Monohybrid Crosses

1. Monohybrid Crosses Objective Use the punnett square method to determine the possible genotypes of different monohybrid crosses Determine the phenotypic ratio of a monohybred cross. Explain how the process of meiosis and fertilization account for the transmission of inherited characteristics.

2. Inheritance - Mendel • The transfer of characteristics (traits) from one generation to another • Your inherited characteristics (traits) are determined by the genes located on your chromosomes

4. Gene • a short segment of a chromosome coding for one trait

5. Homologous chromosomes: • chromosomes are found in pairs • in humans, there are 46 chromosomes, made up of 23 pairs of homologous chromosomes • a pair of chromosomes consists of two homologous chromosomes which look alike and carry genes for the same traits • you receive one homologous chromosome of the pair from each parent, therefore you receive 2 genes for each trait

6. Phenotype • This term is used to describe the physical or visible appearance of an individual as determined by the gene combination you inherit from your parents. • Example: in the case of eye colour, having brown eyes is the phenotype and is dominant

7. Genotype • the gene combination that produces the trait • these genes may be the same or different • genotypes are represented by upper and/or lower case letters e.g. Bb • there are three possibilities for genotypes for a single trait. • e.g. BB or Bb or bb

8. Allele • alternate forms of a gene • same location on a each chromosome of pair • affects the same trait but differently • alleles are different forms of a gene that carry different instructions • eg. Brown eyes or blue eyes

10. Dominant Trait/Allele • Characteristic that is always expressed its gene is present • only one dominant allele needs to be present for the dominant trait to be expressed • an upper case letter designates a dominant allele • eg. Tall is dominant. It is represented with a “T”

11. Recessive Trait/Allele • Characteristic that is only expressed when two genes for that trait are present • two recessive alleles need to be present for a recessive trait to be expressed • if a dominant allele is present it “masks” the recessive allele • eg: Short is recessive. It is represented with a • “t”

12. Homozygous Genotype • “homo” means the “same” • the condition where both alleles for a trait are the same • there are two conditions for homozygous genotype: • homozygous recessive genotype “aa” • homozygous dominant genotype “AA”

13. Heterozygous Genotype • “hetero” means “different” • the condition where both alleles for a trait are different • the genetic information inherited for a trait from both parents is different • example: heterozygous genotype “Aa”

14. Summary (example eye colour)

15. Mendel’s Experiment • Crossed two purebred parents (One tall -T and one short -t) • Pure breeding plants always produce identical offspring. • All offspring were tall!! Mendel concluded that some traits were ___________ and some traits were ___________.

16. Principal of dominance • When individuals with different traits are crossed, the offspring (F1 generation) will express only the dominant trait. Parents: Tall x Short F1 (Offspring): Tall Tall Tall Tall What must the genotype of the offspring be if the parents were pure breeding?

17. What happened next?? • Mendel crossed the F1 generation. • He crosses two hybrid pea plants. • Hybrids have contrasting traits. • F1: Tall plants x Tall plants • F2 (Offspring of F1): Tall Tall Tall Short • F2 generation = 3:1 ratio • What must the genotype of these offspring be if the parents were hybrids (Tt and Tt)?

18. Law of Segregation • Each F1 parent starts with two hereditary factors (alleles); one is dominant and one is recessive • Each parent contributes only one factor (allele) • Each offspring inherits one factor (allele) from each parent • If the dominant factor (allele) is present it will be expressed. • If the recessive factor (allele) is present it will only be expressed if only recessive factors are present.

19. Monohybrid Cross • Mendel crossed two purebred plants • TT – tall plant • Tt – short plant • Monohybrid cross – only one trait is being tested

20. PunnettSquares • Punnett Square • Is a grid system resembling a checkerboard, used in computing possible results of various genetic combinations • Simply stated, it is a way of representing the possible combinations of genes when an egg and sperm unite in fertilization

21. Predicting genotype and phenotype possibilities using Punnett Squares  Characteristic hair texture – • Dominant allele (gene) is curly hair - C. • Recessive allele (gene) is straight hair - c • Mother • Phenotype – straight hair • Genotype- homozygous recessive - cc • Father • Phenotype – curly hair • Genotype – homozygous dominant – CC

22. Punnett Square All of their children will have curly hair. They all have a different genotype from their parents.

23. Fig. 18.5

24. Fig. 18.6

25. Fig. 18.13

26. Fig. 18.14

27. Your Turn Bikini Bottom Genetics – Monohybrid Crosses HOMEWORK: Find out what the following words mean… Hybrid Pure-breeding/Purebred Why was it important that Mendel used purebred plants in his experiment? How does meiosis ensure that you inherit your mothers and fathers characteristics?

28. Objectives: Explain what co dominance and incomplete dominance is. Give an example of each. Use a Punnett square to solve basic incomplete and co dominance crosses Use a Punnett square to solve basic dihybrid crosses. Codominance, Incomplete dominance and Dihybrid Crosses

29. Terms • Incomplete Dominance: When two alleles are equally dominant, they interact to produce a new phenotype. • Codominance: When both alleles are dominant and are expressed at the same time. • Pg 145 # 1-4

30. Codominance • When a red bull is crossed with a white cow, the offspring will be roan.

31. Incomplete Dominance When a red flower is crossed with a red flower, the resulting offspring will be pink.

32. Dihybrid Crosses • A type of cross that involves two genes, each consisting of non identical alleles.

33. What happened when Mendel crossed two pea plants that different in TWO traits? In his second experiment Mendel crossed a pea plant with round/yellow (RRYY) seeds with a pea plant with wrinkled/greed seed (rryy). What genotypes resulted? What phenotypes resulted? Genotype: RrYy F1 - Generation Ratio 100% Phenotype: Round/Yellow Ratio 100%

34. Crossing F1(RrYy) x F1(RrYy) Genotype: RRYY RRYy Rryy RrYY RrYy rrYY rrYy rryy Ratio: 1 2 2 2 4 1 2 1 Phenotype: Round/Yellow Round/Green Wrinkled/Yellow Wrinkled/Green Ratio: 9 3 3 1 In the resulting F2 – Generation, the phenotypic ratio will always be 9:3:3:1 In other words, the likelihood of obtaining each of the above phenotypes from crossing two hybrid pea plants is 9:3:3:1. What genotypes resulted? What phenotypes resulted?

35. Law of Independent Assortment • The inheritance of alleles for one trait ________ (does/does not) affect the inheritance of alleles for another trait.

36. Plenary • Bikini Bottom • Questions