Dihybrid Crosses

1. 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

2. Dihybrid Crosses

3. Homework

4. 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

5. Dihybrid Crosses • Popcorn reading: The first two pages of the handout, telling the story of the Morgan fruit fly experiments.

6. 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.

7. 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?

8. 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…

9. 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…

10. 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.

11. 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?

12. 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!

13. 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.

14. 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?

15. 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.

16. Dihybrid Crosses • This person’s genotype is • B b H h • BH

17. Dihybrid Crosses • This person’s genotype is • B b H h • BH Bh

18. Dihybrid Crosses • This person’s genotype is • B b H h • BH Bh bH

19. 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?

20. 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.

21. 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.

22. Dihybrid Crosses • JJYy = • JY JY JY Jy • Another practice problem, figure out the four gametes for: • ttGg

23. 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

24. Dihybrid Cross • Try this cross, find genotype and phenotype ratios & probabilities. • FFjj x ffJJ

25. 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%.)

27. 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