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Chapter 15

Chapter 15. Chromosomal Basis of Inheritance. Discovery. 1900 – cytology and genetics converge: correlation between chromosomes and Mendelian genetics. Mendelian genes have certain loci on chromosomes that undergo segregation and independent assortment. Thomas Hunt Morgan.

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Chapter 15

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  1. Chapter 15 Chromosomal Basis of Inheritance

  2. Discovery • 1900 – cytology and genetics converge: correlation between chromosomes and Mendelian genetics. • Mendelian genes have certain loci on chromosomes that undergo segregation and independent assortment

  3. Thomas Hunt Morgan • 1st to associate a specific gene with a _____________. • Worked with Drosophila melanogaster (__________) • Hundreds of offspring, can be bred every two weeks • Only four chromosome pairs; 3 ___________ and one _____ pair (F-XX M-XY)

  4. Terms • Wild type: most common in a population (+) • Mutant type: alternative to wild type

  5. Example • Red eyes (+) X White eyes • Red is dominant to white • Only males had white eyes • Morgan concluded that the gene must be on the X chromosome and not the Y • Females: XRXW _______ • Males: XWY  _________ • Sex linked genes: genes located on a sex chromosome.

  6. Linked Genes • Genes located on the same chromosome that tend to be inherited together. • Ex. Drosophila body color and wing size • b+ gray b black vg+ normal vg vestigial • b+ b vg+ vg X b b vg vg • Expected: 4 phenotypes in equal ratios • Found: More gray normal and black vestigial than gray vestigial and black normal • Conclusion: Body color and wing shape are inherited together on the same chromosome (found more parental phenotypes than nonparental)

  7. Genetic Recombination • Parental types: match a parent • Recombinant types: don’t match a parent (when ½ of the offspring are recombinants, there is a 50% frequency of recombination). • 50% recombination is observed for any 2 genes located on different chromosomes (due to independent assortment of alleles at Metaphase I of meiosis). • Recombination frequency = #/total * 100%

  8. Crossing Over • If genes on the same chromosome, we would expect them to always be linked. However, we saw that this is not true (we saw some gray vestigial and black normal). This incomplete linkage can be accounted for by crossing over in meiosis I. (SHOW MEIOSIS)

  9. Using recombination for genetic mapping • Alfred H. Sturtevan (student of Morgan’s) created a genetic map. • Recombinant frequencies reflects distances on chromosomes: greater distances = higher chance of crossing over • Ex. Body color (b), wing size (vg) and cinnabar (cn) • Recombination frequency: cn and b is 9%, cn and vg is 9.5% and b and vg is 17% • Order on chromosomes must be: • Map unit = 1% recombination or 1 centimorgan • Maximum value = 50% recombination, so far apart they could be on different chromosomes.

  10. Types of Maps • Linkage map: sequence of genes, not actual locations • Cytological map: locate genes with respect to chromosomal features (staining patterns) • DNA nucleotide map

  11. Sex chromosomes • Mammals: Males are _____ females are ____ • Males: in meiosis there is little crossing over between homologous chromosomes because they are different (X and Y). Development: embryo starts differentiating at about 2 months: Y chromosome has ____: sex determining region of the Y discovered in 1990. In the absence of ____, gonads develop into ____________. (many other genes responsible)

  12. Sex linked genes • Humans: usually X linked • Fathers pass X linked genes to daughters, not sons • Mothers pass X linked genes to both sons and daughters • If recessive: female must be homozygous to express trait. Males are ____________ (only one X); any male receiving a trait from mom will express it. Consequences: more __________ than _______________ are affected, although both can be affected.

  13. Human Disorders • Duchenne Muscular Dystrophy: (1/3500 _________ in the US) Progressive weakening of muscles, loss of coordination, rarely live past 20’s (absence of muscle protein dystrophin, gene on X chromosome) • Hemophilia: on X, absence of protein required for _____________________, excessive _____________ (used to ________________ from small _______). Royal Family: starting with Queen Victoria, spread to Prussia, Spain and Russia (Anastasia).

  14. X Inactivation • Females inactivate one X, which condenses into a Barr body inside the nucleus and most genes are not expressed. • Mary Lyon: discovered the Barr body assignment is random, females are a mosaic of the two types of cells (active X from mom and active X from dad) • Ex. Calico Cat: heterozygous for black and orange

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