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?