Exception To Mendelism
- 1. A Seminar on Exception to Mendelism
- 2. The search for hereditary characters dates back nearly a century; 19th century. 1866 Mendel: Introduces concept of heredity 1900 Bateson: Introduced the term Genetics1906 Wilhelm Johannsen: Coins the term Gene1909 DeVries, Correns and Tschermak rediscovered Mendel's work
- 3. Mendel's Postulates Revisited Inheritance of biological characters were determined by Genes. Genes come in pairs (one from each parent). During sexual reproduction, genes are passed from parents to offspring. Genes come in different versions, now called alleles. When an organism has two different alleles of a gene, one (the dominant allele) will hide the presence of the other (the recessive allele) and determine appearance.
- 4. During gamete production, each egg or sperm cell receives just one of the two gene copies present in the organism, and the copy allocated to each gamete is random (law of segregation). Genes for different traits are inherited independently of one another (law of independent assortment). Cont…
- 5. Mendel’s Laws Not Perfect Shortly people began to notice that not all traits are “Mendelian” This means, they do NOT follow Mendel’s laws
- 6. Altering Mendel’s Ratios Two different types of complications: 1. Genotypic ratios follow Mendel’s laws, but phenotypes do not Somehow the underlying genotypic ratios are hidden 2. Mendel’s laws do not apply Both genotypes and phenotypes are not following Mendel’s laws
- 8. In incomplete dominance, neither allele is dominant so there is a blending of traits when two different alleles for the same trait occur together. Colors blend together Heterozygous individuals = 3rd phenotype Phenotypic and Genotypic ratio: 1:2:1 red whitepink Incomplete Dominance or Blending Inheritance A cross between red and white flowered plants produced plants with intermediate flower colour i.e. pink colour in F1 and a modified ratio of 1 red: 2 pink: 1 White in F2.
- 10. Co-dominance Both alleles contribute to the phenotype of the organism by showing up simultaneously (at the same time) in heterozygous individuals. Neither alleles are dominant. Heterozygotes expresses both alleles equally. Phenotypic and Genotypic ratio are equal; 1:2:1 In cattle and horses, if a pure red (RR) is crossed with a pure white(WW), roan (RW) is obtained. These cattle or horses actually have both red and white hairs intermixed, or are spotted. Roan is a third phenotype.
- 11. (WW) (RR) (RW)
- 12. Polygenic Inheritance Some traits are determined by the combined effect of two or more pairs of genes. These traits are called polygenic traits. These traits are not controlled by a single gene locus, but by the combined interaction of many gene loci. Characters show a range of continuous phenotypes instead of discrete, defined phenotypes In this case many genes have an additive effect. The characteristic or trait is the result of the combined effect of several genes. Because so many alleles contribute to the final phenotype, a variety of phenotypes can occur.
- 13. Ex: human skin color, eye color, hair color and height. Note: Traits all result from the interaction of the genes with environmental factors Fig: Pigmentation in humans is controlled by at least three (3) separately inherited genes
- 17. Human Eye Color
- 18. 0 1 2 3 4 5 6 Black Dark Brown Light Brown Blue Light Blue ProportionofVariants Skin Pigmentation × AaBb AaBb 1 16 4 16 6 16 4 16 1 16 Fig: Histogram representation of skin pigmentation
- 19. Multiple Alleles Multiple alleles are gene that are members of the same gene pair and are located on the same locus. All of them control same character but each of the allele effects that character somewhat differently than the others. Traits are determined by the combined effect of two or more alleles. Blood type in humans is an example of this inheritance pattern. The four different blood groups: A, B, O, and AB Produced by three different alleles: A, B, and O
- 20. Bloodtype Alleles Allele IA is dominant Allele IB is dominant Allele IO is recessive Genotypes Phenotypes (blood types) IAIA A IAIO A IBIB B IBIO B IAIB AB IOIO O Note: The phenotypes varies in every different crosses
- 21. Examples of Blood type crosses
- 22. Other examples Coat color in rabbits Four phenotypes and four alleles Allelic series is C > cch > ch > c (which is most dominant) Phenotype Genotype Full Color CC, Ccch, Cch, Cc Chinchilla cchcch, cchch, cchc Himalayan chch, chc Albino cc
- 23. Lethality Gene, which causes the death of its carrier when in homozygous condition is called lethal gene. The fully dominant lethal alleles kill the carrier individuals. Mendel’s findings were based on equal survival of all genotypes. In normal segregation ratio of 3:1 is modified into 2:1 ratio. E.g: In mice, yellow coat colour is dominant to grey. Mice that have YY (pure yellow) coat colour do not survive
- 24. 2:1 ratio from cross between two yellow mice results from a lethal allele.
- 25. Sex-linked Inheritance (x linked)
- 26. In the nucleus of every body cell there are 46 chromosomes 22 homologous pair and one pair of sex chromosomes Normal Cells
- 27. Some traits are found on the sex chromosomes instead of autosomes and are more likely to show up in one gender. So termed as x linked traits These traits generally affect males more than females. EX: colorblindness & hemophilia x linked
- 28. Haemophiliacs cannot make the blood clotting protein Factor VIII. It caused by a recessive allele carried on the X but not the Y chromosome Hence is sex-linked Haemophilia
- 30. Red Green Colour Blindness Inability to distinguish between red and green. A colour blind person cannot distinguish the number clearly. In humans normal vision is completely dominant to colour blindness. Heterozygous females are the carriers.
- 31. Genetics of Colour Blindness
- 32. Are you a colour blind?
- 33. References Genetics – Veer Bala Rastogi Concepts of Genetics Pearson– Klug, Cummings, et al. Introduction to genetic analysis W. H. Freeman and Company – Griffiths, Wessler,.. Genetics - A Conceptual Approach, Benjamin A. Pierce.
- 34. Thank You