROM THE MENDELIAN INHERITANCE

  2. Epistasis • DEVIATIONS FROM THEMENDELIAN INHERITANCE

  3. What is Epistasis? • Epistasis come from the Greek – • “epi” means “upon” • “histani” means “to place” • So it means to place upon or to stand upon

  4. Definition “The situation in which the alleles at one gene cover up or alter the expression of alleles at another gene” (Genetics, Weaver and Hedrick, Wm. C. Brown Publishers,1989)

  5. Or… • Epistasisis a form of gene interaction in which one gene masks the phenotypic expression of another • There are no new phenotypes produced by this type of gene interaction

  6. Epistatic versus Hypostatic • The alleles that are masking the effect are called epistatic alleles • The alleles whose effect is being masked are called the hypostatic alleles

  7. How is epistasis different from dominance? Dominance is when an allele suppresses the expression of another allele at the same locus “A” and “a” are alleles to each other. That means they are alternate forms of the gene at the “A” locus. If “A” suppresses the expression of “a” then “A” is dominant to “a.” Pair of Chromosomes “A” Locus A a

  8. Epistasis involves two gene pairs Epistasis is when an allele at one locus masks (covers up) or alters the expression of an allele at a different locus Pair of Chromosomes If the “A” allele at the A locus alters or masks the expression of the “B” allele at the B locus, then A is epistatic to B. (The B locus could be on the same chromosome as the A locus, or it could be on a different chromosome.) “A” Locus A a “B” Locus B B

  9. Recessive or Dominant? • Epistasis can be described as either recessive epistasisor dominant epistasis • Let’s look at an example of recessive epistasis….

  10. Labrador Retrievers • Fur color in Labrador Retrievers is controlled by two separate genes • Fur color is a polygenic trait! Gene 1 : Represented by B : Controls color Gene 2 : Represented by E : Controls expression of B

  11. Labrador Retrievers • If a Labrador retriever has a dominant B allele, they will have black fur • If they have two recessive alleles (bb) they will have brown fur

  12. Labrador Retrievers • If a retriever receives at least one dominant “E” allele, they will remain the color that the “B” allele coded for • Either black of brown • However, if a dog receives a pair of homozygous recessive “e” alleles, they will be golden regardless of their “B” alleles!

  13. Labrador Retrievers • BBEE and BbEe --> Black retrievers • bbEE and bbEe --> Brown retrievers • BBee, Bbee, or bbee --> Golden retrievers

  14. Try this cross… • You have decided to cross your golden retriever (bbee) with the neighbor’s chocolate retriever (bbEe). What color pups will they have?

  15. bbee x bbEe • Gamete/FOIL : be • Gamete/FOIL : bE or be • Genotypes of F1 generation: bbEe and bbee • Pups phenotypes:Brown and golden

  16. Dominant Epistasis • Let’s have a look at dominant epistasis… • Squash fruit color is controlled by two genes • Gene 1 is represented by a W • Gene 2 is represented by a G

  17. Squash Fruit Color • Genotypes and Phenotypes: • W-G- white • W-gg white • wwG- green • wwgg yellow

  18. Squash Fruit Color • Which allele is epistatic in squash color? • How do you know? The dominant W allele is epistasis Because every time a dominant W allele shows up in a squash genotype, the squash fruit color is white

  19. Try this cross…. • Cross a green squash (wwGg) with a white squash (Wwgg). • What color are the offspring?

  20. Wwgg x wwGg • FOIL: Wg or wg • FOIL: wG or wg • F1 generation genotypes: • Phenotypes:

  21. DEVIATION FROM THE MENDELIAN INHERITANCE • 4.2.1 Codominant alleles

  22. Codominant Alleles • The situation in which the both alleles of a pair are fully expressed in the phenotype of the heterozygous (F1) • The heterozygote offspring has the characteristics of both the homozygous parents • Phenotypic ratio 1:2:1 instead 3:1

  23. Eg : human MN blood groups system in humans are due to the presence of two specific molecules on the surface of red blood cells

  24. P : Blood group M Blood group N MM X NNG : M NF1 : MN X MNG : M N M N F2 : MM MN MN NN Genotypic ratio: 1 MM : 2 MN : 1 NN Phenotypic ratio: 1 blood group M : 2 blood group MN : 1 blood group N

  25. DEVIATION FROM THE MENDELIAN INHERITANCE • 4.2.2 Incomplete dominant alleles

  26. Incomplete Dominant Alleles • The situation in which the phenotype ofheterozygotes (F1) isintermediate between the phenotypes of individuals homozygous for either allele (dominant or recessive) (P generation) • The offspring shows partial expression of both alleles • Phenotypic ratio 1:2:1 instead of 3:1

  27. Eg :Color of snapdragon flower (Antirrhinum sp.) • A cross between individuals that are homozygous with red flowers (RR) with individuals that are homozygous with white flowers (WW) • will produced, Heterozygote (offspring) with pink flowers which intermediate between red and white • The phenotypic ratio becomes 1 red:2 pink: 1 whiteinstead of 3:1

  28. Color of Snapdragon flower Antirrhinum sp. Parent : Red x White RR WW Gamete : R W F1 genotype: RW F1 X F1 : RW x RW Gamete: R W R W F2 generation : RR RW WW Phenotypic ratio: 1 red : 2 pink : 1 white

  29. DEVIATION FROM THE MENDELIAN INHERITANCE • 4.2.3 Multiple alleles

  30. Multiple Alleles • A condition when more than two alleles occupying the same gene locus on a pair of homologous chromosomes • However, only 2 alleles can be present in a single organism • Eg : ABO blood group in humans • There are 4 blood types : A, B, AB and O • The ABO locus has three common alleles : IA , IB , IO

  31. Phenotype (Bloodgroup) Genotype A IAIA , IAIO B IBIB , IBIO AB IAIB O IOIO / ii ABO Blood Group • i /IO is a recessive allele • IA & IB are both dominant (codominant) alelles

  32. Multiple alleles control the ABO blood groups.

  33. DEVIATION FROM THE MENDELIAN INHERITANCE • 4.2.4 Polygenes/ • polygenic inheritance

  34. Polygenes / polygenic inheritance • An additive effect of two or more gene loci on a single phenotypic character (a character is controlled by the cumulative effect of more than one gene) • Eg: heightin human and human skin color • These are called quantitative characters • For example, height in human; skin pigmentation in humans is controlled by at least three (probably more)genes

  35. Height in human and other animal show a continuous variation • Traits show continuous variation are not determined by a single gene, but a large number of genes at different loci so this is called polygenic inheritance • The overall expression of an polygenic traits depends upon the sum of the influences of all the genes involved

  36. Height in Humans • Range of phenotypes resulting from polygenic trait

  37. Let’s consider three genes controlling skin colour: - A, B, C = dark-skin - a ,b, c = fair skin • An AABBCC person would be verydark, while an aabbcc individual would be verylight • An AaBbCcperson would have skin of an intermediateshade • Because the alleles have a cumulative effect, the genotypes AaBbCc and AABbcc would make the same genetic contribution (3 dominant alleles & 3 recessive alleles) to skin darkness

  38. This polygenic inheritance resulted a bell-shaped curve, called a normal distribution, with a mean value and extremes in either direction • Environmentalfactors, such as exposure to the sun, also affect the skin-color phenotype

  39. 0 darkness Phenotype Genotype - Very fair pphh X Fair ppHh ; Pphh XX Quite dark PPhh; ppHH; PpHh XXX Dark PpHH; PPHh XXXX Very dark PPHH Polygenic InheritanceIndividuals based on degrees of skin darkness

  40. Differences between multiple alleles and multiple genes Multiple alleles Polygenetic traits while polygenetic traits are controlled by multiple genes. A polygenic trait refers to any inheritable trait that is controlled by multiple genes, and each of these genes can have multiple alleles. For example, eye color in humans is a polygenic trait. There are at least three different genes, each with multiple alleles, that determine eye color in humans. Polygenic traits don't follow patterns of mendelian inheritance. • Multiple alleles are the presence of more than two alleles for a trait within a gene pool, • Alleles refer to different versions of the same gene. So a single gene can have multiple alleles. • For example in fruit flies there is a single gene that controls eye color, and the eye color of the fly depends on the alleles they have for that gene (since they have two copies of every gene, being diploid).

  41. DEVIATION FROM THE MENDELIAN INHERITANCE • 4.2.5 Lethal genes

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