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Mendelian Genetics

Mendelian Genetics. Key Terms in Mendelian Genetics. Phenotype: observable traits Examples: brown eyes, yellow flower Genotype: actual alleles; describes the genetic characteristics The combination of alleles inherited Examples: BB, dd , Ff. Phenotype: brown eyes

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Mendelian Genetics

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  1. Mendelian Genetics

  2. Key Terms in Mendelian Genetics • Phenotype: observable traits • Examples: brown eyes, yellow flower • Genotype: actual alleles; describes the genetic characteristics • The combination of alleles inherited • Examples: BB, dd, Ff Phenotype: brown eyes This is the trait that is expressed! Genotype: could be BB or Bb This is the 2 alleles that were inherited from mother & father!

  3. Key Terms in Mendelian Genetics • Homozygous: having 2 identical alleles for a trait • TT or tt • “homo” means “same” • Heterozygous: having 2 different alleles for a trait • Tt • “hetero” means “different” Genotype: could be BB or Bb This person could be homozygous dominant (BB) or heterozygous (Bb) for the eye color trait!

  4. What color hair will Kate Middleton’s & Prince William’s baby have? • We can use our knowledge of genetics & meiosis to make a prediction!

  5. Meiosis • Remember! • During meiosis, our bodies make egg & sperm cells. • Egg & sperm cells contain ½ the amount of DNA as other cells. Why?

  6. What color hair will Kate Middleton’s & Prince William’s baby have? • We now know that Prince William can only pass on the “b” allele. • Kate can pass on a “B” or a “b” to the baby. • To make our prediction, we have to draw a Punnett Square!

  7. Punnett Squares • Diagrams we can draw to help us predict the genotypes & phenotypes of offspring

  8. Start with a blank box with 4 empty squares. Add the genotype of each parent to the top & left side of the box (outside). Father is on top & mother is on the left side. b b B b

  9. Fill in the squares! Remember that each parent only passes on 1 allele for the hair color gene! b b B Bb Bb bb bb b

  10. Remember, alleles are a version of a gene.

  11. What do the results of the Punnett square mean? Kate and William will have 4 children Two children will look like Kate, two will look like William There is chance that a baby will have brown hair, and a chance that it will have blonde hair

  12. B=brown, b=blonde What color hair will the offspring have if they inherit these alleles? Bb=?

  13. What are the chances that the baby will have blonde hair? 25% 75% 50% No chance 50%

  14. 3:1Phenotype = tall or shortThere’s a 75% chance the offspring will be tall & a 25% chance the offspring will be short! What’s the phenotypic ratio in the offspring? • 1:2:1 • 1:1:1:1 • 3:1 • What?!

  15. Mendelian inheritance reflects the rules of probability! • Mendel’s laws of segregation & independent assortment reflect the same laws of probability that apply to tossing coins or rolling dice. • The probability of tossing heads with a normal coin is 1/2.

  16. When tossing a coin, the outcome of 1 toss has no impact on the outcome of the next toss! • Each toss is an independent event, just like the distribution of alleles into gametes. Like a coin toss, each egg cell from a heterozygous parent has a ½ chance of carrying the dominant allele & ½ chance of carrying the recessive allele! (The same odds apply to sperm.)

  17. Law of Segregation This parent is carrying 2 alleles for a gene. Genotype = Ss During meiosis, the alleles are separated into 4 gametes. Each gamete has a 50% chance of carrying S & a 50% chance of carrying s.

  18. By the law of segregation, the 2 alleles for a gene are packaged into separate gametes. • Here, the F1 hybrids all have purple flowers.

  19. When Mendel crossed the F1 plants together, the F2 generation included both purple & white flowered plants! • The white flowered allele was absent in the F1 plants, but reappeared in the F2 plants! Based on a large sample size, Mendel recorded 705 purple plants & 224 white plants in the F2 generation. What’s the ratio of purple to white plants?

  20. Mendelian Ratios ¼ of the F2 offspring will inherit 2 white flower alleles & produce white flowers. (p + p = pp) ½ of the F2 offspring will inherit 1 white flower allele & 1 purple flower allele & produce purple flowers. (p + P = Pp) ¼ of the F2 offspring will inherit 2 purple flower alleles & produce purple flowers. (P + P = PP) This represents a 3:1 ratio in the F2 generation!

  21. What’s the phenotypic ratio for these offspring? • 1:2:1 • 3:1 • 2:1 • 1:1 1:1 Phenotype = yellow or green pea pod The chances of the pea pods being yellow are equal to them becoming green!

  22. The Genotypic Ratios 50% Yy 0% YY 50% yy

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