Chapter 15 The Chromosomal Basis of Inheritance

Chapter 15 The Chromosomal Basis of Inheritance - Chapter...

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Unformatted text preview: Chapter 15: The Chromosomal Basis of Inheritance 15.1: Mendelian inheritance has its physical basis in the behavior of chromosomes − Scientists began to see similarities between Mendel’s genes and chromosomes which led to the chromosome theory of inheritance o Mendelian genes have specific loci along chromosomes and it is the chromosomes that undergo segregation and independent assortment Morgan’s Experimental Evidence: Scientific Inquiry - Tho m a s Hu n t Morg a n c h o s e t o d o his e x p e ri m e n t s o n t h e rosophila melanogaster, because D fruit fly, t h e of its advantages o Fruit flies produce many offspring with one mating o They can be bred every two weeks o They only have 4 pairs of chromosomes - At firs t, Morg a n h a d t o find a m u t a n t fly t o br e e d . Aft er t w o y e a r s, h e b r e d a si n gl e m al e fly e y e s ins t e a d of t h e u s u al r e d o Th e p h e n o t y p e for a c h a r a c t e r m o s t c o m m o nly o b s e r v e d in n a t u r al p o p ul wild n s is c all e d a tio type (red eyes) Alternative traits are called mutant phenotypes (white eyes) o Morgan mated this white eyed male with a red eyed female The F2 generation had the 3:1 ratio that Mendel observed. However only males had white eyes • Morgan concluded that the fly’s eye color was somehow linked to its sex chromosome, specifically the X chromosome • The male could have white eyes if only one copy of the allele was transferred, but the female would have to have both alleles for the trait 15.2: Sex-Linked genes exhibit unique patterns of inheritance The Chromosomal Basis of Sex - X-0 system: grasshoppers, cockroaches, and some other insects o There is only one type of sex chromosome, X Females are XX, males are just X, with no other sex chromosome - Z-W system: birds, some fish, and some insects o Sex chromosomes in the egg and not the sperm determine sex Females are ZW, males are ZZ - Haplo-diploid system: most species of ants and bees o There are no sex chromosomes Females develop from fertilized eggs and are diploid Males develop from unfertilized eggs and are haploid and have no father - In h u m a n s , XX is fe m al e, XY is m al e o An a t o mic al si g n s of s e x b e gi n t o e m e r g e o n c e t h e e m b r y o is 2 m o n t h s old o Th er e is a g e n e o n t h e Y-c hro m o s o m e t h a t c o d e s for t h e d e v elo p m e n t of t e s t e s c all e d SR Inheritance of Sex-Linked Genes - S e x c hr o m o s o m e s h a v e g e n e s for m a n y c h a r a c t e ris tic s u nr el a t e d t o s e x - Any g e n e s loc a t e d o n ei t h e r s e x c hr o m o s o m e a r e c all e d s e x-link e d g e n e s o Us u ally, t h e s e g e n e s a r e o n t h e X c hr o m o s o m e - Th e s e g e n e s follow t h e s a m e p a t t e r n of in h e rit a n c e t h a t fruit flie s h a v e o Far m or e m al e s will h a v e s e x-link e d r e c e s siv e dis or d e r s - Ex a m pl e s of s e x-link e d di s or d e r s o Color blin d n e s s o Duchenne muscular dystrophy 1/3500 males will have progressive weakening of the muscles and loss of coordination due to the absence of a muscle protein called dystrophin o Hemophilia Due to the absence of one or more of the proteins required for blood clotting X Inactivation in Female Mammals On e X c hr o m o s o m e in e a c h fe m al e c ell will b e c o m e al m o s t c o m pl e t ely in a c tiv a t e d d u rin g e m d e v elo p m e n t t o a v oi d d u al c o pi e s of pr o t ei n s o Th e in a c tiv e X will c o n d e n s e Barr body int o a o In the ovaries, the Barr bodies will be reactivated so that every female gamete has an active X - Mary Lyon s h o w e d t h a t t h e s el e c tio n of w hic h X c hr o m o s o m e will for m t h e Barr b o d y o c c u r s i n e a c h e m b r y o nic c ell o Th er e will b e a mix of c ells wit h t h e a c tiv e X fro m t h e fa t h e r a n d t h e a c tiv e X fro m t h e m All mi t o tic d e s c e n d a n t s of t h a t c ell will h a v e t h e s a m e X c hro m o s o m e in a c tiv a t e d • So if a fe m al e is h e t e r ozy g o u s for a s e x link e d tr ait, h alf of t h e c hr o m o s o m e s will e a ll el e a n d t h e o t h e r s will e x pr e s s t h e o t h e r o n e • This mosaicism is observed in the sweat glands of some humans - Wh e n a n X c hr o m o s o m e is in a c tiv a t e d , s o m e c h a n g e s in t h e DNA m u s t b e m a d e o Met hyl gr o u p s a r e a t t a c h e d t o o n e of t h e nitro g e n o u s b a s e s of DNA n u cl e o tid e s o Sci e n tis t s h a v e di s c ov e r e d a g e n e , XIST, t h a t is a c tiv e o nly o n t h e Barr b o d y RNA pr o d u c t s of t hi s g e n e will a t t a c h t h e m s elv e s t o t h e X c hr o m o s o m e o n w hic h t h e y 15.3: Linked genes tend to be inherited together because they are located near each other on the same chromosome - G e n e s loc a t e d o n t h e s a m e c hr o m o s o m e t h a t t e n d t o b e in h e rit e d tlo g e t h e r a r e c all e d inked genes How linkage affects inheritance - Wild t y p e flie s h a v e g r a y b o di e s a n d n or m al-siz e d win g s o Morg a n al s o br e d flie s wit h bl a ck b o di e s a n d win g s t h a t w e r e m u c h s m all er t h a n n or m al, v e s tigi al win g s - Wh e n h e w a s cro s sin g flie s, h e fo u n d t h a t t h e r e w a s a hi g h e r pr o p o r tio n of p a r e n t al p h e n o t y w o ul d b e e x p e c t e d if t h e t w o g e n e s a s s o r t e d in d e p e n d e n tly o He c o n cl u d e d t h a t b o d y c olor a n d win g siz e a r e u s u ally in h e rit e d t o g e t h e r Genetic Recombination - Th e pr o d u c tio n of offs prin g wit h c o m bi n a tio n s t h a t diff er fro m t h o s e fo u n d in eit h e r p a r e n t o Us u ally, t h e r e is a 5 0 % fr e q u e n c y of r e c o m bi n a tio n in t e s t cr o s s e s o b s e r v e d for a n y t w o g l oc a t e d o n diff er e n t c hr o m o s o m e s This is d u e t o t h e r a n d o m ori e n t a tio n of h o m olo g o u s c hr o m o s o m e s a t m e t a p h a s e I of 1 7 % of Morg a n’ s offs prin g w e r e r e c o m bi n a n t s o Morg a n c o n cl u d e d t h a t t h e r e m u s t b e s o m e pr o c e s s t h a t br e a k s t h e p h y sic al c o n n e c tio n s p e cific all el e s of g e n e s o n t h e s a m e c hr o m o s o m e This is c all e d cro s si n g o v e r a n d it o c c u r s d u rin g p r o p h a s e of m ei o sis I w h e n r e plic a t e d h o m olo g o u s c hro m o s o m e s a r e p air e d This pr o c e s s b rin g s n e w all el e s t o g e t h e r in n e w c o m bi n a tio n s Mapping the Distance Between Genes Using Recombination Data - St ur t e v a n t s e t o u t t o c o n s t r u c t a g e n e tic m a p , a n or d e r e d list of g e n e tic loci alo n g a p a r tic u o He a s s u m e d t h a t cr o s si n g o v e r is a r a n d o m e v e n t, s o t h a t t h e c h a n c e of cro s sin g o v e r a t t h e c hr o m o s o m e is e q u al So, t h e fart h e r a p a r t t h e t w o g e n e s a r e , t h e hi g h e r t h e pr o b a bility t h a t a cro s s o v e r wi b e t w e e n t h e m a n d t h e r efor e t h e hi g h e r t h e r e c o m bi n a tio n fr e q u e n c y o A g e n e tic m a p b a s e d o n r e c o m bi n a tio n fr e q u e n ci e s is c all e d a link a g e m a p Ma p u nit s a r e e q uiv al e n t t o a 1 % r e c o m bi n a tio n fr e q u e n c y o How e v e r, s o m e g e n e s a r e s o far a p a r t t h a t a cr o s s o v e r is al m o s t g u a r a n t e e d So, d e s pit e p h y sic ally b ei n g o n t h e s a m e c hr o m o s o m e , t h e y a r e g e n e tic ally u nlink e d Th e s e a r e m a p p e d b y a d di n g t h e r e c o m bi n a tio n fr e q u e n ci e s fro m cro s s e s involvin g a p air s of g e n e s lyin g b e t w e e n t h e t w o dis t a n t g e n e s - Link a g e m a p s d o n o t p or tr a y t h e a c t u al di s t a n c e s b e t w e e n g e n e s , b u t t h e y d o p o r tr a y t h e o - Th er e a r e o t h e r m e t h o d s t h a t e n a bl e u s t o m a k e cy t o g e nic m a p s , w hic h loc a t e g e n e s wit h r c hr o m o s o m al fe a t ur e s , like s t ai n e d b a n d s - ...
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This note was uploaded on 04/21/2009 for the course BISC 120Lg taught by Professor 11:00-01:50pm during the Fall '06 term at USC.

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