1 Theoretical Genetics copyright cmassengale

2 Gregor Mendel ( ) Responsible for discovering the Inheritance of Traits copyright cmassengale

3 Gregor Johann Mendel  Austrian monk  Studied the inheritance of traits in pea plants  Developed the laws of inheritance  Called the “Father of Genetics" copyright cmassengale

4 Genetic Terminology  Trait - any characteristic that can be passed from parent to offspring  Heredity - passing of traits from parent to offspring  Genetics - study of heredity copyright cmassengale

5 Genetic Terminology  Locus – a particular position on homologous chromosomes.  Gene – a section of DNA that controls a specific characteristic and is found on a specific locus.  Allele – one specific form of a gene that occupies the same gene locus as other alleles of the gene. copyright cmassengale

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7 Genetic Terminology  Dominant Allele – an allele that “shows up” in the phenotype whether the individual has one copy or two. Represented by an upper case letter (T or R)  Recessive Allele – an allele that only “shows up” in the phenotype when the dominant allele isn’t there. Represented by a lower case letter (t or r). copyright cmassengale

8 Genetic Terminology  Genotype – the pair of alleles that an organism has for a particular trait.  E.g. Tt or AA or I A I B  Phenotype – the outward appearance of a genotype.  E.g. tall or attached earlobes or Type AB blood. copyright cmassengale

9 Genotype & Phenotype in Flowers Genotype of alleles: R = red flower r = yellow flower All genes occur in pairs, so 2 alleles affect a characteristic Possible combinations are: GenotypesRR Rrrr PhenotypesRED RED YELLOW copyright cmassengale

10 Genotypes  Homozygous genotype - gene combination involving 2 dominant or 2 recessive genes (e.g. RR or rr); also called pure  Homozygous genotype - gene combination involving 2 dominant or 2 recessive genes (e.g. RR or rr); also called pure  Heterozygous genotype - gene combination of one dominant & one recessive allele (e.g. Rr); also called hybrid copyright cmassengale

11 Genetic Terminology  Carrier– A heterozygous individual that has one copy of a recessive allele that causes a genetic disease in an individual that is homozygous for the allele. copyright cmassengale

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13 Types of Genetic Crosses  Monohybrid cross - cross involving a single trait e.g. flower color  Dihybrid cross - cross involving two traits e.g. flower color & plant height copyright cmassengale

Test cross – testing a suspected heterozygote by crossing with a known homozygous recessive. copyright cmassengale 14

15 Punnett Square Used to help predict the possible offspring of a testcross copyright cmassengale

male parent genotype female parent genotype T t t t

Then, write the possible combinations of alleles from the parents inside the boxes. t T t

TTTT Tt (Male) homozygous short (Female) homozygous tall x

TTTT t Tt Offspring genotype % 100% Tt Phenotype% 100% tall

20 Trait: Seed Shape Alleles: R – Roundr – Wrinkled Cross: Round seeds x Wrinkled seeds RR x rr P 1 Monohybrid Cross R R rr Rr Genotype:Rr Genotype: Rr PhenotypeRound Phenotype: Round Genotypic Ratio:1:0 Genotypic Ratio: 1:0 Phenotypic Ratio: 1:0 copyright cmassengale

21 P 1 Monohybrid Cross Review  Homozygous dominant x Homozygous recessive  Offspring all Heterozygous (hybrids)  Offspring called F 1 generation  Genotypic & Phenotypic ratio is ALL ALIKE or 1:0 copyright cmassengale

22 Trait: Seed Shape Alleles: R – Roundr – Wrinkled Cross: Round seeds x Round seeds Rr x Rr F 1 Monohybrid Cross R r rR RR rrRr Genotype:RR, Rr, rr Genotype: RR, Rr, rr PhenotypeRound & wrinkled Phenotype: Round & wrinkled G.Ratio:1:2:1 G.Ratio: 1:2:1 P.Ratio: 3:1 copyright cmassengale

23 F 1 Monohybrid Cross Review  Heterozygous x heterozygous  Offspring: 25% Homozygous dominant RR 50% Heterozygous Rr 25% Homozygous Recessive rr  Offspring called F 2 generation  Genotypic ratio is 1:2:1  Phenotypic Ratio is 3:1 copyright cmassengale

Practice Worksheet copyright cmassengale 24

25 Mendel’s Laws copyright cmassengale

26 Results of Monohybrid Crosses Inheritable factors or genes are responsible for all heritable characteristics Phenotype is based on Genotype Each trait is based on two genes, one from the mother and the other from the father True-breeding individuals are homozygous ( both alleles are the same) copyright cmassengale

27 #1 Law of Dominance In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. All the offspring will be heterozygous and express only the dominant trait. RR x rr yields all Rr (round seeds) copyright cmassengale

28 Law of Dominance copyright cmassengale

29 #2 Law of Segregation During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring. copyright cmassengale

30 Applying the Law of Segregation copyright cmassengale

31 #3 Law of Independent Assortment Alleles for different traits are distributed to sex cells (& offspring) independently of one another. This law can be illustrated using dihybrid crosses. copyright cmassengale

32 Dihybrid Cross Traits: Seed shape & Seed color Alleles: Alleles: R round r wrinkled Y yellow y green RrYy x RrYy RY Ry rY ry All possible gamete combinations copyright cmassengale

33 Dihybrid Cross RYRyrYry RYRy rY ry copyright cmassengale

34 Dihybrid Cross RRYY RRYy RrYY RrYy RRYy RRyy RrYy Rryy RrYY RrYy rrYY rrYy RrYy Rryy rrYy rryy Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 9:3:3:1 phenotypic ratio RYRyrYryRY Ry rY ry copyright cmassengale

35 Dihybrid Cross Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 9:3:3:1 copyright cmassengale

Dihybrid Cross Practice copyright cmassengale 36

37 Summary of Mendel’s laws LAW PARENT CROSS OFFSPRING DOMINANCE SEGREGATION INDEPENDENT ASSORTMENT copyright cmassengale

38 Summary of Mendel’s laws LAW PARENT CROSS OFFSPRING DOMINANCE TT x tt tall x short 100% Tt tall SEGREGATION Tt x Tt tall x tall 75% tall 25% short INDEPENDENT ASSORTMENT RrGg x RrGg round & green x round & green 9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods copyright cmassengale

39 Codominance copyright cmassengale

40 Genetic Terminology  Codominant Alleles – a pair of alleles that both affect the phenotype when present in a heterozygote. copyright cmassengale

Codominance copyright cmassengale 41

42 Codominance Two alleles are expressed (multiple alleles) in heterozygous individuals. Example: blood type 1.type A= I A I A or I A i 2.type B= I B I B or I B i 3.type AB= I A I B 4.type O= ii copyright cmassengale

43 Codominance Problem Example:homozygous male Type B (I B I B ) x heterozygous female Type A (I A i) IAIBIAIB IBiIBi IAIBIAIB IBiIBi 1/2 = I A I B 1/2 = I B i IBIB IAIA i IBIB copyright cmassengale

44 Another Codominance Problem Example:Example: male Type O (ii) x female type AB (I A I B ) IAiIAiIBiIBi IAiIAiIBiIBi 1/2 = I A i 1/2 = I B i i IAIA IBIB i copyright cmassengale

45 Codominance Question: If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents? boy - type O (ii) X girl - type AB (I A I B ) copyright cmassengale

46 Codominance Answer: IAIBIAIB ii Parents: genotypes genotypes = I A i and I B i phenotypes phenotypes = A and B IBIB IAIA i i copyright cmassengale

47 Sex-linked Traits Traits (genes) located on the sex chromosomes Sex chromosomes are X and Y XX genotype for females XY genotype for males Many sex-linked traits carried on X chromosome copyright cmassengale

48 Sex-linked Traits Sex Chromosomes XX chromosome - femaleXy chromosome - male fruit fly eye color Example: Eye color in fruit flies copyright cmassengale

49 Sex-linked Trait Problem Example: Eye color in fruit flies (red-eyed male) x (white-eyed female) X R Y x X r X r Remember: the Y chromosome in males does not carry traits. RR = red eyed Rr = red eyed rr = white eyed XY = male XX = female XRXR XrXr XrXr Y copyright cmassengale

50 Sex-linked Trait Solution: X R X r X r Y X R X r X r Y 50% red eyed female 50% white eyed male XRXR XrXr XrXr Y copyright cmassengale

51 Female Carriers copyright cmassengale

52 Genetic Practice Problems copyright cmassengale

53 Breed the P 1 generation tall (TT) x dwarf (tt) pea plants T T tt copyright cmassengale

54 Solution: T T tt Tt All Tt = tall (heterozygous tall) produces the F 1 generation tall (TT) vs. dwarf (tt) pea plants copyright cmassengale

55 Breed the F 1 generation tall (Tt) vs. tall (Tt) pea plants T t Tt copyright cmassengale

56 Solution: TT Tt tt T t Tt produces the F 2 generation 1/4 (25%) = TT 1/2 (50%) = Tt 1/4 (25%) = tt 1:2:1 genotype 3:1 phenotype 3:1 phenotype tall (Tt) x tall (Tt) pea plants copyright cmassengale

57copyright cmassengale