genetics

• Gregor Mendel - studied pea plants, looked at traits.

• Heritable features (characters) with different variants (traits).

• Pea plants self-fertilize; Mendel cross-fertilized to study traits.

• Mendel cross-pollinated (hybridized) 2 true-breeding pea varieties.

• True-breeding parents - P generation; hybrid offspring - F1 generation.

• F1 hybrids then self-pollinate to produce F2 generation.

http://nitro.biosci.arizona.edu/courses/EEB320-2005/Lecture02/pics/pea.jpeg

• Thought genes blended - purple flower crossed with white flower result would be light purple flowers.

• All the flowers purple.

• F1 self-fertilized, white flower reappeared in next generation.

• Ratio of purple to white in F2

generation was 3:1.

• Mendel’s hypotheses:• 1Alternative versions of genes

(alleles) account for variations.• 2Organism inherits 2 alleles, 1

from each parent - can be same or different.

• 32 alleles differ - dominant allele fully expressed in organism.

• Recessive - no noticeable effect on organism’s appearance.

• 42 alleles segregate (separate) during gamete production.

http://discover.edventures.com/images/termlib/d/dominant_allele/support.gif

• Mendel’s laws:• 1Dominance and recessiveness -

one gene is dominant over the more recessive gene.

• 2Segregation - alleles separate during meiosis.

• 3Independent assortment - alleles organize in gametes regardless of other alleles.

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http://www.switchbackfair.co.uk/encyclopedia/images/genetics_peas.png

• Punnett square predicts results

of genetic cross between individuals.

• Testcross - breeding homozygous recessive with dominant phenotype but unknown genotype. QuickTime™ and a

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• Started with monohybrid crosses meaning (one trait at a time)

• Then did dihybrid crosses (two different traits)

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• Based on probability.• Toss coin 4 times - ½ chance -

heads every time.• Tosses independent of each

other.• Coming up heads all four times -

½ * ½ * ½ * ½ = 1/16 - rule of multiplication.

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12.2

• Law of addition - chances of an event happening in different ways.

• 2 ways F1 gametes can combine to form heterozygote; dominant allele from sperm, recessive from ovum (= 1/4).

• Dominant allele from ovum, recessive from sperm (= 1/4).

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• Incomplete dominance - heterozygotes have completely different phenotype than homozygotes (can happen in snapdragons)

• Homozygous recessive flowers - white; homozygous dominant - red; heterozygotes - pink.

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• Codominance - two alleles affect phenotype in separate, distinguishable ways (example blood type)

• Inherit A allele and B allele - blood type will be AB; AA or AO - blood type will be A.

• A is dominant to O, B is dominant to O, but A is codominant to B.

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http://blogsci.com/images/ABO_blood_type.jpg

• Type A - anti-B antibodies.• Exposed to B blood - clump

together causing a transfusion reaction.

• Type O - both antibodies - can donate to any other blood type.

• AB - neither antibodies - can receive from any blood type.

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• Most genes do not control only one trait but are pleiotropic - affecting more than one phenotypic character.

• Epistasis - gene at one locus alters phenotypic expression of gene at second locus.

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http://courses.bio.psu.edu/fall2005/biol110/tutorials/tutorial5_files/figure_14_11.gif

• Example - In mice, one gene determines whether or not there will be coat color.

• If gene is turned off, mouse will be white; if turned on, another locus will determine what color is (brown or black).

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• Quantitative characteristics vary along continuum - polygenic inheritance - more than one gene controls single trait.

• Skin color controlled by at least three different genes - responsible for variety of skin colors.

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• Phenotype depends on environment.

• Humans - nutrition influences height, exercise alters build, sun-tanning darkens skin, experience improves performance on intelligence tests.

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• Geneticists use pedigrees to look at traits found in families.

• Family tree created showing absence or presence of specific trait to determine how it is passed.

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• Ethnicity plays role in genetic disease patterns.

• Sickle-cell anemia found predominately in African-Americans.

• Causes red blood cells to be sickle shaped instead of normal shape causing cells to get stuck in vessels.

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• Cystic fibrosis affects mostly Caucasians.

• Cystic fibrosis - multi-system disease - causes mucous to build up in various organs, especially lungs.

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• Tay-Sachs affects people of Jewish descent.

• Tay-Sachs affects brains of small children causing death of child prior to 5 years old.

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• Dwarfism - child 50% chance of inheriting disease because one parents has it.

• Huntington’s affects nervous system.

• Most dominant diseases not lethal (Huntington’s disease is).

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• Genetic counseling works with people that have history of genetic disease in family.

• Child with recessive disease can be born to phenotypically normal parents.

• Tests are performed to determine couple’s risk.

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• 1Amniocentesis - cells extracted from amniotic fluid surrounding fetus; then analyzed to search for potential problems through karyotyping.

• Karyotyping - mapping out chromosomes of individual.QuickTime™ and a

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• 2Chorionic villus sampling (CVS) - faster karyotyping - extracts sample of fetal tissue from chorionic villi of placenta.

• 3Ultrasound detects only physical abnormalities present.

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http://www.bookwood.com/triplets/Images/12WEEKS_ALL3.JPG

• Thomas Morgan - fruit fly eye color.

• Wild type (normal) eye color - red; mutant - white.

• Discovered mutant eye color appeared more often in males - some traits sex-linked (carried on sex chromosomes).

• Chromosomes have hundreds or thousands of genes.

• Genes located on same chromosome, linked genes, inherited together - chromosome passed as unit.

"A" and "B" are linked due to their occurrence in the same chromosome.  Similarly, "a" and "b" are linked in the other chromosome.

http://anthro.palomar.edu/biobasis/images/linked_genes.gif

• Production of offspring with new combinations of traits inherited from 2 parents - genetic recombination (can occur during crossing over)

• Genetic map - list of the loci along the chromosomes.

• Further apart genes are, higher probability they will switch places.

• 2 sex chromosomes - X and Y.• Males – XY, females - XX. • Other species - X-0 system, Z-W

system, haplo-diploid system.• Humans - X-Y system like

normal chromosomes - 50/50 chance of having male or female.

• Until embryo is 2 months old - fetus female.

• If fetus XY - SRY gene turned on making fetus male.

http://www.expectalipil.com/images/fetal_dev_2.jpg

• Sex chromosomes, have genes for traits other than sex.

• Trait recessive - female will only inherit it if both parents pass it on.

• Males - 50% chance of inheriting it (only have 1 X chromosome)

• Males have higher rate of sex-linked diseases than females.

http://www.emc.maricopa.edu/faculty/farabee/biobk/hemophb.gif

• Muscular dystrophy - sex-linked disease.

• Affects far more males than females.

• Hemophilia (excessive bleeding) - sex-linked.

http://upload.wikimedia.org/wikipedia/en/a/a3/XlinkRecessive.jpg

• Only 1 X in females turned on.• Other - Barr body - reactivated

in ovaries during egg production (to pass genes on).

• Females exhibit characteristics from mother, some from father (sex chromosomes only).

http://www.columbia.edu/cu/biology/courses/c2005/images/barr_body.19.gif

Barr body replicated,not transcribed

• Pattern responsible for mosaic of effects (tortoiseshell cats)

• Due to patches of cells expressing orange allele, others have non-orange allele (almost always female cats)

http://www.cas.muohio.edu/~wilsonkg/gene2005/images/f3p27.jpg

• 1Nondisjunction - homologous chromosomes fail to separate during meiosis I, or chromatids - during meiosis II.

• Some gametes receive 2 of same type of chromosome; another gamete receives no copy.

• Abnormal # of chromosomes - aneuploidy.

• Trisomy - gamete receives 3 of same chromosomes (2n + 1).

• Monosomy - gamete receives 1 of same chromosome (2n – 1).

• Earlier in development - more profound effect - those cells go through mitosis.

http://www.musckids.com/health_library/genetics/images/chromosome_j.gif

• More than 2 complete sets of chromosomes - polyploidy (happens more with plants)

• Species with polyploidy usually more normal than aneuploidy (no missing chromosomes)

http://emedia.leeward.hawaii.edu/millen/bot130/learning_objectives/lo15/15b_p37b.gif

A rodent species that is the result of polyploidy

• 2Deletion - piece of chromosome broken off.

• 3Duplication - fragment becomes attached as extra segment to sister chromatid.

• 4Inversion - piece breaks off, turns around, reattaches (backwards).

• 5Translocation - chromosomal fragment joins nonhomologous chromosome.

• Down syndrome - trisomy (Trisomy 21).

• Chromosome 21 - smallest chromosome - individual can survive.

• Aneuploidy can occur in sex chromosomes.

• Klinefelter’s syndrome - male XXY (nondisjunction)

• Trisomy X (XXX) can occur in females.

• Monosomy X - Turner syndrome - nondisjunction.

http://www.biology.iupui.edu/biocourses/N100/images/klinefelter.gif

http://www.tokyo-med.ac.jp/genet/kry/xok.jpg

• Deletion - cri-du-chat - chromosome 5.

• Chronic myologenous leukemia - translocation between chromosome 9 and chromosome 22.

http://learn.genetics.utah.edu/units/disorders/karyotype/images/criduchat_karyotype.jpg

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Ph1Karyotype.gif

• Some traits dependent on who passes alleles (mother or father)

• Prader-Willi syndrome caused by deletion on chromosome 15 (father)

• Deletion from mother - Angelman syndrome.

• Genomic imprinting - gene on 1 homologous chromosome silenced, allele on homologous chromosome expressed.

http://www.cytopix.com/ImageResizeCache/723578_t2001.5.25.11.38.0_q50_600x450.jpg

• Some eukaryotic genes located in mitochondria.

• These genes all passed from mother to offspring; none of father’s mitochondrial genes passed on.

Coloration due to mitochondrial genes