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Chapter 10
Sexual Reproduction and Genetics
11-2 Probability and Punnett
Squares
Genetics and Probability
How do geneticists use the principles of probability?
 Probability – the likelihood that a particular event will
occur
 The principles of probability can be used to predict the
outcomes of genetic crosses.
Chapter 10
Sexual Reproduction and Genetics
Punnett Squares
• Punnett Squares – a tool used to determine the
combinations from a genetic cross
• Shows all possible combinations of gametes and
likelihood each will occur
Chapter 10
Sexual Reproduction and Genetics
Setting up a Punnett Square
• Draw the punnett square – size of your square depends on
how many traits you are using (Ex. 1 trait = 4 boxes, 2
traits = 16 boxes)
• Gametes produced by each parent are shown along the
top and left side.
• Possible gene combinations for the offspring appear in
the four boxes.
Chapter 10
Sexual Reproduction and Genetics
Mendel’s Experiment
• Gametes from the F1
generation parents are
used to get the offspring
for the F2 generation.
• Mendel only looked at
one trait at a time in his
first experiments
• Monohybrid cross – a
cross involving one trait
Probablility and Segregation
• One fourth (1/4) of the F2
plants have two alleles
for tallness (TT).
• 2/4 or 1/2 have one allele
for tall (T), and one for
short (t).
• One fourth (1/4) of the F2
have two alleles for short
(tt).
Chapter 10
Sexual Reproduction and Genetics
10.2 Mendelian Genetics
Mendel’s Law of Segregation
 Two alleles for each trait separate during meiosis.
 During fertilization, two alleles for that trait unite.
 Heterozygous organisms are called hybrids.
Chapter 10
Sexual Reproduction and Genetics
10.2 Mendelian Genetics
Monohybrid Cross
 A cross that involves
hybrids for a single
trait is called a
monohybrid cross.
Chapter 10
Sexual Reproduction and Genetics
10.2 Mendelian Genetics
Dihybrid Cross
 The simultaneous inheritance of two or more
traits in the same plant is a dihybrid cross.
 Dihybrids are heterozygous for both traits.
Chapter 10
Sexual Reproduction and Genetics
10.2 Mendelian Genetics
Law of Independent Assortment
 Random distribution of alleles occurs during
gamete formation
 Genes on separate chromosomes sort
independently during meiosis.
 Each allele combination is equally likely to
occur.
 Law of independent assortment occurs during
metaphase I of meiosis.
Chapter 10
Sexual Reproduction and Genetics
10.2 Mendelian Genetics
Punnett Square—
Dihybrid Cross
 Four types of alleles from
the male gametes and
four types of alleles from
the female gametes can
be produced.
 The resulting phenotypic
ratio is 9:3:3:1.
Chapter 10
Sexual Reproduction and Genetics
10.3 Gene Linkage and Polyploidy
Polyploidy
 Polyploidy is the occurrence of one or more extra
sets of all
chromosomes
in an organism.
 A triploid organism,
for instance, would
be designated 3n,
which means that
it has three complete sets of chromosomes.