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Mendel’s Laws of Heredity
Section 10.1 p. 253 - 262
Chapter 10 Mendel and Meiosis
• Mendel chose which pea plants he would
allow to fertilize or pollinate one
another and then kept records of the
offspring produced.
Before we can go any further…
• There are a bunch of vocab words we
need to start using.
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Alleles
Dominant
Recessive
Homozygous
Heterozygous
Mendel’s ~ The Rule of Unit Factors
• Each organism has two factors that control
each of its traits.
• These factors are now known as genes.
• Genes come in different forms.
• Alternative forms of a gene for each
variation of a trait is an allele.
alleles
• We get two copies of each allele or
directions for a trait.
• One copy comes from each parent.
• We may get a matching set or they may
be different with the dominant one
hiding or masking the recessive version.
dominant
• Observed trait of an organism that
masks the recessive form of a trait.
• If present, this is the trait we see.
• “Dominant” is not always “good”.
recessive
• Trait of an organism that can be
masked by the dominant form of a trait.
• You need two copies, one from each
parent, for a recessive trait to be
expressed.
• Recessive traits are not always “bad”.
Mendel’s ~ The Rule of Dominance
• Even though an organism carries two
alleles for the same trait, only one
trait is observed.
• The dominant trait will be expressed.
Homozygous
• Having two of the same alleles for a
gene
• When there are two identical alleles
for a trait.
Heterozygous
• Having two different alleles for the same
gene.
• Back to the peas, each pea has two height
genes.
• T = tall, t = short
• TT and tt are homozygous.
• Tt is heterozygous.
Phenotype
• Outward appearance of an organism,
regardless of its genes.
• An organism's traits (like green vs.
yellow seeds, or pea plant height)
• What you look like!
Genotype
• Combination of genes in an organism.
• Can not tell from outward appearance
• Set of directions on the inside.
Genotype
• If it is a recessive trait, the genotype
is obvious.
• Dominant traits are expressed whether
the organism is heterozygous (2
different alleles) or homozygous (2 of
same alleles) so you do not know for
sure just from looking.
hybrid
• Offspring formed by parents having
different forms of a trait.
• Mendel’s crossbreed strains were made
breeding know true-breeding plants to
different true-breeding plants.
• Working with his peas, Mendel used 355
cross-bred strains and 12,980
resultant hybrids.
True-breeding or homozygous
• True-breeding means that they only have one
version of a trait to pass on or homozygous.
• Sometimes we refer to homozygous organisms
as being a “pure bred”.
• If a pea plant came from parents that were
tall, it is tall and all of its offspring are tall
when crossed with other pea plants that are
“true-breeding”, we can be fairly certain that
it is homozygous.
Poodles
Wolves
• What would their puppies look like?
• In 1976 a person named C. Schleifenbaum
lead a study to find out.
poodle-wolf hybrids
• They bred wolves to poodles.
• Then looked at the fur marks and coat
color in wolves and poodle-wolf hybrids.
• Here are some pictures of their
results.
1st generation ~ poodle-wolf hybrid
2nd generation ~ poodle-wolf hybrid
Notice how the coats range from wolf-like to poodle-like
Monohybrid cross
• How can we explain the poodle-wolf
hybrid results?
• Poodles are “true-breeding” for one
version of traits.
• Wolves have a different version of
these same traits.
Monohybrid cross
• In a monohybrid cross, the parents
selected are true-breeding for the
trait we will be studying.
• The parents are known as the P1
generation.
• Two P1 parents will be breed or crossed
and the resulting offspring are the 1st
or F1 generation.
1st generation
• First generation is the F1 generation
• “F” stands for filial ~ son or daughter
• In the F1 generation, all of the
offspring should be similar, showing all
the dominant traits that were passed on
by the P1 generation.
st
1
generation
• All of the poodle-wolf hybrids looked
the same.
Mendel’s ~ The Law of Segregation
• Each plant has two different alleles, it
can produce two different types of
gametes.
• During fertilization, male and female
gametes randomly pair to produce four
combinations of alleles.
The Law of
Segregation
Monohybrid Crosses
Homozygous P1 =
All offspring will have
the same genotype
They will all have the
same phenotypes & look
like the dominant trait.
Heterozygous P1 =
The offspring will have a
1:2:1 ration in genotypes
3:1 ration in phenotypes
Homozygous crosses
• Mendel choose plants that were
breeding true.
• His peas must have been homozygous…
• Only able to provide one type of allele
or variety of that particular trait.
Homozygous crosses
• Mendel’s first pea plant crosses were
between tall pea plants and short pea
plants.
• One parent has two tall alleles and the
other parent has two short alleles.
• They are both homozygous for their
trait.
How we represent
the parents & offspring in a cross
• A letter is selected to represent a
trait.
• The dominant version is shown with the
capital.
• The recessive version is shown with the
lower case .
How we represent
the parents in a cross
• With the height of pea plants a capital
T is used to represent the tall version
• A lower case t is used to represent the
short version
Genotypes in the cross
• The tall pea parents will be =
– TT, it can only give T alleles
• The short pea parents will be =
– tt, it can only give t alleles
Punnett Squares
• A punnett square is a
way to predict the
possible outcome of
a breeding cross
• Each box represents
a reproductive
opportunity or
offspring.
Short
parent’s
alleles
t
Tall
parent’s
alleles
T
T
t
Punnett Squares
• The offspring gets one
allele from each parent
so they end up with two
copies of alleles for
each trait.
• Divide each parent’s two
alleles so that each box
has one letter next to
or above it
Short
parent’s
alleles
t
Tall
parent’s
alleles
T
T
t
Monohybrid cross - Homozygous
• If one parent is
homozygous
dominant for their
trait and the other
is homozygous
recessive, the
offspring will all
have the same
phenotype and
genotype.
Short
parent’s
alleles
t
Tall
parent’s
alleles
T
T
t
Monohybrid cross - Homozygous
• The offspring’s
genotype (what the
alleles say) will be =
• Tt
• The offspring’s
phenotype (what we
see) will be =
• The plants will be
tall.
• The ________ is
the dominant trait.
Short
parent’s
alleles
t
Tall
parent’s
alleles
T
T
t
Monohybrid cross
t
t
Genotype
Genotype
Both Homozygous/
1 –Recessive
1 -Dominant
T
T
t
Phenotype =
T
Phenotype =
Dominant
Genotype
T
t
T
t
Phenotype =
Dominant
Genotype
T
t
Phenotype =
Dominant
Dominant
nd
2
generation
• Second generation is the F2 generation
• The second generation all has two
different copies (heterozygous) for
each trait.
• Their offspring will show more variation
because the alleles will regroup in many
ways.
Monohybrid cross - Heterozygous
• If two organisms are selected out of
the F1 generation to be parents for the
next generation, they will each be
heterozygous.
• Sticking with our offspring of a tall pea
plant & and a short pea plant, this group
will have a genotype of Tt.
• They will have a phenotype of tall.
Monohybrid cross - Heterozygous
• Remember, the
offspring get one allele
from each parent so
they end up with two
copies of alleles for
each trait.
Heterozygous
parent’s
alleles
T
Heterozygous
parent’s
alleles
• Both parents are
heterozygous so they
have a genotype of Tt.
T
t
t
Monohybrid cross - Heterozygous
• How many of the
offspring will be
tall?
• How many of the
offspring will be
short?
Heterozygous
parent’s
alleles
T
Heterozygous
parent’s
alleles
T
t
t
Monohybrid cross - Heterozygous
• What are the
possible genotypes
for the offspring?
• What is the ratio of
tall (dominant) to
short (recessive)
plants in the F2
generation?
Heterozygous
parent’s
alleles
T
Heterozygous
parent’s
alleles
T
t
t
Monohybrid cross
T
t
Genotype
Genotype
Both
Heterozygous
T
T
T
Phenotype =
T
Phenotype =
Dominant
Dominant
Genotype
t
t
t
T
Phenotype =
Genotype
t
t
Phenotype =
Dominant
Recessive
2nd generation
• The second generation of poodle-wolf hybrids
looks very different from each other!
Crossing
Homozygous Pea
Plants
Crossing
Heterozygous
Pea Plants