20 Basic Genetics Notes

2 0 .1 Genetic Terms --liliC:il!l!!::..-::=--- -- !l!!!!!!l!!C!!:---.1!~~-=====-~ A . Alleles C' They are different.forms of a gene which occupy th e sa me poc;, it io n on hrm /(Jl ogou, chromosomes. Example: eye colour gene - _/ _ -~~ -l L--:~ ' brown eye _l_} t-----i : allele ' i----, other genes B. Phenotype C, IL refers to t he ap pearance of a tr ait . C. Genotype blue eye allele { It refe rs to the Kenef ic make-up or an organism. 93

"' t ,; and c xe1 ( :iSC 'J N :' ,S 8H"i< 0\ - /1·tG /iS I\ { No e, ; Rn 94 ul/c /1' .,. E. H ete ro~.ygote I . . 1 · 1 c ul ar ch aracteri s tic co nlai n 1;, two dif{ert I ,, 11 ot\'\ JCO a pa1. c ·· 111 lt 1 :-- , \\1 l1 r ~a ni~rn ,, l()\C ~ l - a l/ rlc s. F. Hy brid . . .. 1 1 . d I by crossi na two in di v idua ls w hich are ge ne ti calh \t ,, an 1nd1, 1c ua p1 o u cec . -· c • diffe rent in o ne or more ch arac te ri s ti cs. G. Do m;nant allele 11 ,s the a ll ele whi ch c an ex p ress it se lf in h etero::ygo 11 s co ndit io n. H. Dominant character It is the phenotype w hi ch can s ho w up in hete ro:,ygo us co nd iti on; co nt ro ll ed by t he do mjnant a llel e. I. Recessive allele It is the alle le w hic h effect is mask ed in het ero::,ygo us co ndition, i. e. , it c an o nl y ex pr ess it se lf in honw :: ,ygo us co nditi on. J. Recessive chara ct er C It is th e phen oty pe w hi ch can only s how up in hom crvoo us co i 1 ct ·t· II ! ~- o 1 10 11 : co n tro ec by th e r ec e ss i ve allel e. K. Monohybrid inheritance , It refe r, to the s tud y or lh e i nheritan ce nf a single pair of contrast in g character s. L. Dihybrid inher i tance C The inh er itance nf two pai r ::, or contra-.tin g char;1c 1cr s.

---~="'"""---: Jn ~-:,~n ,v Not, ,nil r~ xucr,es • , . 11 ..,cll in the pr e1.ic n cc of the bro \':n al lele . ( '1 11<' h l1 w :d kk c ,1111wt , xpll '· ' " "'Q i< 96 _ 1 1 , 1 .. 1 · 1 17 ,rcl or c 1h c r eces~ i vc all ele. The blue eye cr,i, 'd h I I h, 1 11 ' , II II ', , c , , <111 " c . , ,u r " tf · . h· t , . 11 ,1 tl' l" occ ur s onlv in homo z yg ou , cond1t 1on. '' n·c e"i\ t \' l' p t• no ~ 'p t: ,1 ~ , - U. Men del's firs t I.o w - The Law of Segregation C ~cndcl \ hn . ·c d tn g c:<. pcrimc nt '-t on ga rd en pe a: 0 '1 r: <11 , tuJi cd : !lo we r co lo ur !two contrasting characters : r ed and v. hi teJ 0 Pur c- hr e~ d in g rcd- ll owcrcd p ea pl anb we re cr os s-po lli nat ed w ith pure-br eedin ,; wh it c- fl o wc rcd pea pl a nt s, ' a· R e Su lts of the breeding experiment: Pa rents: Red X ( pur e- br eeding) l c ros s-pollination all red (hybrid) se lf - pol I ination Red Phenotypic ratio: 3 b. Explanation using genetic d iagram : Defining symbols: White ( pure-breeding) White Let 'R' re pre se nt the dominant allele f d II I f or re flower and ' ' a e e or white flower r represent th • · e recessive Parents: Red (homozy gote) RR t Ga mete s: R F . ) • X Rr (hete ro zyg ote) a ll red White (homozygote) rr t r

Basic Genet F,: Rr X Rr / ~ / ~ G: l{ r I< r ~r--::J F2: RR Rr Rr rr Genotypic ratio: 2 Phen otypic ratio: red 3 ~ This ratio is known as monohybrid ratio. Significance of the ratio: wh it e 0 It indicates the proportion (or %) of offspring having a part ic ul ar phenotype e.g. 3/4 or 75% of the F 2 plants will produce red flower. 0 It indicates the probability of appearance of a particular phenotype in an offspring. e.g. the chance for an F 2 plant to produce red flower is 75%. C) Mendel's first law of inheritance (in modern terms) - for a pair of alleles. o nl y ONE can be passed to the offspring through gamete . C) The Mendel's law can be applied as follows: If the two phenotypes of the offspring from a certain cross are in the 3 : l ratio , according to Mendel's law of inheritance, bo th parents must be heteroz) 1 gous. 97

10 - --- □ 2 6, Key: D mak tonguc-rullcr D rnalc non- ro l le r Q female tongue- ro ll er Q female non-ro ll er ~~\ - - ~ ( , C:- 1 Informa ti on obta in ed from th e above pedigr ee: C:, Tongue-rolling is domina nt . C:, Both individuals 1 and 2 are hete ro zygous. F. Solving genetic proble,ns involving monohybrid inheritance a. The phenotypes and I or genotypes of the parents are gi ven, C:, deduce the phenotypes and / or genoty pe s of th e offsp ri ng and th e ir pheno l~ pie ratio and / or genotypic ratio; C:, predict the proportion of off spring th at will have a c ena in phen ol~ pe genotype; C:, determine the probability that an offspring will have a ce rt ain phenotype genotype. Example: A white-flowered pea pl a nt is se lf-c ro ssed. W ith th e help of a ge ne tic di~1gr~m. predict th e p ro portion of th e F 1 pl a nt s w hi ch w ill p rod u ce red fl cn wr ~. (Gi ve n : Red is domina nt over white.) Solution: Le t , R · rep re se nt the domina nt al le le for red fl ower and ·r· represl'n l the re cessive a ll ele for wh ite flo wer. 99 -i:::!fd

d E e1c1se s I nf :lf1SIV(I NSS B1< 1 lt 1 Q) - t;:: N otes an -x White IT it 100 P: Whit e rr t r G: rr l <' • \ ' a ll w hit e . . · oducino r ed fl ower s === 0 The μro p or tt on of F, pl an ts p1 c .d d deduce the phenotypes and I (Jr . es are provt e ' b. Tire result of certam cr oss ge notypes o.f the parents. CASE 1 h t 0 ·ven that both parents show the dominant Deduce the oe notypes of t e paren s 0 1 0 ,ffi • h the rec essive character. character and o ne / some of the OJJ spring s ow Example : ln mice, black fur is dominant over brown fur. Two black mice were mated gi\ ing 3 brown and 1 black mice. Assuming that the fur colour is con tr olled by a single pair of alleles, deduce th e genotypes of the par ents . Deduction: The brown offspring must be homo zy gous recessive. Therefore, th ey must haw received one recessive allele from each parent. Since both of th e parents should have a dominant allele for being black, their genotypes mu st be hetero z.ygo us. CASE2 Deduce the ge notypes (and phenotype s) of the parents o- i ven tt 1 at tl 1 t . h ::o c e wo p enotypes in the offspring are in the ratio of 3: 1. Exa mple: A mai ze plant wa s se lf-pollinated and cobs bearing both pur\Jle a 11 c1 ,, II ~ Jc ow corn grains were produce d. The number of purpl e grnins and ye llo w grains 0 11 one of the c ob~ were 53 and 18 re sp ec ti ve l y.

es an d E xe rc ises NSS Biology- lnt e nstve Nol , 10~ Kram pl e I: _ . " . . . . II · I by a pair ol allel e~ . Jn n b1 ced 1n g ex pe rim ent • 1 _ r n 1 ._ , s co 111 , o cl . 1 , •. f ur L'U 1.) Llt 11 " · · 1 . . , 1 , g ave birth to 1hr ee gr ey and fi ve bJ, 1 k .,, I · 11 1d 11l c c 1nc1 L , c r· 1lf ('\ J':l l'i wer e 1n atc 1. ' , ') r, I . I ell \ ::- . • . , • 1 ,- ev 0 ,- h/ <1r ·k ('(J/0111 . c XJJ mn _ \O llr an swer H · 1, ; (11 i., rhc· r<'<' c ' SS/\ '< ' ('/1(/1 u, f ti · -~ · lkduction: Rlac A 1:-- n:c~ :-- s i\' c h ec ausL ': )h ,th parents h ave gre y .fur. th ere fore e ach of th em mu 5t po ssess at i rnl/ on e gre y allele. S om1.: of the ir o ff s pri ng ha ve black.fur sh ow in g that th ey mu st ha ve recei ved w lc1as1 one a ll ele for bl ack f ur fr om e ith er of th e parent s. At le a st one of the pa ren ts is hetero zygous. in hete r ozygo us co nditi on, th e rece ss ive all ele is m as k ed. Therefo re the black allele is recessive . Example 2: ln man. the ab ilit y to roll tongue is determined by a pair of alleles. D ma le t ong ue- ro ll er D ma le non - ro ll er Q fema le tongue - ro ll er Q female non -ro ll er Wi th rc }N ef/('e I ti 1 0 U' 0 Jn 1 1 e /'Jcdi[lrec .. ,,1 11 ·(·/ • , 1 · '" • • I I,\ f I l' r/ > · or tnr,hility to ro ll 10 11 > .,,.., .- · • < 111111r1111 clwracr e. . . . .~ lf t . L.rp l mn yo 11 r c 111 s 11 ·er . , • the ahtl 1( r

Basic Genetics Deduction: The abi!i(y to roll 1011 ' " ) . . I . ,I< ( ' " l 0 11111 1. 111[ h L'l' ] ll '-;t': Both p,ircnts ; 1rc tou ,. , . I , . , . ~ 11 ' ,n h ' ·' · tlwrl'll>1t · L'i 1L ·h 1 i1 · 1l1 c 111 m11 q p o<.,<., cs., u/ /co st one to11g1w-rol/i,,,:: :i lkk . Some or !heir cllildre 11 'l .. , . . , • , • < I L notHo/ler s , how 111 1~ 1l1 ; 1t 1l1 cy n,11 , 1 h;1vc received or ha. \/ 0 11 c alklc for llOJ _ 1 n , . . . . . 1 Ul\ , .. LI L 1oll111 g trnrn c ilh er 1)1 Ili c pcin·n h. At kast o ne of the parents is heterozygous. In hetcro zy , ious . r · _ . ~ c- ' co nuti o 11 , only th e dum1nc1nt allele will h <: n p r(' ,w' rl. ·r l 1crclorc.: the tongue-rolling allele is dominant. CASE2 Deduce th e dominant / recess iv e a. ll ele / c hara ct er given that the parents have d~fferent ph enotypes but o ff s prin g are of the same pheno(vpes.* Example: Wing length of fruit flies is controlled by a pair of alleles. In a br eeding experiment. long-winged flies were crossed with short-winged fli es . a ll the F 1 fli es , vere long- winged . Which of the par e nts shows the dominant character? Explain ru11r m1s1t ·er. Deduction: Some offsprjng must have rece iv ed a long-winged allele and a short-winged allek from their parents/ so me offspring must be heterozygous. A ll offspring have long wings. Therefore. th e long-winge<I a ll ele must be dominant over the short-winged allek. · . - - -· -· ·- . -- . - --- ~ .. . 1 lun· a Li r<'C' 111 1111h cr 11f offsprin g. l '11 , 1s nnlv 1ru i.: lu 1 11rt• . a111 o.;111-; , ... 111l Ii i.: ,11 1 P 1 ( 1 t, . ~ 103

Basic Genet/ TliC' pn 1p,1rt1t11 is. lll. J l1 - I l' llnl ,, ,, ,, .., ., 1 dih,h ri< .1 • ·· •· . , 1p 1u '<. 11 11 ; 11 c d 111 ii rn 110 , ,r <J : J: 3 : I - the • .I • ' 10 . h. Explanation 11 .\·inx R<' 11 eli<' dia ~ra 111 : I l'I : I } . ' IL'j)t°l.'sc nt s the I · . . L um 111ant allele fur rc ,ullll .:; ccd r n .' ]ll"l' :--. cnts I he re , , . . · 1 . · l L:SS I\ c a k le lor wrinkled see d '\ represe nt s th e domi1nnt a ll ele f . II d ~ - 0 1 ye ow see y represents th e re ce ss i ve · 1l lelc f . d • c 0 1 green see P: ro und. ye ll ow seed x wrinkled. green seed G: R RYY rr yy t RY ry RrYy F, seeds are all round and yellow ( dih y brid s) t self-pollination RRYY RRYy Rr YY round & yellow round & y ell ow ro und & ye ll ow RR Yy RRy y R rYy round & ye J low rou nd & green round & ye ll ow Rr YY Rr Yy rr YY R rY y round & yell ow Rry y ro u nd & green rrY y round & yellow ro und & ye ll ow w ri nk le d & ye ll ow wrinkled & yellow RrY y Rryy IT Yy rr yy round and ye lJ ow round a nd green wri nk led & yellow wrinkl ed & gr ee n Punnett square s ho w in g all po ss ib le c omb ina ti o ns<~ { ga met es to form F 1 105 ;:_;::zd

c'e~ ,··d E~erc ses i.) ---- RE \1 \RI\.\ : J' ff crcnl ch rom osome s. Uni~ true !0 1 ge ne" ho rn c on I 20.4 I nherita nce in Humans 106 A. Multiple alleles C Th ey a n. .: a se ri es of 3 or more alleles (alternative form s) of a gene which occ ur at t he s am e locus on homologous chromo so mes. C; On ly 2 of them can be present together in a cell. C Exa mpl e: human ABO blood gro up s / 1 - domi n ant all e le r es pon s ibl e for produc in g a nti gen A on red blood ce ll s. t'1 - dom in a nt all e le r es ponsible for produ ci ng antigen B on red blood cell s. , -- recessive alIde produ c in g neith er antigen . / 1 t.1 / 11 d . d . . . b all arc co omuiant an , 1s recess iv e to oth. 0 (;e1101,·pe !"!° : s in ce th e two alleles arc equally dominant , they can express th emselves at th e sa me ti111 e in heterozyg ous co ndition. 0 Human bloot.l gro up ge not yp es : A 1:3 AB 0 Genotype \' t'rt . t 'i t11 J'i , t i r'/'1 ii A B both no ne

IU I ., \C"fL f...) \ .,..,.'l vgy - ,r 11 cr 11'"' 11 C' /'ll l l /C.) ,;JI - QC U>vC , 108 1, ,, 11 «I 61 4 5 2 3 3\&l~HHU lh 6 7 l\ U AA 13 14 15 U )UC 19 20 8 9 11 Xl 21 22 10 11 12 1°C II H 16 17 18 ( t- XY )) ,i 2 31 &I 6 7 n A6 AA 13 14 15 U XJ 19 20 11 3 it u 8 9 11 XI 21 22 «i H 4 5 n ax lh 10 11 12 re ix 11 16 17 18 ,, xx Ch romosomes of male Chromosomes of female c:, The chance of a child being a boy or a girl is 50% . P: Father XY /~ G: X C. Sex-linkage in man xx 0 irl 0 y X XY boy Mother xx ! X ~ Genes carried on the sex chromosomes are said to be sex-linked. In human male. th e re is a portion of the X chromosome for which there is NO homologous region of th e y chromosome. Genes carried on th e non-homologous por ti on of rh e X ch ro mo so me there .f<Jre can expre ss th e ms e lv es even if th ey are recessive . CJ Example: c ol o ur blindn ess and ha emophilia a. Red-green colour blindness = Pers on s suffe rin g. from red -g r ee n co lour blindn ess ca nnot di s tingui sh be t, veen re d and gr ee n colo ur s.

Basic Genetic e A higher !) lT l' Cllt a μ c 01 · l lll ' I .. , ' . . . . . ~ · 1 dl t.: et il()u1hl111d 111 a pr,pula 11 on. c:, Explanation: Th C g C 11 (' ro r C () I l) ll r h 1 · I . . , ,. lll l ll C...,..., I ', a r ct.: c-,..., , vc ; 1l le lc l, 1r r1 cd r;n th e X Lh1omosnn 1 e Th ey . 1 • 1... 1 rnmn -, om c ()I" 111..il c c ,rric ., ncil ht r th e c () l <>U rhli ncJ gene nor it s normal al lclc . C) Exampl e: A normal \.\ ' . . . L om ...in 1s married to a colourblinrJ man Let: X rep ·e x 1 se nt s chromosome w ith th e normal a ll ele X' represents X chromo so me w ith colourblind a ll ele y represe nt s y chromosome Parental phenotype: Normal female X Parental genotype: XX Gametes: X F 1 genotype: xx c F I phenotype: normal female (carrier) F1: Normal female X (carrier) xx c /~ Gametes: X Xe F . 2· XY CoJourblind male xcy xc XY normal male Normal male XY y /~ X y xx normal female normal normal colour-blind mal e female (carrier) male e Cris s -cross pattern of inheritance - a colourblind man transmits his colo ur blind gene through hi s daughter who is phenotypically normal but carries the colourblin cl g en e to half of her sons, i.e., the se x-linked recessive character of the g ra nd fat he r reappears in the grandson. 109 -:::J?J!

1 200 1 100 1 000 900 800 C C'il E 700 ::i .c. 0 (/) 600 u C C'il t/) ::i 5 00 0 .c I- 400 300 200 100 I 14 ~0 ~~ 1 ~ 45 ~-: 1-:=-5 :::--- 0 -: 1~ ~-=-5 - 1 J_ 6_0 _ 1 J_ 6_5 _ 1 j_ ;- o _ 117_5 _1180 ::::::~1 :i::: 85= __, =1 :i..__ 90 Height (cm) B. Causes of genetic variation C) / ndependent assortment of chromosomes and crossing over at meiosis e to give gametes with different combinations of alleles. ~ Random fertilisation C, Mutation e a sudden, inheritable change in the genetic material (DNA) . e Cause: exposure to ionising radiation and chemicals C. Effects of environment on gene expression Ex amples : a. Effect of light on chlorophyll production c:, The all ele for c hl orophyll production (dominant) can express it self o nl y when light is present. 111 ~

d E crus es logy- lntenswo Notes ,m x _ i.1; .... ....... __,~~ ---""~- l F .riu , rim ents 0 11 twin s J. - . . . .. . , , , I 10 s wd y env ir on me nt al ef fect s on ge n (: , 0 I Jc 11t1 ca l 1w 1I1 s t11 c u scl L:X p rl'. \\ 11 l . t li cv , 11 .,, "t' J ze li c a/lu id e nt ica l. 'rt 1 1 , rn an K ' ca u sc . , -- - ,., - J "' 7 , 1 . H ·d · ii n rl in Jiffcrc nt environments wi l!1 diff er ent nut . . .., i cy arc 1 e; c , • rit1 cJn . rtn(j L 'du ca l io n. 0 Resu lt s sh ow th at 0 char ac ters sh ow in g discontinuous variation are n ot affected 0 ch ar acters sho win g continuous variation ar e aff ected to di ff crc nt cxtc nh- body we ight is significantly affected by nutrition wh il e hei oht aw' fQ · b u arc l ess affec ted. "'il,,m::: 112