CH 11 Gregor Mendel, Probability, And Punnett Squares

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CH 11 Gregor Mendel,Probability, and Punnett SquaresVocabulary: Genetics True-breeding Trait Hybrid Gene Allele Segregation Gamete Probability Punnett Square Homozygous Heterozygous Phenotype GenotypeKey Concepts: What is the principle ofdominance? What happens duringsegregation? How do geneticists use theprinciples of probability?

Background Mendel was anAustrian monk whowas in charge of themonastery garden. His work with peaplants has led to himbeing considered the“Father of ModernGenetics.” Genetics: thescientific study ofheredity

Pea plant structure: Reproduction occurs throughflowers Male part of the flower containspollen male sex cells Female part of the flower containseggs female sex cells When pollen fertilizes an egg cell, aseed for a new plant is formed Pea plants normally fertilize bySELF-POLLINATION (pollen andegg are from the same flower)

Background When Mendel took charge ofthe monastery garden, he hadseveral true-breeding plants(if allowed to self-pollinate,offspring would be identical) Some would produce onlygreen seeds, others onlyyellow, some tall, and someonly short

Mendel’s Experiments Mendel controlled the reproduction of pea plants He would cross-pollinate plants (pollen andegg from different pea plants)– Two different pea plant parents– Prevented plants from self-pollinating

GENES & DOMINANCE Mendel studied several different pea plant traits– Trait: a specific characteristic (ex: seed color orplant height) that varies from one individual toanother Mendel’s Labels for pea plant generations– Original pair of plants: P (parental) generation– Offspring of “P” generation: F1 (first filial isLatin for “first son”) generation– Offspring of crosses between parents with differenttraits (ex: yellow x green seeds): HYBRIDS

What were the F1 hybridslike? Did the characters ofthe parent plants blend inthe offspring? NO! All of the offspringhad the character of onlyONE parent; the characterof the other parent seemedto have disappeared

Mendel’s 2 ConclusionsConclusion #1 Biological inheritance is determined by factorsthat are passed from one generation to the next– “factors” GENES (determine traits)– Each of the traits Mendel studied was controlledby one GENE that occurred in two contrastingforms EX: GENE seed color– 2 contrasting forms yellow or green– 2 contrasting forms: ALLELES ALLELE #1: yellow ALLELE #2: green

Mendel’s 2 ConclusionsConclusion #2 Principle of Dominance: some alleles aredominant and others are recessive–Inherit 2 dominant alleles: dominant allelewill show– Inherit 2 recessive alleles: recessive allele willshow– Inherit 1 dominant and 1 recessiveallele, the DOMINANT allele will show!

Dominant vs. Recessive

Segregation This led Mendel toask: Had the recessivealleles disappearedforever? To answer this heallowed the F1 hybridplants to self-pollinateto produce an F2generationThe recessiveallele reappeared!

reappeared!

When each F1 plant flowers, the 2 allelesare segregated (separated) from each otherso that each gamete (SEX CELL)carries only a single copy of each gene Therefore, each F1 plant produces 2 types ofgametes– those with an allele for yellow seeds– those with an allele for green seeds

Segregation of gametes:

Genes are on chromosomes

ALLELES: versions or copies ofa gene

A little more background info Every time Mendel performed a cross with hispea plants, he carefully counted the offspring Out of the over 20,000 plants he counted henoticed there was a PATTERN!– Whenever he crossed 2 plants that were hybrid forstem height (Tt), about ¾ of the resulting plantswere TALL and ¼ were short (3:1 ratio)– He realized that the principles of probability couldbe used to explain his results!

PROBABILITY: Definition: the likelihood that a particularevent will occur– Ex: flip a coin: ½ or 50% chance it will come upheads If you flip a coin 3 times in a row, what is theprobability it will come up heads 3 times in a row?– Each coin flip is an independent event therefore itis:½ x ½ x ½ 1/8– Past outcomes do not influence future outcomes The principles of probability can be used to predict theoutcomes of genetic crosses

PUNNETT SQUARES: The gene combinations that MIGHTresult from a genetic cross can bedetermined by drawing a PunnettSquare The parents go on the outside of thesquare along the side and top The possible outcomes of the crossare on the inside of the squares The letters in the Punnett Squarerepresent ALLELES

Alleles in a Punnett Square: “T” represents the DOMINANT allele: TALL “t” represents the RECESSIVE allele: short– Organisms with 2 identical alleles (TT or tt) for atrait are said to be HOMOZYGOUS Mendel called these: truetrue-breeding– Organisms with 2 different alleles for a particulartrait are said to be HETEROZYGOUS Mendel called thesehybrids

MORE VOCABULARY! Phenotype: the physical characteristics of theorganism (what the organism looks like)– Ex: Tall; short Genotype: the actual genetic makeup of theorganism (the 2 alleles the organism inherited)– Ex: TT; Tt: tt

Probabilities Predict Averages: Probabilities predict the outcomes of aLARGE number of events Probabilities cannot predict preciseoutcomes of an event– Ex: flip a coin twice you may get 100% heads– You need to flip the coin many, many times toget close to 50%

What is the expected ratio for this cross? Tt x ttttTTtTtttttt

Punnett Square Rules:1. Choose a letter to represent the dominant allele andcapitalize it (choose a letter that is easy to distinguishbetween upper-case and lower-case).2. Use the same letter but use lower case to represent therecessive allele.3. Put the male’s alleles down the left side of the squareand the female’s alleles across the top.4. Write "genotype" and "phenotype" below the square.5. Use "x" to indicate a cross.6. Write the dominant allele first if you are writing out ahybrid.

What is the expected ratio for this cross? Tt x ttttTTtTtttttt

What is the expected ratio for this cross? Tt x ttttTTtTttttttGenotype ratio: 2 Tt : 2 ttPhenotype ratio: 2 tall : 2 short

More Mendelian Genetics Key Concepts:– What is the principle of independentassortment? Vocabulary:– Independent assortment

Independent Assortment Two-factor cross / dihybrid cross of Parent Generation:– Crossed true breeding plants (Parents) Round Yellow x Wrinkled Green– F1 phenotypes all yellow, round Which alleles are dominant?– Round shape and Yellow color. Why is this called a DIHYBRID or two-factor cross? Because the experiment tests two characteristicscontrolled by two factors genes.

Summary of Mendel’s two factorcross (F1 Generation) Mendel crossed plants that were homozygousdominant for round yellow peas (RRYY) andplants that were homozygous recessive forwrinkled green peas (rryy). All of the F1 offspring were heterozygousdominant for round yellow peas (RrYy).

The Two-Factor Cross F2 Mendel knew all of the F1 plants were RrYy– but how would these alleles interact if the F1 plants werecrossed? Would the two dominant alleles stay together (R andY) or would they segregate independently?– In his experiment, F2 plants produced 556 seeds. 315 were round and yellow (looked like Parental) 32 were wrinkled and green (looked like Parental) 209 had combinations of the phenotypes Therefore, the alleles for seed shapesegregated independently of the allelesfor seed color.

This phenomenon is knownas Independent assortmentDefinition: genes for different traitssegregate independently during theformation of gametes (meiosis).

Making Gametes If the genotype of a plant is RrYy,(round, yellow seeds) what are thefour possible combinations ofgametes? RY, Ry, rY, ry These are FOUR possible gametes,each with two alleles!!!

Independent Assortment.inother words Does the segregation of one pair of allelesaffect the segregation of another pair ofalleles?– EX: does the gene that determines seed shape haveanything to do with the gene that determines seedcolor? The answer: NO If genes are not connected, then they shouldseparate (segregate) independently:Independent Assortment

F2 generation

CROSS: RrYy x RrYYRrYyrrYYrrYyryRrYyRryyrrYyrryy

CROSS: RrYy x RrYyRYRyrYryRYRyrYryRRYYRRYyRrYYRrYyRound, yellowRound, yellowRound, yellowRound, yellowRRYyRRyyRrYyRryyRound, yellowRound, greenRound, yellowRound, greenRrYYRrYyrrYYrrYyRound, yellowRound, yellowWrinkled, yellowWrinkled, yellowRrYyRryyrrYyrryyRound, yellowRound, greenWrinkled, yellowWrinkled, green

F2 GenerationPhenotype ratio:9:3:3:1Can you seethe ratio of phenotypes inthe Punnett square?9 round, yellow:3 round, green:3 wrinkled,yellow:1 wrinkled, green

A Summary of Mendel’s Principles:1. Inheritance is determined by GENES2. Some genes are DOMINANT and some areRECESSIVE3. Each sexually reproducing adult has 2 copies of agene. These genes are segregated during gamete formation.4. Alleles for different genes USUALLY segregateindependently (independent assortment)

3. Put the male ’s alleles down the left side of the square and the female ’s alleles across the top. 4. Write "genotype" and "phenotype" below the square. 5. Use "x" to indicate a cross. 6. Write the dominant allele first if you are writing out a hybrid.

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