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Dihybrid Crosses

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

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Dihybrid Crosses

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  1. Dihybrid Crosses

  2. Homework

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

  4. 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?

  5. 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. • At each table, please tear a piece of paper out of a notebook, and then tear it into four strips of roughly equal size.

  6. Dihybrid Crosses • Suppose that a person is heterozygous for eye color (Bb). One 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. • This person is also heterozygous for hitchhiker’s thumb. • Write H on the third strip, and h on the fourth. • These are their chromosomes in a diploid cell.

  7. 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?

  8. 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.

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

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

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

  12. Dihybrid Crosses • This person’s genotype is • B b H h • BH Bh bH bh • Of the four gametes, how many have B? How many have b? H? h?

  13. Dihybrid Crosses • This person’s genotype is • B b H h • BH Bh bH bh • Notice that we keep the genes in the same order, regardless of capitalization. For example, bH.

  14. Dihybrid Crosses • Now practice figuring out the four possible gametes from this genotype: JJYy • Note, some of them may be the same as each other.

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

  16. 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.

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