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Mendelian Genetics. I. A Historical Perspective tall x short  medium A. Genetics has always been suspected. B. There are records of trait selection in.

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Presentation on theme: "Mendelian Genetics. I. A Historical Perspective tall x short  medium A. Genetics has always been suspected. B. There are records of trait selection in."— Presentation transcript:

1 Mendelian Genetics

2 I. A Historical Perspective tall x short  medium A. Genetics has always been suspected. B. There are records of trait selection in almost all early civilizations C. Early genetic debates: 1. Who contributed more to offspring? Male? Female? 2. What is actually passed down? Fluid? Life Forces? Pre-formation? E. Theory of Blending: Combined traits lost their “identities” thus were not discrete units A Homunculus Uterus D. Pangenesis: Many sperm with many eggs form one organism Offspring traits are always a blending of parental traits

3 II. Gregor Mendel: His Background A. Austrian Monk B. Educated at U of Vienna C. High school teacher D. Worked with common “garden pea” E. Published his work but died an “Unknown”

4 III. Why pea plants were a good choice A. Control reproduction B. Large # of offspring to provide good ratios C. Short generation span D. Easy traits to distinguish from one another

5 IV. Mendel’s Experimental Methods A. Established “Pure Lines” Tall X Tall (self-pollination) Short X Short (self-pollination) B. Cross the pure lines of the contrasting traits Pure Tall X Pure Short (P1 X P1) Results : F1 (First Family) 100% Tall C. Cross the F1 offspring Hybrid Tall (P2) X Hybrid Tall (P2) Results: 75% Tall 25% Short (F2) Ratio 3 Tall : 1 Short

6 Results from Mendel Crossing 7 Different “Characters” in Pea Plants

7 V. Mendel’s Conclusions (Laws) Known as Law of:Mendel’s Conclusions Alternate versions of factors account for inherited characteristics For each character an organism inherits 2 factors Unit Characters One member of the 2 inherited factors may mask over the expression of the other factor The 2 factors segregate (separate) in the formation of gametes When two or more paired factors form gametes by segregation, one member will segregate independently with any other member of a different factor Dominance Segregation (Meiosis) Independent Assortment

8 VI. Modern Genetic Terminology Factor Character Paired Factors Pure Hybrid MENDEL MODERN DEFINITION EXAMPLES The “stuff” that produces a trait One member of the gene pair Gene Allele Phenotype Genotype Homozygous Heterozygous The trait produced by a gene The visible expression Brown hair Blue eyes The interacting gene pair that produces a trait Genes of a pair are identical Genes of a pair are different The dominant gene is ex- pressed, recessive is hidden TT = (tall) tt = (short) TT tt Tt

9 VII. Punnett Squares A. Definition: B. Example: Symbols: Phenotype: Genotype: Gametes: P = Purple p = white Purple X Purple P p X P p P p P p Punnett Square P p P p PP Pp pp Answer % Purple = 75% % White = 25% Ratio = 3:1 A probability “tool” used to predict possible offspring using Mendel’s laws In pea plants, purple flowers are dominant to white flowers. A heterozygous purple flower plant is crossed with another heterozygous purple flower plant. What % of the offspring would be expected to be purple? What % would be white? What would be the expected ratio of purple offspring to white offspring?

10 VII. Mendel’s Law Independent Assortment A. Definition: When genes of different gene pairs make gametes (meiosis), the gene pairs separate and assort independently of other gene pairs B. Application 1. Only applies when keeping track of more than one gene pair 2. Must follow the patterns of meiosis C. Example of law A A A a a a B B B b b b 4 Chromosomes (Diploid) 2 Chromosome (Haploid) Meiosis

11 D. Forming gametes following Independent Assortment AaBB AB, aB aaBB aB AaBb AB, Ab, aB, ab AaBbCc ABC, ABc, AbC, Abc aBC, aBc, abC, abc E. Mendel’s Laws of Independent Assortment and Punnett Squares All possible gametes must be used to make an accurate punnett square

12 F. Punnett Squares that Require the Law of Independent Assortment In pea plants purple flowers are dominant to white and yellow seeds are dominant to green. A heterozygous purple flowered, heterozygous yellow seeded plant was crossed with another heterozygous purple, heterozygous yellow plant. What would be the phenotypic ratio of the traits found in the offspring? Symbols Phenotypes Genotype Gametes Punnett Square P = Purple, p = white, Y = Yellow seeds, y = green seeds Purple flowers, yellow seeds X Purple flowers yellow seeds PpYy X PpYy PY Py pY py PY Py pY py PPYYPPYy PpYY PpYy PPyy PpYy Ppyy PpYy ppYY ppYy Ppyy ppYy ppyy Possible Traits of Offspring Purple and Yellow Purple and Green White and Yellow White and Green P_Y_ = ________ P_yy = ________ ppY_ = ________ ppyy = ________ Ratio: 9 : 3 : 3 : 1

13 A. Recessive genes VIII. Human Genes that follow Mendialian PatternsMendialian Patterns 1. Examples -overproduction of mucus resulting in breathing and digestive complications. Missing a membrane bound chloride ion pump a.Cystic FibrosisCystic Fibrosis b. Tay-Sachs -lethal nervous system disorder. Lethal by the age of 4. Missing a lipid metabolizing enzyme. d. PKU - lacks an enzyme to metabolize phenylalanine. Increasing levels of this amino acid becomes toxic to brain cells resulting in mental retardation. Special diet can control the problem. c. Sickle Cell Anemia -deformed red blood cells clog capillaries. Oxygen deprived cells result in a severe form of anemia. Gene produces faulty hemoglobin.

14 2. Characteristics N n N n NN Nn nn b. Many have ‘high risk” ethnic groups c. Some bad genes may have become established to give a carrier a “heterozygous advantage” a. Requires 2 carriers (heterozygous) as parents to have a child expressing the trait N = Normal gene n = Bad gene Normal (carrier) X Normal (carrier) Nn X Nn 1 chance out of 4 to have an affected child

15 3. Typical Recessive Pedigree a)Tend to “appear out of nowhere” -parents do not show trait but a child does b)Does not show a sex bias - Found in both males and females c)Marriage into family tends to hide the gene Male Female “Married” Shows trait “out of nowhere” “males and females” “marriage hides the gene”

16 B. Dominant 1. Examples a.Huntington’s disease – Neurological degeneration that begins later on in life (mid 20s-45) b.Achondroplasia – Dwarfism; “Little People” long bones do not grow properly c. Polydactyly – Extra fingers and toes 2. Characteristics Only one bad gene is required to express the trait B = bad gene b = good gene Affected parent X Normal Parent Bb X bb Children have a 50% chance to express the trait B b b Bb bb Bb bb

17 3. Typical Dominant Pedigree a) Tends to be expressed in every generation b) Marriage into the family does not hide the trait “every generation” “marriage does not hide the trait”

18 VIII. Genetics Since Mendel “There is no such thing as a dominant or recessive gene” A.Since proteins have many different functions, the gene that makes the proteins will show a wide range of expression Protein Functions: Structural, Enzymes, Transport, Hormones B. Gene expression is due to the type of protein made by the gene Produces a protein that is phenotypically observable when only one of the genes is present (Heterozygous) 1. A dominant gene: Usually the type of protein produced is a structural protein 2. A recessive gene: Results of gene is only observable in the homozygous condition Usually the type of protein produced is an enzyme OR no protein product at all

19 IX. Non-Mendelian Gene Patterns A.Incomplete Dominance: RW = Pink Flower (1/2 as much red) One gene produces an observable protein while the other gene does not make any protein, a THIRD phenotype results Example: Snap Dragons R gene = Makes Red Pigment W gene = Makes no pigment RR = Red Flower (Red Pigment) WW = White Flower (no pigment) RRWW RW RW R RR RR RW WW WW W Ratio: 1 red : 2 pinks: 1 white Does this support the theory of Blending?

20 B. Codominance : X Protein produced by both alleles are observable, both alleles are expressed’ co means “together” 1. Example: Cattle fur color W gene = White furR gene = Red fur WW = white furRR = red fur WR = roan fur (red and white) Cross a white cow and red bull 1. Both the red and white genes are expressed 2. Does this support the theory of Blending? X Results? Does this support the theory of Blending? Cross roan cow and roan bull

21 Example: Blood Types: A B AB O ABOABO I Ior A B i Codominant Recessive AA, AO BB, BO AB OO C.Multiple Alleles: Blood Type Genes GenotypesPhenotypes More than two genes of a pair are found in a population of which an organism can only have two Proteins found (or not found) on red blood cells 1. Cross a person with AB blood with a person with O blood Example Problems 2. A parent with A blood and a parent with B blood have a child with O blood? Note: Other blood proteins are found on blood cells as well. Examples: 1. M and N group (M and N genes are codominant) 2. Rh factor group Having the factor(+) is dominant to not having the factor (-) Example

22 D. Epistatic genes: Example: Fur color in mice B = gene for black fur color (Dominant to brown) b = gene for brown fur color (Recessive to black) C = gene to make color in fur (Dominant and epistatic to black or brown) c = gene can not make color in fur (Recessive) Important Points: 1.A capital “C” must be present in order to show color 2.Unique ratios result from epistasis 9 black : 3 brown : 4 white One gene pair influencing the expression of other gene pairs Cross 2 mice heterozygous for both genesWhat ratio would you expect? (9:3:3:1) Usually due to genes “upstream” in a shared metabolic pathway

23 E. Polygenic Traits: 1.Characteristics a. Show a wide distribution of traits over a gradient b. Most human traits are polygenic c. Examples: height, hair color, hair texture, eye color, skin color Example: Skin Color aabbccAabbcc aaBbcc aabbCc AaBbcc AAbbcc aaBBcc aabbCC AabbCc aaBbCc AaBbCc AABbcc AaBBcc AAbbCc aaBBCc aaBbCC AabbCC AABbCc AaBBCc AaBbCC AABBcc AAbbCC aaBBCC AABBCc AaBBCC AABbCC AABBCC Capital letters Produces Pigment Lower case produces little pigment # of CapitalsPhenotype 6 Darkest 5 Darker 4 Dark 3 Medium 2 Light 1 Lighter 0 Lightest Many gene pairs work together to produce a trait

24 Multifactorial Traits Several genes AND environmental factors influence the phenotype Examples in humans – height, heart disease, alcoholism

25 Slide 15

26 Example Blood Problem A man with heterozygous A and heterozygous “+” blood has a child with woman with O “–” Blood. What could be the possible blood types of their children? Symbols Phenotypes Genotypes Gametes Square Slide 15 R = + r = - A = A gene B = B gene o = O gene A + X O- AoRr X oorr AR, Ar, oR, oror Answer Question AR AoRrAorr Ar ooRr oRor oorr A +A -o +o -


28 Slide 13

29 Alleles Slide 8Slide 13

30 Make own pathway to show epistasis, search human examples

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