Download Chapter 11 Section 11_2 Applying Mendel_s Principles

Chapter 11: Introduction to genetics
11.2 – Applying Mendel’s Principles
Mr. M. Varco
Saint Joseph High School
Probability and Punnett Squares
• How can we use probability to predict traits?
• Probability is the likelihood that a particular event will occur (ex: coin flip)
• Whenever Mendel crossed two plants that were hybrids for stem height
(Tt), about three-quarters of the offspring were tall.
Hybrids
Probability and Punnett Squares
• Why were 25% of the F2 offspring short if both F1 plants were tall?
• If each F1 plant had one tall allele and one short allele (Tt), and if the alleles
segregated, then ½ of gametes produced would carry the short allele (t)
• The only way to produce a short (tt) plant is
two gametes, each carrying the “t” allele, to
combine
is for
to
• F2 Ratio: 3 dominant to 1 recessive
Probability and Punnett Squares
• Organisms that have two identical alleles (TT or tt) for a particular gene
are said to be homozygous.
• Organisms that have two different alleles (Tt) for the same gene are called
heterozygous
Probability and Punnett Squares
• Mendel noticed that all of the tall pea plants had the same phenotype, or
physical traits
• They did not, however, have the same genotype, or genetic makeup.
• The genotype of an organism is inherited,
while the phenotype is determined by the
genotype
Probability and Punnett Squares
• How can we easily predict the outcomes of a genetic cross?
• Punnett squares use mathematical probabilities to help predict the
genotype and phenotype combinations in genetic crosses.
STEP 1:
Punnett Squares
STEP 2:
STEP 3:
STEP 4:
Punnett Squares
Punnett Squares
STEP 5:
Independent Assortment
• Mendel wondered if the segregation of one pair of alleles for a particular
gene affected another pair for a different gene.
• Mendel followed two different genes as they were passed down from one
generation to the next.
• Mendel’s experiment is known as a
two-factor” or dihybrid cross.
“
Independent Assortment
Two-Factor Cross: F1
•First, Mendel crossed true-breeding plants that produced only round yellow
peas with plants that produced wrinkled green peas.
•Round yellow peas had the genotype: RRYY
•Wrinkled green peas had the genotype: rryy
Independent Assortment
Two-Factor Cross: F1
•All of the F1 offspring produced round yellow peas, showing that these
traits are dominant.
•The F1 offspring provided Mendel with plants that
were all
heterozygous for seed shape and color
•The purpose of this cross was to provide
hybrids.
Mendel with
Independent Assortment
Two-Factor Cross: F2
•Mendel now crossed the F1 hybrid plants to produce F2 offspring.
•The F2 plants produced 556 seeds in total, of which 209 had combinations
of phenotypes and therefore combination of alleles not found in either
parent.
•These results indicated that the alleles
seed shape segregated independently
for seed color.
for
of those
Independent Assortment
• Mendel had discovered the Principle of Independent Assortment.
• The Principle of Independent Assortment states that genes for different
traits can segregate independently during the formation of gametes.