1. 5.2 Studying Genetic Crosses

2. Agenda Take up homework Lesson 5.2: Studying Genetic Crosses Read pages 208-218 Vocabulary Learning Check page 212 # 7. 8, 10-12

3. Learning Goals Students will define the following terms: Punnett square, test cross, dihybrid cross, law of independent assortment, chromosome theory of inheritance. Students will analyze a monohybrid cross using a Punnett square. Students will analyze a dihybrid cross using a Punnett square. Students will describe the chromosome theory or inheritance.

4. Analyzing Genetic Crosses: Punnett Squares Reginald Punnett (1875 – 1967) devised a visual technique to help analyze the results of genetic crosses. Punnett Square- A grid used to illustrate all possible genotypes and phenotypes of offspring from genetic crosses The punnett square use Mendel’s law of segregation to illustrate all of the possible offspring that could be formed from the gametes of the parents. From the punnett square the following can be determined: the probability of inheriting a specific genotype and phenotype percentage outcome can be determined. Each square represents a 25% chance of occurrence for that trait

5. A.When working with one gene make a box and divide it into four squares B.Above the squares, write the genotypes of the gametes from one parent. Often the female parent’s gametes are put here. C. Beside the squares on the left write the genotypes of the gametes form the other parent. D.Inside each square, write the symbols for the allele above and the allele beside it. The upper-case letter is always placed first. The genotypes inside the squares are the genotypes of the offspring in the ratio that would be expected for the cross.

6. Using Punnett Squares A monohybrid cross is used to study only one trait at a time. Using a Punnett square, you can visually determine the possible outcomes between a cross of two parents. Possible F 1 outcomes from a cross between two heterozygous parents. The phenotype ratio is 3:1 for flower colour (purple to white) This is a monohybrid cross

8. Test Crosses A cross between an individual of unknown genotype and homozygous recessive for that trait. Analyzing the phenotype should provide insight into the unknown genotype. In a test cross, if any of the offspring show the recessive phenotype, the unknown genotype of the parent must be heterozygous.

9. Mendel’s Dihybrid Crosses He crossed yellow, round seeds with green, wrinkled seeds YYRR x yyrr All F 1 generation displayed both dominant traits- yellow and round When a F1 generation was allowed to interbreed or self pollinate, the F2 generation expressed the recessive forms. Mendel designed a second set of experiments that involved following two traits. Dihybrid cross: is a cross of two individuals that differ in two traits due to two different genes.

10. Parents: yellow, round seeds with green, wrinkled seeds All F 1 yellow and round F2 generation expressed the recessive traits again: green and wrinkled

11. The Punnett Square on the previous page modelled Mendel’s Results. When a parent, with four gametes is crossed with a parent with four gametes, the F2 generation, will have 16 outcomes and four phenotypes and 9 genotypes. Phenotypes: 9 displaying the dominant forms for both traits 3 displaying one dominant form and one recessive form for each trait 3 displaying the other dominant/recessive pair of traits 1 displaying both recessive forms of each trait

12. Dihybrid Cross 1. Cross each trait separately and use probability to determine the outcomes

13. 2. Use the Punnett Square to determine the outcomes. This is the easier option

14. The Law of Independent Assortment Mendel found a 9:3:3:1 ratio for every dihybrid cross he performed. This law states that the alleles for one gene segregate or assort independently of the alleles for other genes during gamete formation.

15. The Chromosome Theory of Inheritance By early 1900s scientists finally began to see the link between meiosis & inheritance Walter Sutton (1902) proposed that genes were located on chromosomes. Chromosome Theory of Inheritance: traits determined by genes are inherited through the movement of chromosomes during meiosis.

16. Success Criteria I am able to define the following terms: Punnett square, test cross, dihybrid cross, law of independent assortment, chromosome theory of inheritance. I am able to analyze a monohybrid cross using a Punnett square. I am able to analyze a dihybrid cross using a Punnett square. I am able to describe the chromosome theory or inheritance.