Download Independent Assortment

WEB TUTORIAL 11.2
Independent Assortment
Text Sections
Section 11.4 Mendel's Generations Illustrated, p. 173
Introduction
What is the principle of independent assortment, and how does it work? This tutorial illustrates the principle in a genetic cross, then demonstrates how it occurs during the process of meiosis.
Learning Objectives
•
•
Know what is meant by the phrase "independent assortment."
Understand what types of chromosomes follow the law of independent
assortment.
Narration
Independent Assortment: A Cross
To understand the principle of independent assortment, let's consider one of Gregor
Mendel's classic crosses. In this cross, Mendel mated a plant grown from a round,
yellow pea to a plant grown from a wrinkled, green pea. The offspring of this cross
appear in equal proportions of shape and color combinations.
The wrinkled, green parent is homozygous for the recessive r and y alleles and produces only one gamete genotype.
The round, yellow parent is heterozygous (Rr and Yy) for both genes and produces
equal proportions of four gamete genotypes.
The gametes fuse with each other in the cross to yield equal proportions of offspring genotypes.
Note that in the gametes from the round, yellow parent, the dominant R allele is
just as likely to be found with the dominant Y allele as it is to be found with the
recessive y allele. In other words, the R and Y alleles enter the gametes independently of each other during gamete formation. Mendel referred to this independence
as the principle of independent assortment.
Today we know that independent assortment is explained by the independent alignment and distribution of nonhomologous chromosomes during meiosis.
Independent Assortment: Meiosis
These identical diploid cells have just begun meiosis. Their progress through the
cell divisions will illustrate the principle of independent assortment.
Each cell is heterozygous for the shape and color genes. One chromosome contains
the dominant R allele, and the homologous chromosome contains the recessive r
allele. Another pair of chromosomes contains the dominant (Y) and recessive (y)
alleles of the color gene.
During meiosis, the homologous chromosomes pair up and migrate to the middle
of the cell. Notice that in one cell, the chromosomes containing dominant alleles
are lined up on the same side of the cell; whereas in the other cell, they are on
opposite sides.
This arrangement at metaphase I determines which chromosomes segregate to
which pole of the cell during division. In the first case, the dominant R and dominant Y alleles end up in the same cell. In the second case, the dominant R and dominant Y alleles end up in different cells.
The alleles of the shape gene have segregated from each other independently of the
alleles of the color gene, and the shape and color genes are assorted randomly into
the daughter cells. This is the basis of Mendel's principle of independent assortment.
In meiosis II, the cells divide again. Notice that there are equal proportions of
gamete genotypes. One fourth are dominant R, dominant Y; one fourth are recessive s, recessive y; one fourth are dominant R, recessive y; and one fourth are recessive r, dominant Y. On average, half the cells that undergo meiosis will produce the
genotypes shown on the left, and the other half will produce the genotypes shown
on the right.
Genes located on different chromosomes, as in our example, always assort independently. It is important to note, however, that genes located on the same chromosome will assort together more often.
You should now be able to…
•
•
•
•
Give an example of a cross that illustrates Mendel's principle of independent assortment.
Identify the phase in meiosis that is the basis for the principle of independent assortment.
Draw a diagram to show the events in meiosis that lead to independent
assortment of chromosomes.
Describe conditions under which genes might not assort independently.