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• Chromosomal Inheritance
• Human X-Linked Disorders
• Gene Linkage
Genetics – Part II Spring 2013 - Althoff Reference: Mader & Windelspecht Ch. 11)
Lec16
• _______________________________________: 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
Beyond Mendel…
• ______________ to determining just sex of the individual
• Other traits may be attributable to the “sex” chromosome.
• _____________________ refers to genes carried on the __________________. The Y-Chromosome does NOT carry these genes.
• Early (and even current work) to determine this enabled by using Drosophila (fruit flies)
Sex Chromosomes
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
Eye Color•“Y” chromosome only
from male•No allele for eye color
on Y chromosome•Thus, only pairing that
results in white eyedfly is
XrY
X-linkedAlleles
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 mapshows the relative positions of some of the genes on Drosophila chromosome 2. the distances between the genes (thenumbers = map units) are equivalent to the percentage of crossing-over events that occurs between various alleles. ForExample, the crossing-over frequency between gray body and long legs would be 48.5 – 31.0 =17.5%. This means that17.5% of all gametes would carry recombinant gametes.