Chromosomal Basis of Inheritance Lecture 13 Fall 2008

The Chromosomal Basis of Inheritance 1 Chromosomal Theory of Inheritance Genes are found in specific locations on chromosomes The behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns Mendel published in 1866 Mitosis worked out in 1875 Meiosis worked out in 1890s Early 1900’s, Chromosomal Theory of Inheritance 1953 – Structure of DNA (and how it accounts for the ability of cells to pass on genes) discovered

The Chromosomal Basis of Inheritance 2 Fig. 15.2

Sex-linked Genes Thomas Hunt Morgan Ideal model organism 3 Thomas Hunt Morgan 1907 established fruit fly lab Drosophila melanogaster Ideal model organism Fast breeding Many offspring Chromosomes easy to see with light microscope Four chromosomes 3 autosomes Sex chromosomes Female = XX Male = XY Fig. 15.3

Sex-linked Genes Created first mutant after 2+ year of breeding 4 Created first mutant after 2+ year of breeding White eyed male Naming conventions in flies Wild type Trait that is found most often in nature Mutant phenotypes Alternatives to the wild types Gene takes its symbol from the first mutant discovered w = white eyes (mutant) w+ = red eyes (wild type) Fig. 15.3

Sex-linked Genes Red eyed female and white eyed male crossed 5 Red eyed female and white eyed male crossed F1 = all red eyes F2 = 3:1 ratio White eyed flies were all males Fig. 15.4

Sex-linked Genes Allele for white eye must be on X chromosome 6 Sex-linked Genes Allele for white eye must be on X chromosome No corresponding allele on Y chromosome First experiment linking a gene to a specific chromosome Fig. 15.4

Sex Linked Genes Sex linked genes Y chromosome has few genes 7 Sex linked genes Any gene located on a sex chromosome Female XX (mammals, flies) Male XY Y chromosome has few genes Most genes related to “maleness” (testes formation, sperm production) X has many more genes than Y Only small regions at end of Y homologous with X Most sex linked genes on X Many recessive sex linked traits show up more often in males Example: white eyed fruit flies Fig. 15.5

Sex Linked Genes 8 Hemizygous: individual has only one member of a chromosome pair

X chromosome inactivation 9 One X chromosome inactivated in every cell Chromosome condensed, so unavailable for transcription of DNA Barr body Random and independent Female heterozygous for sex-linked trait, half of cells express one trait and half the other trait Fig. 15.8

Linked Genes & Crossing Over 10 Linked Genes Genes located on the same chromosome that tend to be inherited together Often found close together on a chromosome Violates Mendel’s Law of Independent Assortment

Linked Genes & Crossing Over 11 High frequency of parental genotypes suggested that genes are linked Presence of other combinations suggested that genes only partially linked

Linked Genes & Crossing Over 12 Crossing over Exchanging of genetic material between non-sister chromatids Crossing over results in genetic recombination Parental types Phenotype matches parent Recombinant Type Phenotype different from parents Can prevent linked genes from traveling together Review Fig. 15.10 in text

Linkage Maps Genetic map 14 Genetic map Ordered list of the genetic loci along a particular chromosome The farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency Assumes crossing over random event Linkage map Genetic map based on recombination frequencies Fig. 15.11

Linkage Maps Limitations to linkage maps 15 Limitations to linkage maps Genes may be so far apart they are considered unlinked even though they travel on the same chromosome Portrays order along a chromosome, but not precise location Crossover frequency not uniform over length of chromosome Fig. 15.12

Genetic Disorders Nondisjunction 16 Nondisjunction Members of a pair of homologous chromosomes do not separate properly during meiosis 1 Sister chromatids fail to separate during meiosis 2 Fig. 15.13

Genetic Disorders Aneuploidy Polyploidy (3n, 4n) 17 Aneuploidy Zygote with abnormal number of chromosomes Due to aberrant gamete uniting with normal gamete Monosomic (2n-1) Zygote missing a chromosome Trisomic (2n+1) Zygote has an extra chromosome Downs syndrome – extra chromosome 21 Polyploidy (3n, 4n) More than two complete chromosome sets in all somatic cells Common in plants Fig. 15.13

Alterations of Chromosome Structure 18 Deletion Fragment of chromosome lost Duplication If fragment attached to sister chromatid, causes repeated section Can occur during meiosis Results from unequal crossing over Fig. 15.15

Alterations of Chromosome Structure 19 Inversion Fragment detached, reattached to same chromosome but in reverse Translocation Segment from one chromosome moved to a nonhomologous chromosome Reciprocal translocation Nonhomologous chromosomes exchange segments Fig. 15.15

Alterations of Chromosome Structure 20 Can also occur in Mitosis Chronic myelogenous lukemia Reciprocal translocation Daughter cells now have “Philadelphia chromosome” Fig. 15.17

Genomic Imprinting Genomic imprinting 21 Genomic imprinting Variations in phenotype depending on whether an allele is inherited from the male or female parent Occurs during gamete formation One allele silenced Species specific for particular gene E.g., insulin-like growth factor Only paternal allele is expressed Fig. 15.18

Genomic Imprinting 22 Fig. 15.18

Inheritance of Organelle Genes 23 Inheritance of Organelle Genes Extranuclear genes Mitochondria and chloroplasts Organelles reproduce independently Binary fission Organelles passed down through maternal line E.g., variegated leaves Mutations in plastid genes controlling pigmentation E.g., defects in electron transport chain Mitochondrial genes code for proteins in ETC Fig. 15.19