This example illustrates the Law of Independent Assortment , which states that, if two genes are on different chromosomes, then the alleles for these genes separate independently of each other during the formation of eggs or sperm. Therefore, the traits determined by these two genes are inherited independently. For example, the wing gene and the horn gene are located on different chromosomes so they are inherited independently. Genes on different chromosomes are inherited independently of each other because each pair of homologous chromosomes lines up independently of the others when the chromosomes line up in the center of the cell near the beginning of the first meiotic division. Consequently, when the pairs of homologous chromosomes separate during the first meiotic division, the chromosome that has an H allele is equally likely to end up in the same egg with the chromosome that has the W allele or with the chromosome that has the w allele. (This is illustrated in the figure on page 2.) In this activity, when you dropped the two chromosome Popsicle sticks, each stick independently landed with one particular side up, and this corresponds to the independent assortment of chromosomes and their alleles during meiosis. 4. To illustrate how the Law of Independent Assortment applies to humans , consider the inheritance of the recessive allele for sickle cell anemia ( s , located on chromosome 11) and the SRY gene. The SRY gene is located on the Y chromosome and the SRY gene results in male development. This explains why a person who has both an X chromosome and a Y chromosome in each cell is a male, and a person who has two X chromosomes and no Y chromosome is a female. Suppose that a father and mother are both heterozygous for the allele for sickle cell anemia ( Ss ). The following Punnett square shows the inheritance of the sickle cell and SRY genes, with X representing the X chromosome with no SRY gene and Y representing the Y chromosome which does have an SRY gene. Complete the Punnett square. Mother (SsXX) SX sX SX sX Father (SsXY) SX sX SY sY Based on this Punnett square, what fraction of the sons will have sickle cell anemia? What fraction of the daughters will have sickle cell anemia? Is there any sex difference in the risk of inheriting sickle cell anemia? The Law of Independent Assortment applies to genes which are located on different chromosomes, but it does not apply to genes which are located near each other on the same chromosome, as you will see in the next activity. 6
Genetic Linkage -- Inheritance of Genes which are Close Together on the Same Chromosome Obviously, real chromosomes have more than one gene each. In this activity, you will analyze the inheritance of multiple genes which are close together on the same chromosome. We will consider three genes on Chromosome 1 and one gene on Chromosome 2, as indicated in the following table.
- Winter '16
- Matthew Van Kouwenberg