Category Archives: Genetics
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Test 1 on February 13 over 2, 3,4, & 5
-Most phenotypic traits such as height, weight, and pigmentation are the result of many genes interacting.
-Mendel was very fortunate to have picked single gene influenced traits.
Two ZGene Interaction
(same results) 9:3:3:1
(diference: interaction of 2 genes influencing a single trait
R rose comb is dominant to r
P pea comb is dominant to p
R and P are codominant (walnut comb)
rrpp produce a single comb
Bateson and Punnet bred rose comb (RRpp) to a pea comb (rrPP). All the F1 offspring had a walnut comb (RrPp).
-Nearly a dozen enzymes interacting in this single pathway resulting in a single phentypic product
-Each enzyme the result of the expression of one or multiple genes.
Epistasis (simplified example)
-Alleles of one gene masks the phenotypic effects of the alleles of another gene.
Can have two or more genes coding for different protein in an enzymatic pathway producing a single product.
Colorless precursor > Enzyme C > Colorless Intermediate > Enzyme P > Purple Pigment
-If one piece falls out, the whole process falls down
C (enzyme C) dominant to c
P (enzyme P) dominant to p
cc or pp mask the P or C alleles producing white
CCpp (variety 1 white) x ccPP (variety 2 white) = CcPp all purple
Ch 5 p101-117
Linkage and Genetic Mapping in Eukaryotes
-Thousands of different genes, but relatively few chromosomes
-Genes located to close together on the same chromosome violates the law of independent assortment
-Any method used to determine the relative positions of genes on a chromosome and the distance between them.
-Gene mapping allows us to determine if genes are “linked”
“linkage” is used in two ways
-Two or more genes located on the same chromosome [i.e. linkage group]
-*Genes that are close together on the same chromosome that tend to be transmitted as a unit are also referred to as linked.
Genes of the smae linkage group (if they are located far apart) can independently assort due to crossing over during meiosis I
Note: gene A & B assorted independently of C despite being on the same chromomsome
Haploid cells with new allele combination are called nonparentals or recombinants vs. those having same combination as parents called parentals or non-recombinants
The phenomenon of crossing over was discovered when crosses failed to follow simple Mendelian patterns of inheritance
Genetic Linkage Mapping
-The closer two genes are on a chromosome, the less likely they will become separated during a “crossing over” event
-Thus: *there is a correlation between the number of recombinant gametes and the distance between two genes on the same chromosome.*
Which genotype would you expect to find more often among gametes from this heterozygote individual?
If you did a dihybrid crosses involving genes R&B and a separate dihybrid cross involving genes B&H, which cross would produce more recombinant offspring?
Expected ration in F2 generation of a dihybrid cross; if the alleles independently assorted
-Bateson and Punnet’s dihybrid cross in F2 Generation
Expected a 9:3:3:1 ratio, observed a 15.6:1.0:1.4:4.5 ration (offspring had a much greater proportion of the parental phenotypes)
Explanation: these two genes must not be assorting independently. These two genes must be linked together (assorting as a unit)
But how did they know that the difference in the rations they observed and expected was not due to sampling error?
Chi Square Analysis
Hypothesis must be stated in the negative; hence, Null Hypothesis (H0).
There is no difference between the observed ratios and the expected ratios.
P=<5% reject P=>5% fail to reject
Bateson and Punnet’s Offspring P<1% thus rejected H0
Genetic Linkage Map
The percentage of recombinant offsprings correlated to the distance between 2 genes
Map distance=Number of recombinant offspring/total number of offspring x 100
Units of distance are called map units (mu) or centiMorgans (cM)
One map unit is equivalent to a 1% recombination frequency
Figuring distance between A and B
AaBb x aabb
Crossing over in the case of this homozygous individual could not produce a recombinant gamete
What would be the linear order of these three genes given the following map distances?
Map distance between Sh & bz = 2.0cM
Map distance between C & bz = 5.0cM
Map Distance between C & Sh = 3.0cM
Caution: as percentage of recombinant offspring approaches a value of 50%, this value becomes a progressively less accurate measurement of map distance. This is due to multiple crosses in regions between genes that are far apart.
5 chiasmata in a grasshopper tetrad.
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-Two alleles produce intermediate results; heterozygotes have intermediate phenotype
-50% of the normal protein is not sufficient to produce the same phenotype as a homozygote
Multiple alleles vs. single pair alleles
Cch=chinchilla pattern (allele produces partial defect)
Ch=himalayan pattern (temp sensitive; extremities only)
c= albino (defective allele that cannot produce pigments)
C dominate to cch, ch, and c
cch recessive to C but dominant to c and ch
ch is recessive to C and cch but dominate to c
c is recessive to C, Cch and Ch
Other examples of multiple allele
-Siamese cats (Himalayan pattern)
- Brown Swiss Cattle (opposite of himalayan pattern)
Alleles of ABO blood group
-Can be dominant, recessive, or codominant
- ii= type O blood
- IAIA or IAi = type A blood
- IBIB or IBi = Type B Blood
- IAIB = type AB blood (codominant)
- Codominance: a phenomenon in which two alleles are both expressed in the heterozygous individual
Incomplete Dominance vs Codominance
-In incomplete dominance a blending of the two alleles produces a third phenotype (white x red = pink)
-Codominance is the equal expression of both alleles (white x red = white spotted) ex. AB blood type equal expression of both alleles
Overdominance (heterozygous advantage)
-Due to two alleles that produce proteins with slightly different amino acid sequence
-Allows individual to function under broader range of environmental conditions (more adaptable)
-Less susceptible to specific pathogens/disease (more difficult to recognize and compete)
Hybrid Vigor (heterosis)
-Different from overdominance because of the involvement of many genes
-Hybrids display traits (size, weight, growth rate, disease resistance) that are superior to parental strains
-Intraspecific (within a species) crossing of sub-species, varieties, breeds, stains, or populations.
Interspecific (between species)
Interspecific hybrids usually sterile. Why?
-Phenomenon in which a dominate allele does not always “penetrate” into the phenotype of the individual
-Described at the POPULATION level (ex 60% penetrant trait = 60% of heterozygotes express this phenotype)
-Individual either has the trait or does not have the trait
-Expression of dominant trait may skip generations
Example of Incomplete Penetance & Variable Expressivity
-An allele may not be expressed to the same degree in all individuals: Polydactyly
Environmentally influenced traits; Expression influenced by Environment
-Remember the Himalayan pattern? (temperature influenced)
-Hydrangea, blue or pink? Expression of pigment color influenced by pH of soil
Sex influence inheritance
-Allele can be dominant in one sex and recessive in the opposite sex
-Baldness: autosomal trait (not X-linked)
BB Bald Bald
Bb Bald Nonbald
bb nonbald nonbald
Sex limited traits
-Trait occurring in only one of the sexes (ex breast development, beard growth, bright plumage in birds, etc).
-Difference between sex limited or sex-influence vs X-linked inheritance is where the gene is located (autosomal vs sex chromosome)
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Cytokinesis (Division of cytoplasm)
- animal cell
- begins shortly after anaphase
- contractile ring
- myosin motor proteins
- actin filaments squeezed by the myosin motor proteins tighter and tighter to make the contractile ring
Golgi-derived vesicles carry cell wall material to cell plate (figure 8-8)
Vesicles follow tracks or microtubules and actin to site
Fusion of vesicles forms plate toward outer walls
- Gamete (n): egg or sperm/haploid
- Fertilization: fusion of gametes
- Zygote (2n): 1st cell resulting from union of gametes
- Isogamous: gametes of equal size, structure and motility
- Heterogamous: gametes different in size, structure and motility
- Oogamous: larger non-motile egg; smaller motile sperm (humans)
Object of Meiosis
Diploid cell (2n) to Haploid cell (n)
Not just any half will do! One homologous chromosome from each set
Superficially similar to mitosis, but involves two successive divisions after G1, S, G2: Phase I & Phase II
Focus on Prophase I (5 subdivisions)
-Sister chromatids condense and become visible
-Often take on a horse shoe shape; ends associated with proteins on the nuclear lamina
Synapsis: homologous sister chromatids join along their lengths
-The two joined sister chromatids are now knows as “bivalents”
2 pair of sister chromatids
4 total chromatids
Also known as “tetrad”
-Joined by synaptonemal complex
can begin anywhere
homologous genes must be aligned
-Crossing over occurs; recombinant DNA- major place where exchange occurs
-breakage and rejoining of chromosomal strands
-Specific Enzymes Involved:
-DNA ligases (glue ends)
-DNA polymerases (repair)
Chiasma: site of crossing; shaped like the greek letter chi (X)
-Synaptonemal Complex dissolves
-Bivalents come apart
-In females, meiosis is arrested at this stage when female embryos are 5 months and will not resume process until puberty
-All that remains of Synaptonemal Complex is gone
-Nuclear membrane disappears completely
-Nucleolus (dark stained area of nucleus, where ribosomes are produced) disappears
-Official end of Prophase I
Meiotic Division, DNA replication and recombination Bivalents or tetrads
Cell division 1 Separation of homologous bivalents (2n=46)
Meiotic division 2 No replication
Cell division 2 Separation of sister chromatids (n=23)
Gametes (n) not genetically identical
We call this gametogensis because general production of a gamete
Oogenesis in Humans
Dictyate: arrested at primary oocyte (diploid) will not continue until female enters puberty
Polar Bodies: Genetic material viable
Polar Body Twins? 50% identical
monozygotic twin (identical) dizygotic twin
PGD & PGS (preimplantation genetic diagnostics / screening)
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Checkpoints: Controls That Ensure the Order of the Cell Cycle Event
Leland Hartwell and Ted Weinhert
Science : Nov. 3 1989, pp. 629-634
DUE ONE WEEK FROM TODAY. NEXT WEDNESDAY
-G0 Phase (aka quiescent phase): cell arrested in permanent resting state
-Cell spends most time in interphase- not cell division
-During mitotic phase, there is no gene expression occurring
-The chromosomes have became so compacted, (able to see in microscope) can’t get mechanisms (polymerases) down there to even transcribe those genes.
Interphase: G= gap & S = synthesis
-G1- increases cell size, RNA, & proteins
-S- chromosomes doubles (sister chromatids)
-G2- further cell growth &protein synthesis
G1 =46 chromosomes
G2= 92 chromatids
Sister chromatids are joined together at centromere
Kinetochore(protein) outside, microtubule attachment site
-Coined term “Mitosis” (Greek mitos= thread)
Mitotic Spindle Apparatus
Protein units “tubulin” assemble and disassemble.
3 types of Mictotubules
-Asters- (star look) Microtubules going opposite direction of chromatids. Help position centrosomes secure where they need to be
-Kinetochore Microtubules- Grab kinetochore and pull chromatids apart – see microtubule structure fig.
-Polar Microtubule- Doesn’t grab chromatid. Actually goes past sister chromatids and goes to the opposite pole. Pushes poles apart to have separation (oblong shape in division).
Division of Nucleus
Subdivision of Mitosis:
-Nuclear membrane begins to disassociate into smaller vesicles
-Centrosome migrate to opposite poles
-Mitotic spindles can begin to form
-Centrosomes reach the opposite poles (future daughter cells)
-All sister chromatids are captured at kinetochore by microtubules from opposite poles
-Dynamic instability- microtubules growing and shrinking
-Sister chromatids pulled in opposite direction by depolymerizing kinetochore microtubules
-Sister chromatids aligned at plate facing opposite poles
-Polar microtubules are pushing poles apart
-Sisters are pulled apart as can now be called chromosomesDepolymerization of kinetochore microtubules more chromosomes to opposite poles.
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Mendel postulated 3 important ideas from his results.
Dominate and recessive traits vs “blending theory”
Particulate theory of Inheritance (genes that govern traits as discrete units and remain unchanged) – because the white that was completely lost in the F1 generation reappeared in F2.
Genetic determinants segregate from each other during the process that gives rise to gametes.
Mendelian Genetics Today
-Chromosomes contain short segments of DNA that function as units of heredity called genes.
-Most eukaryotes species have pairs of chromosomes (one from each parent)
-Each pair of chromosomes may have identical or different forms of a gene (alleles). Eye color, skin pigment and many others have multiple alleles. (multiple genes acting together)
-Homozygous: individual with the same form (allele) of a gene on both chromosomes dominant(BB) or recessive (bb)
-Heterozygous: individual with different forms (allele) of a gene on each chromosome (Bb). Multiple alleles = write allele as exponent
-Genotype: genetic makeup or composition of an individual (BB, Bb, bb)
-Phenotype: observable trait of an individual (ex Brown hair)
-Two genotypes = BB, Bb
-One phenotype= brown hair
Law of Independent Assortment
“Two different genes will randomly assort their alleles during the formation of haploid reproductive cells.” Ex. Crossing over, random paternal/maternal metaphase alignment
Punnett Square- Reginald Crundall Punnett (1875-1967)
-English Zoologist who helped establish a genetics program at Cambridge university
-Worked with William Bateson to incorporate statistics into Mendelian genetics (eg. Punnett square)
-Worked with mathematician G. H. Hardy inspiring what later became as the Hardy-Weinberg equation.
-Genophagy??- eventually will all become one dominant trait. Blondes extinct?
Chapter 3 Reproduction of Chromosome Transmission
-Cytologist vs. Geneticist
-Cytologist- completely with chromosomes, bigger picture, disorders having to do with chromosomes (forest)
-Geneticist- wants to look at the genes itself, nucleotide sequence, mutations, transposons (trees)
-Prokaryotic vs. Eukaryotic
-Prokaryotic- chromosomes are circular, nucleoid
-Eukaryotes- linear chromosomes, nucleus
-Somatic vs. Gamete
-Somatic- 2n, body, diploid
-Gametes- n, sperm or egg, haploid
-Karyotype: photographic representation of the chromosomes found within actively dividing cell. In karyotype, the homologues are arranged by largest to smallest.
Homologues: nearly identical members of paired chromosomes. Mom and dad.
Locus (plural= loci)- physical location of a gene
-p represents short arm of chromosome
-q represents long arm of chromosome
Example: Sex determining region: Yp11.3
-Y- which chromosome
-p- short arm
-11.3- position on chromosome
Chromosomes must be:
Cell division is necessary in asexual reproduction & for muilticellularity (growth)
Binary Fission (Bacteria)
-Does not involve the fusion of gametes.
Eukaryotic Cell Division
-Goal: Two daughter cells with the same number and types of chromosomes as mother cell.
-In 1882 first to describe chromosomes and mitosis. (looked like colored pieces of thread)
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Pre-Mendelian concepts of heredity
-Pangenesis- Hippocrates 400 B.C.
-Spermists vs. Ovists—spermists believed in homunculus (man inside sperm)
-Blending theory- idea that each parents/plants blended together to produce offspring
Mendel’s Opposition to Evolution and to Darwin
the Journal of Heredity, 1996,
1 page synopsis due by wednesday, January 25
Important Plant Breeding Terms
-Hybridization – crossing of two individuals with distinct characteristics
-Hybrid – an offspring resulting form the cross between parents of different species
-Self-fertilization- fusion of male and female gametes produced by the same individual
-Cross-fertilization- fusion of male and female gametes produced by different individuals
-True Breeding Line- a kind of breeding in which the parent with a particular phenotype produce offspring only with the same phenotype.
-Single Factor Cross- offspring are known as monohybrids
-Parental (P) generation > F1 generation- first > F2 generation- second
Is there a natural (predictable) pattern to the regularity with which certain parental trait appeared among offspring?
If there is a pattern, who established the pattern? if there is design, who was the designer… battled with Darwin and his theory of evolution
Basically crossed two true breeding plants (purple and white) and NEVER got a blend. This disproves the blending theory.