Download Monohybrid Crosses

Monohybrid Crosses
Gregor Mendel
• Austrian monk
who started the
study of genetics
in his monastery’s
garden in the
1860s
• Studied heredity
in garden peas
Why did he study peas?
1. They had a variety of characteristics that
occur in two different forms or
.
Why did he study peas?
1. They had a variety of characteristics that
occur in two different forms or alleles .
Why did he study peas?
1. They had a variety of characteristics that
occur in two different forms or alleles .
Why did he study peas?
Why did he study peas?
2. He could easily control which plants pollinated each
other.
Why did he study peas?
2. He could easily control which plants pollinated each
other.
– Pea plants can
, which is when a
plant’s pollen, which contains the sperm, is used to
fertilize the same plant’s egg. If he wanted pure
breeding plants, he could allow the plants to selfpollinate over many generations.
Why did he study peas?
2. He could easily control which plants pollinated each
other.
– Pea plants can self-pollinate , which is when a
plant’s pollen, which contains the sperm, is used to
fertilize the same plant’s egg. If he wanted pure
breeding plants, he could allow the plants to selfpollinate over many generations.
Why did he study peas?
2. He could easily control which plants pollinated each
other.
– Pea plants can self-pollinate , which is when a
plant’s pollen, which contains the sperm, is used to
fertilize the same plant’s egg. If he wanted pure
breeding plants, he could allow the plants to selfpollinate over many generations.
– Pea plants can also
, which is when
the pollen of one plant is used to fertilize another
plant. He could do this by removing stamens or
male parts of the flowers. He could brush pollen
from one flower to the female parts of another
flower.
Why did he study peas?
2. He could easily control which plants pollinated each
other.
– Pea plants can self-pollinate , which is when a
plant’s pollen, which contains the sperm, is used to
fertilize the same plant’s egg. If he wanted pure
breeding plants, he could allow the plants to selfpollinate over many generations.
– Pea plants can also cross-pollinate , which is when
the pollen of one plant is used to fertilize another
plant. He could do this by removing stamens or
male parts of the flowers. He could brush pollen
from one flower to the female parts of another
flower.
Why did he study peas?
Why did he study peas?
3. Peas produce a lot of offspring quickly, so he
could obtain lots of data quickly.
Why did he study peas?
3. Peas produce a lot of offspring quickly, so he
could obtain lots of data quickly.
– P generation-
Why did he study peas?
3. Peas produce a lot of offspring quickly, so he
could obtain lots of data quickly.
– P generation- 1st generation; parental (p)
generation
Why did he study peas?
3. Peas produce a lot of offspring quickly, so he
could obtain lots of data quickly.
– P generation- 1st generation; parental (p)
generation
– F1 generation-
Why did he study peas?
3. Peas produce a lot of offspring quickly, so he
could obtain lots of data quickly.
– P generation- 1st generation; parental (p)
generation
– F1 generation- The first generation of
offspring
Why did he study peas?
3. Peas produce a lot of offspring quickly, so he
could obtain lots of data quickly.
– P generation- 1st generation; parental (p)
generation
– F1 generation- The first generation of
offspring
• F stands for filial or son.
Why did he study peas?
3. Peas produce a lot of offspring quickly, so he
could obtain lots of data quickly.
– P generation- 1st generation; parental (p)
generation
– F1 generation- The first generation of
offspring
• F stands for filial or son.
–F2 generation-
Why did he study peas?
3. Peas produce a lot of offspring quickly, so he
could obtain lots of data quickly.
– P generation- 1st generation; parental (p)
generation
– F1 generation- The first generation of
offspring
• F stands for filial or son.
–F2 generation- The second generation of
offspring; offspring of the F1 generation
3 Steps of Mendel’s Experiments
1. Produce a true-breeding P generation
Self Pollination
Long stemmed plant
P generation
Long stemmed plant
Self Pollination
Short stemmed plant
Short stemmed plant
P generation
3 Steps of Mendel’s Experiments
2. Producing an F1 generation
C
r
o
s
s
P
o
l
l
i
n
a
t
i
o
n
Long stemmed
F1 Generation
3 Steps of Mendel’s Experiments
3. Producing an F2 Generation
C
r
o
s
s
P
o
l
l
i
n
a
t
i
o
n
¾ Long Stemmed
¼ Short Stemmed
F2 Generation
Mendel’s Law of Segregation
• Genes for different traits can segregate
(or separate) independently during the
formation of gametes
• This is because the two different alleles
that an organism has are on different
chromosomes in a homologous pair.
Mendel’s Law of Segregation
Parent Cell
Meiosis I
Meiosis II
Gametes
Practice
• What are the possible genetic contents of the
gametes made by a person with the genotype
BB?
• What are the possible genetic contents of the
gametes made by a person with the genotype
Tt?
Practice
• What are the possible genetic contents of the
gametes made by a person with the genotype
BB? B or B
• What are the possible genetic contents of the
gametes made by a person with the genotype
Tt?
Practice
• What are the possible genetic contents of the
gametes made by a person with the genotype
BB? B or B
• What are the possible genetic contents of the
gametes made by a person with the genotype
Tt? T or t
Making Sense of Mendel’s Findings
• Probability-
Making Sense of Mendel’s Findings
• Probability- the likelihood that a particular
event will occur
Making Sense of Mendel’s Findings
• Probability- the likelihood that a particular
event will occur
–Scientists use probability to predict the
phenotypes and genotypes of the
offspring.
Making Sense of Mendel’s Findings
• Probability- the likelihood that a particular
event will occur
–Scientists use probability to predict the
phenotypes and genotypes of the
offspring.
• Punnett Square-
Making Sense of Mendel’s Findings
• Probability- the likelihood that a particular
event will occur
–Scientists use probability to predict the
phenotypes and genotypes of the
offspring.
• Punnett Square- a diagram that shows gene
combinations that might result from a
genetic cross
Making Sense of Mendel’s Findings
• Probability- the likelihood that a particular
event will occur
–Scientists use probability to predict the
phenotypes and genotypes of the
offspring.
• Punnett Square- a diagram that shows gene
combinations that might result from a
genetic cross
–Punnett squares show predicted results,
not actual results.
Punnett Square
Parent 1
Gamete 1
Gamete 1
Offspring 1
Gamete 2
Offspring 2
Parent 2
Gamete 2
Offspring 3
Offspring 4
Let’s take a look at Mendel’s crosses that he completed to determine his
laws of heredity. We will use L to represent the allele for long stems and l
to represent the allele for short stem.
CROSSES INVOLVING ONE TRAIT
(MONOHYBRID CROSSES)
Homozygous x Homozygous
• When he crossed two pure-breeding plants for
the same version of the trait, all of the
offspring shared the same phenotype as the
parents.
Homozygous x Homozygous
LL
Parent 1
L
Gamete 1
LL
Parent 2
*Each box is filled in
with
the
letter
Gamete
L 1 appearing
Offspring
1
LLabove
it and
to the left. These are
the GENOTYPES of the
offspring.*
L
Gamete 2
LL
Offspring 3
L
Gamete 2
LL
Offspring 2
LL
Offspring 4
Homozygous x Homozygous
Possible Genotypes
Genotypic Ratio
Possible Phenotypes
Phenotypic Ratio
LL
1
Long stems
1
Homozygous Dominant x Homozygous Recessive
LL
Parent 1
L
Gamete 1
l
Gamete 1
Ll
Offspring 1
L
Gamete 2
Ll
Offspring 2
ll
Parent 2
l
Gamete 2
Ll
Offspring 3
Ll
Offspring 4
Homozygous Dominant x
Homozygous Recessive
Possible Genotypes
Genotypic Ratio
Possible Phenotypes
Phenotypic Ratio
Ll
1
Long stems
1
Heterozygous x Heterozygous
Ll
Parent 1
L
Gamete 1
L
Gamete 1
LL
Offspring 1
l
Gamete 2
Ll
Offspring 2
Ll
Parent 2
l
Gamete 2
Ll
Offspring 3
ll
Offspring 4
Heterozygous x Heterozygous
Possible Genotypes
Genotypic Ratio
Possible Phenotypes
Phenotypic Ratio
LL Ll Ll ll
1:2:1
Long stems Short stems
3:1
A good path to follow when
completing a genetics problem is:
1. Pick a letter to represent the gene, if one
isn’t given to you.
2. Record what characteristics the dominant
and recessive alleles represent.
3. Determine the genotype of the parents.
4. Complete a Punnett Square.
5. Determine your results.
SAMPLE PROBLEMS
1. In peas, wrinkled seeds (W) are dominant to smooth
seeds (w). If a plant that is homozygous dominant is
bred with a plant that is heterozygous, what
phenotypic and genotypic ratios would be expected
in the offspring?
W = wrinkled seed
w = smooth seed
Parents
•Parent 1: Homozygous
dominant: WW
•Parent 2: Heterozygous: Ww
W
w
W
W
WW
WW
Ww
Ww
Possible Genotypes
Genotypic Ratio
Possible Phenotypes
Phenotypic Ratio
WW
Ww
1:1
Wrinkled
1
2. In peas, purple flowers are dominant to white
flowers. If a plant that is heterozygous for purple
flowers is bred with a plant that has white flowers,
what percentage of the offspring will have white
flowers?
P = Purple flowers
p = White flowers
Parents
•Parent 1: Heterozygous: Pp
•Parent 2: White flowers: pp
p
p
P
p
Pp
pp
Pp
pp
50%
Test Crosses
Test Crosses
• We can’t tell by looking at an individual with a
dominant phenotype if the individual’s genotype
is homozygous dominant (BB) or heterozygous
(Bb). What can we do to determine the genotype
of an individual with a dominant phenotype?
• Test cross:
Test Crosses
• We can’t tell by looking at an individual with a
dominant phenotype if the individual’s genotype
is homozygous dominant (BB) or heterozygous
(Bb). What can we do to determine the genotype
of an individual with a dominant phenotype?
• Test cross: crossing an organism with a dominant
phenotype, but unknown genotype, with a
homozygous recessive organism, then observing
the phenotypes of the offspring to determine the
unknown genotype
Example: In guinea pigs, the dominant allele B
codes for black fur, while the recessive allele b
codes for white fur. Jimmy has a black guinea
pig and wants to determine its genotype. He
decides to do a test cross to determine his
guinea pig’s genotype. What phenotypic ratio
would he expect in his offspring if his guinea
pig was homozygous? If it is heterozygous?
B = Black fur
b = White fur
Parents
•Parent 1: Homozygous Recessive: bb
•Possible Parent 2: Homozygous Dominant: BB
•Possible Parent 2: Heterozygous: Bb
Test Cross #1: bb x BB
b
B
Bb
Bb
B
b
Bb
Bb
Possible Genotypes
Genotypic Ratio
Bb
Possible Phenotypes
Black fur
Phenotypic Ratio
1
1
B = Black fur
b = White fur
Parents
•Parent 1: Homozygous Recessive: bb
•Possible Parent 2: Homozygous Dominant: BB
•Possible Parent 2: Heterozygous: Bb
Test Cross #2: bb x Bb
b
B
Bb
bb
b
b
Bb
bb
Possible Genotypes
Genotypic Ratio
Bb
Possible Phenotypes
Black fur
Phenotypic Ratio
bb
1:1
1:1
White fur
SAMPLE PROBLEMS
1. In a species of lizard, green coloring is
dominant to brown coloring.
A. Sally wants to determine the genotype of her
green lizard. What type of lizard should she mate
her green lizard with to determine its genotype?
Brown
B. Sally carries out this mating. Three of the baby
lizards were green and four were brown. What is
the genotype of her lizard? Explain your
reasoning.
Her lizard is heterozygous. If you perform a test
cross, the Punnett square demonstrates that the
cross with the heterozygous individual is the only
one resulting in some brown lizards.
2. In a species of plant, red flowers are
dominant to white flowers. Billy crosspollinates a red flowered plant with a white
flowered plant. All of the offspring plants
have red flowers. What is the genotype of
the red flowered plant? Explain your
reasoning.
The plant is homozygous dominant. If you perform
a test cross, the Punnett square demonstrates that
the cross with the homozygous dominant individual
is the only one resulting in only red flowered plants.