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Ch3 Mendelian Genetics

Ch3 Mendelian Genetics. You are responsible for reviewing the content of this chapter. Chapter CONCEPTS

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Ch3 Mendelian Genetics

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  1. Ch3 Mendelian Genetics You are responsible for reviewing the content of this chapter.

  2. Chapter CONCEPTS • Inherited characteristics are under the control of discrete particulate factors called genes that are transmitted from generation to generation on vehicles called chromosomes, according to rules, first described by Gregor Mendel The outcomes of crosses subject to these rules are affected by chance deviation and can be evaluated using statistical analysis. Human traits are initially studied using the pedigree method .

  3. 3.1 Gregor Mendel • 3.2 Monohybrid cross • 3.3 Dihybrid cross • 3.4 Trihybrid cross • 3.5 Rediscovery of Mendel’s Work • 3.6 Independent Assortment and genetic Variation • 3.7 Probability and Genetic Event • 3.8 Evaluating genetic data :Chi-square • 3.9 Human Pedigree

  4. what organism to study? • Mendel used garden pea (Pisum sativum) • available cheap from local merchants • easy to grow, small space, fast generation time • lots of phenotypic variation • could be manipulated for controlled pollination: self and cross fertilization

  5. 1. Gregor Mendel • Genotype and Phenotype (study figure 3.1)

  6. Mendel’s Experimental Design • Understand the difference between self-fertilization and cross-fertilization. • Understand the organism used in Mendel’s design. • Understand what true-breeding implies. • testcross • Understand the 7 characteristics or traits that Mendel examined & alternative forms of each.

  7. 3.2 Monohybrid cross • Monohybrid Crosses & Law of segregation • Understand what the notations for the different genotypes are. • Understand what monohybrid crosses are and what reciprocal crosses are. • Understand all the terminology in this section Understand what the Law of Segregation means and what genotypic & phenotypic ratios are. • Natural segregation

  8. 3.3 Dihybrid cross • Dihybrid Crosses & Law of Independent Assortment • Understand what the Law of Independent Assortment means and predicts in terms of dihybrid phenotypic & genotypic ratios. • Understand how to use the branch diagram for dihybrid crosses & how to do dihybrid testcrosses. • Understand how to predict results of trihybrid or greater crosses using the branch diagram.

  9. How Mendel's Peas Become Wrinkled: A Molecular Explanation • Only recently, well over a hundred years after Mendel used wrinkled peas in his ground-breaking hybridization experiments, have we come to find out how the wrinkled 皱缩gene makes peas wrinkled. • The wild-type allele of the gene encodes a protein called starch-branching enzyme (SBEI). This enzyme catalyzes the formation of highly branched starch molecules as the seed matures. Wrinkled peas, which result from the homozygeus presence ofthe mutant form of the gene, lack the activity of this enzyme.

  10. The production of branch points is inhibited during the synthesis of starch within the seed. • This in turn leads to the accumulation of more sucrose and to a higher water content while the seed develops. • Osmotic pressure inside rises, which causes the loss of water internally and ultimately the wrinkled appearance of the seed during its maturation. • In contrast, developing seeds that bear at least one copy of the normal gene (being either homozygous or heterozygous for the dominant allele) synthesize starch, and reach an osmotic balance that minimizes the loss of water • the end result is a smooth-textured outer coat

  11. The SBEI genehas been cloned and analyzed. Interestingly, the mutant gene contains a foreign, sequence of some 800 base pairs that disrupts the normal coding sequence. • This foreign segment closely resembles other such sequences, calledTransposable elements . • They have the ability to move from place to place in the genome of organisms. • Transposable elements similar to this one have been found in maize (corn), parsley, and snapdragons. • Study of the SBEi gene provides greater insight into the relationship between genotypes and phenotypes. • We cover transposable elements greater detail in Chapter 14.

  12. 3.4 Trihybrid cross • Three factor cross

  13. Understand how to use Punnett squares and the branch diagrams. • Understand what probability is and understand when to use the product rule and sum rule of probability. • Understand what a testcross is and the expected Mendelian phenotypic & genotypic ratios produced in testcrosses.

  14. MENDELIAN ANALYSIS: 1  n GENES • 3 methods of working out expected outcomes of controlled breeding experiments: 1. Punnet square 2. tree method (long) – genotypes . tree method (short) – phenotypes 3. (a+b)N

  15. MENDELIAN ANALYSIS: PUNNET SQUARE • all pairings of ♀ and ♂ gametes • = probabilities of all pairings • some pairings occur >1 •  different Ps for different genotypes & phenotypes • 1 gene, 2 x 2 = 4 cells • 2 genes, 4 x 4 = 16 cells • 3 genes, 8 x 8 = 64 cells... • too much work !

  16. MENDELIAN ANALYSIS: LONG TREE    1 gene, alleles A, a GENOTYPE PHENOTYPE ¼ A/A 1/4 A/A 3/4 A ½ A/a 1/2 A/a ¼ a/a 1/4 a/a 1/4 a

  17. MENDELIAN ANALYSIS: LONG TREE          2 genes, alleles A, a, B, b... GENOTYPE PHENOTYPE ¼ B/B 1/16 A/A B/B 9/16 AB ¼ A/A ½ B/b 1/8 A/A B/b ¼ b/b 1/16 A/A b/b 3/16 Ab ¼ B/B 1/8 A/a B/B ½ A/a ½ B/b 1/4 A/a B/b ¼ b/b 1/8 A/a b/b ¼ B/B 1/16 a/a B/B 3/16 aB ¼ a/a ½ B/b 1/8 a/a B/b ¼ b/b 1/16 a/a b/b 1/16 ab

  18. MENDELIAN ANALYSIS: SHORT TREE     2 genes, alleles A, a, B, b... PHENOTYPE ¾ B 9/16 AB ¾ A ¼ b 3/16 Ab ¾ B 3/16aB ¼ a ¼ b 1/16 ab much easier... can extend to >2genes

  19. MENDELIAN ANALYSIS: 1  n GENES formulae for n genes, dominance, n hybrid crosses … 1.# possible different types of gametes = 2n 2. # possible different genotypes of progeny = 3n 3.#frequency of least common genotype = ¼ n 4.# possible different phenotypes of progeny = 2n

  20. STATISTICS: BINOMIAL EXPANSION • diploid genetic data suited to analysis (2 alleles/gene) • product and sum rules apply • examples... coin flipping • define symbols...

  21. From 3.7 Probability and Genetic Event • e.g., outcomes possible from monohybrid cross

  22. STATISTICS: BINOMIAL EXPANSION • Q: True breeding black and albino cats mate and have a little of all black kittens. If these kittens grow up and breed among themselves, what is the probability that at least two of the F2 kittens will be albino?

  23. STATISTICS: BINOMIAL EXPANSION • Q: True breeding black and albino cats mate and have a little of all black kittens. If these kittens grow up and breed among themselves, what is the probability that at least two of the F2 kittens will be albino? • A: First, define symbols and sort out basic genetics... one character – black > albino, one gene B > b ...

  24. STATISTICS: BINOMIAL EXPANSION

  25. STATISTICS: BINOMIAL EXPANSION • possible outcomes...

  26. STATISTICS: PASCAL’S TRIANGLE

  27. STATISTICS: BINOMIAL EXPANSION • binomial expansion of (p + q)n = • ... if p = ¾ and q = ¼ , ... • expansion of (p + q)3 = (¾ + ¼)(¾ + ¼ )(¾ + ¼ ) =

  28. Binomial Expansion • 1、遗传分析中后代组合的概率推算 Aa X aa杂交——二个后代出现Aa和aa的组合如下： No1. No2 概率 Aa Aa ¼ Aa aa ¼ Aa Aa ¼ aa aa ¼ 分布方式1：2：1， 正是（p+q)2=1p2+2pq+1q2 [(Aa) + (aa)]2 = 1(Aa)(Aa):2(Aa)(aa):1(aa)(aa)

  29. Aa X aa产生三个后代 • （p+q)3 = (1/2 + ½ )3 基因型及组合 概率 Aa aa 3 0 P3=(1/2)3=1/8 2 1 3p2q=3(1/2)2(1/2)=3/8 1 2 3pq2=3(1/2)(1/2)2=3/8 0 3 q3=(1/2)3=1/8 一般说，设p为某一基因型（或表型）出现的概率，q或（1-p)为另一基因型（或表型）出现概率，那么有 p+q=1。这些事件每一特定组合的概率都可以用二项式展开说明，二项式展开的系数表示在总共可能出现的组合数中的比率。

  30. 杨辉三角：则为二项式展开的系数 1 1 1 1 2 1 2 1 3 3 1 3 1 4 6 4 1 4 1 5 10 10 5 1 5 1 6 15 20 15 6 1 6

  31. n!/s!(n-s)psqn-s • 如果不考虑出现顺序，可有如下通式计算表型或基因型某一特定组合的概率： • n——子代数目 • S——某一基因型（或表型）数 • n-s——另一基因型（或表型）数 • P——某一基因型（或表型）出现的概率 • q——另一基因型（或表型）出现的概率 • 例：某医院一天出生6个婴儿，三男三女的概率是多少？ • （6！/3！3！）（1/2）3（1/2）3=5/16

  32. 2、孟德尔遗传的分离比率的二项式 • F2基因型比为（1∶2∶1）2，即（1/4+2/4+1/4）2三项式展开式的各项系数； • F2表型比为（3∶1）2 ，即（3/4+1/4）2二项式展开式的各项系数。 （3：1）n（n—基因对数），既（a+b)2=a2 + 2ab + b2 • 以3代a,以1代b，则有， • 当两对基因时 (3+1)2=(3)2 + 2(3)(1)+ (1)2 • 当三对基因时 (3+1)3=(3)3 + 3(3)2(1) + 3(3)(1)2 + (1)3 9:9:9 3:3:3 由此可见任何对基因杂种自交后代表型都可以用二项式展开计算。

  33. N对基因的二项式展开 • 随着基因对数的增加，二项式展开将变得复杂。当有n对基因时： • (a+b)n=Cn0 an+Cn1 a n-1 b + Cn2a n-2 b2…Cnka n-k bk +Cnnbn • Cnk = n!/K!(n-k)! 同样可以用杨辉三角推得

  34. 3.5 Rediscovery of Mendel’s Work

  35. MENDELIAN ANALYSIS: 2 LAWS • particulate hereditary determinants (genes) • alleles  different phenotypes; dominance • alleles of a gene segregate into different gametes • gametes receive 1 allele with equal probability • union of gametes at fertilization is random • alleles of different genes* assort independently into different gametes * gene pairs on different chromosomes

  36. STUFF TO THINK ABOUT • Mendel’s choice of characters was critical • did he chose 7 characters that appeared to reside on different chromosomes by chance? • what would happen if some were linked? • all exhibited dominant/recessive relationships

  37. 3.6 Independent Assortment and genetic Variation • One major consequence of independent assortment is the production by an individual of genetically dissimilar gametes. Genetic variation results because the two members of any homologous pair of chromosomes are rarely if ever genetically identical. Therefore, because independent assortment leads to the production of al1 possible chromosome combinations, extensive genetic diversity results.

  38. We have seen that the number of possible gametes, each with different chromosome compositions, is 2n, where n equals the haploid number. Thus, if a species has a haploid number of 4, then 24 or l6 different gamete combinations can be formed as a result of independent assortment. • Although this number is not high, consider the human Species, where n = 23, if 223 is calcu1ated, we find that in excess of 8 x l06. or over 8 million, different types of gametes are represented. Because fertilization represents an event involving only one of approximately 8 x l06 possible gametes from each of two parents. each offspring represents only one of (8 x 106)2. or 64 x 1012, potential genetic combinations!

  39. No wonder that except for identical twins, each member of the human species demonstrates a distinctive appearance and individuality. This number of combinations is far greater than the number of humans who have ever lived on earth ! • Genetic variation resulting from independent assortment has been extremely important to the process of evolution in all organisms.

  40. 3.7 Probability and Genetic Event • review essential Mendel • genetic ratios & rules • statistics • chi-square • binomial expansion • Poisson distribution • pedigree analysis to some front page

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