1 Nov 1 6:30 PM Ch. 14 Mendel and the Gene Idea
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Gregor Mendel = Father of Genetics 182218841843 entered monasteryfailed teacher examstudied under other scientists (Doppler and Unger), importance of math and science1857 started breeding peas
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Why peas???easy to growfast growinghave many varietiescould easily control cross fertilization
normally self pollinatorsstamen and carpels are in same flower
had easy traits to studycharacter = heritable feature
ex. flower colortrait = variant for a character
ex. purple or white color flower
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Traits Mendel chose to study:
seed color (yellow vs. green) seed shape (round vs. wrinkled) seed coat color (gray vs. white) pod shape (smooth vs. constricted) pod color (green vs. yellow) flower position (axial vs. terminal) plant height (tall vs. short) flower color (purple vs. white)chose traits that had no intermediates
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How did he conduct his experiments?
used true breeding plants = if allowed to self pollinate, would only produce offspring identical to themselves
cut the immature stamen out and cross fertilized pea plants using a paint brushcrossed two contrasting plants = hybridization
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parent plants are called P generation
hybrid offspring called F generation (Filius/Filia)
F generation crossed with another F generation= F generation
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Mendel's studies revealed two principles of heredity
1. Law of Segregationbased on four concepts:
a. alternative versions of genes (called alleles) account for variations in inherited characters
alleles are DNA variations
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b. for each character, an organism inherits two alleles, one from 1 from each parent
if true breeding alleles are sameif hybrids alleles are different
c. if the two alleles at a locus differ, then one is dominant (determines organism's appearance), the other is recessive
(no effect on appearance)
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d. Law of segregation the two alleles for a heritable character separate during gamete formation and end up in different gametes (think meiosis!)
sperm or egg only one of two alleles from somatic cell
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Terminology relating to genetics
homozygous = identical alleles for a character can be dominant or recessive
heterozygous= two different alleles for a genenot true breeders
phenotype = physiological/appearance traitsgenotype = genetic makeup
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How can we tell if a plant is a true breeder or a hybrid?
do a testcrosscross pollinate the plant with a homozygous recessive
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2. Law of Independent AssortmentEach pair of alleles segregates independently of other pairs of alleles during gamete formation (think meiosis!)
figured this out by following two characters dihybrid plants
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the law of segregation and the law of independent assortment reflect the probability of offspring
Rules of Probability if event is absolutely certain to happen probability = 1if event is certain not to happen, probability = 0otherwise, probability is between 0 and 1
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to determine the possibility of two events happening
ex. tossing two coins, each is independent of the otherWhat is probability that both will be heads?
1/2 x 1/2 = 1/4for monohybrid crosses: if heterozygote (Tt)crossed with heterozygote (Tt), what is the probability of TT?
1/2 x 1/2 = 1/4
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called the rule of multiplication*usually used when the word "and" is involved
ex. What would the probability be of a child having blue eyes and blond hair?
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Rule of Addition probability that an event will happen in two or more different ways
ex. in a dihybrid cross of heterozygous F generation (SsYy) S= spherical, s = wrinkledY = yellow, y = green
probability offspring are spherical = 3/4 (do monohybrid cross of Ss x Ss)
1/4 SS + 1/2 Ss = 3/4
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To figure out probability of spherical and yellow:
3/4 x 3/4 = 9/16 (multiplication rule)
probability offspring are yellow = 3/4(do monohybrid cross of Yy x Yy)
1/4 YY + 1/2 Yy = 3/4
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rule of addition usually uses the word "or"
ex. What would be the possibility of having a blue eyed or green eyed child?
1/4 + 1/4
*key to using these rule is that alleles segregate independently so can look at them separately
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Probability problems:What is the probability of each of the following pairs of parents will produce the indicated offspring?
a. AABBCC x aabbcc AaBbCcb. AABbCc x AaBbCc AAbbCCc. AaBbCc x AaBbCc AaBbCcd. aaBbCC x AABbcc AaBbCc
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What fraction of offspring from the cross PpYyRr x Ppyyrr would be predicted to exhibit recessive phenotypes for at least two of three characters?
P = purple flowersp = white flowersY = yellow seedsy = green seedsR = round seedsr = wrinkled seeds
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Inheritance patterns can be more complex than what Mendel studied
1. Complete dominance one allele dominates over another, phenotypes of heterozygous and homozygous dominant are the same
2. Codominance two alleles affect the phenotype in separate ways
ex. blood type AB both A and B are dominant
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3. Incomplete dominance phenotype is in between both parent phenotypes
ex. red snapdragons crossed with white snapdragons = pink snapdragons
*keep in mind that dominance refers to the ability to produce something (synthesis of something like an enzyme), recessive = defect in production of enzyme heterozygous individuals produce equal amounts of normal and dysfunctional enzymes
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TaySachs Disease cause recessive allele
unable to metabolize lipids due to missing enz., causes seizures, blindness, motor/mental problems have disease if have two recessives no disease if heterozygote
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Other conditions caused by dominant allelespolydactyly (extra fingers or toes)tongue rolling
http://www.midwestrocklobster.com/cultbucket/hand6.gifhttp://www.chkd.org/Images/CSSG/plasticsurgery/polydactyly2.jpg
http://www.und.nodak.edu/dept/biology/introlab/bio100/tongue.jpg
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Multiple Alleles for a single geneex. ABO blood types
Four types A,B, AB, Obased on enzymes that adds a carbohydrate to a red blood cell
I allele codes for enzyme that adds A carbohydrate
A
*critical for blood transfusions
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Pleiotropygenes that have multiple phenotypic effects
ex. in Siamese cats light body with dark extremities and crossed eyes
http://www.cfainc.org/breeds/profiles/siamese.html
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Situations of two or more genes1. Epistasis gene at one location affects phenotypic expression of gene at a second locationex. black fur (B) is dominant over brown fur (b)
for mouse to be brown = bbsecond gene determines whether pigment will be deposited in hair (C = dominant) if mouse is cc, then coat is white no matter what other genotype is
so, the gene for pigment deposition is epistatic to the gene coding for black or brown pigment
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2. Polygenic Inheritance an additive effect of 2 or more genes on a single phenotype character
ex. skin coloration (3 or more genes) height
polygenic skin color
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Can the environment affect gene expression?Yes!
ex. nutrition differences, exercisemost genes have a phenotypic range called a "norm of reaction"
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How are human traits affected by inheritance?
can understand them by looking at a pedigree= a diagram of a family tree showing heritable characters over many generations
*helps us predict future probability of inheriting a trait
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Recessive disorders:homozygous recessive to have the disorder if heterozygous = carrier (does not have disorder but could potentially give it to offspring)
ex if both parents = heterozygous1/4 chance of getting disorder2/3 will of possible 3 "normals" will be carriers)
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Cystic Fibrosis normal allele codes for protein that helps chloride transport between cells and extracellular fluid
if missing protein no chloride channelscauses thick and sticky mucus in lungs and digestive systemleads to poor nutrient absorption, chronic bronchitis, bacterial infectionslethal, die in 20's or 30's
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Sickle cell diseaseblood disorder in African Americans (1 in 400)
cause 1 amino acid change in hemoglobin protein
if oxygen is low red blood cells form sickle shape, clog blood vesselsheterozygotes healthy, but could suffer if oxygen is lowadvantage resistant to malaria
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Probability of passing on a disease causing trait increases the closer related you are
why we don't marry first cousins!!
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Dominant Inherited disorders
Achondroplasia form of dwarfism (1 in 25,000)homozygous dominant and heterozygous are dwarfs99.99% of people are homozygous recessive
dominant disorders causing lethal disease are less common than recessive disorders
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Huntington's Diseasedegenerative nervous system diseasedoes not affect person until 3545 years oldalready may have passed it to offspring before a person knows if they have itif one parent has the allele 50% chance child will (1 in 10,000)can now test to see if have allele
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Multifactorial Disordersgenetic component plus environmental influence
Ex. heart disease, diabetes, cancer, alcoholism, mental illness
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How can we prevent the risk of getting a genetic disorder?
genetic counseling (determining probabilities)testing ethical problemsfetal testing amniocentesis
chorionic villus samplingimaging techniques ultrasoundnewborn screening ex. PKU