Genetics › Mendelian Inheritance

Independent Assortment

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Notes

Independent Assortment

Sections

INDEPENDENT ASSORTMENT

Homozygous

Having a pair of identical alleles

Heterozygous

Having two different alleles for a gene

Phenotype

Organism's traits

Genotype

Organism's genetic makeup

Monohybrid

Organisms heterozygous for only one character

Dihybrid

Organisms heterozygous for two characters

Dependent Assortment vs Independent Assortment

Dependent Assortment

  • Different traits stay together through the generations (ie, alleles inherited together)
  • If true, expect a 3:1 phenotype ratio of the offspring of a dihybrid cross

Independent Assortment

  • Alleles inherited independently of one another
  • If true, expect at 9:3:3:1 phenotype ratio of the offspring of a dihybrid cross

Mendel observed a 9:3:3:1 ratio of offspring so concluded that alleles are inherited independently of one another (though exceptions have been found to exist)

Full-Length Text

  • Here we'll learn about Independent Assortment.

Before we begin, let's review some key terminology.

  • Denote that homozygous means having a pair of identical alleles for a gene.
  • Denote that heterozygous means having two different alleles for a gene.
  • Denote that phenotype is an organism's traits.
  • Denote that genotype is an organism's genetic makeup. We will discuss the differences between these two shortly.

Finally, let's explore Mendel's Law of Independent Assortment.

  • Before we begin, denote that our previous crosses have been monohybrid.
    • That is, the organisms were heterozygous for only one character (in this case, flower color).

To derive the law of independent assortment, Mendel performed dihybrid crosses.

  • Denote that dihybrids are organisms that are heterozygous for two characters (in the following cases, we will be looking at seed color and seed shape together).
  • Write that we use capital Y to describing the dominant yellow seed color allele.
  • Write that we use lower case y to describe the recessive green seed color allele.
  • Write that we use capital R to describe the dominant round seed shape allele.
  • Write that we use lower case r to describe the recessive round seed shape allele.
  • Mendel was testing whether alleles for different traits would stay together throughout various generations (dependent assortment) or if alleles were inherited independently of one another (independent assortment).
  • Now draw the yellow, round seed and write its genotype as big Y, big Y, big R, big R.
  • Draw the green, shriveled seed and write its genotype as little y, little y, little r, little r.
  • Write that a cross between these two would generate offspring with the big Y, little y, big R, little r genotype.

Allowing this plant to self-pollinate would help answer the question of dependent or independent assortment. Let's look at what the predicted offspring would be in each case.

  • For dependent assortment, the big Y and big R alleles would stay together so they can be treated as a single allele.
  • Draw a Punnett square.
  • Write that our alleles will be big Y, big R and little y, little r.
  • Write the expected allele combinations in the boxes.
  • Note that we would expect a 3 to 1 ration of yellow round seeds to green shriveled seeds in this example.
  • For independent assortment, each allele would randomly be found with the other, so there are four possible combinations: big Y, big R; big Y, little r; little y, big R, or little y, little r.
  • Draw a four by four Punnett square and write the combinations above and to the side.
    • We'll also draw the appropriate colored and shaped seeds in each box as a visual reminder of what phenotypes the genotypes stand for.
  • In the first row, write big Y, big Y, big R, big R; then big Y, big Y, big R, little r; then big Y, little y, big R, big R; and finally big Y, little y, big R, little r.
  • In the second row, write big Y, big Y, big R, little r; then big Y, big Y, little r, little r; then big Y, little y, big R, little r; and finally big Y, little y, little r, little r.

Now, stop and see if you can fill out the Punnett squares.

Now check your work against ours …

  • In the third row, write big Y, little y, big R, big R; then big Y, little y, big R, little r; then little y, little y, big R, big R; and finally little y, little y, big R, little r.
  • In the fourth row, write big Y, little y, big R, little r; then big Y, little y, little r, little r; then little y, little y, big R, little r; and finally little y, little y, little r, little r.
  • Write that we would expect a 9 to 3 to 3 to 1 ratio of yellow round seeds to green round seeds to yellow shriveled seeds to green shriveled seeds.
  • Write that it was this ratio that Mendel observed in his experiments which showed that alleles sort independently (though exceptions exist that we will discuss elsewhere).

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