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Punnett Squares: Genetic Crosses

Punnett Squares: Genetic Crosses. Making Genetic Predictions. Mendel found that you could predict the traits of a percentage of the offspring. He invented and used the tool of a Punnett square to illustrate these percentages. Punnett Squares. The Law of Dominance

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Punnett Squares: Genetic Crosses

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  1. Punnett Squares: Genetic Crosses

  2. Making Genetic Predictions • Mendel found that you could predict the traits of a percentage of the offspring. • He invented and used the tool of a Punnett square to illustrate these percentages.

  3. Punnett Squares • The Law of Dominance Traits are shown with a Capital letter for dominant and a lower case for recessive – Brown Eyes (B) Blue Eyes (b)

  4. Punnett Squares • Eye Color – Brown eyes B – Blue eyes b • Purebred individual with brown eyes (BB) – Homozygous • Purebred individual with blue eyes (bb) – Homozygous • Hybrid individual with brown eyes (Bb) – Heterozygous

  5. Punnett Squares • Eye Color – Father is (BB) – Mother is (bb) • Their Genes Are Crossed – BB x bb – Since we are looking at only 1 gene, it is called a monohybrid cross.

  6. Law Of Segregation • Law of Segregation – Alleles for the same trait must be in separate sex cells. – Father is (BB) as his diploid genotype – Mother is (bb) as her diploid genotype • Somatic Cells BB bb • Sex cells B Bbb

  7. Monohybrid Crosses • Monohybrid cross deals with one gene – We are working with only eye color • Setting up a punnet square • The sex cells go on the outside of the square

  8. Punnett Squares B B Bb = Brown All of the offspring will have brown eyes, but they will be heterozygous individuals. b b

  9. Punnett Squares • Eye Color – Father has Brown eyes and is heterozygous (Bb). – Mother has Brown eyes and is heterozygous (Bb).

  10. Punnett Squares B b BB = Brown Bb = Brown bb = Blue B b

  11. Ratios • Genotype Ratio – 1 BB to 2 Bb to 1 bb • Phenotype Ratio – 3 Brown eyes to 1 blue Eyes • What is the chance of this couple having a child with blue eyes? – 1/ 4 or a 25 % chance

  12. Punnett Squares • One parent is heterozygous for the trait the other parent is homozygous • Eye Color – Father (Bb) Brown – Mother (bb) blue

  13. Punnett Squares B b b b

  14. Ratios • Genotype Ratio – 0 BB to 2 Bb to 2 bb (reduce) – 0 BB to 1 Bb to 1 bb • Phenotype Ratio – 2 Brown eyes to 2 blue eyes (reduce) – 1 Brown eye to 1 blue eye • What is the chance of this couple having a child with blue eyes? – ½ or 50%

  15. Mendel’s Work • Pea Color: • Yellow: Y • Green: y • P Generation • YY x yy

  16. Mendel’s Work • Crossed two of the F1 to see what the F2 would look like.

  17. Multiple Characteristics • Punnett Squares work with multiple characteristics. • Crosses can be done with an assortment of genes.

  18. Dihybrid Crosses • The Parents are heterozygous for both hair and eye color (BbEe: Brown hair and Brown Eyes). – B: Brown hair - E: Brown eyes – b: Blonde hair - e : Blue eyes • Cross = BbEe x BbEe

  19. Mendel’s Laws • The law of segregation – You can’t have the same letters in a Sex cell • The law of independent assortment – The two genes will have their alleles placed into the same sex cells randomly. • Work with one parent at a time – BbEe (B b) (E e) – Possible sex cells – (BE) (Be) (bE) (be)

  20. BE Be bE be BE Be bE be

  21. Phenotype Ratios • Phenotypes – Brown Hair Brown Eyes _________ – Brown Hair Blue Eyes _________ – Blonde Hair Brown Eyes _________ – Blonde Hair Blue Eyes _________

  22. Phenotype Ratios • Phenotypes – Brown Hair Brown Eyes ____9____ – Brown Hair Blue Eyes ____3____ – Blonde Hair Brown Eyes ____3____ – Blonde Hair Blue Eyes ____1____

  23. Phenotype Percentages • Phenotypes – Brown Hair Brown Eyes ____56%___ – Brown Hair Blue Eyes ____19%___ –Blonde Hair Brown Eyes ____19%___ – Blonde Hair Blue Eyes ____6%____

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