Transcript Fruit Flies…

Genetics – Part II
Spring 2013 - Althoff Reference: Mader & Windelspecht Ch. 11)
• Chromosomal
Inheritance
• Human X-Linked
Disorders
• Gene Linkage
Lec
16
Beyond Mendel…
• _______________________________________:
chromosomes are carriers of genetic information
• ________________— any chromosome that does NOT
determine sex of the individual
• ________________— chromosomes “active” in
determining the sex of an individual
• For humans and fruit flies, the sex chromosomes in
females are XX and in males XY
Sex Chromosomes
• ______________ to determining just sex of the
individual
• Other traits may be attributable to the “sex”
chromosome.
• _____________________ refers to genes
carried on the __________________. The YChromosome does NOT carry these genes.
• Early (and even current work) to determine this
enabled by using Drosophila (fruit flies)
Fruit Flies
• Drosophila melanogaster
• A “________” organism
• They share 75% of the
genes that cause disease in
humans
• Cheap and easy to maintain
• Small, but don’t need a
microscope to see
Fruit Flies…con’t
• Some “simple” differences:
long wings (L)
vs.
short wings (l)
gray bodies (G)
vs.
Black bodies (g)
• Two-trait testcross
(2 of 4 outcomes shown)
LlGg
llgg
1 PAIR
SEX
CHROMOSOMES
3 PAIRS
AUTOSOMES
=8
CHROMOSOMES
X-linked
Alleles
Eye Color
•“Y” chromosome only
from male
•No allele for eye color
on Y chromosome
•Thus, only pairing that
results in white eyed
fly is
XrY
Examples of X-linked traits
• ______________. Factor VIII is a protein, that is
essential to blood clotting. People who do not have
factor VIII have a condition called hemophilia a
• __________________________ (more common in
males…only 1 X chromosome)
• _________________________ Absence of protein
called dystrophin causes this disorder
• Generally, X-linked disorders more common in _____
than ____________
____________ syndrome
• X-linked disorder
• 2nd leading cause of
mental retardation
• Individuals
hyperactive or
autistic
• Name comes from
fragile “tip” held on
by a ‘thread’
Some other Human X-linked disorders
• Menkes Syndrome – kinky hair syndrome caused by
defective allele on X chromosomes. Effects
movement of copper in an out of cells. Symptoms:
poor muscle tone, seizures, brittle steely hair, etc.
• Hemophilia (again) – inability of blood to clot
efficiently externally or internally. Two types: A and
B distinguished by missing or minimal presence of
clotting fctor VIII or IX, respectively.
Gene Linkage
• Defined: ___________ on the same chromosome
• LINKAGE GROUP – genes on the same chromosome
are “linked” because they tend to be _____________
together.
• Example of Drosophilia…(handout) where some
traits have more likelihood of crossing-over together
than others.
• LINKAGE MAP - a way to examine order of gene loci
(same handout)
LINKAGE GROUP.
Each homologous pair of chromosomes carries a number of genes. The alleles on a
chromosome form a linkage group because they tend to go together into the gametes. This simplified chromosome map
shows the relative positions of some of the genes on Drosophila chromosome 2. the distances between the genes (the
numbers = map units) are equivalent to the percentage of crossing-over events that occurs between various alleles. For
Example, the crossing-over frequency between gray body and long legs would be 48.5 – 31.0 =17.5%. This means that
17.5% of all gametes would carry recombinant gametes.
Gene Linkage – Noting an exception!
• Linked genes do ______ obey Mendel’s law of
independent assortment because they tend
to go into the same gamete together.