1. Mendelian Patterns of Inheritance Chapter 9

2. Introduction Gazelle always produce baby gazelles, not bluebirds

3. Poppy seeds always produce poppies, not dandelions

4. Introduction Everyone who observes this phenomenon reasons that the parents must pass this hereditary information to their offspring

5. Introduction It also occurs that offspring can appear markedly different than either parent, however The laws of heredity must be able to explain not only the stability, but the variation that is observed between generations of offspring

7. Gregor Mendel: “Mendelian Genetics” Gregor Mendel, an Austrian monk in the 1860s, formulated two fundamental laws of heredity He is known as the Father of Genetics

8. Gregor Mendel Mendel experimented with the garden pea, Pisum sativum, in the gardens of the monestary, to test and formulate his hypotheses about inheritance

9. Mendel and Inheritance: “blending concept” Before Mendel, it was thought that both sexes contribute equally to an individual, and that parents of contrasting appearance should always produce offspring of intermediate appearance--the “blending concept” of inheritance

10. Blending concept: not always true! If this were the case, then crossing red and white flowers should always produce pink flowers We know that this is not always the case This discrepancy—when white and red flowers would show up in further generations—was explained by some instability in the breeding system

11. Mendel’s Experimental Procedure Mendel chose to work with the garden pea They were easy to cultivate and had a short generation time They could easily be pollinated by hand Many varieties were available

12. Mendel’s experimental procedure Mendel chose 22 varieties for his experiments When these varieties self-pollinated, they were referred to as “true breeding”— meaning that no offspring were like the parents and like each other

13. Some characteristics of the pea plants

14. Mendel’s experimental procedure Mendel studied simple and discrete traits of the peas—seed shape, seed color, and flower color He observed no intermediate characteristics among the offspring

15. One trait inheritance For his first experiment, Mendel crossed the tall and the short plant through cross pollination

16. One trait inheritance Mendel called the original parents the P generation He called the first-generation offspring the F1 generation

17. One trait inheritance: the first test cross If the blending theory were correct, the offspring should have the intermediate trait: all medium height plants

18. One trait inheritance His result of crossing the Tall and the Short plants: ALL TALL PLANTS! Not medium sized plants So, therefore, the F1 generation were all tall plants—resembling only one parent Did the characteristic for shortness disappear?

19. One trait inheritance Mendel then allowed these F1generation plants to self-pollinate with each other This next generation is referred to as the F2 generation The result?

20. One trait inheritance In the F2 generation, ¾ of the plants were tall, while ¼ of them were short, a 3:1 ratio So, the F1 plants were able to pass on the factor for shortness and it just didn’t disappear Perhaps the F1 plants were tall because tallness was dominant to shortness?

21. One trait inheritance Mendel explained why the short plants showed up in a 3:1 ratio in the F2 generation and not the F1 generation The F1 parents contained 2 separate copies of each hereditary factor, one being dominant and the other recessive

22. One trait inheritance These factors separated when gametes were formed, and each gamete carried only one gamete of each factor And random fusion of all possible gametes occurred upon fertilization

23. The Law of Segregation Each individual has 2 factors for each trait The factors segregate (separate) during the formation of gametes Each gamete contains only one factor from each pair of factors Fertilization gives each individual two factors for each trait

24. As viewed by modern genetics Each trait in a pea plant is controlled by two alleles, alternate forms of the gene—in this case, that control the length of the stem for tallness and shortness

25. The dominant allele is so named because of its ability to mask the expression of the other allele, called the recessive allele

27. Dominance and recessiveness The dominant allele is identified by an uppercase (capital) letter The recessive allele is identified by a lowercase (small) letter So, in this reference, the allele for tallness (the dominant allele) is “T”, and the allele for shortness (the recessive gene) is “t”

28. alleles These alleles occur on a homologous pair of chromosomes at a particular location that is called the gene locus

29. alleles One allele for each trait is located in each gamete because of the division of chromosomes during gamete formation in meiosis

30. Homozygous alleles When an organism has two identical alleles, we say it is homozygous For instance, in Mendel’s P generation of Tall plants, the parents were homozygous for tallness: TT. The short plants were homozygous for shortness: tt

31. Heterozygous alleles When an organism has two different alleles at the same gene locus, we say that it is heterozygous Therefore, these F1 plants all had the alleles “Tt”

32. Genotype vs. Phenotype Two organisms with different allelic combinations for a trait can give the same outward appearance: for example, TT and Tt plants are both tall We distinguish between the alleles present in an organisms and the appearance of that organism

33. Genotype vs. phenotype The word genotype refers to the alleles an individual receives at fertilization Genotype may be referred to by letters or by short descriptive phrases Genotype TT is called homozygous dominant, and genotype tt is called homozygous recessive Genotype Tt is called heterozygous

34. Genotype vs. phenotype Phenotype refers to the physical appearance of the individual The homozygous dominant (TT) individual and the heterozygous (Tt) individual both show the dominant phenotype of being tall The homozygous recessive phenotype is short

35. Genotype vs. phenotype

36. Exceptions to simple Mendelian Inheritance 1) incomplete dominance: the offspring have an intermediate phenotype compared to the parents with two different phenotypes 2) multiple alleles: the offspring inherits 2 of several possible alleles 3) codominance: two inherited alleles are expressed equally 4) polygenic inheritance: This occurs when a single physical trait is governed by two or more sets of alleles

37. 1. Incomplete Dominance Incomplete dominance is exhibited when the heterozygote has an intermediate phenotype between that of either homozygote For example (next slide), a red and a white flower will produce a pink flower

39. 2) Multiple Allelic Traits When a trait is controlled by multiple alleles, the gene exists in several alleleic forms But, each person can only have two of the possible alleles Blood types are an example of this

40. Multiple Allelic traits and co- dominance: ABO blood types Three alleles for the same gene control the inheritance of ABO blood types These alleles determine the presence or absence of antigens on red blood cells: –IA = A antigen on red blood cells –IB = B antigen on red blood cells –i = neither A nor B antigen on red blood cells

41. 3) Co dominance An example of co-dominance is: If a white flower and a red flower were crossed, the resulting offspring would be flowers with red and white stripes (not pink)

42. 4) Polygenic inheritance This occurs when a trait is governed by two or more sets of alleles Each dominant allele has a quantitative effect on the phenotype, and these effects are additive The result is a continuous variation of phenotypes, resulting in a distribution that resembles a bell-shaped curve

43. Skin color and height are examples of polygenic inheritance

46. Polygenic inheritance and epistasis Epistasis: this occurs when a gene at one locus interferes with a gene at a different locus Albinism is an example of this: no matter what genes for skin color are inherited from the parents, the gene for albinism interferes with the expression of alleles for skin color

47. Environment and phenotype Nutrition plays a part in height determination Temperature can effect the color of primroses and Himalayan rabbits Soil acidity effects the color of certain flowers

48. Ex. hydrangea

49. Sex-linked inheritance We have two types of chromosomes: 1. sex chromosomes (X and Y: XX for a female, XY for a male) 2. autosomal chromosomes: all of our other chromosomes not including sex chromosomes

50. Males produce 2 types of gametes: those that have an X and those that have a Y Females produce only one type of gamete: those that have an X Therefore, sex of the offspring is determined by the FATHER

52. Sex chromosomes Not only are sex-specific traits carried on sex chromosomes, but genes that have nothing to do with sex of the individual are carried here as well These are termed SEX-LINKED TRAITS i.e., X-linked traits are carried on the X chromosome