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Chapter 16

BIOL 243 Chapter Notes - Chapter 16: Zygote, Chlorophyll, Chromosome

Course Code
BIOL 243
Gordon Chua

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Chap te r 16 Note s
- Section 16.2: The Founda tions of Mode rn Transmis s ion Gene tics
- Mendel’s e xperimenta l a pproach wa s s imila r to tha t of a fe w botanis ts of the eighteenth
and nine te e nth centurie s who s tudied the res ults of hybridiza tion, or interbre e ding
be tween two diffe re nt va rietie s or s pecie s of a n organism
- Where Me ndel differe d from his pre dece ssors was in pa ying close atte ntion to a sma ll
numbe r of e as ily classified traits with contra sting cha ra cteris tics. For e xample , where
one s tra in of pe a s had ye llow s e e ds, a nothe r ha d gre en s e e ds; and where one ha d
round se e ds , a nother had wrinkle d s e e ds
- Altoge ther, Me nde l s tudie d s e ve n physica l fe a ture s expres s e d in contra s ting fa s hion
among the stra ins: see d colour, s e ed s hape, pod colour, pod s ha pe, flower colour,
flower position, a nd plant he ight
- The e xpre s s ion of e a ch tra it in e ach of the original s tra ins that Me ndel obtained was true
bre e ding, which me ans tha t the physical a ppe a ra nce of the offs pring in e ach s ucce ssive
ge ne ra tion is ide ntical to the pre vious one
- The obje ctive of Mendel’s e xpe rime nts : by mea ns of cross e s be tween the true-bre e ding
stra ins a nd cros s e s a mong the ir proge ny, Me ndel hoped to de te rmine whe ther there are
sta tistica l patterns in the occurre nce of the contras ting chara cteris tics , s uch a s yellow
se e ds or gre e n s e e ds. If such patte rns could be found, he would s e e k to de vis e a
hypothe s is to e xplain them a nd the n us e his hypothesis to product the outcome of
further cros s e s
- In de s igning his e xperime nts , Me nde l de viated from othe r pla nt hybridize rs of the time in
thre e importa nt wa ys:
- Mende l studied true -breeding stra ins, unlike ma ny othe r pla nt hybridizers , who us ed
comple x a nd poorly define d mate rial
- Mende l focus s e d on one trait, or a sma ll number of traits , a t a time , with
cha ra cteris tics tha t we re e a s ily contra s te d among the true-bree ding strains . Othe r
pla nt hybridize rs crossed s tra ins diffe ring in ma ny tra its a nd trie d to follow a ll the tra its
at once. This res ulte d in a ma zingly comple x inheritance , a nd no underlying pa tte rns
could be dis ce rne d
- Mende l counte d the proge ny of his cros s e s , looking for s ta tistica l pa tte rns in the
offs pring of his cros s es. Others typica lly note d only whe ther offs pring with a particula r
cha ra cteris tic were pre sent or a bsent, but did not ke e p tra ck of a n count a ll the
proge ny of a particula r cros s
- Pea flowers include both male a nd fema le re productive organs e nclos e d toge the r within
pe ta ls. Be caus e of this a rra ngement, pea pla nts us ua lly fe rtilize the mse lve s (s elf-
- For each of the s even pairs of contra s ting tra its, true -bre e ding stra ins differing in the tra it
were cros se d. A typica l re s ult in the ca s e of a cros s be twee n a pla nt producing ye llow
se e ds and a pla nt producing gre e n s e e ds. In thes e kinds of cros s es, the pa re nta l
ge ne ra tion is refe rre d to a s the P 1 ge neration, a nd the first offs pring, or filial, gene ra tion
is referred to a s the F1 genera tion
- Cros s e s in which the e xpre s s ion of the tra it in the fe male and ma le pa re nts are
inte rcha nged, a re known a s re ciproca l cros s e s . Me nde l s howe d that re ciproca l cros ses
yie lde d the s a me re sult for e ach of his s e ve n pairs of contrasting traits
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