• 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

SEX CHROMOSOMES

AUTOSOMES

3 PAIRS1 PAIR= 8

CHROMOSOMES

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.

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.