Linked Genes, Down Syndrome, and Non-disjunction Describe what is meant by a linked gene and explain how linked genes can separate. What is down syndrome and what causes down syndrome. Aneuploidy Trisomy, Monosomy Polyploidy Test on Thursday. Study Outline Due on Exam Day. (Extra Credit) Do Now Take out homework. Which two questions were the easiest? …the hardest?

Chromosome Theory of Inheritance Mendelian genes have specific loci along chromosomes, and it’s the chromosomes that undergo segregation and independent assortment.

Thomas Hunt Morgan 1933 – Nobel Prize Winner for Chromosomal Heredity (Columbia University) First to associate a specific gene with a specific chromosome Drosophila melanogaster = fruit fly

Alfred Henry Sturtevant Sturtevant was always interested in inheritance and genetics. One of Sturtevant's earliest publications was a pedigree analysis of horses owned by his father. In 1909, while an undergraduate at Columbia University, Sturtevant attended a lecture given by Thomas Hunt Morgan. It was one of the few undergraduate classes that Morgan ever taught. Morgan's passion for science and discovery interested Sturtevant so much that he approached Morgan about working for him. Sturtevant became one of Morgan's first students in the "Fly room" to work on Drosophila melanogaster.

body color and wing shape are usually inherited together because their genes are on the same chromosome

linkage map used recombination frequencies from fruit fly crosses to map the relative position of genes along chromosomes frequency of recombinant offspring reflected the distances between genes on a chromosome

map units One map unit is equivalent to a 1% recombination frequency

“Disorders” Examples Duchenne muscular dystrophy Hemophilia Absence of an X-linked gene for a key muscle protein, called dystrophin Characterized by a progressive weakening of the muscles and loss of coordination Hemophilia Absence of one or more clotting factors Prolonged bleeding because clots form slowly

Linked Genes, Down Syndrome, and Non-disjunction What is down syndrome and what causes down syndrome. Aneuploidy Trisomy, Monosomy Polyploidy Chromosomal Mutations Test on Thursday. Study Outline Due on Exam Day. (Extra Credit) Do Now What is a linked gene? How can linked genes be “unlinked”? How do we measure the frequency of genes becoming unlinked?

Sex Linked Genes Heterozygous females will be carriers Any male receiving the recessive allele from his mother will express the trait

What information does this karyotype give us? Down syndrome 3 copies of chromosome 21 affects 1 in 700 children born in the US

Nondisjunction Problems with the meiotic spindle cause errors in daughter cells

Aneuploidy Trisomic cells  three copies of a particular chromosome type 2n + 1 total chromosomes Monosomic cells  only one copy of a particular chromosome type 2n - 1 chromosomes

Polyploidy Organisms with more than two complete sets of chromosomes Relatively common among plants and much less common among animals fishes and amphibians have polyploid species

Aneuploidy vs. Polypoidy Aneuploidy – ONE OR MORE EXTRA CHROMOSOMES Polyploidy – AN EXTRA SET OF CHROMOSOMES

Chromosomal Mutations Barr Bodies Describe four different types of chromosomal mutations. Describe what a barr body is and why it only occurs in females. Test on Friday. Study Outline Due on Exam Day. (Extra Credit) Do Now What is down syndrome? What process causes down syndrome and when does this process occur?

Sex Chromosome Disorders Klinefelter’s syndrome XXY male Occurs 1 in every 2000 live births have male sex organs, but are sterile Turner’s syndrome XO female (monosomy) occurs 1 in every 5000 births produces phenotypic, but immature females

changes in chromosome structure deletion  a chromosome fragment lacking a centromere is lost during cell division duplication  a fragment becomes attached as an extra segment to a sister chromatid

changes in chromosome structure inversion  a chromosomal fragment reattaches to the original chromosome in the reverse orientation translocation  a chromosomal fragment joins a nonhomologous chromosome

Barr body During female development, one X chromosome per cell condenses into a compact barr body This inactivates most of its genes Reactivated in ovarian cells that produce ova Females consist of a mosaic of cells some with an active paternal X others with an active maternal X