Biology 1001 Spring 2008 (B. Fall), Class notes, topic #9—Genes within individuals II:
monohybrid cross review; dihybrid crosses; linkage
complete the reading assignments in your text (Freeman, Biological Science, 2
ed.): pp. 276-
281. I recommend that you work through the two tutorials on the text companion website for chapter 13 (see
part V-A in the outline below); these are also available on the course WebVista page.
Appreciate what a pedigree is and how to interpret it.
Understand the process of independent assortment and its application to gamete formation and
Learn how to predict the expected outcome of a dihybrid cross.
Appreciate what linkage is and its effect on genetics crosses.
(from class topic #5) single-character crosses and concepts (alleles, homologous
chromosomes, homozygous, heterozygyous). For a single gene with two alleles (e.g., A and a), there
are only 6 possible different kinds of crosses: AA x AA; AA x Aa; AA x aa; Aa x Aa; Aa x aa; aa x
aa. You should understand the expected outcome (offspring) of each (e.g., Aa x Aa
1/4 AA, 1/2
Aa, 1/4 aa).
diagram parents and offspring over several generations, and are used to study the
heredity of genetic diseases.
Consider this example (circles represent females, squares males, and darkened symbols show
individuals exhibiting a particular genetic trait). Individuals 1 and 2 are the parents of daughter 3,
and the grandparents of her children 5-8. Individuals 5 and 7 exhibit the trait. What are the
genotypes of all 8 individuals?
Start with 3 and 4, which share the same phenotype. Some of their offspring (6, 8) have the same
phenotype as 3 and 4; others (5, 7) have a different phenotype. The only cross consistent with
this is Aa x Aa. Thus we conclude that 3 and 4 = Aa, 5 and 7 = aa. For 1, 2, 6, and 8, we only
know they exhibit the dominant phenotype; genotype could be either AA or Aa.
III. Dihybrid crosses
involve individuals that differ in
independent (unlinked) traits.
(Mendel's Second Law)—the alleles of one gene will
segregate or assort into gametes independently of those of a different gene [this is true only for
genes that are on different pairs of homologous chromosomes, or far apart on the same
homologous pair]. This results from independence of the orientation of different tetrads during