Biology: Exploring the Diversity of Life
Chapter 11: Chromosomes and Inheritance (PAGES 234-254)
- Thomas Hunt Morgan
o First to associate a specific gene with a specific chromosome in
the early 20th century.
o Morgan uses a fruit fly, Drosophila melanogaster, that eats fungi
Small and versatile
Short life cycle (2 weeks)
A female fertilizes hundreds of fertilized eggs during her
brief life span. The resulting large populations make
statistical analysis easy and reliable.
Large (“polytene”) chromosomes
o He spent years looking for variant individuals
Discovered a male with white eyes instead of the usual
Wild type: An individual having the normal phenotype;
that is, the phenotype generally found in a natural
population of organisms.
Mutant: An individual having a phenotype that differs
from the normal phenotype.
o His findings:
When Morgan crossed his white-eyed male with a red-
eyed female, all the F1 offspring had red eyes,
The red allele appeared dominant to the white allele.
Crosses between the F1 offspring produced the classic
3:1 phenotypic ratio in the F2 offspring.
Surprisingly, the white-eyed trait appeared only in males.
All the females and half the males had red eyes.
Morgan concluded that a fly’s eye color was linked to its
o Linked genes tend to be inherited together because they are
located on the same chromosome
o Results of crosses with linked genes deviate from those expected
according to independent assortment.
o According to independent assortment, this should produce 4
phenotypes in a 1:1:1:1 ratio.
o Surprisingly, Morgan observed a large number of wild-type (gray-
normal) and double-mutant (black-vestigial) flies among the
offspring. These phenotypes correspond to those of the parents.
o Morgan observed this linkage and its deviations when he followed
the inheritance of characters for body color and wing size. The wild-type body color is gray (b+) and the mutant
The wild-type wing size is normal (vg+)and the mutant
has vestigial wings (vg).
- Independent assortment of chromosomes and crossing over produce
o The production of offspring with new combinations of traits
inherited from two parents is genetic recombination.
o Genetic recombination can result from independent assortment
of genes located on nonhomologous chromosomes or from
crossing over of genes located on homologous chromosomes.
Can ‘un-link’ genes
- Mendel’s dihybrid cross experiments produced some offspring that had a
combination of traits that did not match either parent in the P generation
o If the P generation consists of a yellow-round parent (YYRR)
crossed with a green-wrinkled seed parent (yyrr), all F1 plants
have yellow-round seeds (YyRr).
o Half are be parental types, with phenotypes that match the
original P parents, either with yellow-round seeds or green-
o Half are recombinants, new combination of parental traits, with
yellow-wrinkled or green-round seeds.
o In contrast, linked genes, genes located on the same
chromosome, tend to move together through meiosis and
o Under normal Mendelian genetic rules, we would not expect
linked genes to recombine into assortments of alleles