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Chapter 8
Section 3: Studying Heredity
Grade 10 Biology
Spring 2011
Objectives
Predict the results of monohybrid
genetic crosses by using Punnett squares
 Apply a test cross to determining the
genotype of an organism with a dominant
phenotype
 Predict the results of monohybrid
genetic crosses by using probabilities
 Analyze a simple pedigree

Punnett Square

Punnettt Square: diagram that predicts
the outcome of a genetic cross by
considering all possible combinations of
gametes in the cross
Punnett Square

Monohybrid Cross:
◦ Homozygous plants
Punnett Square

Monohybrid Cross:
◦ Heterozygous plants
Punnett Square
Dihybrid
cross:
 RrYy x
RrYy

Test Cross

Test Cross: an individual whose
phenotype is dominant, but whose
genotype is not known, is crossed with a
homozygous recessive individual
Test Cross

Example:
◦ Determine the genotype of a dominant purple
flower. Cross the purple flower with a
homozygous recessive white flower.
◦ Dominant purple flower could have the
genotypes of: PP or Pp
◦ Homozygous recessive flower has genotype
of: pp
Test Cross
Probability

Probability: the likelihood that a specific
event will occur
◦ Expressed as: words, decimals, percentages,
fractions
Probability
Number of one kind of possible outcome
Total number of all possible outcomes
Probability

Example: if a pea plant has 2 alleles for
seed colour (yellow and green), what is
the probability the gamete will carry the
allele for green seed colour? Yellow seed
colour?
◦ Green seed colour = ½
◦ Yellow seed colour = ½
Probability

Example: Two pea plants are heterozygous for
seed shape (Rr).
◦ R = dominant, round
◦ r = recessive, wrinkled
◦ Probability of each parent carrying gametes with R or
r alleles = ½
◦ Probability of offspring with RR alleles:
 ½x½=¼
◦ Probability of offspring with rr alleles:
 ½x½=¼
◦ Combination of Rr alleles can occur in two possible
ways. One parent can contribute the R allele, and the
second parent the r allele, or vice versa.
◦ Probability of offspring with Rr alleles:
 ¼ + ¼ = 2/4 = ½
Inheritance of Traits

Pedigree: family history that shows how
a trait is inherited over several
generations
◦ Helpful with: genetic disorders

Carriers: individuals who are
heterozygous for an inherited disorder
but do not show symptoms of the
disorder
Inheritance of Traits

Pedigree:
Inheritance of Traits
If a gene is autosomal: it will appear in
both sexes equally
 If a gene is sex-linked: its effects are
usually seen in only males

Inheritance of Traits
Autosomal Dominant: every individual
with the condition will have a parent with
the condition
 Autosomal Recessive: individual with
the condition can have one, two, or
neither parent exhibiting the condition

Inheritance of Traits
Homozygous or Heterozygous Dominant: their
phenotype will show the dominant allele
 Homozygous Recessive: their phenotype will
show the recessive allele
 Heterozygous Carriers of Recessive Mutations:
will not show mutation, can produce
children who are homozygous for recessive
allele if mate with another heterozygous
carrier

Inheritance of Traits

Pedigree
◦ Be able to do evaluating a pedigree activity
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