Chromosomal Inheritance Human X-Linked Disorders Gene Linkage Genetics – Part II Spring Althoff Reference: Mader & Windelspecht Ch. 11) Lec 16

_______________________________________: 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 eyed fly is X r Y X-linked Alleles

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 2 nd 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.