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Lesson Four

Lesson Four. Foundations of Genetics.

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Lesson Four

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  1. Lesson Four Foundations of Genetics

  2. Throughout history, the inheritance of traits has fascinated humans. While awareness of inheritance has existed since prehistoric times, few ideas were put forward to explain heredity. Genetics as a science did not arise until the 1850s and 1860s, when Gregor Johann Mendel systematically conducted a series of experiments to demonstrate the basic principles of inheritance. Mendel’s work set the stage for the development of modern genetics. Because genetic processes are fundamental to the understanding of life itself, genetics is regarded by many to be at the center of biology.

  3. Genetics [dVi5netiks] • (used with a sing. verb) The branch of biology that deals with heredity, especially the mechanisms of hereditary transmission and the variation of inherited characteristics among similar or related organisms. • (与单数动词连用)遗传学：研究遗传的生物学分支，尤其是遗传性的遗传机能和相似或相关有机体间非遗传特征的变化

  4. What is a gene • The material that controls which traits are expressed in an organism. • Genes come in pairs and offspring inherit one copy of each gene from each parent .

  5. Allele 等位基因 • Definition: An allele is an alternative form of a gene (one member of a pair) for a particular characteristic that is located at a specific position on a specific chromosome. • Earlobe [5IElEJb] n.耳垂 • The gene for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r).

  6. Alleles - two forms of agene (dominant & recessive) • Dominant - stronger of two genes expressed in the hybrid; represented by acapital letter (R) • Recessive - gene that shows up less often in a cross; represented by alowercase letter (r)

  7. Dihybrid cross双因子杂种杂交，双因子杂交 • Traits: Seed shape & Seed color Alleles: • R round; r wrinkled • Y yellow; y green RrYy x RrYy RY Ry rY ry RY Ry rY ry All possible gamete combinations

  8. RY Ry rY ry Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy 9:3:3:1 phenotypic ratio Dihybrid cross

  9. Dominant and recessive • Dominant: the member of a pair of alleles that shows its effect in the phenotype what ever other allele is present. • A trait that covers over, or dominates, another form of that trait • Trait that always shows up, even when only one of the two alleles is in the dominant form • Shown by a capital letter • Recessive: the member of a pair of alleles that does not show its effect in the present of any other allelic partner.

  10. Homozygous and heterozygous • Homozygous refers to having identical alleles for a single trait. • Both alleles [forms of the gene] are the same. • When offspring inherit two dominant genes, (one dominant gene from each parent) they are said to be homozygous dominant. • When offspring inherit two recessive genes, (one recessive gene from each parent) they are said to be homozygous recessive. • Heterozygous refers to having two different alleles for a single trait.

  11. Homozygous and heterozygous • Examples: The gene for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r). A homozygous plant would contain the following alleles for seed shape: (RR) or (rr). A heterozygous plant would contain the following alleles for seed shape: (Rr).

  12. 同源染色体分别带着控制同一性状的两个等位基因同源染色体分别带着控制同一性状的两个等位基因 • Organisms have two alleles for each trait. When the alleles of a pair are heterozygous, one is dominant and the other is recessive. Using the previous example, round seed shape (R) is dominant and wrinkled seed shape (r) is recessive. Round: (RR) or (Rr), Wrinkled: (rr). homozygote heterozygote 显性等位基因 纯合子 隐性等位基因 纯合子 杂合子

  13. 公元前五世纪希波克拉底（Hippocrates）提出了第一个为人所知的遗传理论，他认为子代具有亲代的特性那是因为在精液或胚胎里集中了来自身体各部分的微小代表元素，按照这一观点，他相信后天获得的性状是遗传的。公元前五世纪希波克拉底（Hippocrates）提出了第一个为人所知的遗传理论，他认为子代具有亲代的特性那是因为在精液或胚胎里集中了来自身体各部分的微小代表元素，按照这一观点，他相信后天获得的性状是遗传的。 • 100年后，亚里斯多德（Aristotle）认为亲代残缺，下一代并不残缺。因而提出：精液不是提供胚胎组成的元素，而是提供后代的蓝图。生物的遗传不是通过身体各部分样本的传递，而是个体胚胎发育所需的信息传递，可惜的是这一精辟而深刻的见解未能引起人们的重视。

  14. 1866年达尔文（Darwin）提出了泛生论（hypothesis of pangenesis） • 认为身体各部分细胞里都存在一种胚芽或“泛子（pangens）”，它决定所在细胞的分化和发育。各种泛子随着血液循环汇集到生殖细胞中。受精卵发育过程中，泛子又不断地流到不同的细胞中，控制所在细胞的分化，产生一定的组织器官。

  15. 1883年, 德国生物学家魏斯曼 (Weismann) 认为：多细胞生物可分为“种质” (germ plasm) 和“体质” (somatoplasm) 两部分,种质是独立的、连续的、能产生后代的种质和体质。体质是不连续的、不能产生种质。种质的变异将导致遗传的变异,而环境引起的体质的变异是不连续的。 • 魏斯曼做了连续22代剪断小鼠的实验，来与“泛生论”论战。

  16. 1869年达尔文的表弟高尔顿（Galton, F.）用数理统计的方法研究人类智力的遗传，发表了“天才遗传（Hereditary genius）”，认为变异是连续的，亲代的遗传性在子女中各占一半，并彻底混合，即“融合遗传论”。 • 由于他所选择的研究性状是数量性状，所以虽然他的结论是完全正确的，但只适合数量性状，而不能作为遗传的普遍规律。

  17. August Weismann’s germ plasm theory 种质学说 • According to his theory, germ plasm, which is independent from all other cells of the body (somatoplasm), is the essential element of germ cells (eggs and sperm) and is the hereditary material that is passed from generation to generation. • Although the details of the germ plasm theory have been modified, its premise of the continuity of hereditary material is the basis of the modern understanding of the process of physical inheritance.

  18. Pangenesis 泛生论 • PANGENESIS is a theory of a process of hereditary transmission according to which all parts of the organismcontribute to the formation of the entire organism.First propounded in ancient Greece, the hypothesis hascontinually reappeared (often in different and increasingly more sophisticated terms and occasionally underdifferent names) in both popular and scientific literature up to recent times.

  19. Incomplete dominance • F1 hybrids have an appearance somewhat in between the phenotypes of the two parental varieties. • Example:snapdragons (flower) • red (RR) x white (rr) • RR = red flower rr = white flower • produces the F1 generation: All Rr = pink (heterozygous pink)

  20. Law of Independent Assortment • Alleles fordifferenttraits are distributed to sex cells (& offspring) independently of one another. • This law can be illustrated usingdihybrid crosses.

  21. 孟德尔自由组合律 黄圆 绿圆 黄皱 绿皱

  22. Law of segregation • During theformation of gametes(eggs or sperm), thetwo allelesresponsible for a traitseparatefrom each other. • Alleles for a trait are then"recombined" at fertilization, producing the genotype for the traits of the offspring.

  23. 孟德尔分离律

  24. Nondisjunction 不分离

  25. Punnett square 庞纳特方格 • A tool to predict the probability of certain traits in offspring that shows the different ways alleles can combine; • A way to show phenotype & genotype; • A chart that shows all the possible combinations of alleles that can result when genes are crossed;

  26. Test cross 测交

  27. Text 1 Early theories of inheritance Hippocrates (约公元前460-约公元前370) Weismann (1834-1914)

  28. 2 Gregor Mendel and the birth of Genetics 孟 德 尔 (1822-1884)

  29. 2 Gregor Mendel and the birth of Genetics Gregor Mendel(1822-1884) Responsible for the Laws governing Inheritance of Traits

  30. 2 Gregor Mendel and the birth of Genetics • Austrian monk • Studied the inheritance of traits in pea plants • Developed the laws of inheritance • Mendel's work was not recognized until the turn of the20th century • Between 1856 and 1863, Mendel cultivated and tested some 28,000 pea plants • He found that the plants' offspring retained traits of the parents • Called the“Father of Genetics"

  31. Mendel stated that physical traits are inherited as “particles” • Mendel did not know that the “particles” were actually Chromosomes & DNA

  32. 3 Mendel’s classic experiments The famous garden of the world

  33. pea plants- why? • structure (male and female parts on same plant) • distinctive traits • rapid reproduction • ability to control pollination and fertilization

  34. 孟德尔研究的七对性状 豌豆杂交操作

  35. Punnett square 庞纳特方格 • A tool to predict the probability of certain traits in offspring that shows the different ways alleles can combine; • A way to show phenotype & genotype; • A chart that shows all the possible combinations of alleles that can result when genes are crossed;

  36. 孟德尔分离律

  37. Test cross 测交

  38. 4 Mendel’s ideas and the law of independent assortment

  39. 5 Chromosomes and Mendelian Genetics Morgan (1866-1945) 果蝇有 4 对染色体

  40. Sex-linked Traits • Traits (genes) located on the sex chromosomes • XX genotype for females, XY genotype for males • Many sex-linked traits carried on X chromosome

  41. fruit fly eye color XX chromosome - female Xy chromosome - male Sex-linked Traits Example: Eye color in fruit flies

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