Download Sect 10.2

Section 10.2
Mendelian Genetics
Objectives
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Explain the significance of Mendel’s experiments to the study of genetics.
Summarize the law of segregation and the law of independent assortment.
Predict the possible offspring from a cross using a Punnett Square
Main Idea
• Mendel explained how a dominant allele can mask the presence of a recessive allele.
How genetics began
• Gregor Mendel
◦ an Austrian monk and plant breeder
◦ in 1866 published his findings on
 the method and mathematics of inheritance
 in green peas
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inheritance
 = passing of traits to the next generation
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he chose pea plants because they are
 easy to grow
 true-breeders
□ consistently produce one trait
Pea plants
◦ usually reproduce by self-fertilization
 occurs when
□ ♂ gamete
• from the stamen of the flower
□ fertilizes the ♀ gamete
• from the pistil of a flower on the same plant
◦
they can also be cross-pollinated
 quite easily by hand
 gametes from different plant combine
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Pea plants can also be cross-pollinated
◦ gametes from different plants combine
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Mendel cross-pollinated the plants
◦ by transferring the ♂ gamete of one plant
 to the pistil of another plant
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Mendel kept meticulous records
◦ analyzed the results
◦ formed hypotheses
 how traits were inherited
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this was the beginning of genetics
 Mendel is considered the father of genetics
The Inheritance of Traits
• Genetics
◦ = the science of heredity
◦ Mendel cross-pollinated
 a true-breeding plant with yellow seeds
 a true-breeding plant with green seeds
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to prevent self-pollination
 he removed the ♂ reproductive organs from the flower
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he called this first cross
 the parent generation
□ aka P generation
He called the offspring fm his P gen
◦ the first filial-generation
 aka F1 generation
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all of the F1 seeds were yellow
 the green seed trait seemed to have disappeared
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when he crossed plants fm the F1 gen
 the green seed trait reappeared
 he called this the second filial-generation
□ aka F2 generation
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Mendel collected all of the seeds from the F2 generation
◦ he collected 8023 seeds
 6022 were yellow
 2001 were green
 almost a perfect 3:1 ratio
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Mendel studied 7 different traits
◦ seed shape
◦ seed color
◦ flower color
◦ seed pod shape
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seed pod color
flower
stem length
Genes in pairs
◦ Mendel concluded that there are 2 forms seed color
 each is controlled by a factor
 today the factor is called an allele
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allele
 = an alternative form of a single gene
□ passed from gen to gen
◦
therefore
 gene for yellow seeds
 gene for green seeds
 are diff forms of a single gene
Mendel
◦ concluded 3:1 ratio could be explained
 if the alleles were paired in the plants
◦ the trait that appeared in the F1 gen
 was dominant
◦
it masked the other F1 trait
□ which was recessive
◦
in pea plants
 the yellow seed trait is dominant form
 the green seed trait is recessive form
Dominance
◦ Mendel concluded that in the F1 gen
 the yellow seed allele
□ represented by the letter ‘Y’
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
masked the green seed allele
□ the recessive allele
□ represented by the letter ‘y’

upper case indicates the dominant allele
so a yellow seeded plant could have
 ‘YY’ or ‘Yy’
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If a pea plant has 2 of the same alleles
◦ its called homozygous for that trait
◦ ex: ‘YY’ – homozygous dominant
 ‘yy’ – homozygous recessive
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a green seeded plant would have
 ‘yy’
If a pea plant has both forms of the allele
its called heterozygous for that trait
 ex: ‘Yy’ -- heterozygous
Genotype and phenotype
◦ genotype
 = the genetic makeup of an organism
◦
phenotype
 = the outward appearance of the trait
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ex: if a pea plant has yellow seeds its
 phenotype is yellow seeds
 genotype could be either ‘YY’ or ‘Yy’
◦
ex: a pea plant with green seeds its
 phenotype is green seeds
 genotype can only be ‘yy’
Law of segregation
◦ states that two alleles for each trait
 separate or segregate during meiosis
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during fertilization
 two alleles for this trait
□ reunite in the zygote
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the resulting pea plant is the F1 gen
 all of these plants are heterozygous
□ i.e. ‘Yy’ with yellow seeds
□ heterozygous organisms are called
~ hybrids
Monohybrid cross
◦ = a cross that involves hybrids for a single trait
◦ dominant allele is always written first
 followed by the recessive allele
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Law of independent assortment
◦ = states that a random distribution of alleles occurs during gamete formation
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Mendel suggested
◦ that the alleles for tallness and shortness in the F1 plants
 segregated from each other during the formation of the sex cells
□ gametes
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When each F1 plant produces gametes
◦ the two alleles segregate from each other
 so that each gamete carries only a single copy of each gene
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each F1 plant produces two types of gametes
 those with the allele for tallness
 those with the allele for shortness
Dihybrid cross
◦ once Mendel determined patterns for a single trait
 he started looking at what would happen if two traits were inherited
together
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another trait he looked at was seed shape
 round seeds – represented by ‘R’
□ this allele was dominant
 wrinkled seeds – represented by ‘r’
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He performed a cross using traits for
◦ seed color
◦ seed shape
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He crossed a
◦ homozygous yellow, round seed plant
◦ homozygous green, wrinkled seed plant
◦ the P genotypes would be ‘YYRR’ and ‘yyrr’
◦ the F1 gen’s genotype would be
 ‘YyRr’
 yellow, round seed
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Dihybrid cross
◦ = the simultaneous inheritance of two or more traits in the same plant
◦ in the pea plant
 round seeds (R) is dominant to
□ wrinkled seeds (r)

yellow seeds (Y) is dominant to
□ green seeds (y)
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Mendel crossed
 a homozygous round yellow seed plant
□ genotype –RRYY
 a homozygous wrinkled green seed plant
□ genotype –rryy
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F1 generation
 all plants would be RrYy
 gametes can be
□ RY Ry rY ry
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In this dihybrid cross
◦ the phenotypes that will be seen are
 9 will have round yellow seeds
 3 will have round green seeds
 3 will have wrinkled yellow seeds
 1 will have wrinkled green seeds
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Law of independent assortment
◦ = a random distribution of alleles occurs during gamete formation
◦ genes on separate chromosomes
 sort independently during meiosis
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each allele combination is equally likely to occur
Punnett square
◦ tool used to predict the possible offspring of a cross
 between two known genotypes
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monohybrid cross
 place the gametes
□ across the tope
□ down the left side
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bring the alleles
□ down from above
□ across from the left
upper case letter represents the dominant allele for tall
lowercase letter represents the recessive allele for short
in this example,
 T = tall

t = short
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gametes produced by each F1 parent are shown
 along the top
 down left side
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bringing the genes
 down & across
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possible F2 gene combinations
 appear in the four boxes
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phenotypic ratio of 3 : 1
 3 tall to 1 short
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the genotypic ratio is
 1 TT : 2 Tt : 1 tt
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the chance for each square is 25%
 1 in 4
Probability
• Probability
◦ = the chance that something will occur
 in a prescribed number of trials
 ex: flipping a coin
□ probability of getting heads is 1 out of 2
~ or ½
◦ in a Punnett square
 the data may not match the predicted ratios
□ but it will be close
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the greater the number of trials
□ the closer the data will approach the probability
In our pea plant dihybrid cross
◦ the expected phenotypic ratios are
 9/16 – both dominant traits (R_Y_)
 3/16 – one dominant & one rec trait (R_yy)
 3/16 – opposite traits (rrY_)
 1/16 – both recessive traits (rryy)
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phenotypic ratio is
 9 : 3 : 3 : 1