punnett-square-notes.ppt
- 2. What is Genetics? Genetics is the scientific study of heredity
- 3. What is a Trait? A trait is a specific characteristic that varies from one individual to another. Examples: Brown hair, blue eyes, tall, curly
- 4. What is an Allele? Alleles are the different possibilities for a given trait. Every trait has at least two alleles (one from the mother and one from the father) Example: Eye color – Brown, blue, green, hazel Examples of Alleles: A = Brown Eyes a = Blue Eyes B = Green Eyes b = Hazel Eyes
- 5. What are Genes? Genes are the sequence of DNA that codes for a protein and thus determines a trait.
- 6. Gregor Mendel Father of Genetics 1st important studies of heredity Identified specific traits in the garden pea and studied them from one generation to another
- 7. Mendel’s Conclusions 1.Law of Segregation – Two alleles for each trait separate when gametes form; Parents pass only one allele for each trait to each offspring 2.Law of Independent Assortment – Genes for different traits are inherited independently of each other
- 8. Dominant vs. Recessive Dominant - Masks the other trait; the trait that shows if present Represented by a capital letter Recessive – An organism with a recessive allele for a particular trait will only exhibit that trait when the dominant allele is not present; Will only show if both alleles are present Represented by a lower case letter R r
- 9. Dominant & Recessive Practice TT - Represent offspring with straight hair Tt - Represent offspring with straight hair tt - Represents offspring with curly hair T – straight hair t - curly hair
- 10. Genotype vs. Phenotype Genotype – The genetic makeup of an organism; The gene (or allele) combination an organism has. Example: Tt, ss, GG, Ww Phenotype – The physical characteristics of an organism; The way an organism looks Example: Curly hair, straight hair, blue eyes, tall, green
- 11. Homozygous vs. Heterozygous Homozygous – Term used to refer to an organism that has two identical alleles for a particular trait (TT or tt) Heterozygous - Term used to refer to an organism that has two different alleles for the same trait (Tt) RR Rr rr
- 12. Punnett Squares Punnett Square – Diagram showing the gene combinations that might result from a genetic cross Used to calculate the probability of inheriting a particular trait Probability – The chance that a given event will occur
- 14. How to Complete a Punnett Square
- 16. You Try It Now! Give the genotype and phenotype for the following cross: TT x tt (T = Tall and t = Short)
- 17. TT x tt Step One: Set Up Punnett Square (put one parent on the top and the other along the side) T T t t
- 18. TT x tt Step Two: Complete the Punnett Square T T t t Tt Tt Tt Tt
- 19. TT x tt Step Three: Write the genotype and phenotype T T t t Tt Tt Tt Tt Genotype: 4 - Tt Phenotype: 100% Tall Remember: Each box is 25%
- 20. You Try It Now! Give the genotype and phenotype for the following cross: Tt x tt
- 21. Tt x tt Step One: Set Up Punnett Square (put one parent on the top and the other along the side) T t t t
- 22. Tt x tt Step Two: Complete the Punnett Square T t t t Tt tt Tt tt
- 23. Tt x tt Step Two: Complete the Punnett Square T t t t Tt tt Tt tt Genotype: Tt - 2 (50%) tt - 2 (50%) Phenotype: 50% Tall 50% Short Remember: Each box is 25%
- 24. Some Terminology P1 – Original parents F1 – First generation F2 – Second generation P1 X P1 = F1 F1 X F1 = F2
- 25. Incomplete Dominance Incomplete Dominance - Situation in which one allele is not completely dominant over another. Example – Red and white flowers are crossed and pink flowers are produced.
- 26. Codominance Codominance - Situation in which both alleles of a gene contribute to the phenotype of the organism. Example – A solid white cow is crossed with a solid brown cow and the resulting offspring are spotted brown and white (called roan). +
- 27. Multiple Alleles Multiple Alleles- Three or more alleles of the same gene. Even though three or more alleles exist for a particular trait, an individual can only have two alleles - one from the mother and one from the father.
- 28. Examples of Multiple Alleles 1. Coat color in rabbits is determined by a single gene that has at least four different alleles. Different combinations of alleles result in the four colors you see here.
- 29. Examples of Multiple Alleles 2. Blood Type – 3 alleles exist (IA, IB, and i), which results in four different possible blood types 3. Hair Color – Too many alleles exist to count There are over 20 different shades of hair color.
- 30. Multiple Alleles There Are Always Multiple Alleles! Genetic inheritance is often presented with straightforward examples involving only two alleles with clear-cut dominance. This makes inheritance patterns easy to see. But very few traits actually only have two alleles with clear-cut dominance. As we learn more about genetics, we have found that there are often hundreds of alleles for any particular gene. We probably know this already - as we look around at other people, we see infinite variation.
- 31. Polygenic Trait Polygenic Trait - Trait controlled by two or more genes. Polygenic traits often show a wide range of phenotypes. Example: The wide range of skin color in humans comes about partly because more than four different genes probably control this trait.