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Dihybrid Crosses. Predicting TWO traits at once!. Mendel’s Law of Independent Assortment. What is the Law of Independent Assortment?. Genes for different traits can segregate independently during the formation of gametes. What does that really mean?. - PowerPoint PPT Presentation
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Dihybrid CrossesPredicting TWO traits at once!
Mendel’s Law ofIndependent Assortment
What is the Law of Independent Assortment?
Genes for different traits can segregate independently during the formation of gametes.
What does that really mean?
The inheritance of one gene does not influence the inheritance of another gene.
Just because you got your dad’s “tall” gene doesn’t mean you will also necessarily get his “blue eye” gene –
the two aren’t connected!
Just because you got your dad’s “tall” gene doesn’t mean you will also necessarily get his “brown eye” gene –
the two aren’t connected!
We said…
But what if we want to know the chances that you DO get both genes?
NO!!!
Or what if we want to know the chances of not one particular gene, but TWO particular genes?
Would a regular Punnett Square work?
In order to predict TWO traits at once,you will need to do a...
DIHYBRID CROSS
Four possible combinations of genes on the top and side (parental gametes)
Sixteen total possible gene combinations (offspring)
Now lets try our own example….
People like to eat green, round peas – not wrinkled and not yellow! So a farmer would
want to know, if he was breeding plants, what the chances are that he would get the
ideal crop (and make the most money!).
The Facts:R = round
r = wrinkled
Y = yellowy = green
Both plants he is starting with are RrYy
So we need to make a GIANT Punnett Square to cross RrYy x RrYy…and then figure out the “chances” that the farmer will
get round, green offspring!
RrYyParent 1:
RrYyParent 2:
When you are predicting only one trait, each parent has two possible genes to give. When you are predicting two
traits, each parent has FOUR possible genes to give!
RY
Ry
rY
ry
So Parent 1 could give the following four combinations of genes to their offspring…
RY
Ry
rY
ry
And Parent 2 could give the following four combinations of genes to their offspring…
RrYyParent 1:
RrYy
Pare
nt 2
:RYR
YRy
Ry
rY
rY
ry
ryRRYY RRYy RrYY RrYy
RRYy RRyy RrYy Rryy
RrYY RrYy rrYY rrYy
RrYy Rryy rrYy rryy
RrYyParent 1:
RrYy
Pare
nt 2
:RYR
YRy
Ry
rY
rY
ry
ryRRYY RRYy RrYY RrYy
RRYy RRyy RrYy Rryy
RrYY RrYy rrYY rrYy
RrYy Rryy rrYy rryy
Round, Yellow Round, Yellow Round, Yellow Round, Yellow
Round, Yellow Round, green Round, Yellow Round, green
Round, Yellow Round, Yellow wrinkled, Yellow wrinkled, Yellow
Round, Yellow Round, green wrinkled, Yellow wrinkled, green
So what do th
ey
look like?
RrYyParent 1:
RrYy
Pare
nt 2
:RYR
YRy
Ry
rY
rY
ry
ryRRYY RRYy RrYY RrYy
RRYy RRyy RrYy Rryy
RrYY RrYy rrYY rrYy
RrYy Rryy rrYy rryy
Round, Yellow Round, Yellow Round, Yellow Round, Yellow
Round, Yellow Round, green Round, Yellow Round, green
Round, Yellow Round, Yellow wrinkled, Yellow wrinkled, Yellow
Round, Yellow Round, green wrinkled, Yellow wrinkled, green
So what do th
ey
look like?
NOW LET’S MAKE SOME
PREDICTIONS!
RY
RY
Ry
Ry
rY
rY
ry
ryRRYY RRYy RrYY RrYy
RRYy RRyy RrYy Rryy
RrYY RrYy rrYY rrYy
RrYy Rryy rrYy rryy
Round,Yellow
Round, Yellow
Round,Yellow
Round,Yellow
Round,Yellow
Round,green
Round,Yellow
Round,green
Round,Yellow
Round,Yellow
wrinkled,Yellow
wrinkled,Yellow
Round,Yellow
Round,green
wrinkled,Yellow
wrinkled,green
How many of the farmer’s plants will produce the desired Round, green peas?
3 out of 16(just 18.75%)
How many will produce the yellow peas, which no one will want to buy?
12 out of 16or 75%
So would it be wise for the farmer tocross these two RrYy plants?
NO!
Ready for another example?!?
Circus Freaks!!!This husband and wife want the ultimate circus freak kid
– one with elephant skin and a hairy face!
If they have a baby, what are the chances that they’ll have the ideal circus freak? And what are the chances that they will just have a
normal kid?!?!
Dog-faced Man
The Facts:Dog-faced Man Normal Face = F
Hairy Face = f
Normal Skin = QElephant Skin = q
Dad must be ff for face.Genetic tests show he is Qq for skin.
Mom must be qq for skin.Her dad was also “dog-faced”, so we
know that she must be Ff for face.
The Facts:So Mom is FfqqAnd Dad is ffQq
Which means that Mom and Dad each can give FOUR possible combination of the two genes….
Mom could give .
Dad could give .
Fq or Fq or fq or fq
fQ or fQ or fq or fq
If they each can give FOUR possible combination of genes, how many boxes is our PunnettSquare going to have? 16!
Mom = Ffqq
Dad
= ff
Fq Fq fq fq
fQ
fq
fQ
fq
FfQq
Ffqq
FfQq ffQqffQq
Ffqq
FfQq
FfqqFfqq
FfQq
ffqq
ffQq
ffqq ffqq
ffQq
ffqq
Fq Fq fq fq
fQ
fq
fQ
fq
FfQq
Ffqq
FfQq ffQqffQq
Ffqq
FfQq
FfqqFfqq
FfQq
ffqq
ffQq
ffqq ffqq
ffQq
ffqq
Normal face, normal skin
Normal face, normal skin
Normal face, normal skin
Normal face, normal skin
Normal face, elephant skin
Normal face, elephant skin
Normal face, elephant skin
Normal face, elephant skin
Hairy face, normal skin
Hairy face, normal skin
Hairy face, normal skin
Hairy face, normal skin
Hairy face, elephant skin
Hairy face, elephant skin
Hairy face, elephant skin
Hairy face, elephant skin
Mom = Ffqq
Dad
= ff
NOW LET’S MAKE SOME
PREDICTIONS!
Fq Fq fq fq
fQ
fq
fQ
fq
FfQq
Ffqq
FfQq ffQqffQq
Ffqq
FfQq
FfqqFfqq
FfQq
ffqq
ffQq
ffqq ffqq
ffQq
ffqq
Normal face, normal skin
Normal face, normal skin
Normal face, normal skin
Normal face, normal skin
Normal face, elephant skin
Normal face, elephant skin
Normal face, elephant skin
Normal face, elephant skin
Hairy face, normal skin
Hairy face, normal skin
Hairy face, normal skin
Hairy face, normal skin
Hairy face, elephant skin
Hairy face, elephant skin
Hairy face, elephant skin
Hairy face, elephant skin
How many of the circus freaks children will be “ideal” circus freaks themselves, with elephant skin and a hairy face?
4 out of 16 (25%)
How many of their kids will be perfectly normal?
4 out of 16 or 25%How many will have just one circus freak trait? 8 out of 16How many will carry a recessive gene that they may pass on to their children? 100%!
