Heredity (part 1): Monohybrid cross & test cross

1. Heredity Chapter 19

2. Lesson Objectives At the end of the lesson, you should be able to (a) Define the gene as a unit of inheritance and distinguish clearly between the terms genes and allele (b) Explain the terms dominant, recessive, co- dominant, homozygous, heterozygous, phenotype and genotype (c) Predict the results of simple crosses with expected ratios if 3:1 and 1:1, using the terms homozygous, heterozygous, F1 generation and F2 generation (d) Explain why observed ratios often differ from expected ratios, especially when there are small numbers of progeny (e) Use genetic diagrams to solve problems involving monohybrid inheritance

3. Whose child is it?

4. Heredity • Passing of characteristics/traits e.g. eye colour, height and intelligence to us from our parents • As traits can be inherited, we could selectively breed varieties of animals and plants with desirable qualities • e.g. cows that produced more milk OR rice plants that produced more rice

5. http://alltopix.com/gallery/index.php?n=13

6. Relationship between DNA, Chromosomes and Genes

9. Gregor Mendel

10. Background information • Gregor Mendel was a monk who lived during the mid-late 1800’s, and he is considered the “father” of genetics. • Mendel was interested in discovering how traits are inherited in the offspring of sexually- reproducing organisms. • Mendel devised what is now called a monohybrid cross to answer the first question of how a single trait is passed from parent to offspring.

11. Monohybrid Cross • Inheritance involving only ONE pair of contrasting characters 1) tallness or shortness of the plant 2) plants that had either red or white flowers 3) plants that produced seeds that were either yellow or green OR round or wrinkled

12. Mendel’s Insight into Patterns of Inheritance • Mendel did his experiments with the garden pea plant - Pollen (male gamete) released from anthers at ends of stamens - Egg (female gamete) is in ovule at base of carpel

15. Terms to familiarise Phenotype: the expressed trait, the outward appearance or visible character of an organism e.g. tallness and dwarfness of a plant • Genotype: the genetic make-up of an organism e.g. dwarf pea plants have genotype tt while a tall plant may have genotype either TT or Tt • Dominant: An allele that is almost always expressed, even if only one copy is present e.g. TT or Tt • Recessive: A gene that is phenotypically manifest in the homozygous state e.g. tt but is masked in the heterozygote by the presence of a dominant allele • Homozygous: having the same two alleles at a given locus on homologous chromosomes (e.g. TT, tt) • Heterozygous: Having two different alleles at a given locus on a pair of homologous chromosomes e.g. Tt •

16. Examples: Let T represent the allele for tall plants. (dominant) And t represent the allele for dwarf plants. (recessive) Phenotype Genotype Tall plant TT or Tt Pure-bred tall plant TT Homozygous tall plant TT Heterozygous tall plant Tt Dwarf plant tt

17. Can we determine the offspring of a genetic cross? • Doing genetic crosses using the Punnett Square female gametes a A A a A a A a male gametes A Aa AA Aa A A A Aa aa a a aa Aa aa Aa aa a a 1. 2. 3. 4.

19. Mendel’s Monohybrid Cross

20. Mendel’s Monohybrid Cross

21. ***Explaining Mendel’s Monohybrid Cross Let T represent the alleles of the pea plant with purple flower. Let t represent the alleles of the pea plant with white flower. Parental phenotype: Parental genotype: Gametes: purple x TT x T T white tt t t Random fertilisation: (using the Punnett Square) t t T T Tt Tt Tt Tt F1 genotype: F1 phenotype: All Tt All purple

22. ***Explaining Mendel’s Monohybrid Cross F1phenotype: F1genotype: Gametes: purple x Tt x T purple Tt t t T Random fertilisation: (using the Punnett Square) T t TT Tt tt T t Tt TT, Tt, Tt, tt purple, purple, purple, white 3:1 (purple:white) F2 genotype: F2 phenotype: Phenotypic ratio:

23. Traits/ character Phenotypic ratio of 3:1 Condition: Only when the sampling size is large then will we obtain a phenotypic ratio of 3:1

24. Summary of Mendel’s monohybrid crosses

25. Question: • A homozygous red flowered hibiscus is crossed with a homozygous white flowered hibiscus. What is the phenotypic ratio that will result from the cross between the offspring resulting from the F1 generation, given that red is dominant.

26. Let T represent the alleles of the hibiscus with red flower. Let t represent the alleles of the hibiscus with white flower. Parental phenotype: Parental genotype: Gametes: red x TT x T white tt t T t Random fertilisation: (using the Punnett Square) t t T T Tt Tt Tt Tt F1 genotype: F1 phenotype: All Tt All red

27. F1phenotype: F1genotype: Gametes: red x Tt x T red Tt t t T Random fertilisation: (using the Punnett Square) T t TT Tt tt T t Tt TT, Tt, Tt, tt red, red, red, white 3:1 (red:white) F2 genotype: F2 phenotype: Phenotypic ratio:

28. ***Testcross to figure out an unknown genotype