Hour 4


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Hour 4

  1. 1. LECTURE 4 <ul><li>2.2 Crosses that deviate from the </li></ul><ul><li>Mendelian inheritance. </li></ul><ul><li>2.2.1 Codominant allele </li></ul><ul><li>2.2.2 Incomplete dominant allele </li></ul><ul><li>2.2.3 Multiple alleles </li></ul><ul><li> 2.2.4 Polygenes/polygenic inheritance </li></ul><ul><li> </li></ul>
  2. 2. OBJECTIVES <ul><li>At the end of the lesson, students should be able to : </li></ul><ul><li>Explain codominant alleles. </li></ul><ul><li>Calculate the genotypic and phenotypic ratio (1:2:1). </li></ul><ul><li>Explain incomplete dominant alleles. </li></ul><ul><li>Calculate the genotypic and phenotypic ratio (1:2:1). </li></ul><ul><li>Explain multiple alleles </li></ul><ul><li>Explain polygenes/polygenic inheritance </li></ul>
  3. 3. Codominant allele <ul><li>both alleles of a pair are fully expressed in a heterozygous form </li></ul><ul><li>Eg : The existence of three different human blood groups called the M,N and MN blood groups. </li></ul><ul><li>Another example is ABO human blood group. </li></ul><ul><li>Genotypic ratio 1:2:1 </li></ul>
  4. 4. ABO Blood Group I O I O / ii O I A I B AB I B I B , I B I O B I A I A , I A I O A Genotype Phenotype (Blood group )
  5. 5. e.g: Cross between L M L M and L N L N <ul><li>P : L M L M X L N L N </li></ul><ul><li>G : L M L M L N L N </li></ul><ul><li>F1 : L M L N </li></ul>
  6. 6. <ul><li>F1 X F1 </li></ul><ul><li>P : L M L N X L M L N </li></ul><ul><li>G : L M L N L M L N </li></ul><ul><li>F2 : L M L M L M L N L M L N L N L N </li></ul><ul><li>Phenotypic ratio:- 1 producing M antigen: 2 producing both antigens </li></ul><ul><li>: 1 producing N antigen </li></ul><ul><li>Genotypic ratio:- 1 L M L M :2 L M L N :1L N L N </li></ul>
  7. 7. <ul><li>Different from Mendel’s law because the phenotypic ratio is 1:2:1 in F 2 generation instead of 3:1. </li></ul>
  8. 8. Incomplete Dominant Allele <ul><li>One allele is not fully dominant over its partner, so in the heterozygous condition, the total product is intermediate between that of the dominant and recessive alleles. </li></ul><ul><li>Eg :color of snapdragon flower Antirrhinum . </li></ul><ul><li>Heterozygotes for color alleles have pink colors in contrast to red ( dominant homozygotes) and white (recessive heterozygotes). </li></ul><ul><li>The phenotypic ratio for the monohybrid cross then becomes 1:2:1 instead of 3:1. (for F2) </li></ul>
  9. 10. Incomplete dominance in snapdragons
  10. 11. Summary of dominance relationships
  11. 12. Multiple Alleles <ul><li>When more than two different forms of alleles, they are referred to as multiple alleles . </li></ul><ul><li>Eg : ABO blood type alleles in humans. </li></ul><ul><li>There are 4 blood types : A, B, AB and O. </li></ul><ul><li>The ABO locus has three common alleles : I A , I B , I O . </li></ul>
  12. 13. Multiple alleles control the ABO blood groups.
  13. 14. Poligenes <ul><li>In many characters such as human skin color and height, an either-or classification is impossible, because the characters vary in the population gradually. </li></ul><ul><li>These are called quantitative characters . </li></ul><ul><li>Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotypic character . </li></ul><ul><li>For example, skin pigmentation in humans is controlled by at least three (probably more) separately inherited genes. </li></ul>
  14. 15. <ul><li>Let us consider three genes, with a dark-skin allele for each gene (A,B,C) contributing one ‘unit’ of darkness to the phenotype and being incompletely dominant to the other alleles (a,b,c). </li></ul><ul><li>An AABBCC person would be very dark , while an aabbcc individual would be very light . </li></ul><ul><li>An AaBbCc person would have skin of an intermediate shade . </li></ul><ul><li>Because the alleles have a cumulative effect, the genotypes AaBbCc and AABbcc would make the same genetic contribution (three units) to skin darkness. </li></ul>
  15. 16. <ul><li>This polygenic inheritance could result in a bell-shaped curve, called a normal distribution , for skin darkness among the members of a hypothetical population. </li></ul><ul><li>Environmental factors , such as exposure to the sun, also affect the skin-color phenotype. </li></ul>
  16. 18. Individuals based on degrees of skin darkness . PPHH Very dark XXXX PPHh; PpHh Dark XXX PPhh; ppHH; PpHh Quite dark XX ppHh ; Pphh Fair X pphh Very fair - Genotype Phenotype 0 darkness
  17. 19. Differences between multiple alleles and polygenes <ul><li>- Phenotypes that controlled by polygenes can be influence by environmental factors while multiple alleles is not influence by environmental factors </li></ul><ul><li>For multiple alleles, phenotypes control involved genes that have multiple alleles (exist more than two allelic forms) while in polygenes the phenotype is controlled by more than one gene </li></ul>
  18. 20. <ul><li>Polygenic traits involved continuous variation/ quantitative variation while multiple alleles traits are expressed as discontinuous variation. </li></ul>