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Mendelian Genetics. Vocabulary. dominant gene: F 1 generation: F 2 generation: gene: genetic trait: genotype: monohybrid cross:. Hybrid: phenotype: principle of independent assortment: principle of segregation: Punnett square: Recessive gene:. Study Questions.

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Mendelian Genetics

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Mendelian Genetics

KMarsh


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Vocabulary

  • dominant gene:

  • F1 generation:

  • F2 generation:

  • gene:

  • genetic trait:

  • genotype:

  • monohybrid cross:

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  • Hybrid:

  • phenotype:

  • principle of independent assortment:

  • principle of segregation:

  • Punnett square:

  • Recessive gene:

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Study Questions

  • 1. List and explain the four principles of genetics established by Mendel.

  • 2. Complete a monohybrid cross between a pure breeding tall plant (TT) and a pure breeding dwarf plant (tt). Carry the cross through to the second generation (F2 generation) by letting the plants of the first cross (Tt) self-fertilize. Describe the phenotypes and genotypes of both generations.

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  • 3. Show how your knowledge of meiosis provides tangible evidence concerning Mendel’s Principles of Segregation and Independent Assortment.

  • 4. Complete a monohybrid cross between (Tt X tt). List all the potential types of gametes produced by each parent and then combine them into all possible combinations in the offspring. List the various phenotypes and ratios from this cross.

  • 5. List and discuss several of the approaches which made Mendel’s work successful.

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Review Meiosis

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  • Mendel’s Work with Pea Plants

  • First Steps to Genetics

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Mendel’s Studies of the Pea Plant

  • A. Characteristics studied by Mendel (7 traits)

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Dominant vs Recessive

  • 1.Form of ripe seed2.Color of seed coat

    SmoothYellow

    WrinkledGreen

  • 3.Flower Color 4.Form of ripe pods

  • Purple Inflated

  • White Constricted

  • 5.Color of unripe pods 6.Position of flowers

  • Green Axial

  • YellowTerminal

  • 7.Length of stem Tall & Dwarf

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Why Pea Plants?

  • 1- Many true breeding varieties available

  • 2- The flower is self-fertile

  • 3- Generation time is very short

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Mendel’sProblems

  • 1- No concept of DNA or chromosomes

  • 2- No concept of meiosis

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Some wise (educated guesses) decisions made by Mendel

  • 1- Selection of well-defined, contrasting traits

  • 2- Extensive groundwork completed prior to establishment of final experimental design

  • 3- Extensive replication of crosses

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Mendel’s Test Evaluating the Inheritance of Seed Shape in Pea Plants

  • Recognized two different traits for the seed shape character in pea plants: round versus wrinkled seeds.

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Established true-breeding varieties for each of these traits.

RR and rr

Offspring (F1 generation) are called “hybrids”.

Rr

Testing continued

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100% of these hybrids produced round seeds.

Why?


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Crossed these F1 generation hybrids among each other (individual crosses being selected at random).

Rr X Rr

Results for their offspring (F2 generation):

5474 (74.7%) were plants that produced round seeds.1850 (25.3%) were plants that produced wrinkled seeds.Ratio of round:wrinkled = approx. 3:1

The Next Step

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The Explaination

  • For every character (e.g., seed shape) an individual possess two instruction sets (alleles).

  • One of these alleles was originally derived from the individual’s mother, the other allele being originally derived from the individual’s father.

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In true-breeding individuals, both alleles are the same.

Homozygous

RR or rr

Hybrids, on the other hand, one of each kind of allele.

Heterozygous

Rr

True vs Hybrid

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Mendel’s Further Explanation

  • Mendel believed that only two alleles were possible for a given genetic character, and that one of the alleles (the dominant one) masked the expression of the other (the recessive one) in the hybrid.

  • When the hybrid formed gametes, only one of the two possible alleles would end up in a gamete.

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When the hybrid formed gametes, only one of the two possible alleles would end up in a gamete.

However, both alleles possessed an equal chance of appearing in a gamete.

Law of Segregation

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Law of Segregation con’t

  • In the formation of gametes, two members of a gene pair (alleles) segregate into different haploid gametes with equal probability.

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Dominant and Recessive

  • Defining alleles:

  • R = the round allele (dominant).

  • r = the wrinkled allele (recessive).

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Phenotype & Genotype

  • Defining genotypes and their respective phenotypes:

  • RR genotype (homozygous dominant) = round phenotype.

  • rr genotype (homozygous recessive) = wrinkled phenotype.

  • Rr genotype (heterozygous) = round phenotype

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Round Round

RR

Rr

rr

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P Generation and F1 Generation

Parental (P) Cross

  • Round X Wrinkled

    RR rr

  • Gametes:

    R r

    F1 (first felial) Generation Hybrids

    100% Round phenotype

    100% Rr genotype

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F1 and F2 Generations

  • Mendel’s Explanation for his Results

  • F1 Generation Cross Round X Round Rr RrGametes: R r R rF2 Generation OutcomeRound WrinkledRR Rr Rr rrNote: 3:1 ratio of round:wrinkled

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Punnett Square Diagram

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  • Mendel’s Genetics Experiments With Pea Plants continued…

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  • Law of Segregation

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Genotype Vs. Phenotype

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Test Crosses

When you cross an individual whose genetics you are not sure about with an individual who is homozygous recessive for the trait(s) in question.


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Test Crosses

  • F1 Hybrids (Rr) xF2 Recessives (rr)

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Test Cross Pea Plants

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Mendel’s Law of Independent Assortment

  • Whenever two or more pairs of contrasting characters are brought together in a hybrid, the alleles of the different pairs segregate independently of one another during gamete formation.

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Dihybrid Crosses

  • Define Alleles and Associated Traits:Seed Shape CharacterR = round seed trait (dominant)r = wrinkle seed trait (recessive)Seed Color CharacterY = yellow seed trait (dominant)y = green seed trait (recessive

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First Cross (Dihybrid)

  • Parental (P) cross: crossed true-breeding plants that produce round-yellow (RRYY) seeds with true-breeding plants that produce wrinkled-green seeds (rryy).

  • F1 Genereation: hybrids for both characters (Rr & Yy; or RrYy) were100% round-yellow.

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Results

  • F1 Generation Individuals (RrYy) crossed among each other.F2 Generation Results:315 (56.7%) round-yellow108 (19.4%) round-green101 (18.2%) wrinkled-yellow32 (5.8%) wrinkled-green

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  • With Independent Assortment

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Calculating Expected FrequenciesRound-Yellow

  • Expect 9/16 of the F2 generation offspring to be round-yellow.Therefore, of a total of 556 offspring the expected number (frequency) of round-yellow offspring may be calculated as follows:Frequency = (9/16)*556 = 312.75

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  • Calculating Expected Frequencies

    Round-Green

  • Expect 3/16 of the F2 generation offspring to be round-green.Therefore, of a total of 556 offspring the expected number (frequency) of round-yellow offspring may be calculated as follows:Frequency = (3/16)*556 = 104.25

KMarsh


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Calculating Expected FrequenciesWrinkled- Yellow

  • Expect 3/16 of the F2 generation offspring to be wrinkled-yellow.Therefore, of a total of 556 offspring the expected number (frequency) of round-yellow offspring may be calculated as follows:Frequency = (3/16)*556 = 104.25

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Calculating Expected FrequenciesWrinkled- Green

  • Expect 1/16 of the F2 generation offspring to be wrinkled-yellow.Therefore, of a total of 556 offspring the expected number (frequency) of round-yellow offspring may be calculated as follows:Frequency = (1/16)*556 = 34.75

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ObservedRound-Yellow315Round-Green108Wrinkled-Yellow101Wrinkled-Green32

Expected

Round- Yellow312.75Round-Green104.25Wrinkled-Yellow104.25Wrinkled-Green34.75

Comparing Observed to Expected Results

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Results of Mendel’s Work

  • A. The principle of unit characters (elementum or genes)

  • B. The phenomenon of dominant and recessive genes

  • C.The principle of segregation: alleles separate from one another during

  • D. The principle of independent assortment

KMarsh


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