Dihybrid Crosses

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.

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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