Chapter 15—Gene Linkage and Linkage Maps
1. Draw the G1 cell from a pea that is homozygous for round seeds (alleles R and r) and homozygous for Green
Seeds (alleles G and g). Assume that these genes are on separate chromosomes (that means they will
independently sort).
2. Draw all of the possible gametes from this parent.
3. Draw the G1 cell from a pea that is homozygous for wrinkled seeds (alleles R and r) and homozygous for
yellow seeds (alleles G and g).
4. Draw all of the possible gametes from this parent.
5. Assume fertilization between the gamete in #2 and the gamete in #4. Draw the resulting cell. Indicate the
phenotype.
6. Carry this cell through meiosis showing ALL possible arrangements of chromosomes. Assume that no
crossing over occurs.
The number of genes in a cell far outnumbers the number of chromosomes in a cell. Therefore, more than
one gene (usually hundreds or thousands of genes) are located on the same chromosome. How is inheritance
of genes that are located on the same chromosome different from inheritance of genes that are located on
separate chromosomes?
LINKED GENES—Genes located on the same chromosome that tend to be inherited together in genetic
crosses because the chromosome is passed along as a unit. In other words, they DO NO independently sort.
Morgan found that fruit flies usually had a gray body (wild type, b+) and normal sized wings (wild type, vg+).
However, he found mutant strains of each of the traits—black bodies (b) and vestigial wings (vg).
He mated true-breeding wild-type flies with black, vestigial-winged ones and produced the F1 dihybrids.
7. Write the genotype of the wild-type fly, the mutant fly, and the F1 offspring. What is the phenotype of the
F1 offspring?
8. He then crossed a dihybrid F1 offspring with a double-mutant. Write the genotypes of these two flies and
predict the phenotypic outcome of the F2 generation based on Mendel’s laws of segregation & independent
assortment.
9. In Morgan’s actual experiment, the proportion of F2’s were 965 wild-type, 944 black vestigial, 206 grayvestigial, and 185 black-normal. What phenotypes seemed to occur more often than Mendel’s rules would
suggest?
PARENTAL TYPES—A phenotype that matches one of the parents.
RECOMBINANTS—Offspring that have new combinations of phenotypes different from either parent.
Morgan explained this phenomenon by suggesting that the gene for body color and wing size must be located
on the same chromosome and that these genes were inherited together and DID NOT independently assort
during meiosis.
10. Show the process of meiosis in the two parents described in #8. Draw a single pair of chromosomes that has
BOTH the gene for body color and wing size. What types of gametes are possible for each of the parents?
If random fertilization occurs, what possible phenotypes SHOULD we see in the offspring?
11. How does Morgan’s actual data (in #10) differ from your predicted outcome in the problem above.
12. What phenomenon could account for the appearance of recombinants from a cross that involved linked
genes?
If a 50% frequency of recombination (a total of 50% recombinants) is observed for any two genes they are
determined to be on different chromosomes. If there is a frequency less than 50%, the genes are determined
to be linked (on the same chromosome). The higher the frequency (from 0 to 49%) the more often crossing
over occurs.