KEY CONCEPTS Locating Genes Along Chromosomes
• Morgan showed that Mendelian inheritance has → genes—Mendel’s “factors” are segments of
its physical basis in the behavior of DNA located along chromosomes
chromosomes: scientific inquiry → We can see the location of a particular gene by
• Sex-linked genes exhibit unique patterns of staining with a fluorescent dye that binds
inheritance specifically to the sequences of that gene
• Linked genes tend to be inherited together → began to develop the chromosome theory of
because they are located near each other on the inheritance.
same chromosome → According to this theory, Mendelian genes have
• Alterations of chromosome number or structure specific loci (positions) along chromosomes, and
cause some genetic disorders it is the chromosomes that undergo segregation
• Some inheritance patterns are exceptions to and independent assortment
standard Mendelian inheritance
▪ Morgan showed that Mendelian inheritance has its physical basis in the behavior of
chromosomes: scientific inquiry
→ The first solid evidence associating a specific gene with a specific chromosome came early in the 1900s
from the work of Thomas Hunt Morgan
→ Morgan’s Choice of Experimental Organism:
⤷ selected a species of fruit fly, Drosophila melanogaster, a common insect that feeds on the fungi
growing on fruit
⤷ Another advantage of the fruit fly is that it has only four pairs of chromosomes
⤷ There are three pairs of autosomes and one pair of sex chromosomes
⤷ Morgan was probably the first person to want different varieties of the fruit fly
⤷ the discovery of a single male fly with white eyes instead of the usual red.
⤷ The phenotype for a character most commonly observed in natural populations, such as red eyes in
Drosophila, is called the wild type
⤷ Traits that are alternatives to the wild type, such as white eyes in Drosophila, are called mutant
phenotypes because they are due to alleles assumed to have originated as changes, or mutations, in
the wild-type allele
→ Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair:
⤷ Morgan mated his white-eyed male fly with a red-eyed female
⤷ When Morgan bred the F1 flies to each other, he observed the classical 3:1 phenotypic ratio among
the F2 offspring
⤷ The white-eye trait showed up only in males
⤷ Morgan concluded that somehow a fly’s eye color was linked to its sex. (If the eye color gene were
unrelated to sex, half of the white-eyed flies would have been female.)
⤷ Morgan’s finding of the correlation between a particular trait and an individual’s sex provided support
for the chromosome theory of inheritance: a specific gene is carried on a specific chromosome
⤷ Morgan’s work indicated that genes located on a sex chromosome exhibit unique inheritance
patterns
TJW NOTES
, TJW NOTES
• Morgan showed that Mendelian inheritance has → genes—Mendel’s “factors” are segments of
its physical basis in the behavior of DNA located along chromosomes
chromosomes: scientific inquiry → We can see the location of a particular gene by
• Sex-linked genes exhibit unique patterns of staining with a fluorescent dye that binds
inheritance specifically to the sequences of that gene
• Linked genes tend to be inherited together → began to develop the chromosome theory of
because they are located near each other on the inheritance.
same chromosome → According to this theory, Mendelian genes have
• Alterations of chromosome number or structure specific loci (positions) along chromosomes, and
cause some genetic disorders it is the chromosomes that undergo segregation
• Some inheritance patterns are exceptions to and independent assortment
standard Mendelian inheritance
▪ Morgan showed that Mendelian inheritance has its physical basis in the behavior of
chromosomes: scientific inquiry
→ The first solid evidence associating a specific gene with a specific chromosome came early in the 1900s
from the work of Thomas Hunt Morgan
→ Morgan’s Choice of Experimental Organism:
⤷ selected a species of fruit fly, Drosophila melanogaster, a common insect that feeds on the fungi
growing on fruit
⤷ Another advantage of the fruit fly is that it has only four pairs of chromosomes
⤷ There are three pairs of autosomes and one pair of sex chromosomes
⤷ Morgan was probably the first person to want different varieties of the fruit fly
⤷ the discovery of a single male fly with white eyes instead of the usual red.
⤷ The phenotype for a character most commonly observed in natural populations, such as red eyes in
Drosophila, is called the wild type
⤷ Traits that are alternatives to the wild type, such as white eyes in Drosophila, are called mutant
phenotypes because they are due to alleles assumed to have originated as changes, or mutations, in
the wild-type allele
→ Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair:
⤷ Morgan mated his white-eyed male fly with a red-eyed female
⤷ When Morgan bred the F1 flies to each other, he observed the classical 3:1 phenotypic ratio among
the F2 offspring
⤷ The white-eye trait showed up only in males
⤷ Morgan concluded that somehow a fly’s eye color was linked to its sex. (If the eye color gene were
unrelated to sex, half of the white-eyed flies would have been female.)
⤷ Morgan’s finding of the correlation between a particular trait and an individual’s sex provided support
for the chromosome theory of inheritance: a specific gene is carried on a specific chromosome
⤷ Morgan’s work indicated that genes located on a sex chromosome exhibit unique inheritance
patterns
TJW NOTES
, TJW NOTES
