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Genetic Crosses and Probability Principles

Understand genotype and phenotype, probability principles, Punnett squares, genotypic and phenotypic ratios, genetic crosses, and exceptions to dominance rules in Mendelian genetics.

jwolcott
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Genetic Crosses and Probability Principles

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  1. Genetic Crosses and Probability Chapter 9 Sec. 9.2

  2. Genotype and Phenotype • Genotype- the genetic makeup of an organism (Represented by alleles, capital letters for dominant and 2 lowercase letters for recessive) Ex. PP, Pp, or pp • Phenotype- the observable, physical appearance of an organism (results from the genotype) • Ex. The phenotype of a PP or Pp is Purple • The phenotype of a pp iswhite

  3. Genetics Vocabulary continued… • Homozygous- when both alleles of a pair are the same for a gene • Homozygouse Dominant- PP • Homozygous Recessive- pp • Heterozygous- when the 2 alleles in the pair are different for a gene • Heterozygous for flower color: Pp

  4. Probability is the likelihood that a particular event will occur. It does not ALWAYS happen (think Deal or No Deal) Can be written as a decimal, percentage, ratio, or fraction How do we use the principles of probability in our daily lives? Ex. Horse racing NCAA March Madness Superbowl Coin Toss Atlantic City $$$$$ What is Probability?

  5. Determining probability • Number of times the event is expected Number of times it could have happened • What is the probability that you picked an odd number when picking from 1-10? There are five odd numbers between 1 and 10. The probability is 5:5. • Or you can express it as a fraction: 5/10. Since it's a fraction, why not reduce it? The probability that you will pick an odd number is 1/2. • Probability can also be expressed as a percent...1/2=50% Or as a decimal...1/2=50%=.5

  6. The Principles of Probability • The Principles of probability can be used to predict the outcomes of genetic crosses • Alleles segregate by complete randomness • Similar to a coin flip!

  7. What is a Punnett Square? • A punnett square is a diagram showing the gene combinations that might result from a genetic cross • The letters in punnett squares represent alleles. • A capital letter is used to show the dominant alleles. • A lower case letter is used to show the recessive alleles. • Monohybrid cross- a cross between individuals that involve one pair of traits

  8. How do you fill in a Punnett Square? • Use the genotype to determine the gametes formed. Ex. a Gg parent will have 2 gametes because of segregation: G or g • Parent 1 gametes on the top, parent 2 gametes on the side • If both parents are heterozygous for green color…

  9. How do you fill in a Punnett Square?

  10. How do you fill in a Punnett Square?

  11. How do you fill in a Punnett Square?

  12. Homozygous Dominant Homozygous recessive Heterozygous Dominant  GG gg  Gg What do the alleles mean?

  13. To find the ratio of offspring in a punnett square… • Count the number of different offspring • For a genotypic ratio count the different genotypes • For a phenotypic ratio count the different phenotypes

  14. Genotypic ratio • 1 GG • 2 Gg • 1 gg • So the genotypic ratio is 1:2: 1

  15. Phenotypic ratio • 3 green plants • 1 yellow plant • So the ratio is 3:1

  16. Ex 1: Homozygous x HomozygousLet’s cross a pure bred purple colored flower and a white flower. Purple is a dominant color and white is a recessive color. • Cross: PP x pp P P (purple x white) p p

  17. Ex 2: Homozygous x HeterozygousA guinea pig who is homozygous dominant for black fur crosses with a guinea pig who is heterozygous for this trait (black fur) • Cross: BB x Bb B B B b

  18. Ex 3: Heterozygous x HeterozygousIn rabbits, the allele for black coat color is dominant (B) over the allele for brown coat color (b). Cross 2 rabbits that are heterozygous for coat color B b Cross: Bb x Bb B b

  19. Design your own peas…. An interactive Pea Breading experiment http://www.sonic.net/~nbs/projects/anthro201/exper/ Choose two plants to breed and see the resulting possibilities for the next generation

  20. Independent Assortment • Does the segregation of one pair of alleles affect the segregation of another pair of alleles? • Does the gene that determines round or wrinkled peas have anything to do with pea seed color? NO! • Genes for different traits can separate independently during the formation of gametes • So one pair of alleles do not influence each other’s inheritance.

  21. Complete Dominance • In Mendelian traits if you are homozygous dominant OR heterozygous you will show the dominant trait • This is called having complete dominance • Example, Rolling your tongue: • RR and Rr can roll their tongue • rr cannot roll their tongue

  22. Exceptions to the dominance rules • Incomplete dominance • Codominance

  23. Incomplete Dominance-The phenotypes MIX when the genotype is heterozygous. Two or more alleles contribute to phenotype, resulting in a phenotype in the middle between the dominant and recessive trait. • Neither allele is completely dominant over the. • Ex. Four o’clock flowers RR=RED (when all red flowers crossRED) rr= WHITE (when all white flowers cross WHITE) or rr When a RED crosses with a white we get  PINK or Rr Which allele is dominant? NONE! • Incomplete dominance is like mixing paint, you get a blend of the 2 colors R R r r

  24. Codominance- when both phenotypes/alleles for a gene are shown in heterozygous offspring • Both alleles of the gene contribute to the phenotype of the organism. • Neither allele is dominant or recessive • Codominance is like mixing glitter together, you can still see the individual glitter colors • Example: Horses • The genes for white and red coat color is called ROAN • Homozygous Dominant RR=red coat • Homozygous Recessive R’R’=white coat • Heterozygous RR’= roan (both red and white hairs can be seen)

  25. Dihybrid Crosses • These are done when we want to determine the inheritance of 2 traits at the same time. For example, pea color and pea texture… • If the parents genotypes are heterozygous for both traits… (use Y for color and R for texture) • Parents genotype is: YyRr • Note: Remember to use 1 letter for EACH trait

  26. The heterozygous parents will give off gametes • YyRr parents will give off 4 different gametes because of independent assortment and segregation • YR Yr yR yr

  27. Dihybrid Crosses RY Ry rY ry • Cross: RrYy X RrYy • Dihybrid Cross – a cross involving 2 different traits • Pea Shape: • Round (R) Dominant • Wrinkled (r) recessive • Pea Color: • Yellow (Y) Dominant • Green (y) Recessive RY Ry rY ry

  28. So when we cross 2 heterozygous parents… …you get the same ratio all the time… 9:3:3:1

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