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Understanding Genotypes and Phenotypes through Probability

Learn how genes and alleles relate to genotype and phenotype through the use of Punnett squares and probability calculations. Explore exceptions to Mendel's principles and the influence of environment on traits.

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Understanding Genotypes and Phenotypes through Probability

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  1. DO NOW: 4/25 If you flip a coin what are the chances it will land on heads? Tails? Suppose that you flipped the coin and got heads. What are the chances that you will get heads again? • Objectives: • Explain how genes and alleles are related to genotype and phenotype. • Use the information in a Punnett square.

  2. A Great Idea • Mendel knew that there must be two sets of instructions for each characteristic. • The instructions for an inherited trait are called genes. • The different forms (often dominant and recessive) of a gene are alleles.

  3. A Great Idea, Continued • Phenotype : An organism’s appearance. • Genes affect the phenotype. • Genotype: The combination of inherited alleles together. • A genotype that has 2 dominant OR recessive alleles is homozygous or true breeding (PP or pp). • A genotype that has one recessive and one dominant is heterozygous (Pp).

  4. A Great Idea, Continued • Genotype vs. Phenotype • An example of a genotype that a butterfly can inherit is AA. • The phenotype this offspring will have is dark wings.

  5. Punnett Squares are used to organize all the possible genotype combinations of offspring from particular parents. What are the genotypes? Phenotypes?

  6. What are the Chances? • A gene is made up of 2 alleles. • Probability is the mathematical chance that something will happen and is often written as a fraction or percentage. • Probability is used in genetics to figure out the percentage of offspring that will have a particular phenotype.

  7. DO NOW: 4/26 In pea plants, the genes for tall plants (T) are dominant over the genes for short plants (t). Cross a homozygous dominant tall pea plant (TT) with a short (homozygous recessive) pea plant (tt). What percentage of short pea plants will appear in the offspring of this cross? • Objectives: (Do Not Have to Re Copy) • Explain how probability can be used to predict possible genotypes in offspring. • Describe 3 exceptions to Mendel’s observations.

  8. Calculating Probability

  9. What are the Chances? Continued • Genotype Probability • To have white flowers, a pea plant must receive a lower case p allele from each parent. • Each offspring of a Pp X Pp cross has a 50% chance of receiving either a P allele or p allelle from the parent. • So, the probability of inheriting two p alleles is 1/2 X 1/2, which equals 1/4, or 25%.

  10. Mendel’s Principles • Mendel came to 3 important conclusions from his experimental results: • The inheritance of each trait is determined by “units” or “factors” that are passed on to descendants (these units were later called genes) • An individual inherits one such gene from each parent for each trait. • A trait may not show up in an individual but can still be passed on to the next generation.

  11. More About Traits (Exceptions to Mendel’s principles) • Incomplete Dominance • Researchers have found that sometimes one trait is not completely dominant over another. • When a red and white snapdragon flower breed it makes offspring that are all pink. • Example in humans: Wavy hair

  12. More About Traits (Exceptions to Mendel’s principles) • One Gene, Many Traits • Sometimes one gene influences multiple traits. • Example: A gene for fur on a tiger to be white may also code for the tiger’s eyes to be blue. • Many Genes, One Trait • Some traits, such as the color of your skin, hair, and eyes, are the result of several genes acting together. This is called multiple alleles. • Examples: Eye color

  13. More About Traits, Continued • The Importance of Environment • Genes aren’t the only influences on traits. • A combination of things determine an individual’s characteristics. • Example: Diet affects growth and sun affects skin tone • Lifestyle choices can also affect a person’s traits.

  14. What is the genotype of the offspring represented in the upper left-hand box of the Punnett square? • A RR • B Rr • C rr • D rrr

  15. 2. What is the genotype of the offspring represented in the lower right-hand box of the Punnett square? A RR B Rr C rr D rrr

  16. 3. What is the ratio of Rr (purple-flowered plants) to rr (white-flowered plants) in the offspring? A 1:3 B 2:2 C 3:1 D 4:0

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