Lesson FourFoundations of Genetics

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.

Genetics [](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

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 .

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 [] 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).

Alleles - two forms of a gene (dominant & recessive)

Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R)

Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r)

RrYy x RrYy

RY Ry rY ry

All possible gamete combinations

Traits: Seed shape & Seed color Alleles:R round; r wrinkledY yellow; y green

Dihybrid cross 双因子杂种杂交，双因子杂交

RY Ry rY ry

Dihybrid cross

Round/Yellow: 9

Round/green: 3

wrinkled/Yellow: 3

wrinkled/green: 1

9:3:3:1 phenotypic ratio

RRYY

RRYy

RrYY

RrYy

RRYy

RRyy

RrYy

Rryy

RrYY

RrYy

rrYY

rrYy

RrYy

Rryy

rrYy

rryy

RYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry

Dominant and recessiveDominant: 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 traitTrait that always shows up, even when only one of the two alleles is in the dominant form Shown by a capital letterRecessive: the member of a pair of alleles that does not show its effect in the present of any other allelic partner.

Homozygous and heterozygousHomozygous 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.

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).

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).

同 源染 色体 分别 带着 控制 同一 性状 的两 个等 位基因

显性等位基因 纯合子

隐性等位基因 纯合子

杂合子

homozygote heterozygote

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。

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。

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。

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。

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.

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.

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)

Law of Independent Assortment

Alleles for different traits are distributed to sex cells (& offspring) independently of one another. This law can be illustrated using dihybrid crosses.

孟德尔自由组合律

黄圆绿圆黄皱绿皱

Law of segregation

During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other.

Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

孟德尔分离律

Nondisjunction 不分离

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;

Punnett square 庞纳特方格

Test cross

测交

Hippocrates (约公元前460-约公元前 370)

Weismann (1834-1914)

Text 1 Early theories of inheritance

2 Gregor Mendel and the birth of Genetics

孟 德 尔 (1822-1884)

2 Gregor Mendel and the birth of Genetics

Gregor Mendel(1822-1884)

Responsible for the Laws governing Inheritance of Traits

2 Gregor Mendel and the birth of Genetics

Austrian monkStudied the inheritance of traits in pea plantsDeveloped the laws of inheritanceMendel's work was not recognized until the turn of the 20th centuryBetween 1856 and 1863, Mendel cultivated and tested some 28,000 pea plantsHe found that the plants' offspring retained traits of the parentsCalled the “Father of Genetics"

Mendel stated that physical

traits are inherited as

“particles”

Mendel did not know that the

“particles” were actually

Chromosomes & DNA

3 Mendel’s classic experiments

The famous garden of the world

pea plants- why?

structure (male and female parts on same

plant)

distinctive traits

rapid reproduction

ability to control pollination and

fertilization

豌豆杂交操作

孟德尔研究的七对性状

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;

孟德尔分离律

Test cross

测交

4 Mendel’s ideas and the law of independent assortment

5 Chromosomes and Mendelian Genetics

Morgan (1866-1945)

果蝇有 4 对染色体

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

Sex-linked Traits

XX chromosome - female Xy chromosome - male

fruit flyeye color

Example: Example: Eye color in fruit Eye color in fruit fliesflies