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Understanding Basic Genetics with Punnett Squares

Learn about alleles, heredity, traits, and generations through Punnett squares. Explore genotype and phenotype concepts with practical examples and Filbert the Frog's genetic cases.

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Understanding Basic Genetics with Punnett Squares

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  1. - A monk who studied peas in his garden and developed our basic understanding of heredity. Gregor Mendel

  2. Allele • Alternate forms of a gene • Most traits have 2 alleles • Ex. Mendel’s peas

  3. Blue eye or brown eye allele

  4. Heredity • The transmission of characteristics from parents to offspring

  5. Trait • The visible character that shows Ex. Yellow pea or green pea

  6. Genes • A discrete unit of hereditary information located on a chromosome

  7. Generations • P1 Generation – The original parents • F1 Generation – The first offspring (Children) • F2 Generation – The offspring from the F1 gen. (grandchildren)

  8. Law of segregation • When gametes (sperm and egg) form, only one of the 2 possible genes for each trait will be in each gamete

  9. Dominant vs. Recessive • Dominant – The trait that is stronger or masks the other trait • Recessive – The trait that is weaker or hidden

  10. Dominant gene • Black fur is dominant to white

  11. If there are both a dominant and recessive traits together, only the dominant one appears

  12. Genotype • The genes an organism carries • Ex: Gg = green (G) and yellow (g) genes G g

  13. Phenotype • How the genes are expressed (what it looks like) Ex: Gg has both green and yellow genes but the phenotype is green because green is dominant

  14. Homozygous Dominant • Both Dominant Genes Present Genotype = GG Phenotype = Green GG G G

  15. Heterozygous 1 Dominant gene, 1 recessive gene present Genotype = Gg Phenotype = Green g Gg G

  16. Homozygous Recessive • Both recessive Genes Present Genotype = gg Phenotype = Yellow gg g g

  17. Monohybrid cross • Look at one single character

  18. If we know the alleles of the parents, we can make predictions about the genetic traits of the offspring usingA PUNNETT SQUARE! • Draw a Punnett square • Label 1 side with the genotype of one parent. • Label the other side with the genotype of the other parent. • Do the cross • Describe genotypes including the possible percentages for the offspring • Describe the phenotypes including the possible percentages for the offspring

  19. Punnett Square Practice H h H H h h Genotypes: HH: Hh: hh: Phenotypes: Curly:Straight: Hh H H h h 1. Curly hair (H) is dominant to straight hair (h). Draw a punnett square crossing a heterozygous male (Hh) with a heterozygous female (Hh). Name all possible gentoypes and phenotypes and the percentages.

  20. Punnett Square Practice d d d d d d Genotypes: DD: Dd: dd: Phenotypes: Dimples: No Dimples: D D D D D D 2. Dimples (D) are dominant to not having dimples (d). Draw a punnett square crossing a homozygous dominant female (DD) with a homozygous recessive male (dd). Name all possible genotypes and phenotypes and the percentages.

  21. Punnett Square Practice e e Genotypes: EE: Ee: ee: Phenotypes: Attached:Unattached: E e Ee Ee ee ee 3. Unattached earlobes (E) are dominant to attached earlobes (e). Draw a punnett square crossing a homozygous recessive female with a heterozygous male. Name all possible genotypes and phenotypes and percentages.

  22. Punnett Square Practice F f Genotypes: FF: 50% Ff: 50% PhenotypesFreckles: 100% F F FF Ff FF Ff 4. Having freckles (F) is dominant to not having freckles (f). Draw a Punnett square crossing a heterozygous female with a homozygous dominant male. Name all possible genotypes and phenotypes and percentages.

  23. Punnett Square Practice i i Genotypes: Ii: 100% PhenotypesImmunity: 100% I I Ii Ii Ii Ii 5. Immunity to poison ivy is dominant to being susceptible to getting poison ivy. Draw a punnett square crossing a homozygous recessive female with a homozygous dominant male. Name all possible genotypes and phenotypes and percentages.

  24. Punnett Square Practice W w Genotypes: WW: 25% Ww: 50% ww: 25% PhenotypesImmunity: Widows peak: 75% Normal: 25% WW Ww Ww ww W w 6. A widow’s peak is dominant to a normal hairline. Draw a punnett square crossing a heterozygous female with a heterozygous male. Name all possible genotypes and phenotypes and percentages.

  25. Cases in Genetics with Filbert the Frog and Friends

  26. S: smooth s: rough Y: yellow y: green Dihybrid Punnett Squares SSYY: 1/16 SSYy: 2/16 SSyy: 1/16 SsYY: 2/16 SsYy: 4/16 Ssyy: 2/16 ssYY: 1/ 16 ssYy: 2/16 ssyy: 1/16 Smooth Yellow: 9/16 Rough Yellow: 3/16 Smooth green: 3/16 Rough green: 1/16 Genotypes: Phenotypes:

  27. B = black fur • b = brown fur • S = short fur • s = long fur Now try to make one… BBSS: 1/16 BBSs: 2/16 BBss: 1/16 BbSS: 2/16 BbSs: 4/16 Bbss: 2/16 bbSS: 1/16 bbSs: 2/16 bbss: 1/16 BBSSBBSsBbSSBbSs BBSsBBssBbSsBbss BbSSBbSsbbSSbbSs BbSsBbssbbSsbbss Black, Short: 9/16 Brown, Short: 3/16 Black, Long: 3/16 Brown, Long: : 1/16 • Genotypes: • Phenotypes:

  28. B = black fur • b = brown fur • S = short fur • s = long fur Try this one: BBSs vs. bbSs Genotypes: Phenotypes:

  29. BBSs vs. bbSs BS Bs BS Bs bS bs bS bs

  30. B = black fur • b = brown fur • S = short fur • s = long fur Try this one: BBSs vs. bbSs bS bs bS bs BbSS: 4/16 BbSs: 8/16 Bbss: 4/16 BS Bs BS Bs BbSSBbSsBbSSBbSs BbSsBbssBbSsBbss BbSSBbSsBbSS BbSs BbSsBbssBbSsBbss Black, Short: 12/16 Black, long: 4/16 Genotypes: Phenotypes:

  31. Homework: Complete a dihybrid punnett square using this cross: TtRR x TTrr. Tall (T) is dominant to short (t) and Red (R) is dominant to pink (r). Find the genotype and phenotype percentages.

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