Recombina*on of Linked Genes: Crossing Over. discovered that genes can be linked. the linkage was incomplete

Transcription

1 Recombina*on of Linked Genes: Crossing Over Fig Testcross parents Gray body, normal wings (F 1 dihybrid) Black body, vestigial wings (double mutant) Morgan discovered that genes can be linked the linkage was incomplete evident from recombinant phenotypes proposed that some process must some:mes break the physical connec:on between genes on the same chromosome That mechanism was the crossing over of homologous chromosomes Eggs Testcross offspring Meiosis I Meiosis II 965 Wild type (gray-normal) Replication of chromosomes Parental-type offspring Recombination frequency = b + vg + Recombinant chromosomes b+ vg b+ vg Meiosis I and II + Replication of chromosomes 944 Blackvestigial 206 Grayvestigial 185 Blacknormal + Recombinant offspring 391 recombinants 2,300 total offspring 100 = 17% Sperm Mapping the Distance Between Genes Using Recombina*on Data: Scien'fic Inquiry Alfred Sturtevant Student of Morgan Constructed a gene*c map an ordered list of the gene:c loci along a par:cular chromosome predicted that the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombina8on frequency Linkage map gene:c map of a chromosome based on recombina:on frequencies Map units Linkage Map Expression of distances between genes one map unit, or cen:morgan, represents a 1% recombina:on frequency Map units indicate rela:ve distance and order, not precise loca:ons of genes 1

2 Fig RESULTS Chromosome Recombination frequencies 9% 9.5% 17% Linkage Map Genes that are far apart on the same chromosome can have a recombina:on frequency near 50% Such genes are physically linked, but gene:cally unlinked, and behave as if found on different chromosomes b cn vg Sturtevant Gene Mapping Fig Short aristae Mutant phenotypes Black body Cinnabar eyes Vestigial wings Brown eyes used recombina:on frequencies to make linkage maps of fruit fly genes Chromosomal banding Staining method to iden:fy regions of chromosomes gene:cists can develop cytogene:c maps of chromosomes Cytogene*c maps indicate the posi:ons of genes with respect to chromosomal features Ie bands Long aristae (appendages on head) Gray body Red eyes Normal wings Wild-type phenotypes Red eyes 2

3 Altera*ons of chromosome number or structure cause some gene*c disorders Large scale chromosomal altera:ons omen lead to spontaneous abor:ons (miscarriages) or cause a variety of developmental disorders Nondisjunc*on Abnormal Chromosome Number pairs of homologous chromosomes or sister chroma:ds do not separate normally during meiosis As a result one gamete receives two of the same type of chromosome, and another gamete receives no copy Fig Meiosis I Fig Meiosis I Meiosis II (a) of homologous chromosomes in meiosis I (b) of sister chromatids in meiosis II (a) of homologous chromosomes in meiosis I (b) of sister chromatids in meiosis II 3

4 Fig Meiosis I Meiosis II Gametes Abnormal Chromosome Number Aneuploidy results from the fer:liza:on of gametes in which nondisjunc:on occurred Offspring with this condi:on have an abnormal number of a par:cular chromosome n + 1 n + 1 n 1 n 1 n + 1 n 1 n n (a) of homologous chromosomes in meiosis I Number of chromosomes (b) of sister chromatids in meiosis II Abnormal Chromosome Number Monosomic zygote has only one copy of a par:cular chromosome Trisomic zygote has three copies of a par:cular chromosome Polyploidy Chromosome Number condi:on in which an organism has more than two complete sets of chromosomes Triploidy (3n) is three sets of chromosomes Tetraploidy (4n) is four sets of chromosomes common in plants, but not animals Polyploids are more normal in appearance than aneuploids 4

5 Fig Altera*ons of Chromosome Structure Breakage of a chromosome can lead to four types of changes in chromosome structure: Dele*on removes a chromosomal segment Duplica*on repeats a segment Inversion reverses a segment within a chromosome Transloca*on moves a segment from one chromosome to another Fig (a) A B C D E A B C E Deletion Aneuploidy Down Syndrome (Trisomy 21) (b) A B C D E Duplication A B C B C D E Down syndrome aneuploid condi:on that results from three copies of chromosome 21 (c) A B C D E Inversion A D C B E It affects about one out of every 700 children born in the United States (d) A B C D E Reciprocal translocation M N O C D E M N O P Q R A B P Q R The frequency of Down syndrome increases with the age of the mother, a correla:on that has not been explained 5

6 Fig Aneuploidy of Sex Chromosomes Nondisjunc:on of sex chromosomes Produces a variety of aneuploid condi:ons Klinefelter syndrome XXY male Sterile, small testes, some female secondary sexual characteris:cs Monosomy X, Turner syndrome X0 female sterile it is the only known viable monosomy in humans Structurally Altered Chromosomes cri du chat ( cry of the cat ) Fig specific dele:on in chromosome 5 mentally retarda:on catlike cry usually die in infancy or early childhood transloca:ons of chromosomes Certain cancers, including chronic myelogenous leukemia (CML) Normal chromosome 9 Normal chromosome 22 Reciprocal translocation Translocated chromosome 9 Translocated chromosome 22 (Philadelphia chromosome) 6

7 Excep*ons to the standard chromosome theory Two normal excep:ons to Mendelian gene:cs Some genes located in the nucleus 2 3 dozen traits dependent upon gender of parent Genes located outside the nucleus Genomic imprin*ng Genomic Imprin*ng varia:on in phenotype depends on which parent passed along the alleles for those traits involves the silencing of certain genes that are stamped with an imprint during gamete produc:on Organelle genes Fig Paternal chromosome Maternal chromosome (a) Homozygote Normal Igf2 allele is expressed Normal Igf2 allele is not expressed Mutant Igf2 allele inherited from mother Normal size mouse (wild type) Normal Igf2 allele is expressed Wild-type mouse (normal size) Mutant Igf2 allele inherited from father Dwarf mouse (mutant) Mutant Igf2 allele is expressed Genomic Imprin:ng Genomic Imprin*ng the result of the methyla:on (addi:on of CH 3 ) of DNA thought to affect only a small frac:on of mammalian genes Most imprinted genes are cri:cal for embryonic development Mutant Igf2 allele is not expressed (b) Heterozygotes Normal Igf2 allele is not expressed 7

8 Inheritance of Organelle Genes Extranuclear genes (or cytoplasmic genes) genes found in organelles in the cytoplasm Mitochondria, chloroplasts, and other plant plas:ds carry small circular DNA molecules Extranuclear genes inherited maternally zygote s cytoplasm comes from the egg The first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant Fig Inheritance of Organelle Genes You should now be able to: Some defects in mitochondrial genes prevent cells from making enough ATP and result in diseases that affect the muscular and nervous systems For example, mitochondrial myopathy and Leber s hereditary op:c neuropathy Failure of oxida:ve phosphoryla:on 1. Explain the chromosomal theory of inheritance and its discovery 2. Explain why sex linked diseases are more common in human males than females 3. Dis:nguish between sex linked genes and linked genes 4. Explain how meiosis accounts for recombinant phenotypes 5. Explain how linkage maps are constructed 8

9 6. Explain how nondisjunc:on can lead to aneuploidy 7. Define trisomy, triploidy, and polyploidy 8. Dis:nguish among dele:ons, duplica:ons, inversions, and transloca:ons 9. Explain genomic imprin:ng 10. Explain why extranuclear genes are not inherited in a Mendelian fashion 9