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Mendelian Genetics AP Biology Ch. 14 Ms. Haut. Pre-Mendelian Theory of Heredity Blending Theory —hereditary material from each parent mixes in the offspring.

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Presentation on theme: "Mendelian Genetics AP Biology Ch. 14 Ms. Haut. Pre-Mendelian Theory of Heredity Blending Theory —hereditary material from each parent mixes in the offspring."— Presentation transcript:

1 Mendelian Genetics AP Biology Ch. 14 Ms. Haut

2 Pre-Mendelian Theory of Heredity Blending Theory —hereditary material from each parent mixes in the offspring 1.Individuals of a population should reach a uniform appearance after many generations 2.Once traits are blended, they can no longer be separated out to appear in later generations Problems —inconsistent with observations: 1.Individuals of a population don’t reach uniform appearance 2.Traits can skip generations

3 Modern Theory of Heredity Based on Gregor Mendel’s fundamental principles of heredity 1.Parents pass on discrete inheritable factors (genes) to their offspring 2.These factors remain as separate factors from one generation to the next

4 Mendel’s Experimental, Quantitative Approach Advantages of pea plants for genetic study: –There are many varieties with distinct heritable features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits –Mating of plants can be controlled –Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels) –Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

5 Mendel’s Discoveries Developed pure lines— populations that “breed true” (always produce offspring with the same traits as the parents when parents are self- fertilized) Counted his results and kept statistical notes on experimental crosses

6 Useful Genetic Vocabulary Homozygous —having 2 identical alleles for a given trait (PP or pp) Heterozygous —having 2 different alleles for a trait (Pp); ½ gametes carry one allele (P) and ½ gametes carry the other allele (p) Phenotype —an organism’s expressed traits (purple or white flowers) Genotype —an organism’s genetic makeup (PP, Pp, or pp)

7 Combinations resulting from a genetic cross may be predicted by a Punnett square This law predicts a 3:1 ratio observed in the F2 generation of a monohybrid cross

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9 Mendel’s Principles of Heredity 1.First Law of Genetics: Law of Segregation a)alternate forms of genes are responsible for variations in inherited traits the gene for flower color in pea plants has two alleles, one for purple flowers and the other for white flowers b)for each trait, an organism inherits 2 alleles, one from each parent c)If 2 alleles differ, one is fully expressed (dominant allele); the other is completely masked (recessive allele)

10 Mendel’s Principles of Heredity 1.First Law of Genetics: Law of Segregation d)Each gene resides at a specific locus on a specific chromosome 2 alleles for each trait segregate during gamete production

11 The Testcross How can we tell the genotype of an individual with the dominant phenotype? Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual If any offspring display the recessive phenotype, the mystery parent must be heterozygous Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

12 The Testcross The cross of an individual with dominant phenotype to a homozygous recessive parent Used to determine if the individual is homozygous dominant or heterozygous CAUTION: Must perform many, many crosses to be statistically significant

13 Mendel’s Principles of Heredity 2.Second Law of Genetics: Law of Independent Assortment a)During gamete formation, the segregation of the alleles of one allelic pair is independent of the segregation of another allelic pair b)Law discovered by following segregation of 2 genes

14 Dihybrid Cross

15 Mendelian Inheritance Reflects Rules of Probability Rules of Multiplication: The probability that independent events will occur simultaneously is the product of their individual probabilities.

16 Mendelian Inheritance Reflects Rules of Probability Question: In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will be homozygous recessive? Answer: Probability that an egg from the F1 (Pp) will receive a p allele = ½ Probability that a sperm from the F1 will receive a p allele = ½ Overall probability that 2 recessive alleles will unite at fertilization: ½ x ½ = ¼

17 Mendelian Inheritance Reflects Rules of Probability Question: For a dihybrid cross, YyRr x YyRr, what is the probability of an F2 plant having the genotype YYRR? Answer: Probability that an egg from a YyRr parent will receive the Y and R alleles = ½ x ½ = ¼ Probability that a sperm from a YyRr parent will receive the Y and R alleles = ½ x ½ = ¼ Overall probability of an F2 plant having the genotype YYRR: ¼ x ¼ = 1/16 Works for Dihybrid Crosses:

18 Mendelian Inheritance Reflects Rules of Probability Rules of Addition: The probability of an event that can occur in two or more independent ways is the sum of the separate probabilities of the different ways. Question: In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will being a heterozygote? Answer: There are 2 ways in which a heterozygote may be produced: the dominant allele may be in the egg and the recessive allele in the sperm, or the dominant allele may be in the sperm and the recessive allele in the egg.

19 Mendelian Inheritance Reflects Rules of Probability Probability that the dominant allele will be in the egg with the recessive in the sperm is ½ x ½ = ¼ Probability that the dominant allele will be in the sperm with the recessive in the egg is ½ x ½ = ¼ Therefore, the overall probability that a heterozygote offspring will be produced is ¼ + ¼ = ½

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21 Variations to Mendel’s First Law of Genetics 1.Incomplete dominance —pattern of inheritance in which one allele is not completely dominant over the other Heterozygote has a phenotype that is intermediate between the phenotypes of the homozygous dominant parent and homozygous recessive parent

22 Incomplete Dominance in Snapdragon Color Genotypic ratio: Phenotypic ratio: 1 C R C R : 2 C R C W : 1 C W C W 1 red: 2 pink: 1 white F2

23 Variations to Mendel’s First Law of Genetics 2.Codominance —pattern of inheritance in which both alleles contribute to the phenotype of the heterozygote

24 Codominance in MN Blood Groups MN blood group locus codes for the production of surface glycoproteins on the red blood cell There are 3 blood types: M, N, and MN Blood TypeGenotype MMM NNN MN The MN blood type is the result of full phenotypic expression of both alleles in the heterozygote; both molecules, M and N, are produced on the red blood cell

25 Multiple Alleles Some genes may have more than just 2 alternate forms of a gene. –Example: ABO blood groups A and B refer to 2 genetically determined polysaccharides (A and B antigens) which are found on the surface of red blood cells (different from MN blood groups) –A and B are codominant; O is recessive to A and B

26 Multiple Alleles for the ABO Blood Groups 3 alleles: I A, I B, i

27 Pleiotropy The ability of a single gene to have multiple phenotypic effects (pleiotropic gene affects more than one phenotype) Examples: In tigers and Siamese cats, the gene that controls fur pigmentation also influences the connections between a cat’s eyes and the brain. A defective gene cause both abnormal pigmentation and cross-eye condition Marfan’s syndrome—one gene causes the slender physique, hypermobility of the joints, elongation of the limbs, dislocation of the lens, and susceptibility to heart disease

28 Epistasis Interaction between 2 nonallelic genes in which one modifies the phenotypic expression of the other. If epistasis occurs between 2 nonallelic genes, the phenotypic ratio resulting from a dihybrid cross will deviate from the 9:3:3:1 Mendelian ratio

29 C = Melanin deposition c = No Deposition (Albinism) B = Brown coat color b = Tan coat color A cross between heterozygous brown horses for the 2 genes results in a 9:3:4 phenotypic ratio 9 Black (B_C_) 4 Albino (__cc) 3 Brown (bbC_)

30 Polygenic Traits Mode of inheritance in which the additive effect of 2 or more genes determines a single phenotypic character Skin pigmentation in humans --3 genes with the dark-skin allele (A, B, C) contribute one “unit” of darkness to the phenotype. These alleles are incompletely dominant over the other alleles (a, b, c) --An AABBCC person would be very dark; an aabbcc person would be very light --An AaBbCc person would have skin of an intermediate shade

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32 Pedigree Analysis A pedigree is a family tree that describes the interrelationships of parents and children across generations Inheritance patterns of particular traits can be traced and described using pedigrees Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

33 Pedigree Analysis Analysis of existing populations Studies inheritance of genes in humans Useful when progeny data from several generations is limited Useful when studying species with a long generation time

34 = female = male = affected individual = mating = offspring in birth order I and II are generations I II Symbols : = Identical twins = Fraternal twins

35 Dominant Pedigree : I II III For dominant traits: Affected individuals have at least one affected parent The phenotype generally appears every generation 2 unaffected parents only have unaffected offspring

36 Recessive Pedigree : I II III For recessive traits: Unaffected parents can have affected offspring Affected progeny are both male and female

37 Pedigree Analysis Is widow’s peak a dominant or recessive trait? Dominant

38 Pedigree Analysis Is attached earlobe a dominant or recessive trait? Recessive

39 Recessive Human Disorders Parents are generally unaffected Defective form of a normal trait. Generally, more serious phenotypic affect than dominant genes 2 Heterozygous normal, unaffected parents can have affected offspring Probability the child of 2 carriers will be: – affected = ¼ –Normal, but carriers = 1/2

40 Recessive Human Disorders Cystic Fibrosis: autosomal recessive –Ineffective component of Na + /Cl - ; causes mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine Tay-Sachs: autosomal recessive –Usually fatal by 2 or 3 yrs –Developmental retardation, followed by paralysis, dementia, and blindness –Lack enzyme to breakdown lipids— accumulate in brain so cells lose function

41 Recessive Human Disorders Sickle-cell anemia: autosomal recessive –Caused by single amino acid substitution in hemoglobin –Abnormal hemoglobin packs together to form rods creating crescent-shaped cells –Reduces amount of oxygen hemoglobin can carry

42 Dominant Human Disorders Traits inherited in every generation When there is 1 affected parent; ½ progeny are affected 2 affected parents can have unaffected offspring If prevents survival, then gene is quickly eliminated from population Usually more variable in its effects. If lethal, usually after reproductive age

43 Dominant Human Disorders Huntington’s Disease: autosomal dominant Average onset is 40 yrs. Late acting, presents itself after reproductive age; lethal Affects nervous system, muscle spasms Destroys neurons Located on chromosome 4 Children of an afflicted parent have a 50% chance of inheriting the lethal dominant allele

44 Nature versus Nurture Environmental conditions can influence the phenotypic expression of a gene, so that a single genotype may produce a range of phenotypes One may have a history of heart disease in their family and thus be at risk of heart disease themselves. If this person watches his/her diet, exercises, doesn’t smoke, etc. his/her risk of actually developing heart disease decreases

45 Genetic Testing & Counseling Genetic counselors can help determine probability of prospective parents passing on deleterious genes –Genetic screening for various known diseases alleles (gene markers)

46 Genetic Testing & Counseling Fetal testing Amniocentesis –needle inserted into uterus and amniotic fluid extracted Test for certain chemicals or proteins in the fluid that are diagnostic of certain diseases Karyotype-can see chromosome abnormalities

47 Genetic Testing & Counseling Fetal testing Chorion Villus Sampling –Suctions off a small amount of fetal tissue from the chorionic villus of placenta Karyotype-can see chromosome abnormalities

48 Ultrasound at 12 weeks --can see any physical abnormalities


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