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. 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 7
chromosome. We will consider three genes on Chromosome 1 and one gene on Chromosome 2, as indicated in the following table. Dominant Alleles Recessive Alleles Chromosome 1 W = has wings w = no wings F = fire-breathing f = no fire-breathing N = long fangs n = short fangs Chromosome 2 H = big horns h = small horns For this activity, the father is heterozygous for each of these genes ( WwFfNnHh ), and the mother is homozygous for the recessive alleles ( wwffnnhh ). For the three genes on chromosome 1 for the father, the dominant alleles are all located on one chromosome and the recessive alleles are all located on the other homologous chromosome. The Chromosome 1 pairs for the father and mother look like this. Father Mother ________________________ ________________________ ( W F N ) (w f n ) ______________________ ______________________ (w f n ) (w f n ) Draw the genes for the Chromosome 2 pairs for the father and mother. Father Mother ________________________ ________________________ ( ) ( ) ______________________ ______________________ ( ) ( ) 1. In the figures below, draw the phenotypes of the heterozygous father and the homozygous mother. Father Mother 2. What different combinations of alleles can be found in the different types of sperm that the heterozygous (WwFfNnHh) dragon father can produce?3To answer this
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