Ch. 15 The Chromosomal Basis of Inheritance

Transcription

1 Ch. 15 The Chromosomal Basis of Inheritance Nov 12 12:58 PM 1

2 Essential Question: Are chromosomes the basis of inheritance? Nov 12 1:00 PM 2

3 1902 Walter S. Sutton, Theodor Boveri, et al Chromosome Theory of Inheritance There are specific loci on chromosomes and it is the chromosomes that undergo segregation and independent assortment Nov 12 1:01 PM 3

4 Thomas Hunt Morgan early 20th century first to provide evidence that chromosomes contained heritable factors used fruit flies (Drosphilia melanogaster) quick breeders (offspring in 2 weeks) produce hundreds of offspring only 4 pairs of chromosomes 3 pair autosomes, 1 pair sex chromosomes chromosomes can be seen with light microscope Nov 12 1:06 PM 4

5 Fruit flies wild type= normal phenotype for a character mutant phenotype= alternative trait to wild type w = allele for white eyes in fruit flies w + = allele for wild type (red eyes) the gene takes the symbol from the first mutant discovered Nov 12 1:20 PM 5

6 Morgan discovered sex linked inheritance mated a whiteeyed male vs. red eyed female F1 generation = all red eyed F2 generation = 3:1 ratio, red:white but white eyes only in males therefore: eye color is related to its sex supported evidence of chromosome theory:specific genes carried on specific chromosomes Nov 12 1:28 PM 6

7 Since chromosomes have hundreds of genes on same chromosome, they tend to be inherited together in crosses called linked genes evidence for linked genes Nov 12 1:36 PM 7

8 Genetic recombination and Linkage Recombination of unlinked genes YyRr x yyrr if offspring match the parent phenotypes= parental types if offspring do not match parent phenotypes = recombinant types or recombinants Nov 12 1:42 PM 8

9 if 50% of offspring are recombinants, then there is a 50% frequency of recombination this is seen if any two genes are located on different chromosomes *due to random orientation of homologous chromosomes on metaphase 1 plate of meiosis = independent assortment of alleles Nov 12 1:47 PM 9

10 Determining Recombination Frequency Nov 20 7:13 AM 10

11 Recombination of linked genes what Morgan saw in his flies was crossing over that caused recombination of linked genes happens during prophase 1 in meiosis portions of nonsister chromatids trade places each time crossover occurs bring alleles in new combinations Nov 12 1:51 PM 11

12 Genetic mapping:discovered by Alfred Sturdevant genetic map =ordered list of the genetic loci along a particular chromosome recombination frequencies depend on the distance the genes are apart *the farther apart two genes are, the higher probability that a crossover will occur between them and therefore the higher the recombination frequency Nov 12 1:56 PM 12

13 A partial genetic map of a Drosophila chromosome Nov 20 7:17 AM 13

14 linkage map = a genetic map based on the recombination frequencies is map of order of genes on a chromosome, but not precise locations map units= distance between genes 1 map unit = 1% recombination frequency aka centimorgans do not correspond to actual distances Nov 12 1:59 PM 14

15 cytogenic map = map that locates genes with respect to chromosomal features,like stained bands Nov 12 2:09 PM 15

16 Sex linked genes XX = female, only produces X gametes XY = male, produces 50% X and 50% Y gametes sex of offspring = 50:50 chance SRY gene on Y chromosome codes for testes development if SRY absent = ovaries sex linked if gene is located on sex chromosomes Nov 12 2:10 PM 16

17 Some chromosomal systems of sex determination Nov 20 7:19 AM 17

18 if sex linked trait is recessive, female will have it only if she is homozygous males who receive recessive trait from mother, will have the trait examples colorblindness Duchenne Muscular Dystrophy weakening of muscles and coordination loss lack dystrophin protein Hemophilia blood clotting disorder Nov 12 2:22 PM 18

19 The transmission of sex linked recessive traits Nov 20 7:20 AM 19

20 X inactivation of female mammals of two X chromosomes, one gets in inactivated = Barr body (condensed form) these genes are not expressed reactivated in cells that make eggs Mary Lyon females have a "mosaic" of two types of cells: those with active X from father and those with active X from mother Nov 12 2:20 PM 20

21 Ex. tortoiseshell cats Ex. in humans alleles for sweat glands so can have patches of skin with sweat glands and others without sweat glands Nov 12 2:35 PM 21

22 Calico cats have white areas that are determined by another gene What is this called? Nov 20 7:25 AM 22

23 How can chromosome number be altered? nondisjunction when members of a pair of homologous chromosomes fail to move apart properly during meiosis I or sister chromatids fail to separate in meiosis II one gamete gets two copies other gamete gets none * can also occur during mitosis aneuploidy= abnormal number of a particular chromosome Nov 12 2:38 PM 23

24 nondisjunction Nov 20 7:26 AM 24

25 Trisomic = 2n +1 chromosomes Monosomic= 2n 1 chromosomes polyploidy = when have 2 or more complete chromosome sets (common in plants) triploidy = 3n tetraploidy = 4n *more normal in appearance than aneuploids Nov 12 2:42 PM 25

26 Triploidy in humans %20OLD/Triploidy.doc Nov 20 7:32 AM 26

27 Tetraploid mammal Tympanoctomys barrerae sperm head unusually large may have arisen when ancestor doubled chromosome # by errors in mitosis or meiosis within the reproductive organs Nov 20 7:28 AM 27

28 Alterations of Chromosome structure deletion chromosomal fragment lacking a centromere is lost (missing genes) usually during meiosis duplication when deleted fragment attaches as an extra segment of a sister chromatid usually during meiosis inversion when fragment reattaches to a segment but in reverse order can affect phenotype expression translocation fragment joins a nonhomologous chromosome can affect phenotype expression Nov 12 2:47 PM 28

29 Nov 20 7:35 AM 29

30 Human disorders due to chromosomal alterations Down syndrome aneuploid condition 1 in 700 in U.S. extra chromosome 21 aka trisomy 21 characteristics: short stature, heart defects, respiratory infections, mental retardation, prone to leukemia and Alzheimer's, most sexually underdeveloped and sterile risk increases if mother over 30 Nov 12 2:52 PM 30

31 Down syndrome Nov 20 7:36 AM 31

32 Aneuploidy of Sex chromosomes Klinefelter syndrome = XXY have male sex organs, but small sterile may have breast enlargement normal intelligence Males with XYY no different than normal, except taller Trisomy X (XXX) healthy, only tell by karyotype Nov 12 2:57 PM 32

33 Nov 20 7:37 AM 33

34 Turner Syndrome Monosomy X (XO) phenotypically female sterile sex organs do not mature if treated with estrogen, can develop secondary sex characteristics normal intelligence Nov 12 3:00 PM 34

35 Disorders of Structurally altered chromosomes Deletions Cri du Chat (Cry of the cat) deletion of chromosome 5 mentally retarded small head, round face, sm. receding chin widely spaced eyes, low set ears cry that sounds like mewing cat die in infancy Nov 12 3:02 PM 35

36 Translocations chronic myelogenous leukemia cancer of the blood cells reciprocal translocation chromosome 22 is replaced by tip of chromosome 9 = shortened chromosome 22 and extra long chromosome 9 =Philadelphia chromosome Nov 12 3:04 PM 36

37 Some traits depend on the parent that you receive them from = genomic imprinting mostly autosome genes occurs during formation of gametes one allele is silenced zygote expresses only one allele, transmitted during mitosis ex. gene imprinted for maternal allele will always be imprinted for maternal expression Nov 12 3:07 PM 37

38 genomic imprinting Nov 20 7:42 AM 38

39 ex. insulin growth factor(igf2) paternal allele is expressed Nov 12 3:14 PM 39

40 cause of a genome imprint believed to be methylation of cytosine nucleotides of DNA silences the allele can also work opposite, methylation can activate a gene such as in Igf2 * genomic imprinting seems to be important for development Nov 12 3:14 PM 40

41 Inheritance of organelle genes extranuclear genes genes found in organelles in cytoplasm found in mitochondria and chloroplasts have circular DNA reproduce by themselves genes go to daughter organelles do not display Mendelian inheritance mitochondrial genes come from mother can cause mitochondrial myopathy weakness, muscle deterioration also related to some heart disease or diabetes Nov 12 3:16 PM 41

42 So, how are the chromosomes the basis of inheritance? Nov 24 7:04 AM 42