Patterns of Inheritance 1

2 Gregor Mendel Chose to study pea plants because: 1. Other research showed that pea hybrids could be produced 2. Many pea varieties were available 3. Peas are small plants and easy to grow 4. Peas can self-fertilize or be cross- fertilized

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4 Mendel’s experimental method Usually 3 stages 1. Produce true-breeding strains for each trait he was studying 2. Cross-fertilize true-breeding strains having alternate forms of a trait –Also perform reciprocal crosses 3. Allow the hybrid offspring to self-fertilize for several generations and count the number of offspring showing each form of the trait

5 Stigma Style Anthers (male) 1. The anthers are cut away on the purple flower. Petals Carpel (female) 4. All progeny result in purple lowers. 3. Pollen is transferred to the purple flower. 2. Pollen is obtained from the white flower. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

6 Monohybrid crosses Cross to study only 2 variations of a single trait Mendel produced true-breeding pea strains for 7 different traits –Each trait had 2 variants

7 F 1 generation First filial generation Offspring produced by crossing 2 true- breeding strains For every trait Mendel studied, all F 1 plants resembled only 1 parent –Referred to this trait as dominant –Alternative trait was recessive No plants with characteristics intermediate between the 2 parents were produced

8 F 2 generation Second filial generation Offspring resulting from the self- fertilization of F 1 plants Although hidden in the F 1 generation, the recessive trait had reappeared among some F 2 individuals Counted proportions of traits –Always found about 3:1 ratio

9 PurpleWhite YellowGreen RoundWrinkled GreenYellow 1. Flower Color 2. Seed Color 4. Pod Color DominantRecessive 3.15:1 X X X X 3.01:1 2.96:1 2.82:1 F 2 Generation 705 Purple: 224 White 6022 Yellow: 2001 Green 5474 Round: 1850 Wrinkled 428 Green: 152 Yellow 3. Seed Texture InflatedConstricted X 2.95:1 AxialTerminal TallShort 6. Flower Position 7. Plant Height X X 3.14:1 2.84:1 882 Inflated: 299 Constricted 651 Axial: 207 Terminal 787 T all: 277 Short 5. Pod Shape DominantRecessive F 2 Generation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

10 3:1 is actually 1:2:1 F 2 plants –¾ plants with the dominant form –¼ plants with the recessive form –The dominant to recessive ratio was 3:1 Mendel discovered the ratio is actually: –1 true-breeding dominant plant –2 not-true-breeding dominant plants –1 true-breeding recessive plant

11 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parent generation Self-cross Cross-fertilize Self-cross True- breeding Purple Parent True- breeding White Parent Purple Offspring F 1 generation F 2 generation (3:1 phenotypic ratio) F 3 generation (1:2:1 genotypic ratio) Purple Dominant Purple Dominant Purple Dominant White Recessive True- breeding Non-true- breeding Non-true- breeding True- breeding

12 Conclusions His plants did not show intermediate traits For each pair, one trait was dominant, the other recessive Alternative traits were expressed in the F 2 generation in the ratio of ¾ dominant to ¼ recessive

13 Five-element model 1.Parents transmit factors (genes) to offspring 2.Each individual receives one copy of a gene from each parent 3.Not all copies of a gene are identical –Allele – alternative form of a gene –Homozygous – 2 of the same allele –Heterozygous – different alleles

4.Alleles remain discrete – no blending 5.Presence of allele does not guarantee expression –Dominant allele – expressed –Recessive allele – hidden by dominant allele Genotype – total set of alleles an individual contains Phenotype – physical appearance 14

15 Principle of Segregation Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization Allele segregation is caused by the behavior of chromosomes during meiosis Mendel had no knowledge of chromosomes or meiosis

Punnett square Tool for predicting genetic crosses Example) Cross purple-flowered plant with white- flowered plant P is dominant allele – purple flowers p is recessive allele – white flowers True-breeding white-flowered plant is pp –Homozygous recessive True-breeding purple-flowered plant is PP –Homozygous dominant Offspring are Pp heterozygote purple-flowered plants 16

17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P P p ppp P P p p Pp P P p ppppP P P p ppp Pp pP PpPP a. 1. p + p = pp.2. P + p = Pp. 3. p + P = pP.4. P + P = PP.

18 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. p P p P Pp White parent pp b. P P p p pp Pp Purple parent PP Purple heterozygote Pp Purple heterozygote Pp F 1 generation PP pP F 2 generation 3 Purple:1 White (1PP: 2Pp :1pp )

19 Human traits Some human traits are controlled by a single gene –Some of these exhibit dominant and recessive inheritance Pedigree analysis is used to track inheritance patterns in families Dominant pedigree – juvenile glaucoma –Disease causes degeneration of optic nerve leading to blindness –A dominant trait appears in every generation

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21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Dominant Pedigree Generation I Generation II Generation III Key affected female affected male unaffected female unaffected male 3

Recessive pedigree example  albinism –Condition in which the pigment melanin is not produced –Males and females affected equally –Most affected individuals have unaffected parents 22

Recessive Pedigree Generation I Generation II Generation III Generation IV Heterozygous Homozygous recessive Key male carrier female carrieraffected female affected male unaffected female unaffected male Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. One of these persons is heterozygous Mating between first cousins

24 Dihybrid crosses Examination of 2 separate traits in a single cross RRYY x rryy (true-breeding lines for 2 traits) The F 1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait Mendel allowed F 1 to self-fertilize to produce F 2

25 F 1 self-fertilizes RrYy x RrYy The F 2 generation shows all four possible phenotypes in a set ratio –9:3:3:1 –Round yellow:round green:wrinkled yellow:wrinkled green

26 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cross-fertilization RYRyrYry Meiosis rr yy Parent generation RR YY Rr Yy F 1 generation Meiosis (chromosomes assort independently into four types of gametes)

27 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. RYRyrYry RR yyRr yy rr yy 9/16 3/16 1/16 round, yellow round, green wrinkled, yellow wrinkled, green RY Ry rY ry F 1 X F 1 (RrYy X RrYy) F 2 generation RR YYRR YyRr YYRr Yy RR YyRr Yy rr Yy rr YYRr YyRr YY Rr Yy

28 Principle of independent assortment In a dihybrid cross, the alleles of each gene assort independently The segregation of different allele pairs is independent Independent alignment of different homologous chromosome pairs during metaphase I leads to the independent segregation of the different allele pairs

29 Testcross Cross used to determine the genotype of an individual with dominant phenotype Cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp) Phenotypic ratios among offspring are different, depending on the genotype of the unknown parent

30 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P p P P p p Heterozygous dominant Homozygous recessive Alternative 2: Half of the offspring are white and the unknown flower is heterozygous (Pp) PP or Pp then If Pp Dominant Phenotype (unknown genotype) If PP then Alternative 1: All offspring are purple and the unknown flower is homozygous dominant (PP) Homozygous recessive Homozygous dominant Pp pp

31 Extensions to Mendel Mendel’s model of inheritance assumes that –Each trait is controlled by a single gene –Each gene has only 2 alleles –There is a clear dominant-recessive relationship between the alleles Most genes do not follow these rules

32 Polygenic inheritance Occurs when multiple genes are involved in controlling the phenotype of a trait The phenotype is an accumulation of contributions by multiple genes These traits show continuous variation and are referred to as quantitative traits –For example – human height –Histogram shows normal distribution

′6″' 6′0″ 5′0″ Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Number of Individuals (top): From Albert F. Blakeslee, “CORN AND MEN: The Interacting Infl uence of Heredity and Environment—Movements for Betterment of Men, or Corn, or Any Other Living Thing, One-sided Unless Th ey Take Both Factors into Account,” Journal of Heredity, 1914, 5:511-8, by permission of Oxford University Press Height

34 Pleiotropy Refers to an allele which has more than one effect on the phenotype Pleiotropic effects are difficult to predict, because a gene that affects one trait often performs other, unknown functions This can be seen in human diseases such as cystic fibrosis or sickle cell anemia –Multiple symptoms can be traced back to one defective allele

35 Multiple alleles May be more than 2 alleles for a gene in a population ABO blood types in humans –3 alleles Each individual can only have 2 alleles Number of alleles possible for any gene is constrained, but usually more than two alleles exist for any gene in an outbreeding population

36 Incomplete dominance –Heterozygote is intermediate in phenotype between the 2 homozygotes –Red flowers x white flowers = pink flowers Codominance –Heterozygote shows some aspect of the phenotypes of both homozygotes –Type AB blood

37 Parent generation 1 : 2 : 1 CRCR CWCW Cross-fertilization CWCWCWCW CRCRCRCR F 1 generation CRCWCRCW CRCWCRCW CRCRCRCR CRCR CWCW CRCWCRCW CWCWCWCW C R C R : C R C W : C W C W F 2 generation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

38 Human ABO blood group The system demonstrates both –Multiple alleles 3 alleles of the I gene (I A, I B, and i) –Codominance I A and I B are dominant to i but codominant to each other

39 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Alleles AB None O Galactosamine A Galactose B Blood Type Sugars Exhibited Donates and Receives Receives A and O Donates to A and AB Receives B and O Donates to B and AB Universal receiver Donates to AB Receives O Universal donor Both galactose and galactosamine I A I A, I A i (I A dominant to i) I B I B, I B i (I B dominant to i) I A I B (codominant) ii (i is recessive)

Environmental influence Coat color in Himalayan rabbits and Siamese cats –Allele produces an enzyme that allows pigment production only at temperatures below 30 o C 40 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © DK Limited/Corbis Temperaturebelow 33º C, tyrosinase active, dark pigment Temperature above 33º C, tyrosinase inactive, no pigment

41 Epistasis Behavior of gene products can change the ratio expected by independent assortment, even if the genes are on different chromosomes that do exhibit independent assortment R.A. Emerson crossed 2 white varieties of corn –F 1 was all purple –F 2 was 9 purple:7 white – not expected

42 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ABAbaBab AABBAABbAaBBAaBb AABbAAbbAaBbAabb AaBBAaBbaaBBaaBb AaBbAabbaaBbaabb 9/16 Purple: 7/16 White AB Ab aB ab Cross-fertilization a. b. Parental generation F 1 generation F 2 generation Pigment (purple) Enzyme B Enzyme A Precursor (colorless) Intermediate (colorless) White (aaBB) White (AAbb) All Purple (AaBb)