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Chapter 5
Heredity
Section 1: Objectives
 Explain
the relationship between
traits and heredity.
 Describe
 Explain
the experiments of Mendel.
the difference between
dominant and recessive traits.
Heredity
 Heredity:
passing of traits from
parent to offspring
 Gregor
Mendel was one of the first
scientists to study heredity.
 What
are traits?
Heredity
 Gregor
Mendel was born in 1822 in
Austria.
 He
grew up on a farm where he
learned about flowers and fruit trees.
 He
entered a monestary when he
was 21 years old.
Heredity
 Here,
he was taught science and
performed many scientific
experiments.
 He
put most of his energy into
research.
 He
discovered the principles of
heredity.
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 From
working with plants, Mendel
knew the patterns of inheritance
weren’t always clear.
 For
example, sometimes a trait may
be expressed in offspring that was
not expressed in either parent.
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 Mendel
wanted to learn more about
what caused these patterns.
 To
keep it simple, he studied 1 type
of organism: pea plants.
 Why
would studying 1 organism at a
time be helpful in studying patterns
of inheritence?
Heredity
 Pea
plants were a good choice for
several reasons:
– They grow quickly
– There are many different kinds
– They are self pollinating
– A self pollinating plant has both male
and female structures.
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 Pollen
from one plant can fertilize the
ovule of the same plant.
 Mendel
was able to grow truebreeding plants.
 In
true-breeding plants, the offspring
have the same traits as the parent.
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 Pea
plants can also cross-pollinate.
 In
cross-pollination, pollen from one
plant fertilizes another plant.
 Pollen
can be carried by:
– Insects
– Wind
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 Mendel
studied only 1 characteristic
at a time.
A
characteristic is a feature that has
different forms in a population.
 For
example, hair color is a
characteristic; different forms such
as red hair or brown hair is a trait.
Heredity
 Mendel’s
first experiments:
 Mendel crossed pea plants to study 7
different characteristics.
 The
trait that appeared most of the
time was called the dominant trait.
 The
recessive trait was the trait
that appeared less.
Heredity
 Mendel’s
Second Experiments:
 Mendel allowed the first generation
plants to self-pollinate.
 He
completed the exact same
experiment for each of the 7
characteristics he was studying.
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 Mendel
studied each of the following
characteristics:
– Flower color
– Seed color
– Seed shape
– Pod color
– Pod shape
– Flower position
– Plant height
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A
ratio is a relationship between 2
different numbers.
 Mendel
calculated the ratio for
dominant to recessive traits.
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 Ratio
Example:
Dominant = purple flowers
Recessive = white flowers
705 purple: 224 white
Ratio = 3.15:1
Calculate the Following Ratios

A) 6,002 yellow: 2,001 green

B) 5,474 pink: 1,850 red

C) 428 green: 152 yellow

D) 787 short: 277 tall

What do these ratios have in common?
Complete the Following Cross
In daisies, yellow flower color is dominant
over white. Cross a homozygous yellow
flowered plant with a homozygous white
flowered plant.
 1) Offspring Genotype (include
percentages):
 2) Offspring Phenotype (include
percentages):
 3) Ratio of dominant to recessive traits:

Complete the Following Cross
Pick any 2 offspring from the last slide and
complete a cross. Dominant and recessive
traits remain the same.
 1) Offspring Genotype (include
percentages):
 2) Offspring Phenotype (include
percentages):
 3) Ratio of dominant to recessive traits:

Complete the Following Cross
In crimson clover, red flowers are
dominant over pink. Cross a heterozygous
red crimson clover plant with a
homozygous pink crimson clover plant.
 1) Offspring Genotype (include
percentages):
 2) Offspring Phenotype (include
percentages):
 3) Ratio of dominant to recessive traits:

Complete the Following Cross
Pick any 2 offspring from the last slide and
complete a cross. Dominant and recessive
traits remain the same.
 1) Offspring Genotype (include
percentages):
 2) Offspring Phenotype (include
percentages):
 3) Ratio of dominant to recessive traits:

Complete the Following Cross
In grapes, purple color is dominant over
green. Cross a heterozygous purple
colored grape plant with a homozygous
green colored grape plant.
 1) Offspring Genotype (include
percentages)
 2) Offspring Phenotype (include
percentages)
 3) Ratio of dominant to recessive traits:

Complete the Following Cross
Pick any 2 offspring from the last slide and
complete a cross. Dominant and recessive
traits remain the same.
 1) Offspring Genotype (include
percentages):
 2) Offspring Phenotype (include
percentages):
 3) Ratio of dominant to recessive traits:

Chapter 5 Notebook Quiz
1) __________ was one of the first
scientists to study heredity.
 2) What is the difference between selfpollination and cross pollination?
 3) What type of plants did Mendel study?
 4) What is a characteristic?
 5) What is a ratio?
 6) How many characteristics did Mendel
study?

Section 2: Objectives
 Explain
how genes and alleles are
related to genotype and phenotype.
 Use the information in a Punnett
square.
 Explain how the probability can be
used to predict possible genotypes in
offspring.
 Describe 3 exceptions to Mendel’s
observations.
Phenotype vs. Genotype
 The
first generation carries the
instructions for both dominant and
recessive traits.
 Scientists now call these instructions
genes, one set of instructions for an
inherited trait.
 The different forms of a gene are
alleles.
Phenotype vs. Genotype
 Genes
affect the traits of offspring.
 An
organism’s appearance is known
as phenotype.
 Examples:
Red flower color, white
flower color, yellow flower color
Phenotype vs.Genotype
 Both
inherited alleles together form
an organism’s genotype.
 Heterozygous:
one dominant and
one recessive trait
 Homozygous:
recessive traits
2 dominant or 2
Phenotype vs. Genotype
 Probability:
“The mathematical
chance that something will happen”
 In
a coin toss, you have a 50%
chance of getting heads or tails.
 This
is the case with inherited traits
as well.
Phenotype vs. Genotype
 Probability
is written as a fraction or
percentage.
 If
you toss a coin, the probability of
tossing tails is 50%. (You will get
tails half the time.)
Probability Example
 If
you roll a pair of dice, what is the
probability you will roll 2 three
separate times?
– Count the number of faces on the dice.
Put this number in the denominator. (6)
– Count the number of ways you can roll
3 with one dice. Put this number in the
numerator. (1)
Probability Example
– To find the probability that you will
throw 2 threes, multiply the probability
of throwing the first 3 by the probability
of throwing the second 3: 1/6 X 1/6 =
1/36.
Probability Example # 2
 What
is the probability that you will
roll an even number with one dice?
– Count the number of faces on the dice
(denominator)
– Count the number of ways you can roll
an even number. (numerator)
Phenotype vs. Genotype
Incomplete dominance:
 Sometimes, one trait is not completely
dominant over another.



It is possible to have 2 dominant traits
(co-dominance).
Sometimes genes can affect more than
one trait.
Phenotype vs. Genotype



Genes aren’t the only influence on traits.
In some ways, the environment affects
how you grow and develop.
Example: Your genes make it possible for
you to grow tall, but you need a healthy
diet to reach your full potential.
Incomplete Dominance Example
 In
a rare breed of roses, white and
red flower colors are dominant. What
would happen if a red rose was
crossed with a white rose?
– Genotype:
– Phenotype:
– Ratio:
Chapter 5 Pop Quiz
 1)
What is the probability of rolling
an odd number with one dice?
 2)
In a rare breed of sunflowers,
yellow and orange are both dominant
flower colors. What would happen if
a yellow flower was crossed with an
orange flower? Write all genotypes,
phenotypes, and ratios.
Section 3: Objectives
 Explain
the difference between
mitosis and meiosis.
 Describe how chromosomes
determine sex.
 Explain why sex-linked disorders
occur in 1 sex more often than in the
other.
 Interpret a pedigree.
Meiosis
 Chromosomes
that carry the same
sets of genes are called
homologous chromosomes.
 These
are like a “pair of shoes”
because there are 2.
 Sex
cells are different because they
have 1 chromosome.
Meiosis
 Sex
cells are made during meiosis.
 Meiosis
is a copying process that
produces cells with half the usual
number of chromosomes.
Meiosis
 What
does meiosis have to do with
chromosomes?
 Genes
are located on chromosomes.
 Understanding
meiosis was critical to
finding the location of genes.
Meiosis
 During
mitosis, chromosomes are
copied once.
 During meiosis, the nucleus divides
twice.
 The result is sex cells, with half the
number of chromosomes as a normal
body cell.
Meiosis
 Info.
contained on chromosomes
determine many of our traits.
 Sex
cells carry genes that
determine sex of an individual.
 Females:
XX
Males: XY
Meiosis
A
pedigree is a diagram that shows
the occurrence of a genetic trait in
several generations of a family
 Pedigrees
are helpful in
tracing/predicting diseases and traits
from one generation to the next.
Meiosis
 Useful
in predicting if a person is a
carrier for a certain trait/disease.
 Carriers
do not always show signs of
the trait/disease.
Meiosis
 Cystic
fibrosis causes serious lung
problems.
 People
with cystic fibrosis disease
have 2 inherited recessive alleles.
 Both
parents must be carriers of the
trait for it to show up in their
children.