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 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 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.

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

Started with monohybrid crosses meaning (one trait at a time) Then did dihybrid crosses (two different traits)

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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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). Probability of heterozygote is 1/4 + 1/4 = 1/2.

Incomplete dominance - heterozygotes have completely different phenotype than homozygotes (can happen in snapdragons) Homozygous recessive flowers - white; homozygous dominant - red; heterozygotes - pink.

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.

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.

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.

http://courses. bio. psu 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).

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.

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

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.

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.

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

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

Dominant Diseases 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).

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.

1Amniocentesis - cells extracted from amniotic fluid surrounding fetus; then analyzed to search for potential problems through karyotyping. Karyotyping - mapping out chromosomes of individual.

* 2Chorionic villus sampling (CVS) - faster karyotyping - extracts sample of fetal tissue from chorionic villi of placenta. 3Ultrasound detects only physical abnormalities present.

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.

Sex chromosomes 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).

Barr body replicated, not transcribed 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

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

http://users. rcn. com/jkimball. ma 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 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