Dihybrid Crosses 2013
Monohybrid Crosses – one ______
Monohybrid crosses allow us to look at the offspring when considering just ______set of alleles
flower color or stem length, for instance
Dihybrid – more ______
Now it gets a little more complex if we try to predict the possible offspring if we are looking at two traits at once
flower color and stem length, for instance
These are called dihybrid crosses
Sample Problem
Take a plant that is heterozygous for Tall stem length and purple flowers.
What would it’s genotype be??
Remember that T = tall, t = short, P = Purple and p = white
What gametes could this individual produce?
Now consider taking this individual and self-fertilizing it. What would the cross look like?
Since each individual in that cross makes four different gametes (TP, Tp, tP and tp), the Punnett Square has to be 4X4
Don’t panic!
• Usually these types of crosses are much easier because there are not as many possible gametes.
• Try ttpp X Ttpp
• ttpp – possible gametes ______
• Ttpp – possible gametes ______
• Punnett Square ______
Sample Problem #2
A homozygous purple, heterozygous starchy corn plant is crossed with a homozygous yellow, homozygous sweet corn plant.
What is the cross and what are the gametes that each can produce?
What is the phenotype of the offspring?
Sample Problem #3
A heterozygous purple, heterozygous starchy corn plant is crossed with a homozygous yellow, homozygous sweet corn plant.
What is the cross and what are the gametes that each can produce?
What is the phenotype of the offspring?
Sample Problem #4
A heterozygous purple, homozygous starchy corn plant is crossed with a heterozygous purple, homozygous sweet corn plant.
What is the cross and what are the gametes that each can produce?
What is the phenotype of the offspring?
Using probability
The product rule is a useful principle of probability. Here’s the principle:
the probability of independent events occurring at the same time is the product of each of their probabilities.
Example: If you flip a penny and a nickel at the same time, the probability of both coins coming up heads is
½ X ½ = ¼
If you flipped a penny, a nickel and a dime, the probability that they would all come up heads is
½ X ½ X ½ = 1/8
Now let’s apply it to genetics:
If an organism is AaBb crosses with an individual that is AaBb,
what are the chances you will get an individual that is AaBb
what are the chances you will get an individual that is dominant for both traits?
What are the chances you will get an individual that is Aabb?
If an organism is AaBbCc crosses with an individual that is AaBbCC,
What are the chances you will get an individual that is AaBbCC?
What are the chances you will get an individual that is aabbcc?
A heterozygous purple, homozygous starchy corn plant is crossed with a heterozygous purple, homozygous sweet corn plant. What is the phenotype of the offspring? (hint: the fractions should add up to one)
Dihybrid Crosses 2013
Monohybrid Crosses – one trait
Monohybrid crosses allow us to look at the offspring when considering just ONE set of alleles
flower color or stem length, for instance
Dihybrid – more THAN ONE TRAIT
Now it gets a little more complex if we try to predict the possible offspring if we are looking at two traits at once
flower color and stem length, for instance
These are called dihybrid crosses
Sample Problem
Take a plant that is heterozygous for Tall stem length and purple flowers.
What would it’s genotype be??
TtPp
Remember that T = tall, t = short, P = Purple and p = white
What gametes could this individual produce?
TP, Tp, tP and tp
Now consider taking this individual and self-fertilizing it. What would the cross look like?
TtPp X TtPp
Since each individual in that cross makes four different gametes (TP, Tp, tP and tp), the Punnett Square has to be 4X4
TP / Tp / tP / tpTP / TTPP / TTPp / TtPP / TtPp
Tp / TTPp / TTpp / TtPp / Ttpp
tP / TtPP / TtPp / ttPP / ttPp
Tp / TtPp / Ttpp / ttPp / Ttpp
Phenotype (don’t worry about genotype…it’s crazy hard
Tall and Purple - 9
Tall and white – 3
Short and purple – 3
Short and white - 1
Don’t panic!
• Usually these types of crosses are much easier because there are not as many possible gametes.
• Try ttpp X Ttpp
• ttpp – possible gametes - tp
• Ttpp – possible gametes Tp and tp
• Punnett Square 1 X 2
tpTp / Ttpp
tp / ttpp
Sample Problem #2 Corn plants Starchy is dominant over sweet and Purple is dominant over yellow
A homozygous purple, heterozygous starchy corn plant is crossed with a homozygous yellow, homozygous sweet corn plant.
What is the cross and what are the gametes that each can produce?
PPSs X ppss
Gametes: PS and Ps X ps
What is the phenotype of the offspring?
psPS / PpSs
Ps / Ppss
Sample Problem #3
A heterozygous purple, heterozygous starchy corn plant is crossed with a homozygous yellow, homozygous sweet corn plant.
What is the cross and what are the gametes that each can produce?
PpSs X ppss
Gametes: PS, Ps, pS and ps X ps
What is the phenotype of the offspring?
PS / Ps / pS / psps / PpSs / Ppss / ppSs / Ppss
Sample Problem #4
A heterozygous purple, homozygous starchy corn plant is crossed with a heterozygous purple, homozygous sweet corn plant.
What is the cross and what are the gametes that each can produce?
PpSS X Ppss
PS and pS X Ps and ps
What is the phenotype of the offspring?
Ps / psPS / PPSs / PpSs
pS / PpSs / ppSs
Using probability
The product rule is a useful principle of probability. Here’s the principle:
the probability of independent events occurring at the same time is the product of each of their probabilities.
Example: If you flip a penny and a nickel at the same time, the probability of both coins coming up heads is
½ X ½ = ¼
If you flipped a penny, a nickel and a dime, the probability that they would all come up heads is
½ X ½ X ½ = 1/8
Now let’s apply it to genetics:
If an organism is AaBb crosses with an individual that is AaBb,
what are the chances you will get an individual that is AaBb
Take each cross separately:
Aa X Aa Bb X Bb
Aa is a ½ Bb is a ½
½ X ½ = ¼
what are the chances you will get an individual that is dominant for both traits?
A_B_
¾ X ¾ = 9/16
What are the chances you will get an individual that is Aabb?
½ X ¼ = 1/8
If an organism is AaBbCc crosses with an individual that is AaBbCC,
What are the chances you will get an individual that is AaBbCC?
½ X ½ X ½ = 1/8
What are the chances you will get an individual that is aabbcc?
¼ X ¼ X 0 = 0
A heterozygous purple, homozygous starchy corn plant is crossed with a heterozygous purple, homozygous sweet corn plant. What is the phenotype of the offspring? (hint: the fractions should add up to one)
PpSS X Ppss
Purple and Starchy: P_S_ = ¾ X 1 = ¾
Purple and sweet: P_ss = ¾ X 0 = 0
Yellow and Starchy: ppS_ = ¼ X 1 = ¼
Yellow and sweet: ppss = ¼ X 0 = 0