• Chromosomal Inheritance Human X-Linked Disorders Gene Linkage

1. Genetics – Part II Spring 2013 - Althoff Reference: Mader & Windelspecht Ch. 11) Lec 16 •Chromosomal Inheritance • Human X-Linked Disorders • Gene Linkage

2. 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

3. 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 Y-Chromosome does NOT carry these genes. • Early (and even current work) to determine this enabled by using Drosophila (fruit flies)

4. 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

5. 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

6. 1 PAIR 3 PAIRS = 8 CHROMOSOMES SEX CHROMOSOMES AUTOSOMES

7. 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

8. 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 ____________

9. ____________ syndrome • X-linked disorder • 2nd leading cause of mental retardation • Individuals hyperactive or autistic • Name comes from fragile “tip” held on by a ‘thread’

11. 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.

12. 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)

13. 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.

14. 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.