What is Genetics?

1. What is Genetics? Defined broadly, it’s the branch of biology that deals with heredity and variation.

2. What is Genetics? A key question in genetics is what is the relationship between genotype and phenotype? The question is simple, the answer is complex.

3. Unit 3 Genetics • Every living thing is made up of different traits that make it unique. • i.e. Labrador retrievers. This type of dog and its traits occur because of selective breeding. • Traits can be passed on from generation to generation. This transmission of traits is called heredity and the traits that are passed on are said to be inherited • Genetics is the branch of biology that deals with the principle of variation and inheritance.

4. Gregor Mendel • Born 1822 • Austrian monk • Attended the university of Vienna • Studied botany and mathematics • Conducted experiments on pea plants over a 8 year period from 1853-1861

5. Mendel’s questions were not new – but his approach to understanding heredity was. • Mendel’s experimental innovations included: • choice of model organism • study of discrete traits • use of pure-breeding lines • painstaking control of matings • rigorous mathematical analysis of large numbers of offspring

6. Mendelian Inheritance

7. The Garden Pea Pisum sativum

8. Why did he choose pea plants? • Commonly available in Europe • Were easy to grow and matured quickly • Sexual organs of the plant were entirely enclosed in the flower which allowed him to control reproduction whether through cross pollination or self pollination • Different varieties had different traits that could be observed from generation to generation. Each trait had two variations

9. Mendel's Experiments • How did he conduct his experiments? • His first step was to obtain purebred plants for the trait that he wanted to study • A purebred organism is descended from ancestors of a distinct type or breed. This trait results from previous generations through long term selective breeding • He chose plants that were tall and bred these together. From these he obtained some plants that were tall and some that were short. • He then proceed to breed only the tall plants together again, he did this until he got only tall offspring • He did the same for the short plants as well as 6 other traits

11. Mendel's First experimentMonohybrid Cross • Once he obtained purebred or true bred plants he named them the parental generation or P generation. • He crossed pure tall with pure short the offspring from this cross he designated as the first filial generation or the F1 • The offspring from this cross were hybrids indicating that they were the result of a cross between two different purebreds • This is called a monohybrid cross because it involves only one trait; plant height

12. All offspring from this cross were tall ( there were no short plants) • What happened according to the theory of blended inheritance he should get all medium plants, instead all plants were tall. This lead mendel to conclude that the trait for tall must be dominant and the trait for short must be recessive. • A dominant trait is one that is always expressed . • A recessive trait is a characteristic that is present but inactive. It is only expressed if it is the only trait present • He conducted the experiment many times with the same results and therefore concluded that when plants with two contrasting plants are crossed one is always dominant over the other. This lead to his principle of dominance

13. Mendel Began with Monohybrid Crosses Between Parents Who Bred True for Antagonistic Pairs of Traits

15. White Cross Pollination – One of Mendel’s Experimental Methods Remove stamens from purple flower. Stamens Transfer pollen from stamens of white flower to the stigma of a purple flower. Parental generation Purple Cross-pollinated flower forms seeds. Plant seeds from pod. First- generation plants

16. One of Mendel’s Monohybrid Crosses Regardless of the trait, the pattern remained the same.

17. Mendel Observed Consistent Patterns of Inheritance

18. Second experiment • He bred the F1 generation through self pollination • These produced the second filial generation or the F2 generation • When crossed this way the F2 offspring 75% of the time resembled the dominant parent or P generation and 25% of the time resembled the recessive parent. This ratio of 3:1 is known as the mendelain ratio

19. Why did Mendel get these results from the F2? • He concluded that each parent in the F1 generation starts with 2 hereditary factors. One is dominant and the other is recessive • The factors separate in the parent, only one factor from each parent is contributed to the offspring. • Each offspring inherits one factor from each parent. If the dominant factor is present it will be expressed even if the recessive factor is also present. • The recessive factor will be expressed if only recessive factors are present.

20. Law of segregation • The results from Mendel F2 generation gave rise to the law of segregation • A pair of factors for a trait separate ( segregate ) during the formation of gametes(sperm and egg) and then recombine during fertilization • We know today that these factors that Mendel referred to are genes, the part of the chromosome that governs the expression of a particular trait. • Gene has two forms called alleles. • these factors are sometimes referred to as unit characters.

21. When using letters to designate traits or unit characters a purebred tall plant will have 2 uppercase letter i.e., TT this means the plant is homozygous for tall. • The pure breed short plant has 2 alleles for short designated in lowercase letters as tt. • The product of the cross between a tall and short plant would have the alleles T and t. It would have a dominant and a recessive trait is said to be heterozygous for tall.

22. Principle of Segregation … and these gametes unite at random in fertilization to create a new individual.

23. Principle of Segregation Principle of Segregation: Genes come in pairs that segregate (separate) in the formation of gametes…

24. The Principle of Segregation Coupled with the Postulate of Dominant and Recessive Alleles Explains the Outcome of Monohybrid Crosses

25. Test Cross • Test Cross” a cross in which an individual of unknown genotype is crossed with a homozygous recessive individual. • This is done to determine the genotype of the unknown individual • Two crosses are performed • Cross a homozygous dominant individual with the homozygous recessive individual • Cross a heterozygous dominant individual with a homozygous recessive individual. • Examine the results to tell the genotype of the unknown individual

26. Test cross A - assume the unknown genotype is homozygous dominant • We will use letters TT for homozygous dominant and tt for homozygous recessive • Test cross B- assume unknown genotype is heterozygous dominant • We will use letters Tt for heterozygous dominant and tt for homozygous recessive

27. A Test Cross Reveals the Genotype of a Dominant Individual Test crosses are the most important form of progeny testing. Mendel used test crosses to buttress his proposal of segregation.

28. Punnett Squares Probability Product Rule Test Cross Dihybrid and Trihybrid Crosses Incomplete Dominance—Neither gene is completely dominant over the other. Snapdragons. Codominance—Both alleles are expressed simultaneously. Human ABO blood type More on Mendel

32. Single Factor/ Monohybrid Cross • Single factor cross: A cross that is done for one trait. • Example: Mary is homozygous dominant for straight hair(SS). Steve is heterozygous for straight hair (Ss) what are the possible ratios of offspring for these two individuals • Solution: • Ratios: • Genotypic: 1:1 -- 50% homozygous straight-50% heterozygous straight • Phenotypic :100% straight hair

33. Monohybrid cross is where the parents are heterozygous or hybrid for the trait in question • Ex. Crossing two plants that are heterozygous for tallness (Tt)

34. Results of monohybrid cross • Phenotypic ratio: 3:1 • Tall : short • Genotypic Ratio: 1:2:1 , 1 homozygous dominant, 2 heterozygous dominant: 1 homozygous recessive. • NOTE: In a monohybrid cross the ratios are always as above

35. Product Rule • A rule that uses the principles of probability to determine the possible outcomes of a genetic cross. • Product Rule: The probability or chance that two or more independent events will occur together is the product of their individual probabilities of occurring alone • Example: tossing coins, toss a single coin: chances of getting heads = ½ chances of getting tails= ½ • toss two coins together • Chance of getting two heads= ½ x ½ = ¼ • Chance of getting two tails= ½ x ½ = ¼ • Chance of getting heads and tails; ½ x ½ + ½ x ½ = ½ • The same

36. Monohybrid Cross Example #1: • In a particular type of fish, a "G" allele produces green scales, which is dominant to the "g" allele which produces blue scales. What genotype and phenotype ratios do we get if we cross two Gg fish? • To solve this problem, we must first determine the possible gametes each fish can produce. Recall that during meiosis the chromosome number is reduced by half. Each gamete will have only HALF the number of chromosomes as the individual. • Two green fish, each with a genotype of Gg, can produce gametes that are either G or g.

37. Then we set up a Punnett square like this sperm eggs Then we combine the gametes to fill in the rest of the chart: Results: genotype ratio: 1 GG : 2 Gg : 1 gg phenotype ratio: 3 green fish : 1 blue fish Be sure to write the genotype and phenotype ratios like this! Genotype ratios must have numbers, letters (for the alleles), and colons between them to show it is a ratio. (It is also OK to use percentages for the numbers: 25% GG : 50% Gg : 25% gg) Phenotype ratios must have numbers, words (that describe the trait), and colons between them to show it is a ratio. (It is also OK to use percentages for the numbers: 75% green fish : 25% blue fish)

38. Let's try another example • Let's cross a male heterozygous green fish with a female homozygous recessive blue fish. What genotype and phenotype ratios do we get if we cross these two fish? • To solve this problem, we must first determine the possible gametes each fish can produce. A male heterozygous green fish has a genotype of Gg, and can produce gametes that are either G or g. A female homozygous recessive blue fish has a genotype of gg, and can only produce eggs that are g. Then we set up a Punnett square:

39. Mendel concluded that: 1. Each trait is controlled by 2 genes or alleles. 2. Some alleles are dominant, while other alleles are recessive. Law of Segregation—During meiosis, each allele separates in an orderly manner from its partner, the two always go to different gametes. 4. Law of Independent Assortment—During Metaphase and Anaphase I of meiosis, the alleles for different traits segregate independently. This mixes chromosomes from your father and mother Mendel’s Results and Conclusions

40. www.siskiyous.edu/class/bio1/genetics/monohybrid_v2.html

41. Mendels Crosses • www.sumanasinc.com/webcontent/animations/content/mendel/mendel.html

42. Practice • Recessive male (wrinkled) and Heterozygous Female (round) • P. 533 #’s 1,2, & 3 • P.531 #’s 1 &2 • Assignment 1

43. Does the Segregation of Alleles of One Gene Occur Independently of the Segregation of Alleles of a Second Gene? A dihybrid cross This was Mendel’s next question, and he addressed it using dihybrid crosses.

44. Dihybrid Cross • Mendel wanted to know if the inheritance of one trait influenced the inheritance of another trait • He approached this problem in the same manner as he approached the others • First he produced plants that were purebred for the traits he wanted to examine. He selectively bred plants until the offspring were round and yellow. These plants were homozygous dominant for both seed shape and colour • He breed them with plants that were homozygous recessive for seed colour and shape.(wrinkled and green) • He performed a dihybrid cross. All offspring were heterozygous for both traits. RrYy making them round and yellow.

45. He allowed the F1`s of his dihybrids to self pollinate. Of the 551 plants in the F2 • 320 round yellow • 104 round green • 101 wrinkled yellow • 26 wrinkled green • these results show a phenotypic ratio of 9:3:3:1.

46. Alternative Hypotheses About the Inheritance of Two Traits

47. The result A Dihybrid Cross Between Smooth, Yellow and Wrinkled, Green Strains

48. The Dihybrid Cross and Its Explanation Regardless of the particular traits, the pattern remained the same. These results led Mendel to postulate the Principle of Independent Assortment.

50. Example • In pea plants tall (T) is dominant over short plants (t). Green pods (G) is dominant over yellow pods(g). Mendel crossed a plant heterozygous for both tall and green with a plant that was heterozygous for both tall and green. • What results did he obtain?