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  1. 1. MENDELIAN GENETICS Gregor Mendel Studied inheritance in peas
  2. 2. Mendel’s Garden He artificially crossed plants He became known as the “Father of Genetics”
  3. 3. Genetics : the study of the inheritance of traits 1. Traits are controlled by genes on chromosomes. 2. Genes exist in different forms called alleles. 3. There can be many alleles for a particular trait. 4. All the alleles an organism has is its genotype. 5. The physical expression of alleles is a phenotype. 6. Each parent provides one or more alleles for a trait.
  4. 4. Mendel’s Principle of Dominance: If an allele from one parent is expressed while the matching allele from the other parent is not expressed, that allele is said to be dominant and the allele not expressed is said to be recessive. Capital letters stand for dominant Lower case letters mean recessive In peas, tall ( T ) is dominant to short ( t )
  5. 5. Homozygous : allele from each parent is the same ( pure ) TT = tall tt = short Heterozygous : different allele from each parent ( hybrid ) Tt = tall ( tall dominates )
  6. 6. Mendel’s Law of Segregation : The alleles from each parent are separated during the formation of gametes ( sex cells ). The formation of sex cells ( sperm or egg ) is called meiosis. Meiosis is also known as reduction division During division of cells to form gametes, chromosomes inherited from each parent are separated ( segregated )
  7. 7. The number of chromosomes per cell has been reduced and the chromosomes, ( and therefore the alleles ), from each parent have been segregated.
  8. 8. For particular traits, the results of a cross between two parents can be predicted using the laws of probability. A monohybrid ( one factor ) cross: Female = Tt, so ½ of eggs = T and ½ of eggs = t Male = Tt, so ½ sperm = T and ½ sperm = t
  9. 9. Reginald Punnett developed the Punnett Square to predict outcomes of crosses For each possibility, determine the chance for occurring and then multiply the chances.
  10. 10. Cross: Tt x Tt Possible eggs: T or t Possible sperm: T or t Prediction : chances for offspring TT = ¼ Tt = ½ tt = ¼ From Punnett square: Genotypic ratio = 1:2:1 Phenotypic ratio = 3:1
  11. 11. Mendel’s Law of Independent Assortment Different traits are inherited independently of each other A dihybrid ( two-factor ) cross : Cross: TtBb x TtBb Possible eggs: TB,Tb,tB,tb Possible sperm : TB,Tb,tB,tb
  12. 12. Prediction from Punnett square: Phenotypic ratio = 9:3:3:1 Genotypic ration=1:2:1:2:4:2:1:2:1
  13. 13. Test Cross: A cross to determine if an individual is heterozygous or homozygous for a given trait. Example: NN and Nn look the same due to dominance crossing each with the recessive trait will show the difference: NN x nn = all Nn Nn x nn = ½ Nn and ½ nn The test determines which is pure and which is hybrid.
  14. 14. Non-Mendelian Inheritance 1. Incomplete dominance Neither allele is dominant and the hybrid is a blend
  15. 15. 2. Codominance Both alleles are expressed equally
  16. 16. 3. Multiple alleles More than two alleles for a trait exist in the population Human blood type is controlled by three alleles I (type A) A IB (type B) i (type O) I I and I i = type A A A A IBIB and IBi = type B IA IB = type AB ii = type O
  17. 17. 4. Polygenic Inheritance Several alleles control a trait at the same time.
  18. 18. Polygenic phenotypes have a continuous range of expressions and show a typical bell-shaped curve of values in a population