遺傳學

遺傳學wai4 cyun4 hok6（英文：genetics）係研究生物體嘅遺傳同埋變異嘅科學，係生物學嘅一個重要分支。喺史前時期，人類就已經識得利用生物體嘅遺傳特性通過選擇育種嚟提高穀物同埋牲畜嘅產量。而現代遺傳學，目的係尋求了解遺傳嘅整個過程嘅機制，就係始於19世紀中孟德爾嘅研究工作。雖然，孟德爾並唔知遺傳嘅物理基礎，但喺佢觀察到生物體嘅遺傳特性，某啲遺傳單位遵守簡單嘅統計學規律，呢啲遺傳單位家陣被稱為基因。

基因位於DNA上，而DNA係由四類唔同嘅核苷酸組成嘅鏈狀分子，喺DNA上面嘅核苷酸序列就係生物體嘅遺傳信息。天然嘅DNA係以雙鏈形式存在嘅，兩條鏈上面嘅核苷酸互補，而每一條鏈都可以作為模來合成新嘅互補鏈。呢啲就係形成可以被遺傳嘅基因嘅複製方式。

基因上嘅核苷酸序列可以被細胞轉譯以合成蛋白質，蛋白質上嘅胺基酸序列就係對應基因上嘅核苷酸序列。呢個對應性被稱為遺傳密碼。蛋白質嘅胺基酸序列決定佢點樣摺疊成為一個三維結構，而蛋白質嘅結構就同佢所發揮功能密不可分。蛋白質執行細胞中幾乎所有嘅生物學進程來維持細胞嘅生存。DNA上嘅一個基因嘅改變可以改變佢編碼嘅蛋白質嘅胺基酸，並可能改變呢個蛋白質嘅結構同埋功能，進而對細胞甚至整個生物體造成巨大嘅影響。

雖然遺傳學喺決定生物體外形同埋行為嘅過程中扮演重要嘅角色，但呢個過程係遺傳學同生物體所經歷嘅環境共同作用嘅結果。例如，雖然基因能夠喺一定程度上決定一個人嘅體重，人喺童年所經歷嘅營養同埋健康狀況都會對佢嘅體有重大影響。

繁殖同遺傳

一條 DNA 份子嘅抽象圖解；佢裏面嗰啲份子帶有遺傳資訊，描述隻生物係點。

生物曉繁殖（reproduce），而基因（gene）係所有已知生物將自己嘅特徵傳俾下一代嘅基本媒介：每隻生物嘅細胞裏面都有一啲 DNA 份子喺度，呢啲 DNA 份子成雙螺旋（double helix）噉嘅結構，一條 DNA 裏面有能力決定隻生物嘅特徵嘅某個特定區域就為止一個基因，例如想像人嘅 DNA，如果 DNA 當中嘅某一段決定人嘅頭髮顏色－嗰段有 $A$ 同 $B$ 兩個唔同嘅可能款；身上有 $A$ 嘅人會係金頭髮，而身上有 $B$ 嘅人會係黑頭髮－噉呢段 DNA 就算係決定頭髮顏色嘅基因^[1]^[2]。

基因決定咗一隻生物嘅遺傳資訊：一隻生物內部會靠 DNA 嘅基因產生相應嘅 RNA 基因，而由呢啲 RNA 基因嗰度就會產生出蛋白質（protein），跟手呢啲蛋白質就會俾隻生物攞嚟組成身體嘅各種構造，例如動物嘅肌肉以至神經細胞都係要有蛋白質先組成得到；所以一隻生物身上嘅 DNA 會影響佢嘅各種特徵，由肌肉嘅大細（影響佢大唔大力）以至個腦嘅神經細胞嘅數量（影響佢嘅智能同各種認知特性^[3]）等。好多時，就算係極之唔同嘅生物物種身上都會有相同嘅基因，顯示咗唔同嘅生物好多時都需要相同嘅身體功能，舉個例子說明，如果將喺人類當中負責產生胰島素（insulin）嘅基因植入去第啲生物嗰度嗰時，個基因都仲係會扮演生產胰島素嘅功能^[1]。

基因組

無論一隻生物有細胞核定冇細胞核，佢嘅 DNA 都會聚埋一齊形成染色體（chromosomes）嘅結構，一隻生物體內所有細胞嘅染色體嘅總和就係佢個基因組（genome）：

喺真核生物（eukaryotes；指啲細胞有細胞核，人類係真核生物）當中，基因組嘅 DNA 主要係位於啲細胞嘅細胞核裏面嘅，又有少部份會喺粒線體入面；
喺原核生物（prokaryote；細胞冇細胞核）裏面，DNA 會位於形狀岩岩巉巉嘅擬核（nucleoid）內部^[4]。

舉個例子說明，人類嘅基因組有 23 對染色體，而人類身體差唔多每粒細胞嘅細胞核都有呢 23 對（46 條）染色體喺度。每條染色體都係一條雙螺旋噉形嘅 DNA 份子，而啲 DNA 份子上面有乜嘢基因就因人而異，令到人類個體彼此之間喺身體製造某啲蛋白質嘅能力上有差異，造成身體同心理上嘅個體差異^[5]^[6]。

顯微鏡下睇到、俾研究者上咗色嘅人類染色體

性

稍為複雜少少嘅生物，包括所有已知嘅脊椎動物，絕大多數行有性繁殖（sexual reproduction），即係用公同乸兩個性別做繁殖：喺呢啲生物當中，公嘅會提供精子，而乸嘅就會提供卵子，精子有阿爸嘅部份 DNA，而卵子有阿媽嘅部份 DNA；喺受精（fertilization）嘅過程入面，精子同卵子會結合，於是就產生出一個喺遺傳上有少少似阿爸、又有少少似阿媽嘅新個體^[7]。

對於有性繁殖嘅起源，廿一世紀初嘅生物學界仲有唔少爭議^[8]。其中一個見解係，有性繁殖會賦予物種比較大嘅遺傳多樣性－研究表示，靠無性繁殖（asexual reproduction）所生嘅後代同母體係一個板嘅（想像一粒靠自己分裂嚟繁殖嘅細胞，後代淨係有得由母體嗰度攞遺傳物料），所以如果一個物種係靠無性繁殖嚟生新個體嘅話，個物種嘅遺傳多樣性就會低得好交關，令佢哋好多時一旦有一兩種遺傳病，就搞到成個物種受毀滅性嘅打擊；因為噉，有性繁殖喺進化（睇下面）上對一個物種有利^[9]^[10]。

一個男人（男人係公嘅成年人類）

一個餵緊奶嘅女人（女人係乸嘅成年人類）

顯微鏡下睇到嘅人類精子

一粒人類卵子

生物進化

顯微鏡下嘅血細胞

現代生物學其中一個基礎係進化（evolution）嘅概念^[11]：想像一個生物物種族群，個族群嘅基因庫（gene pool；指個族群身上嘅基因嘅總和）係 $x_{0}$ 噉嘅樣，家吓佢哋做繁殖生下一代，而下一代嘅基因庫係 $x_{1}$ 噉嘅樣，因為物競天擇（睇下面）同基因突變（genetic mutation）等嘅原因， $x_{1}$ 同 $x_{0}$ 會有少少差異，而過咗 $i$ 代後嘅基因庫 $x_{i}$ 可以同 $x_{0}$ 有巨大差異，令第 $i$ 嘅個體同初代個體唔再算係同一物種－進化就係指一個族群嘅基因庫隨時間嘅變化，以及呢啲變化產生新物種嘅過程^[12]^[13]。

進化涉及物競天擇（natural selection）嘅過程：喺 1809 年，法國生物學家拉馬卡（Jean-Baptiste Lamarck）提出咗進化嘅諗法，但佢並冇做得到詳細解釋進化嘅機制（即係「點解一個族群嘅基因庫會隨時間變化」）^[14]；喺佢 50 年後嘅查理斯·達爾文（Charles Darwin）提出咗革命性嘅物競天擇概念，達爾文指出，基因會影響一隻動物生存同繁殖嘅能力，例如跑得快啲嘅馬喺野外冇咁容易俾獵食者殺死，身體生產精子能力弱嘅公冇咁大機會會成功生到仔... 等等，因為噉，喺是但一個時間點，是但搵個基因 $g_{0}$ ， $g_{0}$ 嘅第 $i$ 個複製品叫 $g_{0i}$ ， $g_{0i}$ 傳到去下一代基因庫 $x_{1}$ 嗰度嘅機會率會唔等同於另一個基因嘅一個複製品（ $g_{1i}$ ），即係

\Pr(g_{0i}\subset x_{1})\neq \Pr(g_{1i}\subset x_{1})

^{[註 1]}

於是下一代嘅基因庫會或多或少噉同上一代嘅唔同，而且長遠嚟講，地球嘅環境會係噉變化（例子可以睇氣候變化），令到「乜嘢基因對生存有利」呢點會係噉變化，例如如果一個地區因為氣候變化而搞到旱災多咗，對於住呢個地區嘅生物嚟講，「令身體保水能力強嘅基因」所賦予嘅優勢就會大咗。如是者，地球上嘅生物物種嘅基因庫就會隨地球環境變化而不斷變（進化）^[15]^[16]。而基於呢啲概念嘅進化論預測，隨住一個生物族群散佈並且分做唔同嘅次族群，佢哋會慢慢噉適應各自嘅環境，變到彼此之間愈嚟愈唔同－實驗同觀察嘅數據都撐呢個理論^[17]^[18]。

註釋

↑ 喺數學上， $\Pr(y)$ 係指「事件 $y$ 發生嘅機會率」，而 $a\subset b$ 簡單啲講係指「 $a$ 喺 $b$ 入面（子集）」。

睇埋

攷

↑ ^1.0 ^1.1 Marcial, Gene G. (August 13, 2007) From SemBiosys, A New Kind Of Insulin, Archived 2014-10-29 at the Wayback Machine.. businessweek.com
↑ Hartl, D. L., Clark, A. G., & Clark, A. G. (1997). Principles of population genetics (Vol. 116). Sunderland, MA: Sinauer associates.
↑ Van Der Maas, H. L., Dolan, C. V., Grasman, R. P., Wicherts, J. M., Huizenga, H. M., & Raijmakers, M. E. (2006). A dynamical model of general intelligence: the positive manifold of intelligence by mutualism. Psychological review, 113(4), 842.
↑ Thanbichler, M; Wang, SC; Shapiro, L (October 2005). "The bacterial nucleoid: a highly organized and dynamic structure". Journal of Cellular Biochemistry. 96 (3): 506–21
↑ "Genotype definition – Medical Dictionary definitions 互聯網檔案館嘅歸檔，歸檔日期2014年1月23號，.". Medterms.com.
↑ Belikov, Aleksey V. (January 2019). "Age-related diseases as vicious cycles". Ageing Research Reviews. 49: 11–26.
↑ Bernstein, Harris; Bernstein, Carol; Michod, Richard E. (2011). "Meiosis as an Evolutionary Adaptation for DNA Repair Chapter 19". In Kruman editor, Inna. DNA Repair. InTech.
↑ Hörandl, Elvira (2013). "Meiosis and the Paradox of Sex in Nature". In Bernstein, Carol. Meiosis. InTech.
↑ Otto, SP; Gerstein, AC (August 2006). "Why have sex? The population genetics of sex and recombination". Biochemical Society Transactions. 34 (Pt 4): 519–22.
↑ Agrawal, AF (September 2006). "Evolution of sex: why do organisms shuffle their genotypes?". Current Biology. 16 (17): R696–704.
↑ Dobzhansky, T. (1973). "Nothing in biology makes sense except in the light of evolution". The American Biology Teacher. 35 (3): 125–29.
↑ Futuyma, D.J. (2005). Evolution. Sinauer Associates.
↑ De Duve, Christian (2002). Life Evolving: Molecules, Mind, and Meaning. New York: Oxford University Press. p. 44.
↑ Packard, Alpheus Spring (1901). Lamarck, the founder of Evolution: his life and work with translations of his writings on organic evolution. New York: Longmans, Green.
↑ Darwin, Charles (1859). On the Origin of Species, John Murray.
↑ "The Complete Works of Darwin Online – Biography". darwin-online.org.uk.
↑ Simpson, George Gaylord (1967). The Meaning of Evolution (Second ed.). Yale University Press.
↑ Carroll, Joseph, ed. (2003). On the origin of species by means of natural selection. Peterborough, Ontario: Broadview. p. 15. ISBN 978-1551113371. As Darwinian scholar Joseph Carroll of the University of Missouri–St. Louis puts it in his introduction to a modern reprint of Darwin's work: "The Origin of Species has special claims on our attention. It is one of the two or three most significant works of all time—one of those works that fundamentally and permanently alter our vision of the world ... It is argued with a singularly rigorous consistency but it is also eloquent, imaginatively evocative, and rhetorically compelling."

[15] 喺數學上， $\Pr(y)$ 係指「事件 $y$ 發生嘅機會率」，而 $a\subset b$ 簡單啲講係指「 $a$ 喺 $b$ 入面（子集）」。

[gene2007-1] 1.0 ^1.1 Marcial, Gene G. (August 13, 2007) From SemBiosys, A New Kind Of Insulin, Archived 2014-10-29 at the Wayback Machine.. businessweek.com

[2] Hartl, D. L., Clark, A. G., & Clark, A. G. (1997). Principles of population genetics (Vol. 116). Sunderland, MA: Sinauer associates.

[3] Van Der Maas, H. L., Dolan, C. V., Grasman, R. P., Wicherts, J. M., Huizenga, H. M., & Raijmakers, M. E. (2006). A dynamical model of general intelligence: the positive manifold of intelligence by mutualism. Psychological review, 113(4), 842.

[4] Thanbichler, M; Wang, SC; Shapiro, L (October 2005). "The bacterial nucleoid: a highly organized and dynamic structure". Journal of Cellular Biochemistry. 96 (3): 506–21

[5] "Genotype definition – Medical Dictionary definitions 互聯網檔案館嘅歸檔，歸檔日期2014年1月23號，.". Medterms.com.

[6] Belikov, Aleksey V. (January 2019). "Age-related diseases as vicious cycles". Ageing Research Reviews. 49: 11–26.

[7] Bernstein, Harris; Bernstein, Carol; Michod, Richard E. (2011). "Meiosis as an Evolutionary Adaptation for DNA Repair Chapter 19". In Kruman editor, Inna. DNA Repair. InTech.

[8] Hörandl, Elvira (2013). "Meiosis and the Paradox of Sex in Nature". In Bernstein, Carol. Meiosis. InTech.

[9] Otto, SP; Gerstein, AC (August 2006). "Why have sex? The population genetics of sex and recombination". Biochemical Society Transactions. 34 (Pt 4): 519–22.

[10] Agrawal, AF (September 2006). "Evolution of sex: why do organisms shuffle their genotypes?". Current Biology. 16 (17): R696–704.

[11] Dobzhansky, T. (1973). "Nothing in biology makes sense except in the light of evolution". The American Biology Teacher. 35 (3): 125–29.

[futu-12] Futuyma, D.J. (2005). Evolution. Sinauer Associates.

[13] De Duve, Christian (2002). Life Evolving: Molecules, Mind, and Meaning. New York: Oxford University Press. p. 44.

[14] Packard, Alpheus Spring (1901). Lamarck, the founder of Evolution: his life and work with translations of his writings on organic evolution. New York: Longmans, Green.

[16] Darwin, Charles (1859). On the Origin of Species, John Murray.

[17] "The Complete Works of Darwin Online – Biography". darwin-online.org.uk.

[18] Simpson, George Gaylord (1967). The Meaning of Evolution (Second ed.). Yale University Press.

[19] Carroll, Joseph, ed. (2003). On the origin of species by means of natural selection. Peterborough, Ontario: Broadview. p. 15. ISBN 978-1551113371. As Darwinian scholar Joseph Carroll of the University of Missouri–St. Louis puts it in his introduction to a modern reprint of Darwin's work: "The Origin of Species has special claims on our attention. It is one of the two or three most significant works of all time—one of those works that fundamentally and permanently alter our vision of the world ... It is argued with a singularly rigorous consistency but it is also eloquent, imaginatively evocative, and rhetorically compelling."

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[註 1]

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