Download The Chromosomal Basis of Inheritance

The Chromosomal Basis
of Inheritance
Or Why Siblings Don’t Look
Alike…Or Do They?
Background
• 1900—3 botanists ‘discovered’ Mendel’s
work.
• Each had replicated his experiments and
then found his scientific writings.
• 1875—Mitosis found and documented.
• 1890’s—Meiosis found and documented.
• (Both by cytologists—cell biologists)
• 1900 Cytology and Genetics crashed into
each other
Etc…
• Biologists began to recognize
chromosomes and mendelian ‘factors’
were related.
• 1902 Chromosome theory of inheritance
was developed.
• Early 20th century Thomas Hunt Morgan
was the first to associate a specific gene
to a specific chromosome.
THM
• THM used Drosophila melanogaster ‘fruit
fly’ to study genetics.
• Fruit fly has 4 pairs of chromosomes, and
can be seen with a light microscope
(chromosomes)
• Fruit fly reproduces 100’s of offspring in
about two weeks.
Definitions
• Wild type—Phenotype most common in
wild populations.
• Mutant phenotype—Traits that are
alternative to wild type phenotypes.
THM’s Work
• Morgan found after
extensive breeding a
white eyed fruit fly
that was male.
• Wild type is red eyed.
• Bred with a WT
female F1 generation
all red eyed.
• F2 generation was a
3:1 ratio red eyed to
white eyed.
• White eyed flies were
all male.
• Thus eye color is on
the X chromosome.
• Genes on a sex
chromosome are sexlinked genes.
Linked Genes
• Each chromosome has 100’s to 1000’s of
genes
• Linked Genes—genes that are on the
same chromosome.
• Genes on a chromosome tend to be
inherited together because chromosomes
are inherited as a unit.
• See test cross on pg. 273
• How does independent assortment and
crossing over affect genetic
recombination's?
• Genetic recombination—Production of
offspring with new combinations of traits
from two parents.
• Parental types—Offspring which match
one or the other parent.
• Recombinants—Offspring have new
combinations of phenotypes and thus
genotypes.
• 50% chance of recombination for genes
on separate chromosomes.
• For a dihybred cross of unlinked
heterozygous genes with homozygous
genes the expected ratio would be 1:1:1:1.
• If the two genes were completely linked
the expected ratio would be 1:1:0:0.
• Neither of the previous ratio typically does
not happen for linked genes.
• You can calculate the recombination
frequency with this formula.
# of recombinants
RF = Total offspring
X 100 = ___%
• You can use recombination frequency to
determine/construct a genetic map.