HL 1 Biology

10.3 Gene Linkage & Polygenic Inheritance


<<Gene Linkage>>

  • Exception to law of independent assortment
  • When F1 plants self-pollinated, four phenotypes did not follow 9:3:3:1 ratio
  • gene linkage: pattern of inheritance in which all the genes of a given chromosome are linked

<<Crossing over and recombination of Linked Genes>>

  • autosomes: chromosomes other than the X and Y chromosomes
  • diploid nuclei have 2 of each type of autosome (in humans there’s 22)
  • Autosomal gene linkage: linkage when genes are on the same autosome
  • Sex linkage: linkage when genes are located on the X chromosome
  • Crossing over: incomplete linkage between pairs of genes in a linkage group resulting new allele combinations
  • Recombination: formation of a chromosome or DNA with a new combination of alleles
  • Recombinant: different combination of characters from either of the original parents

<<Polygenic Inheritance and Continuous Variation>>

  • Polygenic inheritance: two or more genes affecting the same character
  • closer to the normal distribution = more than one gene is involved
  • variation in polygenic inheritance is continuous
  • differences in phenotype between the classes are subtle and effects of environment blue differences making it undetectable
  • e.g. human skin color environment (sunlight) and some genes

<<Inheritance of Skin Color>>

  • mutations of few genes can prevent melanin production (black pigment)  =  leads to albinism 
  • mutation SLC4A5 = causes paler skin


Gene Linkage in Zea Mays (p.159)

1) Cc = colored, starchy (dominant) Ss = white, waxy (recessive) The F1 genotype would be CcSs, thus when the F1 is multiplied by F1: 

Screen shot 2013-02-21 at 6.30.57 AM

So the CcSs x CcSs would produce offsprings in the dihybrid ratio of 9 colored, starchy: 3 colored, waxy: 3 white, starchy: 1 white, waxy

2) The genes for colored/white and starchy/waxy are linked because they do not follow the dihybrid typical ratio of 9:3:3:1.

3) F1 = colored and non-shrunken

Screen shot 2013-02-21 at 6.54.40 AM


The ratio is 1 colored, shrunken: 1 colored, shrunken: 1 white, non-shrunken: 1 white, shrunken.

4) Actual frequencies do not become a 1:1:1:1 ratio and thus genes are linked.

5) Starchy/waxy and non-shrunken/shrunken are linked to each other because they are both linked to color.

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