Punnett Square Notes

1. Punnett Square Notes

2. What is Genetics? • Geneticsis 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 • Law of Segregation – Two alleles for each trait separate when gametes form; Parents pass only one allele for each trait to each offspring • 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 RR • 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

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

13. Punnett Square Parent Offspring Parent

14. How to Complete a Punnett Square

15. Y-Yellow y-white Genotype: 1:2:1 (YY:Yy:yy) Phenotype: 3 Yellow 1 White

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

19. TT x tt Step Three: Write the genotype and phenotype T T t t 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

23. Tt x tt Step Two: Complete the Punnett Square T t t t 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 • 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 • Blood Type – 3 alleles exist (IA, IB, and i), which results in four different possible blood types • 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.