7 5 polygenic inheritance

							7.5 Polygenic Inheritance
  By the end of this lesson you
       should be able to…
• Define discontinuous variation.
• Define continuous variation.
• Explain the difference between single-gene
  and polygenic inheritance.
• Describe the relationship between the
  number of genes controlling a characteristic
  and the number of phenotypes.
• Use a Punnett square to work out possible
  genotypes of offspring for polygenic
  inheritance.
• Give examples of polygenic inheritance in
  plants and animals.
Types of Variation
• Characteristics which fall into distinct
  categories are said to show
  discontinuous variation.



• Single-gene inheritance is where
  characteristics are controlled by the
  alleles of only one gene.
                     14                                                   14
                     12                                                   12
Number of students




                     10




                                                     Number of students
                                                                          10
                      8                                                    8
                      6                                                    6
                      4                                                    4
                      2                                                    2

                          Roller        Non-roller                             Right    Left

                          Tongue-rolling ability                                 Handedness
• Characteristics which show a range of
  differences from one extreme to
  another are said to show continuous
  variation.


• Polygenic inheritance occurs when
  characteristics are controlled by the
  alleles of more than one gene.
                     14

                     12
Number of students



                     10

                     8

                     6

                     4

                     2


                          149 - 155 156 - 160 161 - 165 166 - 170 171 - 175

                                    Height (cm)
Example of Polygenic
   Inheritance
        Kernel colour in maize:
     R1= red, R2= red, r1= white, r2= white

P     Phenotype    Very dark red             White


      Genotype      R1R1R2R2         X       r1r1r2r2


      Gametes        All R1R2                All r1r2

F1    Genotype                    R1r1R2r2

      Phenotype                 Medium red
F1   Genotype         R1r1R2r2                   R1r1R2r2


                                       X
     Phenotype      Medium red                Medium red




     Gametes     R1R2 R1r2 r1R2 r1r2       R1R2 R1r2 r1R2 r1r2
                        R 1R 2   R 1R 2

                 R1r2      R1R1R2R2       R 1 r2

                   R1R1R2r2      R1R1R2r2          r1R2
          r1R2

   r1r2      R1r1R2R2                                     r1r2

                                               R1r1R2r2

                           r1r1R2R2




Genotypes?
                                       Phenotypes?
                       14

                       12
Number of phenotpyes




                       10

                       8

                       6

                       4

                       2


                            Dark red   Darkish   Medium   Very        White
                                       red       red      Light red

                                       Colour of maize
What effect does having a greater
 number of alleles have on the
 number of phenotypes for a
 characteristic?
Comparing Single-gene and
  Polygenic Inheritance
Copy and complete the table:

               Type of Inheritance
             Single-gene   Polygenic
Number of
genes
Number of
phenotypes
Type of
variation
Examples of Inheritance

  Single-gene   Polygenic Inheritance
  Inheritance
      Are you now able to…?
• Define discontinuous variation.
• Define continuous variation.
• Explain the difference between single-gene
  and polygenic inheritance.
• Describe the relationship between the
  number of genes controlling a characteristic
  and the number of phenotypes.
• Use a Punnett square to work out possible
  genotypes of offspring for polygenic
  inheritance.
• Give examples of polygenic inheritance in
  plants and animals.