Bio 207

A position on a chromosome; may contain a gene

Alternative versions of the same gene

Two copies of the same allele (in a diploid)

Allele whose phenotype is seen only in one homozygote

Hetz is intermediate between two homz

Hetz has the phenotype of both hoz parents simultaneously

The allele, genotype, or phenotype that is found most often in natural populations

Allele whose phenotype is seen only in one homozygote

One wt allele allele produces enough product to have same phenotype as two wt alleles

One wt allele does not produce enough product for same phenotype as two wt alleles

Many plants and some animals are polyploidy (more than 2 complete sets of homologous chromosomes)

A type of polyploidy that occurs in only some cells and is not inherited, also allows cells to grow very large

Mitochondria and plastids have their own, circular chromosomes; usually many copies of this chromosome in each organelle; usually have uniparental inheritance

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

• Monohybrid cross: (hetz x hetz: Aa x Aa)
• Testcross: (unknown genotype x recessive homozygote: AA x aa OR Aa x aa)

Scientific study of heredity: white flower trait can be inherited even from two purple flowers; particulate (not blending) inheritance); genes are units of inheritance

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)

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

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

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

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

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

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

• 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)

• 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

• Come from different parents
• These usually carry the same genes in the same order (but may have different alleles)

• Are made by replication
• These start out carrying the same genes AND the same alleles

• 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

• 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

• 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

• 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

• 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

• A type of polyploidy that occurs in only some cells and is not inherited
• Allows cells to grow very large

• Mitochondria and plastids have their own, circular chromosomes
• Usually many copies of this chromosome in each organelle
• Usually have uniparental inheritance

• 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

• 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

ZZ and XX.  Males in birds, moths and butterflies and females in humans and drosophila

ZW and XY

Genes on the X chromosome are present as only possible allele in males

• To maintain gene balance, one chromosome in most cells of XX females is (randomly) inactivated (X_i): this is replicated, and is transmitted at mitosis, but does not express most of its genes
• Red gene in cats: red^O makes orange pigment; red^o 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

Don't forget that the phenotype of some genotypes depends on the environment

• 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

• 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

• 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

Yeast

Roundworm

Fruit fly

Mouse

Zebrafish

A small week, the most widely studied plant genetic model organism