Chapter 14 - Gregor Mendel and the Gene Idea

A. Bergeron+AP BiologyPCHS

In fruit flies a recessive mutation in either of two independently assorting genes, brn and prp prevents the synthesis of wild-type (i.e. normal) red eye pigment. Homozygotes for either of these mutations have brownish purple eyes. But heterozygotes for both of these mutations have wild type eyes.

Hint: Let the wild type alleles of each gene be brn+ and prp+,respectively.

Do Now - Now Where Did I Put that Bug Spray?

3) Where would you predict the brn and prp genes to be located in relation to one another?

1) If double heterozygotes are crossed with each other what are the phenotypes and expected phenotypic ratios of the offspring?

2) Which mode of inheritance seems to be at play in this problem?

Mendel’s Conclusions (deduced from a series ofhypotheses that he tested experimentally)

1. Alternative versions of genes (i.e. alleles) account for variations in inherited characters/traits

2. An organism inherits two alleles (two versions of the same gene) from each parent (see next slide)

3. Dominant and recessive alleles for each character/trait may exist (But what does it mean to be dominant? Recessive?)

4. The alleles for each character/trait segregate during gamete production (i.e. meiosis)

Two Alleles for Every Trait (most of the time…)

Recall that offspring inherit one chromosome (of each pair)from Mom and Dad

Together, these chromosomes make-up a homologous pair

The location (i.e. locus) of a particular gene isthe same on bothchromosomes but the allele may be different

Allele: alternative forms of the same gene

Mendel’s Genetic “Laws”1. Principle of SegregationAlleles for different traits segregate independently (as long as the alleles are on SEPARATE chromosomes)Equal probability of passingon either allele to the F2 generation (see example)Segregation of allelic pairs occurs during meiosis (anaphase I)Restoration of paired allelesoccurs at fertilization

Mendel’s Principle of SegregationMendel hypothesized thatthere were two factors (i.e.alleles) for a particular trait

Each parent passes on oneallele to their offspring

But why?

Recall that homologous chromosomes separateinto different cells during meiosis I

As a result, each gametewill receive only only allelefor a particular trait

Useful Genetics Vocabulary1. Homozygous - Having two identical alleles for a particular trait

-Homozygotes are “true-breeding” or pure bred organisms -Can only pass on one type of allele to offspring

2. Heterozygous - Having two different alleles for a particular trait! -Heterozygotes are often referred to as hybrids or! carriers

3. Phenotype - An organism’s traits that are based on its genotype (Note: Phenotype is not necessarily the outward, physical appearance)

4. Genotype - The genetic make-up of an organism! -Genotype determines phenotype

Mendel’s Genetic “Principles”2. Principle of Independent Assortment

Each allele pair segregates independently of the other allele pairs during gamete formation (i.e. meiosis)

Mendel’s Law of Independent Assortment(Assume genes are on separate chromosomes)

MomDad

A aB b

A a

Bb

A

b

a

B

Ab

Ab

aB

aB

AB

ab

AB

AB

ab

ab

Sex cells

Independent Assortment

Important Genotypic and Phenotypic RatiosType of Cross

Parent Genotypes

Genotype Ratio

Phenotype Ratio

Monohybrid Heterozygous 1:2:1 3:1

Dihybrid Heterozygous(for both traits)

Complicated 9:3:3:1

Ratios can be predicted with a Punnett square (but Punnettsquares are evil!) and there is an easier way…

Do Now - Using Probability to Solve Genetics Problems

What is the probability that in a trihybrid cross between two organisms with the genotypes AaBbCc and AaBbCc that the parents will produce an offspring with the genotype aabbcc? (Hint: Consider each pair of alleles as a single “coin flip”)

What is the probability that the two parents will produce anoffspring that is dominant for the traits controlled by genes A and C but recessive for the trait controlled by gene B?

Using Probability to Solve Genetics ProblemsThe probability of an event ranges from 0 (not going to happen)to 1 (certain to occur)

The probabilities of all outcomes of an event must add up to 1

Random events are independent of one another! -The outcome of one event does not influence the ! outcome of a previous or future event

Using Probability to Solve Genetics ProblemsRule of Multiplication

-The probability that independent events will occur simultaneously is equal to the product of their individual probabilities

Question - In a Mendelian cross between pea plants that areheterozygous for flower color (Pp), what is the probability thatthe offspring will be homozygous recessive?

Question - What is the probability that in a trihybrid cross between two organisms with the genotypes AaBbCc and AaBbCc that the parents will produce an offspring with the genotype aabbcc? (Hint: Consider each pair of alleles as a single “coin flip”)

Question - Determine the probability that you will role two “6’s” in a row on a single die. Three “6’s?” Seven “6’s?”

Using Probability to Solve Genetics Problems2. Rule of Addition (Either-or Rule)

-Probability that an event will occur is the sum of the probabilities of each way that it can occur.-Useful when you want to know the probability of an event happening when there are several ways for the event to occur

Question - If pea plants with the genotypes PpYyRr and Ppyyrr are crossed, what is the probability that offspring will be recessive for at least 2 of the 3 traits?

Question - What is the probability of tossing three coins simultaneously and obtaining two heads and one tail?

Question - If two parents are heterozygous for a trait, what is the probability, that their child will be heterozygous as well?

Using Math to Solve Genetics Problems(Wait…Mr. B Knows Math?)

If only two alleles exist for a particular character/trait, the distribution ofphenotypes among the offspring of a cross can be modeled with a binomial distribution.

Question - Imagine that a couple has chosen to have 3 children. How likely is it that two of the children will be boys and one will be a girl?

Family Composition3 boys

2 boys and 1 girl

1 boy and 2 girls

3 girls

Order of birthbbb

bbg bgb gbb

bgg gbgggb

ggg

Calculationp x p x p

p x p x qp x q x pq x p x p

p x q x qq x p x qq x q x p

q x q x q

Prob.p3

3p2q

3q2p

q3

Let p = probability of having a male childLet q = probability of having a female child

p + q = 1p = q = 1/2

3p2q = 3 (1/2)2(1/2) = 3/8

Question - Calculate the probability that two parents heterozygous for the recessive allele producing albinism (a) will have one albino child in a family of five.Question - Imagine that a couple has chosen to have five children. How likely is it that three of the children will be boys and two will be girls?

Genetics Practice Problems

Assume that the ability to hear is controlled by 3 pairs (6 alleles) of independently assorting genes and that normal hearing occurs when an individual has the genotype A_ or B_. The presence of DD, however, causes deafness, no matter what other allelic combinations are present.

Question: Predict the phenotype ratios of the offspring that would be produced if the above parents were to have children.

Question: Write out all of the possible genotypes of the offspring produced from a mother with the genotype AABBDd and a father with the genotype AaBbDd using the underscore format.

Flower color in pea plants is determined by two gene pairs (A/a and B/b). For the first gene pair, the ‘A’ allele is dominant to the ‘a’ allele, while for the second gene pair, the ‘B’ allele is dominant to the ‘b’ allele. If at least one dominant allele from each gene is present, the flowers are purple. All other genotypes have white flowers.

Genetics Practice Problems

1. What is the phenotype of a plant that has the Aabb genotype?

2. Using the symbols provided above, write all possible genotypes for a plant that has purple flowers.

3. If two plants that have the AaBb genotype are crossed, what is the phenotypic ratio of the offspring?

Genetics Practice Problems

Fur color in mice is governed by genes concerned with producing and distributing melanin. At one gene location, a dominant allele (B) specifies dark brown and a recessive gene (b) specifies a tan coat. At another gene location, a dominant allele (C) shuts down melanin production while a recessive allele (c) allows for the deposition of melanin.

4. Show the expected phenotype ratios in the offspring resulting from crossing a true breeding (i.e. homozygous) brown mouse with an albino mouse that is heterozygous at both loci.

5. What are the possible genotypes of the offspring?

Genetics Practice ProblemsPhenylketonuria (PKU) is an inherited disease determined by a recessive allele. If a woman and her husband are both carriers, what is the probability of each of the following?

6. All three of their children will be normal.

7. One or more of the three children will have the disease.

Given two cats with the genotypes llSsdd and LlSsDd, what is the probability of the following? (Short hair is dominant to long hair, white spotted coat is dominant to not spotted, and diluted color is dominant to non-diluted color).

8. A cat having short hair

9. A cat having the genotype llssdd

10.A cat having the short hair, white-spotted, and non-diluted phenotype

Do Now – Non-Mendelian Inheritance Patterns

1. What is the principle difference between codominance and incomplete dominance at the phenotypic level? At the cellular level? At the molecular level?

2. In Labrador retrievers, brown fur color is dominant over yellow fur color. A dog breeder wishes to produce brown puppies rather than yellow puppies. She has two brown males and one yellow female. What could she do to verify that her male “studs” will be more likely to produce brown puppies?

What Does It Mean to Be Dominant? Recessive?Recall that genes encode the instructions for making a particular protein

Dominant alleles can be thought of as the instructions for synthesizing a “normal, functional” protein! Although there are exceptions to this rule...! Example: Dominant negatives

Typically (but not always) recessive alleles encode instructions for making a non-functional protein

If a protein is non-functional, it will affect the organism’s phenotype

Mendelian Genetics Extensions Incomplete Dominance

Neither allele is “completelydominant”

Organisms that possess theheterozygous phenotypehave a phenotype which is intermediate between the two“extremes”

Does not support blendinghypothesis because original phenotypes will reappear ifF1 organisms are crossed

Genetic Disease (Tay Sachs)

Tay Sachs disease is a recessively inherited disease in humans

Brain cells of Tay Sachs babies lack a lipid-metabolizing enyzme.! -Lipids accumulate in brain causing disease symptoms

Heterozygotes are symptom-free! -Normal allele appears completely dominant and Tay! Sachs allele is recessive

However, heterozygotes actually make “equal” amounts of the normal and dysfunctional enzymes! -The presence of the normal enzyme prevents ! heterozygotes from developing the disease (i.e. gene ! dosage effect)

Extensions of Mendelian Inheritance Patterns - Codominance and Multiple Alleles

More than one allele contributes to the phenotype of an organism however phenotype is not intermediate between two“extremes”

Example: Human blood types

Complete Dominance

(A is Dominant)

AA and Aa have the same

phenotype

IncompleteDominance

(A is IncompletelyDominant)

Aa = intermediate phenotype.Phenotype is intermediatebetween the phenotypes of the two homozygotes

Codominance(No “pure” dominance)

Aa = Both alleles areequally expressed

in phenotype

The Dominance Spectrum

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Do Now - Cholesterol Metabolism

1. From which type of biological molecule is cholesterol derived?

2. How do you think cholesterol is transported through the body?

3. What are some of the advantages and disadvantages of cholesterol in the body? Try to be as specific as possible.

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Endocytosis

• Internalization of substances by formation of a vesicle

• Types– Phagocytosis– Pinocytosis– Receptor-mediated

endocytosis

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Pinocytosis and Receptor-Mediated Endocytosis

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HMG-CoA Reductase

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Cholesterol Metabolism

Cholesterol Metabolism

Ingested cholesterol and fatty acids cannot be transported through the bloodstream (Why?)

Cholesterol molecules and fatty acids aretransported in chylomicrons (“fat droplets”) Chylomicron - a small, membrane-bound vesicle composed of phospholipids and protein

Chylomicrons are transported through the bloodstream to adipocytes (i.e. fat cells) where fatty acids are stored

Remaining cholesterol molecules are transported to the liver

Cells in the liver break down the chylomicron releasing the cholesterol

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Most dense (fatty acid composition)

Least dense

Chylomicron

Chylomicron remnant

VLDL

LDL

HDL (scavenger)

High cholesterol(food intake)

Low cholesterol

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Cholesterol Metabolism

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1. In humans, the allele for short fingers (F) is dominant over the allele for long fingers (f). If a person with short fingers who had one parent with long fingers “mates” with a person who has long fingers and this couple have three children, what are the chances that two of their three children will have short fingers?

2. Genes A, B, and B assort independently and are dominant to their respective alleles a, b, and c. Two triple heterozygotes (AaBbCc) are crossed.

What is the probability that a given offspring will be phenotypically dominant for one of the dominant traits?

AP Biology - Genetics Quiz

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AP Biology - Genetics Quiz

3. The recessive allele s causes Drosophila to have small wings and s+ allele results in a normal wing phenotype. This gene is known to be located on the X chromosome.

a. If a small winged male is crossed with a homozygous wild- type female, what ratio of normal to small winged flies can be expected in each sex in the F1?

b. If F1 flies are intercrossed (mated with each other), what F2 progeny ratios are expected?

c. What progeny ratios are expected if F1 females are backcrossed to their father?

TestcrossUsed to determine the genotype of a “parent” with the dominant phenotype but an unknown genotype

Dominant parent “crossed/mated” with a homozygous recessive parent

Genotype of the dominant parent can be determined based onthe offspring produced from the cross

If offspring with ONLY the dominant phenotype are produced, the parent must have the homozygous dominant genotype

If offspring with the dominant and the recessive phenotype areproduced, the parent must have the heterozygous genotype

Sample Testcross

EpistasisExpression of one gene is dependent on the expression of asecond gene

In cats, the dilute gene (d) is responsible for lightening the coat colorDilute gene is recessive to dense gene (D) The presence of dense allele (D) will produce acat with a dark coat color; absence of dense will produce a catwith lightened coat color

Coat color is controlled by one set of genes (For simplicity, let’s useR for black allele and r for brown allele).

Example

Mendelian Genetics Extensions - Epistasis

One would predict 9:3:3:1phenotype ratio in a dihybrid cross

If ratios of offspring do not adhere to predictedratios something elsemust be happening

Epistasis - Ability for one gene to influence the phenotype brought aboutby a second gene

EpistasisExpression of one gene can “mask” or inhibit the expression of a second gene

One human trait in which epistasis may occur is in the occurrenceof deafness in a child

Question: Assume that this trait is controlled by 3 pairs of genes and that normal hearing occurs when an individual has the genotype A_ or B_. The presence of DD, however, causes deafness, no matter what other gene combinations are present

1. Write out all of the possible genotypes of the offspring produced from a mother with the genotype AABBDd anda father with the genotype AaBbDd

2. Predict the phenotype ratios of the offspring that wouldbe produced if the above parents were to have children

Genetic ImprintingDoes it matter which allele is inherited from each parent?

In some cases, the answer is yes.

Genomic imprinting caninactivate one of the two copies(i.e. alleles) of a particular gene in an organism

New imprint corresponds tosex of parent

Addition of methyl groups to DNAnucleotides is responsible for imprinting

Genetic ImprintingSpecial enzymes responsible foradding methyl (e.g. -CH3) groupto nitrogen base