Mendel, Genes, and Inheritance ( 멘델 , 유전자 그리고 유전 )

Chapter 12

털 색깔의 유전적 변이를 보이는 쥐

그림 12.1. 낫 모양 세포 빈혈증 환자의 적혈구

12.1 The Beginnings of Genetics: Mendel’s Garden Peas ( 완두콩 )

Mendel chose true-breeding ( 순계 ) garden peas for his experiments

Mendel first worked with single-character crosses ( 단인자교배 ; 단성교배 )

Mendel’s single-character crosses led him to propose the principle of segregation ( 분리의 법칙 )

Mendel could predict both classes and proportions of offspring ( 자손의 분류와 비율 )from his hypotheses

12.1 (cont.)

Mendel used a testcross ( 검정교배 ) to check the validity of his hypotheses

Mendel tested the independence ( 독립성 ) of different genes in crosses

Mendel’s research founded the field of genetics

Sutton’s chromosome theory ( 염색체설 ) of inheritance related Mendel’s genes to chromosomes

Blending Theory ( 혼합가설 ) of Inheritance

Popular belief until about 1900 • Hereditary traits blend evenly in offspring through

mixing of parents’ blood

Does not explain some observations:• Extremes do not gradually disappear• Offspring sometimes have different traits than

either parent

Gregor Mendel

Founder of genetics

Augustinian monk (1822-1884)

First to use scientific method to study inheritance

Pea Experiments

Garden pea (Pisum sativum)• Easy to grow• Clearly defined characters ( 특성 ) or traits (

형질 )• True-breeding ( 순계 ) varieties• Self-fertilized plants (same trait each generation)

• Easy to cross ( 교배 ; 교잡 )• Cross-pollination ( 타가수분 ) between parents

그림 12.3. 멘델의 실험에 사용된 완두콩 .

그림 12.4. 멘델의 7 가지 특성을 대상으로 한 교배 결과

Single-Character Crosses ( 단성교배 )

P generation (Parents)• Each pea produced contains an embryo

F1 generation (Filial)

• First generation

F2 generation

• Second generation

Flower Color Cross

P generation• Purple flowers crossed with white flowers

F1 generation

• All F1 seeds formed purple flowers

• Purple flower offspring crossed

F2 generation

• Purple flowers (75%)• White flowers reappeared (25%)

Mendel’s First Hypothesis

Genes for genetic characters occur in pairs • One gene inherited from each parent• Alleles are different versions of a gene

Diploid: two copies of each gene

Mendel’s Second Hypothesis

If two alleles of a gene are different, one allele is dominant over the other• Dominant allele is expressed• Recessive allele is masked

Recessive alleles only expressed when two copies of the allele present

Mendel’s Third Hypothesis

Two alleles of a gene segregate (separate) and enter gametes singly• Half the gametes carry one allele, half carry the

other allele (haploid)• Principle of Segregation

Two gametes fuse to produce a zygote that contains two alleles (diploid)

Terminology

Homozygous• Both alleles the same• PP (dominant)• pp (recessive)

Heterozygous• Two different alleles• Pp

그림 12.5. 완두콩의 꽃 색깔에서의 분리의 법칙

Terminology

Genotype ( 유전자형 )• Genetic constitution ( 유전적 구성 ) of an

organism• PP, Pp, pp

Phenotype ( 표현형 )• Outward appearance• Purple flowers, white flowers

Product Rule in Probability ( 확률의 곱셈법칙 )

Probability of two independent events occurring in succession ( 연속적으로 발생하는 두 개의 독립적 사건의 확률 )• Individual probabilities multiplied

Coin flip probabilities• Heads ( 앞면 ) = ½• Tails ( 뒷면 ) = ½• Two heads = ½ × ½ = ¼• Two tails = ½ × ½ = ¼

그림 12.6. 확률의 법칙들

Sum Rule in Probability ( 확률의 덧셈법칙 )

Probability of two different events producing the same outcome ( 같은 결과를 얻는 두 가지 다른 사건들의 확률 )• Individual probabilities added

Probability of a heads and a tails in two tosses:• First possibility: heads then tails • Heads = ½, Tails = ½ (½ × ½ = ¼)

• Second possibility: tails then heads • Tails = ½, Heads = ½ (½ × ½ = ¼)

• Total probability: ¼ + ¼ = ½

Probability in Mendel’s Crosses

Heterozygous cross (Pp × Pp)• Genotype probabilities• PP zygote = ½ × ½ = ¼• pp zygote = ½ × ½ = ¼• Pp zygote = ¼ + ¼ = ½

• Phenotype probabilities• Purple flowers = PP + Pp = ¼ + ½ = ¾• White flowers = pp = ¼

그림 12.7. 퍼넷 사각형으로 유전적 교배에서의 자손들과 그 비율을 예측 .

그림 12.8. 검정교배로 유전자형을 확인

Mendel’s Fourth Hypothesis

Alleles of genes that govern two different characters segregate independently during formation of gametes• Principle of Independent Assortment ( 독립의

법칙 )

Due to independent assortment ( 독립적 분리 ) during meiosis

Dihybrid Cross (1)

Pea shape Pea color• R = round Y = yellow• r = wrinkled y = green

P generation: RR YY × rr yy• RR YY parent produces R y gametes• rr yy parent produces r y gametes

F1 generation• All offspring Rr Yy genotype• All offspring round smooth phenotype

Dihybrid Cross (2)

Two heterozygotes crossed

P generation: Rr Yy × Rr Yy• Rr Yy parents produce 4 kinds of gametes• ¼ R Y, ¼ R y, ¼ r Y, ¼ r y

F1 generation• Offspring have four phenotypes• 9/16 = round yellow• 3/16 = wrinkled yellow• 3/16 = round green• 1/16 = wrinkled green

} 9:3:3:1 ratio

그림 12.9. 독립적 분리로 조합되는 종자 모양과 종자 색깔 형질

Dihybrid Testcross [ 양성 ( 이인자 ) 검정교배 ]

P Generation• Rr Yy × rr yy

F1 Generation

• ¼ = round yellow• ¼ = round green• ¼ = wrinkled yellow• ¼ = wrinkled green

} 1:1:1:1 ratio

Mendel’s Legacy ( 유산 )

Mendel’s results presented in 1866• Only known locally

Mendel died in 1884

Work was rediscovered in early 1900s

Mendel is considered the founder of genetics

Chromosome Theory of Inheritance

Walter Sutton (1903) noted similarities between inheritance of genes and behavior of chromosomes in meiosis and fertilization• Chromosomes occur in pairs in diploid organisms• Chromosomes of each pair are separated and

delivered singly to gametes• Independent assortment of chromosomes• One chromosome of each pair is derived from the

male parent; one from the female parent

그림 12.10. 염색체와 유전자의 행동양상은 서로 평행적 .

그림 12.11. 상동염색체의 특정 위치에 존재하는 염색체 좌(locus)

그림 12.12. 멘델의 유전법칙을 따르는 사람의 유전형질들 .

12.2 Later Modifications and Additions to Mendel’s Hypotheses ( 멘델의 가설에 대한 수정과 추가 )

In incomplete dominance ( 불완전 우성 ), dominant alleles do not completely mask recessive alleles

In codominance [ 공 ( 동 ) 우성 ], the effects of different alleles are equally detectable in heterozygotes

In epistasis ( 상위 ), genes interact, with the activity of one gene influencing the activity of another gene

12.2 (cont.)

In polygenic inheritance ( 다인자유전 ), a character is controlled by the common effects of several genes

In pleiotropy ( 다면발현 ), two or more characters are affected by a single gene

Incomplete Dominance ( 불완전 우성 )

Some or all alleles of gene are neither completely dominant nor recessive

Heterozygote phenotype• Different from either homozygote phenotype

그림 12.13. 금어초 (snapdragon) 꽃 색깔의 불완전 우성

Incomplete Dominance in Human Traits Sickle-cell disease• Homozygote recessive has sickle-cell disease• Heterozygote has milder sickle-cell trait

Familial hypercholesterolemia ( 가계성 과콜레스테롤증 )• Homozygote has severe form of disease• Heterozygote has mild form of disease

Tay-Sachs disease ( 테이 - 작스병 )• Homozygote has serious symptoms• Heterozygote has no symptoms but has

detectable biochemical effects

Codominance

Different alleles of gene have equal effects in heterozygotes• Both alleles expressed

Human M, MN, and N blood types• LMLM = M glycoprotein present; blood type M• LNLN = N glycoprotein present; blood type N• LMLN = both glycoproteins present; blood type MN

Similar inheritance to incomplete dominance

Multiple Alleles

More than three alleles for a gene• Found among all individuals in a population• Diploid individuals only have two of the alleles

Phenotype depends on relationship between different pairs of alleles• Still follows Mendel’s principles • 그림 12.14 ( 없음 ; 교과서 참조 )

Human ABO Blood Group

Antigens • Glycoproteins on surface of red blood cells• IA allele produces A antigen (dominant)• IB allele produces B antigen (dominant)• i allele produces neither A nor B (recessive)

Blood types (phenotypes)• IAIA or IAi = type A blood• IBIB or IBi = type B blood• ii = type O blood• IAIB = type AB blood

Human ABO Blood Group

Immune system produces antibodies against antigens not found on its own red blood cells

그림 12.15. 사람 ABO 혈액군의 혈액형 유전 .

Epistasis

Genes interact• Allele of one locus inhibits or masks effects of

allele at a different locus • Some expected phenotypes do not appear

among offspring

Labrador Retrievers ( 래브라도 리트리버 )

Melanin pigment gene• B allele: black fur color (dominant)• b allele: brown fur color (recessive)

Pigment deposition gene• E allele: pigment deposition normal (dominant)• e allele: pigment deposition blocked (recessive)

Phenotypes• Black fur: BB EE, BB Ee, Bb EE, Bb Ee• Brown fur: bb EE, bb Ee• Yellow fur: BB ee, Bb ee, bb ee

그림 12.16. 래브라도 털 색깔의 상위 유전

Polygenic Inheritance

Several genes at different loci interact to control the same character• Produces continuous variation

Phenotypic distribution: Bell-shaped curve

Often modified by environmental effects

그림 12.17. 다인자유전에 의한 키의 연속적인 변이 .

Pleiotropy

One gene affects more than one character

Sickle-cell disease• Recessive allele affects hemoglobin structure and

function• Leads to blood vessel ( 혈관 ) damage• Damages many tissues, organs, and functions• Many different symptoms result

그림 12.18. 낫세포 빈혈증의 다면발현 효과