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Chapter 14

Chapter 14. Mendel and the Gene Idea. Figure 14.1. What principles of inheritance did Gregor Mendel discover by breeding garden pea plants?. Collection of Genes in an organism. Deck of Cards. VS. Paint. Blending hypothesis (Paint). 수 세대를 건너 재등장하는 형질은 어떻게 설명하나 ?. VS.

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Chapter 14

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  1. Chapter 14 Mendel and the Gene Idea

  2. Figure 14.1 What principles of inheritance did Gregor Mendel discover by breeding garden pea plants?

  3. Collection of Genes in an organism Deck of Cards VS Paint

  4. Blending hypothesis (Paint) 수 세대를 건너 재등장하는 형질은 어떻게 설명하나? VS Particulate inheritance model (Deck of Cards = Gene idea) 유전자는 희석되지 않는 상태로 섞여서 세대를 통해 전달될 수 있다

  5. Augustinian monk Expert plant breeder: majored in physics and plant physiology Discovered the basic principles of genetics in 1866 (just seven years after Darwin’s On the Origin of Species) Gregor Mendel (1822-1884) Fig. 2.2

  6. Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance • Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments 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 can be controlled • Each flower has sperm-producing organs (stamens:수술) and egg-producing organ (carpel:암술) • Cross-pollination (fertilization between different plants) involves dusting one plant with pollen from another

  7. Fig. 14-2 Crossing pea plants TECHNIQUE 1 • Removed stamens from purple flower • Transferred sperm-bearing pollen from stamens of white flower to egg-bearing carpel of purple flower • Pollinated carpel matured into pod • Planted seeds from pod • Examined offspring: all purple flowers in first filial generation offspring (F1) 2 Parental generation (P) Stamens Carpel 3 4 RESULTS First filial gener- ation offspring (F1) 5

  8. Key Components of Mendel’s Mission(1) • Mendel chose to track only those characters that occurred in two distinct alternative forms • He also used varieties that were true-breeding (순종교배plants that produce offspring of the same variety when they self-pollinate) • In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization • The true-breeding parents are the P generation • The hybrid offspring of the P generation are called the F1 generation • When F1 individuals self-pollinate or cross- pollinate with other F1 hybrids, the F2 generation is produced

  9. (1) The Law of Segregation Q: When F1 hybrid pea plants are allowed to self-pollinate, which traits appear in the F2 generation? • When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple • When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white • Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation

  10. Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids • Mendel called the purple flower color a dominant trait and the white flower color a recessive trait • The factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation • Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits • What Mendel called a “heritable factor” is what we now call a gene

  11. Mendel’s Model • Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring • Four related concepts make up this model • These concepts can be related to what we now know about genes and chromosomes • (1) Alternative versions of genes account for variations in inherited characters • For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers • These alternative versions of a gene are now called alleles • Each gene resides at a specific locus on a specific chromosome

  12. The second concept is that for each character an organism inherits two alleles, one from each parent • Mendel made this deduction without knowing about the role of chromosomes • The third concept is that if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance • In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant • The fourth concept (now known as the law of segregation) states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes • Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism • This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis

  13. Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses • The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup • A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele

  14. Useful Genetic Vocabulary • An organism with two identical alleles for a character is said to be homozygousfor the gene controlling that character • An organism that has two different alleles for a gene is said to be heterozygousfor the gene controlling that character • Unlike homozygotes, heterozygotes are not true-breeding 표현형(phenotype) 과 유전자형 (genotype) • Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition • Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup (= composition) • In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes (seenextslide)

  15. The Testcross의 활용도 • 우성인 표현형을 보고 유전자형을 어떻게 예측할 수 있을까? • 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 (검정교배): breeding the mystery individual with a homozygous recessive individual • If any offspring display the recessive phenotype, the mystery parent must be heterozygous

  16. (2) The Law of Independent Assortment • Mendel derived the law of segregation by following a single character • The F1 offspring produced in this cross were monohybrids (단성잡종), individuals that are heterozygous for one character • A cross between such heterozygotes is called a monohybrid cross • Mendel identified his second law of inheritance by following two characters at the same time • Crossing two true-breeding parents differing in two characters produces dihybridsin the F1 generation, heterozygous for both characters • A dihybrid cross, a cross between F1dihybrids, can determine whether two characters are transmitted to offspring as a package or independently

  17. Q: How are two characters transmitted from parents to offspring? • As a package? • Independently? Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character?

  18. Fig. 14-8 EXPERIMENT YYRR yyrr P Generation Gametes yr YR  F1 Generation YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Predictions Sperm or Predicted offspring of F2 generation 1/4 1/4 1/4 yr 1/4 YR yR Yr Sperm YR yr 1/2 1/2 1/4 YR YYRr YYRR YyRR YyRr 1/2 YR YyRr YYRR 1/4 Yr Eggs YYRr YYrr Yyrr YyRr Eggs 1/2 yr YyRr yyrr 1/4 yR YyRR YyRr yyRR yyRr 3/4 1/4 1/4 yr Phenotypic ratio 3:1 Yyrr yyRr YyRr yyrr 3/16 1/16 9/16 3/16 Phenotypic ratio 9:3:3:1 RESULTS Phenotypic ratio approximately 9:3:3:1 315 108 101 32

  19. Proposed biological mechanism of the shuffling: The Law of Independent Assortment • Each pair of alleles assorts independently of gamete formation • The allele for pea shape in a Y-carrying gamete can be either R or r with equal chance and vice versa for a gamete with y • Gametes in F1: YR:Yr: yR:yr = 1:1:1:1 • Possible zygotes in F2: 16 • Genotypes: 9 • Phenotypes: 4 Y y R r Fig. 2.16 Ref: Genetics edited by Hartwell et al.

  20. Pollens         Eggs         Dihybrid cross shows parental and recombinant types • Mating of yellow round & green wrinkled • F1 dihybrid: all dominant phenotypes of yellow round • F1 self-fertilization: production of F2 progeny with parental and recombinant types • 1=2=2=4 (9): Y–R– 1=2 (3): Y–rr 1=2 (3): yyR– 1 (1): yyrr Fig. 2.15 top

  21. Using a dihybrid cross, Mendel developed the law of independent assortment • The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation • Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes • Genes located near each other on the same chromosome tend to be inherited together

  22. 확률의 법칙이 지배하는 멘델법칙!!! • Concept 14.2: The laws of probability govern Mendelian inheritance • Mendel’s laws of segregation and independent assortment reflect the rules of probability • When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss • In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles

  23. The Multiplication and Addition Rules Applied to Monohybrid Crosses

  24. Solving Complex Genetics Problems • We can apply the multiplication and addition rules to predict the outcome of crosses involving multiple characters • A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously • In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied

  25. 유전현상은 멘델법칙의 예측보다 훨씬 복잡하다 • Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics • The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied • Many heritable characters are not determined by only one gene with two alleles • However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance

  26. Extending Mendelian Genetics for a Single Gene • Inheritance of characters by a single gene may deviate (=escape) from simple Mendelian patterns in the following situations: • When alleles are not completely dominant or recessive (Degrees of dominance) • When a gene has more than two alleles (Multiple alleles) • When a gene produces multiple phenotypes (Pleiotropy) 우성의 정도에 따른 경우 (Degrees of Dominance) • Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical • In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties • In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways

  27. The Spectrum of Dominance Summary of dominance relationships

  28. Figure 14.10 Incomplete dominance in snapdragon color (금어초). P Generation White Red CWCW CRCR 금어초/용머리 모양꽃/ Confession Gametes CW CR F1 Generation Pink CRCW 1/2 1/2 CR CW Gametes Sperm F2 Generation 1/2 1/2 CW CR  Unlike the Mendel’s monohybrid, the phenotypic and genotypic ratios are identical 1/2 CR CRCR CRCW Eggs 1/2 CW CRCW CWCW

  29. In codominance • Two dominant alleles affect the phenotype in separate, distinguishable ways • The human blood group MN (another example: blood type AB) • Is an example of codominance • M or N homozygote have red blood cells with only M or N molecules, respectively • MN heterozygote have RBC cells with both M and N molecules (not intermediate between the M and N phenotypes)

  30. The Relation Between Dominance and Phenotype • 대립유전자의 Dominance는 우성대립유전자가 열성대립유전자를 정복하는 방식이 아니라 표현형에서 우성임이 확인되기 때문이다 (A dominant allele does not subdue a recessive allele; alleles don’t interact that way) • Alleles are simply variations in a gene’s nucleotide sequence • For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype 우성와 표현형간의 관계는 관찰대상인 수준에 따라 달라진다 • Tay-Sachsdisease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain • At the organismal level, the allele is recessive (recessive homozygote만 질병증상이 나타남) • At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant (heterozygote의경우 효소활성이 중간 정도임) nosymptom (절반의 효소활성만으로 충분한 기능 발휘) • At the molecular level, the alleles are codominant (heterozygote의 경우 정상효소와 비정상효소분자의 개수는 동일함)

  31. Multiple Alleles • Most genes exist in populations in more than two allelic forms • For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i (seeNext slide) • The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither

  32. Pleiotropy • Most genes have multiple phenotypic effects, a property called pleiotropy • For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease 낫형 빈혈증의 다양한 증상 Multiple defects in the homozygous SSindividual : muscle cramps (근육경련), short breath, fatigue, resistance to Malaria

  33. Extending Mendelian Genetics for Two or More Genes • Some traits may be determined by two or more genes Epistasis • In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus (= One gene’s alleles mask [hide, completely interfere with] the effects of another gene’s alleles) • For example, in mice and many other mammals, coat color depends on two genes • One gene determines the pigment color (with alleles B for black and b for brown) • The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair

  34. Coat color in Labrador retrievers • Coat color depends on the allelic combinations of two coat color genes • The dominant B allele of the 1st gene determines black • The recessive bb homozygote is brown • The dominant E allele of the 2nd gene has no effect on black or brown coat color • The recessive homozygotic alleles (ee) hide the effect of any combinations of the 1st gene alleles to produce golden color • Recessive epistasis : the allele causing the epistasis is recessive Same Mom? Baby adopted? Like or Unlike?

  35. Figure 14.12 An example of Epistasis BbEe BbEe Sperm 1/4 1/4 1/4 1/4 Be BE be bE Eggs 1/4 BE BbEE BBEe BbEe BBEE 1/4 bE BbEE bbEe bbEE BbEe 1/4 Be BBEe BBee Bbee BbEe 1/4 be BbEe bbEe bbee Bbee : 3 9 : 4

  36. Polygenic Inheritance (다원 유전자 유전) 멘델연구의 특징은 either-or basis 임. 그러나 많은 수의 형질 (인간의 키나 피부색 등)은 집단내에서 점진적인 변이를 보이므로 either-or 특징으로분류할 수 없는 정성적인 캐릭터임. • Quantitative characters are those that vary in the population along a continuum (in gradations) • Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype • Skin color in humans is an example of polygenic inheritance (seeNext slide)

  37. Nature and Nurture (천성과 교육): The Environmental Impact on Phenotype • Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype • The norm of reaction (표현형 표준범위) is the phenotypic range of a genotype influenced by the environment • For example, hydrangea flowers (수국) of the same genotype range from blue-violet to pink, depending on soil acidity (seenext slide) • Norms of reaction are generally broadest for polygenic characters • Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype

  38. Concept 14.4: Many human traits follow Mendelian patterns of inheritance • Humans are not good subjects for genetic research • Generation time is too long • Parents produce relatively few offspring • Breeding experiments are unacceptable • However, basic Mendelian genetics endures as the foundation of human genetics

  39. 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 • Pedigrees can also be used to make predictions about future offspring • We can use the multiplication and addition rules to predict the probability of specific phenotypes

  40. Recessively Inherited Disorders • Many genetic disorders are inherited in a recessive manner • These range from relatively mild to life-threatening The Behavior of Recessive Alleles • Recessively inherited disorders show up only in individuals homozygous for the allele • Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal; most individuals with recessive disorders are born to carrier parents • Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair

  41. Cystic Fibrosis (낭포성섬유증) • Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent • The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes • Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine Sickle-Cell Disease: A Genetic Disorder with Evolutionary Implications • Sickle-cell disease affects one out of 400 African-Americans • The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells • In homozygous individuals, all hemoglobin is abnormal (sickle-cell) • Symptoms include physical weakness, pain, organ damage, and even paralysis • Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms • About one out of ten African Americans has sickle cell trait, an unusually high frequency of an allele with detrimental effects in homozygotes • Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous

  42. Dominantly Inherited Disorders • Some human disorders are caused by dominant alleles • Dominant alleles that cause a lethal disease are rare and arise by mutation • Achondroplasia is a form of dwarfism caused by a rare dominant allele

  43. Huntington’s Disease: A Late-Onset Lethal Disease • The timing of onset of a disease significantly affects its inheritance • Huntington’s disease is a degenerative disease of the nervous system • The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age • Once the deterioration of the nervous system begins the condition is irreversible and fatal

  44. Multifactorial Disorders • Many diseases, such as heart disease and cancer, have both genetic and environmental components • Little is understood about the genetic contribution to most multifactorial diseases

  45. Genetic Testing and Counseling • Genetic counselors • Can provide information to prospective parents concerned about a family history for a specific disease • Using family histories • Genetic counselors help couples determine the odds that their children will have genetic disorders Fetal Testing • In amniocentesis, the liquid that bathes the fetus is removed and tested • In chorionic villus sampling (CVS), a sample of the placenta is removed and tested • Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero

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