dihybrid crosses
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Dihybrid Crosses. What are the four possible gametes that the individuals with these genotypes could produce? (Remember to keep the genes in this order, so I/i before L/l, F/f before J/j, etc.) IiLL iiLL FFjj ffJJ YyHh. Dihybrid Crosses. Homework. Objectives. - PowerPoint PPT PresentationTRANSCRIPT
Dihybrid Crosses
What are the four possible gametes that the individuals with these genotypes could produce? (Remember to keep the genes in this order, so I/i before L/l, F/f before J/j, etc.)
– IiLL– iiLL– FFjj– ffJJ– YyHh
Dihybrid Crosses
Homework
Objectives
Be able to perform a dihybrid cross– Be able to identify what each box in the
cross represents– Be able to explain what Mendel’s three
Laws are, and where you can see each one “in action” in a dihybrid cross
Dihybrid Crosses
Popcorn reading: The first two pages of the handout, telling the story of the Morgan fruit fly experiments.
Gregor Mendel
All of these basic principles were first identified by Gregor Mendel, an Austrian monk in the mid-1800s.– Through studying pea
plant breeding, he described three laws of inheritance.
QuickTime™ and a decompressor
are needed to see this picture.
Mendel’s Laws of Inheritance
Law of Dominance: In a cross of parents that are pure (homozygous) for contrasting traits, offspring will only have one of those traits.– What is this describing that you’ve already
learned?– Could you rephrase it to make more sense
to you?
Mendel’s Laws of Inheritance
Law of Segregation: A parent’s allele pair separates during gamete formation, and alleles randomly unite in fertilization.– Let’s figure out how we actually already
learned this when we studied meiosis…
Mendel’s Laws of Inheritance
Law of Independent Assortment: Alleles of different genes separate independently during gamete formation. Therefore, traits pass to offspring independently of each other.– Again, let’s think back to meiosis, the
candy lab, the Baby Lab…
Laws Law of Segregation: A parent’s allele pair separates
during gamete formation, and alleles randomly unite in fertilization.– = Each gamete contains only one of each kind of allele.
Law of Independent Assortment: Alleles of different genes separate independently during gamete formation. Therefore, traits pass to offspring independently of each other.– = The parent makes an equal number of each of the four kind of
gametes. A gamete with an allele from one gene does not force it to have a certain allele from the other gene.
• For instance, for a parent who is BbHh, half their gametes have the H allele. Half of those will have the B allele, and the other half will have the b allele. The B/b alleles assorted independently of H. If they had been dependent, then it could be that gametes with H would only have b.
Dihybrid Crosses
So you know how to figure out the likelihood of having a blue-eyed baby. And you know how to figure out the likelihood of having a baby with a hitchhiker’s thumb.– But what if you want to know the likelihood of
having a brown-eyed baby with a hitchhiker’s thumb?
– A blue-eyed baby with a normal thumb?
Dihybrid Cross
For two genes that are on different chromosomes, we can do a dihybrid cross. (Di=two) – (Scientists can do very large complicated crosses,
but two is the largest one we’ll do.)
The essential difference is that figuring out gametes has a bit of a trick to it.– Baby Lab, the sequel!
Dihybrid Crosses Suppose that a person is heterozygous for eye color
(Bb). On one chromosome #1, they have the B allele. On the other chromosome #1, they have the b allele.– Write B on one strip, and b on another.– Write #1 at the top of both, just like in the baby lab.
This person is also heterozygous for hitchhiker’s thumb.– Write H on the third strip, and h on the fourth.– Write #2 at the top of both, just like in the baby lab.
These are their chromosomes in a diploid cell.
Dihybrid Crosses
When they make gametes, each gamete will get just one from each pair of homologous chromosomes.– Make different gametes. Each gamete should
have just one B/b chromosome, and just one H/h chromosome. Be prepared to answer:
• How many possible combinations are there?• What are the genotypes of those possible alleles?
Dihybrid Crosses
This person’s genotype is B b H h
A gamete from a person who is Bb will have either B or b. And a gamete from a person who is Hh will have either H or h. This is the same situation.
Dihybrid Crosses
This person’s genotype is B b H h
BH
Dihybrid Crosses
This person’s genotype is B b H h
BH Bh
Dihybrid Crosses
This person’s genotype is B b H h
BH Bh bH
Dihybrid Crosses
This person’s genotype is B b H h
BH Bh bH bh
Of the four gametes, what % have B? b? H? h?
Dihybrid Crosses
This person’s genotype is B b H h
BH Bh bH bh
We keep the genes in the same order, regardless of capitalization, like they’re a first and last name.
Dihybrid Crosses
Everyone get a whiteboard…
What are the gametes that can be made from: JJYy– Note, some gametes may be the same as
each other. Write all four of them anyways.
Dihybrid Crosses
JJYy = – JY JY JY Jy
Another practice problem, figure out the four gametes for:– ttGg
Dihybrid Crosses
ttGg = – tG tG tG tg
You set a dihybrid cross up exactly like a monohybrid cross, only now there are four gametes per parent rather than just two.
Example: A cross between IiLL and iill.– Let’s find genotype and phenotype ratios &
probabilities
Dihybrid Cross
Try this cross, find genotype and phenotype ratios & probabilities.
FFjj x ffJJ
Dihybrid Cross
Now try this cross: YyHh x YyHh.
Y = jagged, y = normal. H = square, h = round.
Find genotype and phenotype ratios & probabilities. (Helpful hint: 1/16 = 6.25%, 2/16 = 12.5%.)
• YyHh x YyHh.
• Genotype Probabilities: 6.25% YYHH, 12.5% YYHh, 12.5% YyHH, 25% YyHh, 6.25% YYhh, 12.5% Yyhh, 6.25% yyHH, 12.5% yyHh, 6.25% yyhh
• Genotype Ratios: 1 YYHH : 2 YYHh : 2 YyHH : 4 YyHh : 1 YYhh : 2 Yyhh : 1 yyHH : 2 yyHh : 1 yyhh
• Phenotype Probabilities: 56.25% jagged/square, 18.75% jagged/round, 18.75% normal/square, 6.25% normal/round
• Phenotype Ratios: 9 jagged/square : 3 jagged/round : 3 normal/square : 1 normal/round
YH Yh yH yh
YH YYHH YYHh YyHH YyHh
Yh YYHh YYhh YyHh Yyhh
yH YyHH YyHh yyHH yyHh
yh YyHh Yyhh yyHh yyhh