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Locus (plural loci)
A position on a chromosome; may contain a gene
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Alleles
Alternative versions of the same gene
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Homozygote
Two copies of the same allele (in a diploid)
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Dominant
Allele whose phenotype is seen only in one homozygote
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Semi-dominant ( incomplete dominance)
Hetz is intermediate between two homz
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Co-dominant
Hetz has the phenotype of both hoz parents simultaneously
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Wild-type (wt)
The allele, genotype, or phenotype that is found most often in natural populations
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Recessive
Allele whose phenotype is seen only in one homozygote
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Haplosufficient
One wt allele allele produces enough product to have same phenotype as two wt alleles
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Haploinsufficient
One wt allele does not produce enough product for same phenotype as two wt alleles
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Polyploidy
Many plants and some animals are polyploidy (more than 2 complete sets of homologous chromosomes)
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Endoreduplication
A type of polyploidy that occurs in only some cells and is not inherited, also allows cells to grow very large
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Organellar Genomes
Mitochondria and plastids have their own, circular chromosomes; usually many copies of this chromosome in each organelle; usually have uniparental inheritance
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Mendel's First Law
The Law of Equal Segregation: during gamete formation, the two members of a gene pair segregate from each other; each gamete has an equal probability of containing either member of the gene pair
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Crosses
- Monohybrid cross: (hetz x hetz: Aa x Aa)
- Testcross: (unknown genotype x recessive homozygote: AA x aa OR Aa x aa)
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Mendel:
Scientific study of heredity: white flower trait can be inherited even from two purple flowers; particulate (not blending) inheritance); genes are units of inheritance
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Avery, MacLeod, McCarty
DNA is genetic material: S, R strains of Strep; only S cases lethal infection; heat-killed S could pass genetic information to living R cells to make R pathogenic; what part of S was giving R new traits? DNA (used enzymes)
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Hershey, Chase
Further evidence that DNA is genetic material: phage transfer genetic material to bacteria; 32-P labels DNA, 35-S labels protein; phage labeled with 32-P transfer radioactivity to bacteria; phage labeled with 35-S do not transfer radioactivity to bacteria; therefore, DNA is genetic material
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Beadle & Tatum
Wild-type Neuropsora could grow on minimal media or complete media; some mutants are auxotrophic = can grow ONLY on complete media; find which nutrient will rescue a given mutant; infer mutation in a gene for the synthesis of that mutant
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Genes encode enzymes
Mutant screening can be used to find genes associated with almost any biological process; biochemical pathways can be dissected by providing precursors (= intermediates); different mutants (representing different enzymes) and precursors can be placed in order by inference
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Watson, Crick and others demonstrated the structure of DNA
Chargaff's rules (A=T), (C=G); Antiparallel strands; X-ray crystal image from Rosalind Franklin; Built metal models of bases; Proposed double helix structure of DNA; Also proposed Central Dogma - this connects DNA to phenotype
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Model organisms
APOG (Awesome Power of Genetics/mutational analysis) led to dominance of six model organism; Model organisms have specific practical advantages, including "small genomes"; Many disease genes etc. have ben identified in mutant screens in model organisms; new techniques (including cheap DNA sequencing) are starting to make model organisms less central in genetics
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C-value paradox
C-value is a measure of DNA content (bp or pg); no correlation between and organism's complexity and its c-value; gene number does not vary as much as c-value; repetitive DNA makes up most of the c-value in large genomes
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Chromosome Packaging
- Each chromosome contains a single long DNA molecule; needs to be compacted for cell division
- Chromatin = chromosomal protein + DNA
- Chromosomes are most compact at meiosis & mitosis
- Chromosomes are always partially compacted (Euchromatin: more transcriptionally active; loosely compatces. Heterochromatin: less transcriptionally active; more compact)
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Chromosome morphology & nomenclature
- Centromeres, telomeres = heterochromatic
- Homologs normally carry the same genes in the same order (but may have different alleles)
- Sister chromatids start out carrying the same genes AND the same alleles
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Homologs
- Come from different parents
- These usually carry the same genes in the same order (but may have different alleles)
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Sister chromatids
- Are made by replication
- These start out carrying the same genes AND the same alleles
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Mitosis
- Purpose is to make 2 identical copies of full set of chromosomes
- No information is lost
- Divides sister chromatids of chromosomes at the centromere
- Equation division
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Meiosis
- Purpose: make each daughter cell with HALF of the genetic info
- 2 Stages, meiosis 1 is reductional division (homologous chromosomes pair)
- Meiosis II is an equation division
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Cell cycle
- Interphase (G1, S (synthesis), G2)
- Sister chromatids appear after S phase (but may not be visible in real photos)
- C-value increases during S phase, decreases during mitosis and meiosis II
- N-value increases at fertilization, and decreases during meiosis
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Chromosome abnormalities
- Karyotypes = photograph of a metaphase chromosome;
- Deletions, inversions, insertions, translocations, duplications of large parts of chromosomes are all possible
- Aneuploidy = addition or subtraction from a homologous pair (eg. trisomy 21 = Down's syndrome. Gene balance = biochemistry of the cell is thrown off by abnormal ratios of some gene products
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Polyploidy
- Many plants and some animals are polyploid
- 2N=6c=42 bread wheat, six complete sets of chromosomes
- Stable; chromosomes segregate equally into daughter cells
- Polyploids needs an even number of chromosome sets to be stable
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Endoreduplication
- A type of polyploidy that occurs in only some cells and is not inherited
- Allows cells to grow very large
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Organellar Genomes
- Mitochondria and plastids have their own, circular chromosomes
- Usually many copies of this chromosome in each organelle
- Usually have uniparental inheritance
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Sex determination
- Some organisms have sex chromosomes (although process of sex determination varies widely between species)
- Dropophila superficially similar to humans (XX=females, XY=male)
- Birds, moths, butterflies different (ZZ=male, ZW=female)
- ZZ, XX = "homogametic"; ZW, XY = "heterogametic"
- X and Y pair like homologous chromosomes during meiosis; BUT they do not contain the same loci
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Sex-linkage
- Genes on the X chromosome are present as only one possible allele in males (=hemizygous)
- A single recessive allele can give a mutant phenotype in a hemizygote
- Sex linkage is revealed by recripocal crosses (ie. test each phenotype in M and F parents)
- Sex linkage is an exception to Mendel's First Law
- One (of many) genes that affect Drosophila eye color is White (W)
- All signigicant cases of sex-linked genes involve loci on the X-chromosome; Y chromosome probably encodes only some of the genes required only required for males
- Red-green color blindness is X-linked in humans, so this trait is more common in males - but not exclusive to males
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Homogametic
ZZ and XX. Males in birds, moths and butterflies and females in humans and drosophila
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Hemizygous
Genes on the X chromosome are present as only possible allele in males
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Dosage compensation
- To maintain gene balance, one chromosome in most cells of XX females is (randomly) inactivated (Xi): this is replicated, and is transmitted at mitosis, but does not express most of its genes
- Red gene in cats: redO makes orange pigment; redo does not make pigment. Female hetz are a mosaic of different colors ("tortoiseshell") depending on which allele is inactivated. Homozygotes and hemizygotes all have uniform color.
- Birds, flies have sex chromosomes but do not inactivate whole chromosomes for dosage compensation
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Genotype x Environment
Don't forget that the phenotype of some genotypes depends on the environment
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Incomplete penetrance
- Observed phenotype does not match phenotype that is expected based on genotype
- May involve either the dominant or recessive allele
- May have many different causes
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Variable expressivity
- Phenotype varies in individuals with the same genotype (can sometimes be confused with incomplete penetrance, but there are 2 clear phenotypes in incomplete penetrance, not a range of phenotypes)
- Many different causes for variable expressivity
- Can involve dominant or recessive allele
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Model organisms
- Easy to grow in labs.
- Small
- Short generation times
- Produce lots of progeny
- Mating easily controlled
- Small genomes
- Are diploid
- Yeast, Roundworm, Fruit fly, Mouse, Zebrafish, Arabidopsis thaliana
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Saccharomyces cerevisiae
Yeast
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Caenorhabditis elegans
Roundworm
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Drosophila melanogaster
Fruit fly
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Arabidopsis thaliana
A small week, the most widely studied plant genetic model organism
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