Bio 101Lake TahoeCommunity College
Fall QuarterInstructor: Sue Kloss
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Ch. 14
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I. Mendel used the scientific approach to identify 2 laws of inheritance
A. Mendel’s quantitative, experimental approach
1. character - heritable feature= trait (different from book)
2. bred garden peas
3. advantages to using garden peas - Mendel could control crosses (matings)
a. pollen from stamen
b. eggs in carpel
4. Mendel chose traits that held true in heredity w/ one crossing
5. true breeding - always produce offspring of the same variety
6. hybridization- crossing true breeders of 2 varieties
a. p generation = parental gen.
b. F1 = filial 1 = first offspring gen.
c. F2 = filial 2 = 2nd offspring gen from F1
7. seeing the 2 generations helped Mendel come to his conclusions
B. Law of Segregation
1. if blending occurred, then a purple flowered plant crossed w/ white flower would produce lavender
2. in true breeding parents, F1 is all purple
3. white flowers appeared in F2 generation in a 1 white : 3 purple ratio (Mendel kept track of 1000 plants)
4. he called it the “heritable factor”
5. he called the purples “dominants”, and the whites “recessive”
6. Mendel observed the same patterns with 6 other traits
7. Mendel’s model - 4 related concepts
a. alternative versions of the same genes account for variation in inherited characters
1. different alternatives of the same genes are called alleles
2. each gene resides at same locus on homologous chromosomes
b. for each character, an organism has 2 alleles, one from each parent
1. remarkably, Mendel figured this out before the world knew about chromosomes
2. the 2 alleles for each gene may be the same, or they may be different
c. if 2 alleles at a locus differ, then 1, the dominant, determines the organisms appearance. the
recessive allele has no effect on the organism’s appearance
d. law of segregation states that 2 alleles for heritable characters separate in gamete formation
and end up in different gametes
1. a purple flower gamete has a 50-50 chance of being fertilized by either a purple flowered
or white flowered gamete
2. use a punnett square to illustrate the combinations
a. P = dominant Purple color
b. p = recessive white color
8. genetic vocabulary
a. homozygous - two copies of the same allele - true breeders
b. heterozygous - two different alleles - not true breeders
c. phenotype vs. genotype
d. phenotype ratios - 3:1; genotype ratios - 1:2:1
9. test cross - cross a true breeder with an unknown
C. Law of independent assortment
1. monohybrids - true breeders for 1 cross
2. Mendel followed 2 traits at a time
a. he knew that yellow seeds (Y) are dominant and green seeds (y) are recessive
b. he also knew that round seeds ( R) are dominant and wrinkled ( r) are recessive
c. Will W and R stay together in many generations?
d. dihybrid cross - test plants differ in 2 traits
3. assort independently - each pair of alleles segregates independently of other alleles in gamete
formation
a. phenotype ratios of F2 will be 9:3:3:1
II. Laws of probability govern Mendelian inheritance
A. Laws of Probability govern Mendelian inheritance
1. probabilities range from 0 to 1
2. all probabilities of a particular event must add up to 1
a. the probability of drawing a particular card, say ace of spades from a deck, is 1/52.
b. the probability of flipping a tail when flipping a coin is 1/2
3. each coin toss or other occurrence is an independent event, separate from every other event
4. alleles of one gene segregate into gametes independently of another gene’s alleles (Law of
Independent Assortment)
B. Multiplication and Addition Rules Applied to Monohybrid crosses
1. How do we determine the probability that 2 separate events will occur together, say the probability
that 2 coins tossed simultaneously will both land heads up?
2. Multiplication rule states that we multiply the probability of one event by the probability of the other event
3. prob. that both coins will land heads up is 1/2 x 1/2 = 1/4
4. we can determine the probability for combinations of alleles w/ a Punnett square
5. to determine the probability that a plant from an F1 cross will be heterozygous, we use the addition rule
a. Rr 1/4 + Rr 1/4 = Rr 1/2 of offspring have probability of being heterozygous
6. We can use this info to calculate more complex genetics problems than we can by doing dihybrid crosses
III. Inheritance Patterns are often more complex than predicted by simple Mendelian genetics
Not all genes work in the manner outlined by Mendel
A. Extending Mendelian genetics for a single gene - when genes have more than 2 alleles, or are not
dominant/recessive
1. Alleles can show different degrees of dominance and recessiveness toward each other
2. this range is called the spectrum of dominance
3. Mendel used traits reflecting presence of genes with complete dominance
a. phenotypes of homozygous dominant and heterozygotes are indistinguishable
4. another extreme is codominance - 2 alleles both affect phenotype in separate distinguishable ways
a. both genes expressed in the trait
b. eg. blood type - proteins from both parental alleles are expressed in offspring
5. incomplete dominance - red snapdragons crossed with white = pink snapdragons- neither allele dominates
the phenotype
a. if blending occurred, you would never get the red or white back from subsequent generations, and
this does not occur
B. Relationship between dominance and phenotype
1. alleles are variations in a gene’s nucleotide sequence
2. dominant and recessive genes don’t interact at all, their products manifest themselves in particular ways
a. ex - round seed shape - a normal enzyme that converts sugar to starch;
b. wrinkled seed shape - that enzyme is defective (mutated) - causes seed to swell bc of high sugar
content and osmotic uptake of water, then when it dries out, it wrinkles; round seeds don’t take up
excess water, since sugars converted to starch when produced
c. in heterozygotes, 1 dominant allele makes an enzyme that converts enough of the sugar so that the
seed doesn’t swell so heterozygotes and homozygous dominants have same phenotype
3. observed dominance/recessive, codominance or incomplete dominance is relative depending on whether
you look at level of phenotype or gene expression
a. Tay Sachs disease - heterozygotes have a mix of messed up enzymes and normal ones, but
having only half the normal enzyme is enough to keep you from getting symptoms of the disease
4. Frequency of dominant alleles
a. polydactyly is dominant, 399/400 people have 2 recessive genes for this trait
5. Multiple alleles - Mendel looked at traits that had 2 alleles, but most genes have more than 2 alleles
a. blood type in humans- 4 phenotypes = A, B, AB, O
b. refer to 2 carbohydrates on surface of rbcs
c. I = enzyme that attaches the carbo to the cells
d. 6 genotypes and 4 phenotypes are possible
e. matching compatible blood types is critical; a person w/ type A blood who gets a transfusion of
B or AB blood will have an immune system attack on the “foreign” blood cells
6. Pleiotropy - most genes have multiple phenotypic effects - a single gene affects multiple characteristics
C. Mendelian genetics - Two or more genes affect a particular phenotype
1. Epistasis - a gene at one locus alters phenotypic expression of a gene at another locus
a. in mice, B = black coat, b = brown coat
b. a second gene determines whether coat will have any pigment at all; C = color, c = no color
c. gene for pigment deposition is epistatic to gene for color of pigment
2. Polygenic interaction - many genes affect phenotype, resulting in continuum (quantitative traits)
a. Fig. 14.12 - dominant alleles contribute “units” to darker skin color
b. bell shaped curve with intermediates in most frequency (heterozygotes)
3. Nature + Nurture - an individuals phenotype is result of G + E in many cases.
4. emergent properties of individuals = complex mix of many factors
Lesson Objectives
1. Explain how Mendel’s particulate mechanism differed from the blending theory of inheritance.
2. Define the following terms: true-breeding, hybridization, monohybrid cross, P generation, F1 generation, and F2 generation.
3. List and explain the four components of Mendel’s hypothesis that led him to deduce the law of segregation.
4. Use a Punnett square to predict the results of a monohybrid cross, stating the phenotypic and genotypic ratios of the F2 generation.
5. Distinguish between the following pairs of terms: dominant and recessive; heterozygous and homozygous; genotype and phenotype.
6. Explain how a testcross can be used to determine if an individual with the dominant phenotype is homozygous or heterozygous.
7. Use a Punnett square to predict the results of a dihybrid cross and state the phenotypic and genotypic ratios of the F2 generation.
8. State Mendel’s law of independent assortment and describe how this law can be explained by the behavior of chromosomes during meiosis.
9. Use the rule of multiplication to calculate the probability that a particular F2 individual will be homozygous recessive or dominant.
10. Given a Mendelian cross, use the rule of addition to calculate the probability that a particular F2 individual will be heterozygous.
11. Explain why it is important that Mendel used large sample sizes in his studies.
12. Give an example of incomplete dominance and explain why it does not support the blending theory of inheritance.
13. Explain how phenotypic expression of the heterozygote differs with complete dominance, incomplete dominance, and codominance.
14. Explain why Tay-Sachs disease is considered recessive at the organismal level but codominant at the molecular level.
15 Explain why genetic dominance does not mean that a dominant allele subdues a recessive allele. Illustrate your explanation with the use of round versus wrinkled pea seed shape.
16. Explain why dominant alleles are not necessarily more common in a population. Illustrate your explanation with an example.
17. Describe the inheritance of the ABO blood system and explain why the IA and IB alleles are said to be codominant.
18. Define and give examples of pleiotropy and epistasis.
19. Describe a simple model for polygenic inheritance and explain why most polygenic characters are described in quantitative terms.
20. Distinguish between the specific and broad interpretations of the terms phenotype and genotype.
21. Describe how environmental conditions can influence the phenotypic expression of a character.