Chapter 11:Introduction to GeneticsSection 11-2: Applying Mendel’s Principles

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Introduction• We cannot predict the future – If a parent carries 2 different

alleles for a certain gene, there is no way to be sure which allele will be inherited by its offspring

• The only thing we can do is predict the odds by applying Mendel’s principles

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Probability

• Mendel was very careful to categorize and count all of the offspring each time he preformed a cross

• Whenever he crossed 2 plants that were hybrid for height (Tt) about ¾ of the offspring were tall and ¼ were short

• Mendel realized that he could apply the rules of probability to explain his results

• Probability is the likelihood that a particular event will occur• Think about a coin flip – the probability of flipping heads is the

same as the probability of flipping tails – ½ or 50%• If you flip a coin three times in a row, the probability that it

will land heads up every time is ½ x ½ x ½, or 1/8• Each flip is an independent event - past outcomes do not

effect future outcomes3

Using Segregation to Predict Outcomes

• The way in which alleles segregate during meiosis is just as random as flipping a coin, therefore the rules of probability can be used to predict the outcomes of crosses

• Let’s go through Mendel’s cross:• A true-breeding tall parent (TT) was crossed with a true-

breeding short parent (tt)• Each F1 offspring received a T and a t – meaning they were all

Tt, and tall• When the F1 plants make gametes, ½ of the gametes will

receive the T allele and the other ½ will receive the t allele

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Using Segregation to Predict Outcomes

• The only way to get a short F2 offspring is for 2 t alleles to combine

• The probability of 2 gametes coming together that both carry the t allele is ½ x ½, or ¼

• You would predict ¼ offspring to be short, and the other ¾ to be tall

• This 3:1 ratio of dominant to recessive plants appeared again and again, consistently for each of the seven traits

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Using Segregation to Predict Outcomes

• The reality is that not all organisms that look the same carry the same alleles – for example a plant that is tall could have two T alleles (TT) or is could have one T and one t (Tt)

• All short plants have 2 t alleles (tt)• An organism that has two identical alleles for a gene – for

example TT or tt – is said to be homozygous• Organisms with 2 dominant alleles are homozygous dominant• Organisms with 2 recessive alleles are homozygous recessive

• Organisms that have two different alleles for a gene - for example Tt – are said to be heterozygous

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Probabilities Predict Outcomes• Probabilities are used to predict the outcomes of large events

• In genetics, the larger the number of offspring the closer the results will be to predicted outcomes

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Genotype and Phenotype• An organism’s genotype is its genetic composition – the alleles

it carries for each gene• It can be homozygous dominant, heterozygous, or homozygous

recessive

• An organism’s phenotype is its physical appearance, or how it looks• It can show the dominant phenotype or the recessive phenotype

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Genotype and Phenotype• Genotype determines phenotype

Genotype: HD (TT)Phenotype: Dominant (tall)

Genotype: Het (Tt)Phenotype: Dominant (tall)

Genotype: HR (tt)Phentoype: recessive (short)

• The genotype of an organism is inherited• Phenotype can also be influences by environment 9

Using Punnett Squares• Punnett squares are simple diagrams used to predict the

results of genetic crosses

• Used to predict genotype and phenotype possibilities of offspring using probability

• Refer to sheet for how to make PS for one factor crosses

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Independent Assortment• So far, we have only looked at one trait (1 gene) – called

monohybrid crosses

• Remember that Mendel also experimented to see what would happen if he followed two traits at the same time (2 genes)

• These experiments are called dihybrid crosses

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Dihybrid Cross: F1• Mendel crossed true-

breeding plants with round, yellow peas with true-breeding plants with wrinkled green seeds• One plant HD for both

traits – RRYY• One plant HR for both

traits – rryy12

Dihybrid Cross: F1

• All of the F1 offspring produced round yellow peas• Punnett square

shows that the genotype of each F1 offspring was RrYy, heterozygous for both seed shape and seed color 13

Dihybrid Cross: F2• Mendel used 2 F1 plants to create the F2 generation• Mendel observed that 315 of the F2 seeds were round and

yellow, while another 32 seeds were wrinkled and green—the two parental phenotypes

• But 209 seeds had combinations of phenotypes, and therefore genotypes, that were not found in either parent

• The alleles for seed shape segregated independently of those for seed color

• Genes that segregate independently do not influence each other’s inheritance – The Law of Independent Assortment

• Refer to sheet for how to make PS for two-factor crosses14

Summary of Mendel’s Principles

• The inheritance of biological characteristics is determined by individual units called genes, which are passed from parents to offspring

• Where two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive

• In most sexually reproducing organisms, each adult has two copies of each gene that segregate from each other when gametes are formed

• Alleles for different genes usually segregate independently of each other

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