蔵妬みLα眈飽

hered ity　　　　　拙eies

dominant trait genotype

recessive trait phenotype

gen?S Probabilfty

脇勿-‰解物◆ Expiain the experiments of

Grego「 Mendel,

◆Exp!ain how genes and a11eIes

a「e related to genotypei and

Phenotypes.

◆Use the information in a

Punnett squa「e.

!magine that you are plan-

ning to meet you「 pen pal

at the airpo巾but you have

neve「 met. How wouid γOu

desc「ibe yourself? WouId you

Say that you are taIl or short,

have curly hai「 or straight

hai鴨have b「own eyes o「

green eyes? Make a list・ Put a

Check mark next to t「aits you

think you inhe証ed。

56　　chapter 3

聞雷電甲ea§

丁here is no one else in the world exactly like you" 「ねu are

unique. But what sets you apart? If ysu look aromd youェdass-

room, yOu’11 see血at you share many physical characte血Stics

Wi血your classmates. For example, yOu all have skin in§tead

Of scales and a no慣Ceable lack of antermae. Y()u are a human

being very much like all of your fellow hunan beings・

Yet you are di紙質ent from everyone else in many ways. The

PeOPle you most resemble are your parents and your

bro血ers and sisters. But you probal)ly don’t look exactly like

them either. Read on to航nd out why this is so.

W霊ry Do珊,t「め出回ook離霞e a R離れoceros?

The answer to血is ques瞳On SeemS Simple: Neither of your par-

ents is a rhinoceros. Bu亡there′s more to this answer than meets

血e eye. As it tums out, hered雷同or the passing of traits from

ParentS tO Offspring, is a very complicated

Subject. For example, yOu might have curly

hair, While bo血of your parents have

Straight hair. 1bu might have blue eyes,

even though bo血of your parents have

brown eyes. How does this happen? People

have血vestigated t址s question for a long

time. About 150 years ago, SOme Very impor置

tant experiments were performed血at helped

SCientists begin to find some answers. The person who per-

formed these experiments was Gregor Mendel・

Who W尋s Gregor Mende量?

Gregor Mendel was bom in 1822 in

Heinzendorf, Austria. Grow血g up on

his family’s farm, Mendel leamed

a lot about cul債va慣ng組owers and

fruit trees. A紅er comple也ng his

Studies at a university, he entered

a monastery. He worked in血e

monastery garden, Where he was

able to use plants to study the way

traits are passed from parents to

O練sp血ng.鴫開聞㊧ fl shows an i皿s-

tra亀On Of Mendel in the monastery

garden.

醜聞総官　G着egor Mende書

町批aVe工主軸g the Mvsをe着y

From his experiences breeding plants’Mendel knew that some-

times the pattems of inheritance seemed simple and some-

times they did not・ Mendel wanted to紐Id out why.

Mendel was interested in the way traits are passed from

parents to offspring. For example′ SOmetimes a trait that

appeared in one generation did not show up in any of血e

O播spring in the next generation. In [he third generation’

though, the trait showed up again. Mendel noticed similar

pattems in people′ Plants′ and many other livirlg things.

Tb simplify his investigation, Mendel decided to study only

one kind of organism. He had already done s調dies us血g the

garden pea plant, SO he chose this as his subject了

醐⑮脚圃⑪髄軸脚線駒闘軌e㊥S? Garden peas were a good

choice for several reasons. These plants grow quick切they are

usually self-POllina鯖ng, and they come in many varieties. A

se陶OI協a練n押′an亡contains both male and female reproduc-

也Ve StruCtureS, like the flower in E璃聞胞2. Therefore, POllen

from one組ower or plant can fertilize the eggs of the same

flower or the eggs of another租ower on the same plant.覇翻聡5

illustrates the parts of a観ower and how fe虻田zation takes place

in plants・

軸翻印璽昌Du所g po脇0め旬

po侮n from ure an拓ers

毎aゆ応加onsfe〃edめ的eS的ma (femaゆ屠r細めn

OCCurs when a spe〃n from

肋e j)O施n的ve応through

励e s匂ma md enfers励e

eggわon o胸ね

離郷聡壇柵庵phoめg′叩h of

o伽wer shoM停船e mole and

ねmoね′印仰dud ve紺u観u′eS"

White

軸離聡& 77]eSe are SOme Ofthe

p佃nt ch伽acね証e庇sめdt MendeI

s露Id碇d

A plant that

P「Od uces

wrinkled seeds

Peas Be My巴od職能

Mendel chose to study only one characteristic′ SuCh as.plant

height or pea coIor/ at a time. That way/ he could understand

血e resu虹s. Mendel chose plants that had two forms for each

Of the characteristics he studied. For example, for血e charac-

teristic of plant height′ One form always produced tall plants′

and the other form always produced sho競Plants. Some of皿e

Characteristics inve也gated by Mendel are shown in n軸圃㊥軋

The two di飾erent traits of each characteristic are also shown.

晴間㊨嘲eed嘩g軸軸S Mendel was very careful to use plants

血at were true breeding for each of the仕aits he was s山dying.

When a打o化-bned暗〆ant self-POllinates, it will always pro-

duce offspring wi吐血e same trait the parent plant has. For

example, a tall t叩e・breeding plapt will always produce 6ff-

SPring that are ta虹

Mendel decided to丘nd out what would happen if he crossed

two plants that had di締erent foms of a single trait. Tb do

t址s, he used a method known as crossrpollin融on・ In cross-

pollination′血e an批ers of one plant are removed so血at血e

plant camot self-POllinate. Then pollen from another plant isused to fert組ize也e plant w弛.out anthers. This way, Mendel

could select which po11en would feI劇ize which plant. This

technique is illustrated in醜聞印璽5〇

A plant that

PrOduces

round seeds

醗鍵聡S Mendel removed幼e an拓ers from c,〆o′,t

幻旧t pmdrced round seeくね. 7Y’en he “sed po侮n from

a phnt t厄t pro血‘Ced面hMed seedsめfer施e the

p佃n揃的t produced round seeds.

58　　　Chapte「 3

Anthers are removed,

Me珊de19s離意sを電xpe出離ent

In his紐st experiment, Mendel performed

CrO§SeS tO S調dy seven di飾erent character-

is慣CS. Each of the crosses was between

the two traits of each characteristic. The

results of the cross between plants

血at produce round seeds and plants

that produce wrinkled　§eeds are

Shown in ng醐㊧ ㊧∴me offsp血ng

from this cross are known as the印髄

gg7qemtZon. Do the results surp工ise

you? What do you think happenedto the trait for wrinkled seeds?

Mendel got similar results for

each of the crosses that he made.

One trait always appeared, and

the other trait seemed to vanish.

Mendel chose to call the trait that

appeared the do鵬inant tすait“ The

other trait seemed to recede into

the background, SO Mendel called

this the recessive trait. Tb宜nd out

What might have happened to the reces・

Sive trait, Mendel decided to perform

another experimen亡・

Mende案is §econd

重賞p erime放電

Mendel allowed the紐§t generati壷でuom

each of the seven crosses to self-POllinate.

Thi§ is also i皿§trated in Figure 6. This也me

the plant with the dominant trait for seed’

Shape (Which is round) was allowed to self-

PO11inate. As you can see′ the recessive trait

for w血nkled seeds showed up again.

Mendel performed this same experiment

on the two traits of each of the seven char-

acteristics. No ma壮er which characteristic

Mendel inve§蹟gated, When the first gen-

eration was a11owed to §elf-POⅡinate, the

recessive trait reappeared.

Se`Ond generation

醜聞鵬@ A pねnt跡ot proくれces

M加柄ed seeds応far制度ed wi肋

po施n from a p佃nt肋at pro血/Ces

mund seeds.

Parent gene営ation

First gene冒atio調

round seeds

軸st generation

A seed grows into

a mature p!ant

that is aIIowed to

Self-POllinate.

hree round seeds

wrinkted see

且Di鱒ere調をPoint ofView

Mendel then did some批ng that no one else had done before‥

He decided to count the number of plants with each trait血at

調ned up in the second genera也On. He hoped that this might

help him explain his results. T狼e a look at Mendel′s actual

results, Shown in the table below

Cha ra cteri s章i`

馴ower color

Seed co置o細

Seed shape

70与・

PurPle

Recess ive

trait 京軸i 〇二

1 9《も≦∑2

Unde冒stamding Ra蹄os

A ratio is a wayto compare

two numbers by using divi-

sion. The ratio of plants with

purple flowe「s to plants with

white fIowers can be w「抽en

as 70与to 224 0r 70与:224,

丁his ratio can be reduced, Or

simplified′ bγ dividing the

first numbe「 by the second

as follows:

70う∴3.1う・=　三三　二二-臆・

之24　1

which is the same thing as a

「atio of 3.15:漢. For every three

lants with purp!e flowers,

there wiIl be rough!y one

Plant with white flowe「s・ rty

this proble皿

In a box of chocolates,

there are 18 nougat-軸ed

chocolates and 6 ca「ameト

制ed chocolates. What is

the 「atio of nouga珊!led

chocolates to caramel一軸ed

chocolates?

Pod colo細

Pod shape

書l ower

PO Si章ioれ

277

short

As you can see′ the recessive trait showed up again′ but

not as often as血e dominant債ait showed up. Mendel decided

to calculate the 7融o of dominant traits to receSSive traits foI

each characteris也C. Follow in Mendel′s footsteps by calculat-

ing the dominant-tO-reCeSSive ratio for each characteristic. (If

you need help′ Check ou亡the MathBreak at left) Can you find

a pattem among the ratios?

楽二言、

漆 幸三 ‾ごくミ音 ∴ :∴ ;∴ :∴∴ ∴∴. ∴∴∴ :∴ :∴ 、、黛 ∴ ∴‡

∴∴∴ 　ABri寒量iant竃dea

/、　　Mendelrealizedthathisresultscouldbeexplainedonlyif

eachplanthadtwosetsofinstructionsforeachcharacteris-

≒∴∴債C.Eachparentdonatesonesetofinstructions,nOWknown

ぎ　　asgenes,tOtheo挿spring.Thefertilizedeggwouldthenhave

twofomsofthesamegeneforeverycharacteristic-Onefrom　　　　　Atrue-b「eeding

eachparent.Thetwoformsofageneareknown.　　　　　　　　　　whi ����tef!o ��r ∴∵ 謀 �→ ∴ ∴∴ �′ �

asaIleles.

蒙　　軸㊧靴㊥㊨軸$軸醍醐醐㊤睨軸脚㊨Tb皿der-

ま　　StandMendel′scondusions′We′lluse-adiagram　　　　　　　　　　　```ふ …ニケ　CalledaPumettsquare.AP祝me妨Squ雛,is撃ed ミ¥tovisualizeallthepossiblecombinationsofalleles �� � �分ゝく1. ∴

器 � ��

aeeSOrae・reengPurPe-O　P 、∴∴areW批enasPPTheallelesforatrue-breeding 　White一組oweredplantarewri壮enasfや・Thecross � � ��:二 、∴ � � �∴ ∴ : � � �

十言霊「∴∵∴ �� �� �∴ ∴ � �

千言や∴:∴::∴ 　∴/姦 �� � � � � �

聴聞胎動isthenwr距enasPPX押.Thesquares　i!- containthea11elecombina也OnSthatcouldoccur　′・'¥t臆 ��:;∴繁子∴; �� �� � � �

∴∴ � �� �� �表 ¥∴ :∴ � � �

へ、intheo龍sp血ng.Theinheritedcombinationof 手∴∴∴∴allelesi§knownastheo締spr血g’sgenotype. �� ��十 十∴ ∵丁 子し � �ふ二義 ∴」∴ �∴ ∵ � �

Figure7showsthatalloftheoffspringwill　　　　　　p ��∴∴　　∴ �P∴ � � �∴∴ �∴∴　∴ �

avetesamegenotype:P.　e　Ommant　ee, く∴∴Pineachgenotypeensuresthatalloftheoff-∴∴鴫闘弼柵epossj勤ea焼comb庇蘭ns

/　　springwillbepurple血oweredplants.Anorgan∴　　血的eo施p所gカeMr#,居c,OSSa‘rea〃the

上　　i§m′sappearanceisknownasitsphenotype.The　　some:Pp・

recessiveallele,P,maybepassedontothenext

genera債On. 音二二六・

雪的油田両国間た漢珊冊貴書筋葛.a ��′.′′菓 ���� �� �ー　　謀　、

Drawasquare,anddivideitintofoursections"Next,Write � � � � �� ���∴÷ ∴∴ � ��

ま �thele龍e「sthatrepresentalIeIesfromoneparentalongthe　　, topofthebox,Writethele龍ersthatrepresentallelesfrom �� �� ���

the豊吉嵩霊霊慧嘉,lant,h。t.r。_　ff �� ��費

d　　I　　d　d」剛も　d　Ittht　d　　l �� �� uCeSOn　rOun　See　S′　　　an　a　an　　a　rO　uCeSOn � � �� ��

wrinkIedseeds,〃=Followthearrowstoseehowtheinside ��

Oftheboxwasfi!led.Theresu!tlngalle!eslnSldethebox �� �� �

霊諜豊嵩罵嵩霊ぷ薯謹S　節 � �� � � �

:′へ � ��

: ∵ ∵ �He「ed

A selトpo描nating

Pu「Pie flower

“ナ雑報 ー髭、一類雲 �∴ ∴ ∵ � � � � � � �さヌ: ; � �

∴ � � �

後 �� � � �� � � �∴

′i �敬 �� �∴ �∴ �∵: ∴ �

婆 �漉 �� ��一獲 �※

; � ��∴′ゝ �� � ��/臆臆,ふ¥　二 � �萎嘩 �∴ �

護 � � ��∴ � ��　王 子十∴† � �∵: � �

① �伊 ����牟 � � ��膵 (○ ○ �� � � �.登嘉:/ �� � � �∴ ∴ ∴ �

“石 ∈ 《 � ��∴‾¥臆‾ノミ �∴ ÷ � �� � �∴

∴ �露露透 �� � � �

磨 �� �轟 � ��臆‾二　音シ臆 �∴ 、箪 �∴∴ 霧 �∵ �

軸翻曜8相応Pume櫓Square訪O鵬的e

POSS億,厄resu応舟om拓e c仰SS Pp x Pp置

嶋ke Ybur Chances

Ybu have two guinea pigS

you would like to b「eed・ Each

has brown fur and the geno-

type助L What are the

Chances that their offsp「ing

wil! have white fしIr With the

genotype bかタlty this to find

a piece of masking

Oth sides of同VO

しabel one side of

each quarter with a capital B

and the other side with a

!owercase b. 1bss both coins

5O times, making note of

you「 resuIts each time" How

many times did you get the

心b combination? What is the

PrObabiIfty that the nex=osswilI result in bbタ

62　　Chapter 3

離⑭眼臣諒de鵬e In Mendel′s second experiment,

he allowed the紐st-generation plants to self-

POllinate.晦闘e 8 shows a self-POllination cross

Of a紐st-genera也On Plant with the genotype申タ・

The parental alleles血the cross indicate血at the

egg and.spem can contain either a P a11ele or a

p allele.

What might the genotypes of the offspring be?

No宜Ce that one §quare Shows the坤combina-

tion, While ano血er §how§ the pP combination.

These are exactly the same genotype’eVen though

the letters are written in a di鮎erent order. The

other possible genotypes in the offspring are PP

and脚. The combina也OnS PP坤, and pP have

the same phenotype-Puaple組owers-because

they each contain at least one dominant a11ele

(P). Only one combination, pP, Produces plantswith white組owers. The ratio of dominant to reces・

sive is 3:1, just as Mende工calculated from his data.

Wha宣且着e the Cha輪CeS?

It′s important to understand that o鮎sp血g are equally likely

to inherit ei皿er allele紅om ei血er parent. Think of a coin toss.

There′s a 5O percent chance youIll get heads and a 50 percent

chance youIll get tail§・ Like血e toss of a coin′ the chance of

inheriting one a11ele or ano血er is completely random.

軸o脚的The mathematical chance that an event will o註r

is known as probab盲l叫Probabil担y is usually expressed as a

諾t器言霊慧器霊宝霊誓慧you toss a coin′ yOu Will get tails. Tb express probability as a

percentage’divide瓜e numerator of the fraction by the deno皿i-

natoL and then mul也Ply血e answer by lOO.

去× 100=50%

耽圧ind血e probわility血at you will toss two heads in a roW

multiply the probability of the two events.

去×去=壬

The percentage would be l÷4 X lOO′ Which equals

25 percent・

∈通軸鞄隠避銃随

A cu「Iy eared cat, like the one

at right, mated w軸a ca=hat

had no「ma! ea「s, !f haIf the

kittens had the genotype ct

and curiy ea「s, and the other

haIf had the genotype耽and

normal ea「s′ Which was the a!!ei

for curly ears?

What was the genotype

Of each parent? (Hinを; Use a

Punn餌Square tO桐‖n the

genotypes of the o簡spring, and

then work backwa「d.)

㊨鋼⑱ft幅e EL⑱醍醐画The same me血od is used to caloulate

the probability that an offspring will inherit a certain geno-

type. For a pea plant to inherit the white組oweI trait, it must

receive a p allele from each parent. There is a 50 percent

Chance of inher弛ng either allele from either parent. So血e

写管霊等欝芸荒業S血a坤×印章OSS is

卸eg⑪骨髄軸e且調G㊥珊㊧軸忠N⑬昨㊥『離醜m Good ideas are

Often overlooked or misunderstood when they紐st appear. This

WaS血e fate of Gregor MendelIs ideas.血1865I he published

his findings for血e scien触c commu血ty. Unfortunately his

WOrk didn′t get much a請en髄on. It wa§n′t until a龍er his death,

more than 30 yeaェs lateL that Mendel鰯nally got the recog血一

也On he deserved. Once Mendel′s ideas were rediscovered and

understood′血e door was opened to modem genetics.

1〃 The allele for a deft chin′ C/ is dominant among humans.

What would be the results from a cross between a woman

wi也the genotype Cc and a man wi也the genotype cc?

Create a Pume壮Square Showing this cross'

2〃 Of the possible combinations you found in

question l′ What is the ratio of oftspring wi血

a de節chin to offspring without a cleft chin?

3“ Applying Concepts The Pume壮Square at血ght

Shows the possible combina也ons of alleles for

fur color in rabbits. Black f叫, P, is dominant

OVer White　叫　b. Given the combinations

Shown′ What are the genotypes of the parents?