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Crosses and Chromosomes

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  1. Crosses and Chromosomes Do Now (dihybrid cross) In fruit flies, red eyes are dominant to white, and wings are dominant to no wings Draw a Punnett square representing a cross between two flies heterozygous for both traits. What is the probability of having a white-eyed wingless offspring?

  2. Quiz Review • 1: floppy completely dominant to pointy • 2: yellow + orange = codominant • 3: more common in men = sex-linkage • 4: in between phenotype (blend) = incomplete dominance • 5: 4 phenotypes = >2 alleles

  3. 6: Sex-Linkage Carrier f. 50% of males will be colorblind 50% of females will be colorblind, 50% will be carriers. No females w/ disease Normal m.

  4. 7: multiple alleles Green + Blue 1:1 green spots:blue OO = no spots No spots

  5. Dihybrid Punnett Square PY Py pY py PY Py pY py

  6. Pedigree

  7. TEST WEDNESDAY • COME TO COACH CLASS TOMORROW! • TOPICS: • Chromosomes, cell reproduction, crosses, + today’s topics (Mendel’s Laws + crossing over) • CHAPTER 9

  8. Mendel’s Law of Segregation • A pair of factors is segregated (separated) during the formation of gametes (sex cells) Y y For example: 50% Of Gametes 50% Of Gametes

  9. Mendel’s Law of Independent Assortment • Factors (alleles) for different characteristics are distributed independently from one another. T T t t Y Y y y For example: 25% Of Gametes 25% Of Gametes 25% Of Gametes 25% Of Gametes

  10. Mendel’s “Laws” of Heredity • It is important to note that these “laws” work under certain conditions: • Each gene is on a different chromosome • The organisms are diploid

  11. It’s all about the Chromosomes b B • In meiosis, the chromosome number is reduced by half (2nn; diploidhaploid) • 50% of gametes get one allele, 50% get the other. b b B B

  12. Dihybrid Gametes • Pea plant cross: PpYy x PpYy • Gametes: reproductive haploid cells (sperm and egg) • One individual heterozygous for 2 traits can produce 4 (2n, where n= number of different genes)possible combinations of alleles. • PY • Py • pY • py

  13. Dihybrid Punnett Square PY Py pY py PY Py pY py

  14. One More Twist a A • crossing over: when two homologous chromosomes exchange material during meiosis, leading to a new combination of alleles B b a A B b

  15. Meiosis and Genetics • The 2 alleles a diploid organism has for a trait are located on 2 homologous chromosomes. • The separation of these chromosomes during meiosis leads to the 50/50 chance of a gamete having one version or the other. • Mendel’s Laws of heredity only work for alleles on different chromosomes. • Crossing over creates new combinations of alleles.