Download Ch. 14 Mendel and the Gene Idea

Ch. 14 Mendel and the Gene Idea
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Essential Question:
How do genes affect inheritance?
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Gregor Mendel = Father of Genetics
­ 1822­1884
­1843 entered monastery
­failed teacher exam
­studied under other scientists (Doppler and Unger), importance of math and science
­1857 ­ started breeding peas
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Why peas???
­easy to grow
­fast growing
­have many varieties
­could easily control cross fertilization
­normally self pollinators
­stamen and carpels are in same flower
­had easy traits to study
­character = heritable feature
ex. flower color
­trait = 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 brush
­crossed two contrasting plants
= hybridization
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­parent plants are called P generation
­hybrid offspring called F generation 1
(Filius/Filia)
­F generation crossed 1
with another F generation
1
= F generation
2
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Mendel's studies revealed two principles of heredity
1. Law of Segregation
based 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 same
if 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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Law of Segregation
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Round and wrinkled peas in same pod
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Punnett Squares = diagrams used to predict allele composition of offspring
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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 gene
­not true breeders
phenotype = physiological/appearance traits
genotype = genetic make­up
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Genotype vs. phenotype
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How can we tell if a plant is a true breeder or a hybrid?
do a testcross
cross pollinate the plant with a homozygous recessive
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2. Law of Independent Assortment
Each 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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to test law of Independent assortment
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Dihybrid Crosses
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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 = 1
if event is certain not to happen, probability = 0
otherwise, 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 other
What is probability that both will be heads?
1/2 x 1/2 = 1/4
­for 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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Probability of getting two heads with two coins
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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 1
generation (SsYy) S= spherical, s = wrinkled
Y = 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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probability offspring are yellow = 3/4
(do monohybrid cross of Yy x Yy)
1/4 YY + 1/2 Yy = 3/4
To figure out probability of spherical and yellow:
3/4 x 3/4 = 9/16 (multiplication rule)
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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 AaBbCc
b. AABbCc x AaBbCc AAbbCC
c. AaBbCc x AaBbCc AaBbCc
d. 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 flowers
p = white flowers
Y = yellow seeds
y = green seeds
R = round seeds
r = 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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incomplete Dominance in snapdragons
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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 alleles
­polydactyly (extra fingers or toes)
­tongue rolling
http://www.und.nodak.edu/dept/biology/introlab/bio100/tongue.jpg
http://www.chkd.org/Images/CSSG/plasticsurgery/polydactyly2.jpg
http://www.midwestrocklobster.com/cultbucket/hand6.gif
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Multiple Alleles for a single gene
ex. ABO blood types
Four types A,B, AB, O
­based on enzymes that adds a carbohydrate to a red blood cell
A
I allele codes for enzyme that adds A carbohydrate
*critical for blood transfusions
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Multiple Alleles
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ABO Blood Types
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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 genes
1. Epistasis­ gene at one location affects phenotypic expression of gene at a second location
ex. black fur (B) is dominant over brown fur (b)
for mouse to be brown = bb
second 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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Epistasis
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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, exercise
most 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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Pedigrees
dominant trait
Recessive 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 = heterozygous
1/4 chance of getting disorder
2/3 will of possible 3 "normals" will be carriers) Nov 1 ­ 9:37 PM
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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 channels
­causes thick and sticky mucus in lungs and digestive system
­leads to poor nutrient absorption, chronic bronchitis, bacterial infections
­lethal, 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 vessels
­heterozygotes ­ healthy, but could suffer if oxygen is low
­advantage­ resistant to malaria
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Sickle cell
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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 dwarfs
­99.99% of people are homozygous recessive
dominant disorders causing lethal disease are less common than recessive disorders
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Huntington's Disease
­degenerative nervous system disease
­does not affect person until 35­45 years old
­already may have passed it to offspring before a person knows if they have it
­if 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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pedigree of Huntington's Disease
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Multifactorial Disorders
­genetic 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 problems
­fetal testing ­ amniocentesis
chorionic villus sampling
­imaging techniques ­ ultrasound
­newborn screening ­ ex. PKU
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