Genetics Lecture Notes

Transcription

1 Bilgy Genetics Lecture Ntes Name Per Learning Gals Yu will be able t explain hw ffspring receive genes frm their parents Yu will be able t calculate prbabilities f passing genes frm ffspring t ffspring using Punnett Squares Yu will be able t define and explain the 7 different patterns f inheritance Yu will be able t prvide examples f hw humans shw the 7 patterns f inheritance Heredity The first scientists t study the laws f heredity had sme difficult initial prblems t slve Tw parents have t cntribute equally t make ne child Smetimes ffspring shw similar traits t their parents while in ther ways they shw traits that dn t appear related t their parents in any way Mixed breeds: tw different species can smetimes prduce ffspring Laws f heredity must explain hw tw parents can allw their traits t mix with each ther and make a child (the prcess is stable, with chas). Mendel Gregr Mendel was an Austrian mnk that perfrmed the first and mst imprtant experiments with genetics Mendel was able t use gametes frm pea pds t frm special zygtes, and use these zygtes t study heredity Gametes are male and female sex cells. In humans, gametes are sperm and eggs Zygtes are fertilized cells that frm when gametes unite with each ther Mendel s Experiment Mendel decided t study nly ne trait at a time. The first trait he chse was tall plants vs shrt plants. He tk male pllen frm tall plants and crssed (artificially mated) them with the female parts f shrt plants. Mnths later, when he had grwn mre than 1000 plants, all f the plants were tall and nne were shrt. Mendel wndered what happened t all the shrt plants. S he crssed all f the ffspring frm the first experiment with each ther. In the secnd set f ffspring, Mendel bserved 787 tall plants and 277 shrt plants He realized that the rati f tall plants t shrt plants was surprisingly clse t 3:1 (74% tall, 26% shrt). Mendel had sme prblems t wrk ut 1 st, hw cme when he crssed tall plants and shrt plants nly tall nes were brn? 2 nd, hw cme when he crssed nly tall plants that bth tall and shrt plants were brn?

2 One trait f the parent generatin wuld disappear in the first generatin, but reappear in the secnd generatin. What was ging n? Mendel repeated the experiments with different traits, but each time he gt the same results. Generatins P generatin Parent generatin. The first crss yu start with. (Yur parents) F1 generatin The ffspring f the parent generatin (Yu) F2 generatin The ffspring f the F1 generatin (Yur future kids) Chrmsmes Each chrmsme in an rganism cntains a prtin f the blueprint necessary t build that rganism These chrmsmes are made f sequences f DNA called genes. Each gene cdes fr a specific trait in an rganism. Mendel cncluded that each trait fr an rganism must be cntrlled by tw genes, ne frm the father s chrmsme and ne frm the mther s chrmsme Each gene can have mre than ne ptin (called alleles) fr what it will cde fr. The different ptins fr each gene are alleles Example: Eye clr alleles are Brwn and Blue; Dminance If each gene has tw alleles, why will nly ne shw up? Mendel realized that ne allele fr each gene must be a dminant ne. The dminant allele is the allele fr the trait that shws up. The recessive allele is the allele fr the trait that disappeared In pea plants, the allele fr tall plants is dminant and the allele fr pea plants is recessive. Gentypes and Phentypes A gentype is the cmbinatin f alleles that an rganism has Each allele is assigned a letter. Capital letters mean dminant, lwercase mean recessive T=tall allele. t=shrt allele. Tt, TT, tt are all examples f gentypes. A phentype is the trait that actually appears TT, Tt gentypes will each have a tall phentype tt gentypes will have shrt phentype Phentypes d nt take int accunt that an rganism may have an allele that isn t shwing up

3 Example: A plant with a Tt gentype wuld have a tall phentype, even thugh ne f the alleles cdes fr shrt. When bth alleles are the same, we call it hmzygus. When they are different, we call it heterzygus. TT, tt = hmzygus Tt = heterzygus Punnett Squares When yu understand the vcabulary and the prcess behind genetics, yu re nt nly able t calculate what genes an rganism likely has, but what gene s it s ffspring are likely t have In ther wrds, yu can calculate the likelihd f what yur kids will lk like. We use a tl called Punnett Squares t calculate these dds. Mnhybrid Crsses Use a mnhybrid crss if yu want t calculate the dds f nly ne gene On the tp, put the tw alleles f ne parent On the left side, put the tw alleles f the ther parent Then fill in the bxes accrding t the allele frm each parent Each f the squares represent the ptential sequence f genes in the ffspring f the parents Once the squares are filled in, yu can start calculating ratis Gentypic Ratis 1:2:1 TT : Tt : tt Phentypic Rati 3:1 Tall : Shrt Dihybrid Crsses What if yu want t calculate the pssibility f tw genes? Dihybrid crsses are larger, but the verall cncept is the same.

4 Fr this example, we will use Mendel s experiment: The shape f peas AND the clr f peas Rund (R) is dminant ver Wrinkled (r) Yellw (Y) is dminant ver Green (y) First, find the fur pssible allele cmbinatins fr each parent. Assign ne cmb fr each rw and clumn. Fill in the squares as with the mnhybrid crss When yu calculate the different squares, yu end up with a 9:3:3:1 rati Pattern #1: Simple Heredity Patterns f heredity are the rules that the gene r genes fr a specific trait fllw. As it turns ut, Mendel nly discvered the simplest f these heredity patterns Simple Mendelian heredity fllws three rules Rule #1: There is nly ne gene fr this trait. Rule #2: This trait has nly tw alleles Rule #3: One allele is dminant ver the ther allele. The rest f the fllwing inheritance patterns break at least ne f these rules. Pattern #2: Incmplete Dminance In simple heredity, heterzygtes will display the trait f the dminant allele because it is dminant. With incmplete dminance, heterzygtes will instead shw an intermediate, mixture phentype. In incmplete dminance, there are n dminant traits. Each trait is a blend f the alleles that an rganism has. In Punnett squares, yu can t use capital and lwercase letters fr incmplete dminance. Instead, yu use tw capital letters, ne f which has an apstrphe Red allele in snapdragns: R

5 White allele in snapdragns: R With incmplete dminance, three different gentypes will result in three different phentypes All ther rules f genetics and reading Punnett squares apply Pattern #3: Cdminance Cdminance is when ne allele is nt dminant ver the ther, but bth are equally dminant. If the rganism is hmzygus, the rganism will have nly ne phentype. If the rganism is a heterzygte, they display bth phentypes at nce rather than a mixture f phentypes In Punnett squares, bth cdminant alleles are uppercase and given a different letter. Black-feathered chicken: BB White-feathered chicken: WW Checkered chicken: BW Pattern #4: Multiple Alleles Multiple alleles are when a gene has mre than tw pssibilities There culd be between 3 and 100 different alleles Multiple allele traits fllw all ther patterns f dminant and recessive. Each rganism still nly has tw alleles within their DNA cde (ne frm each parent), but the number f different cmbinatins f alleles culd be numerus. In Punnett squares, fr a multiple allele trait, each allele has a single symbl fllwed by a superscript Y=Yellw Lab; Y b =Black Lab; Y c =Chclate Lab The nly fr sure way t read the dminance is if a key is prvided fr yu. Yellw>Black>Chclate Pattern #5: Sex-Linked Traits Every human has 22 pairs f chrmsmes that are exactly alike, male r female. The final pair f chrmsmes is different fr males and females. Females have tw X chrmsmes, males have an X and a Y chrmsme. The X and Y chrmsmes each cntain different genes A female will nt have any gene lcated n a Y chrmsme What if a gene is lcated n the X chrmsme? Males nly have ne X chrmsme. They will nly have ne allele fr that trait, dminant r recessive. Females have tw X chrmsmes, s they will have tw alleles fr the trait. Because f this, it is harder fr females t have a recessive X-linked phentype than it is fr males. In Punnett squares, sex-linked traits are expressed as an X r a Y. There is als a superscript t describe which allele is being represented Fruit flies: X R =Red eyes. X r =white eyes. Y=Y chrmsme, s n gene is present n the chrmsme

6 Pattern #6: Sex-Determined Traits Smetimes rganisms have a gene fr a specific trait, but the trait is nt expressed because f the rganism s gender. A sex-determined trait is when a trait nly appears in a certain gender Hrmnes prduced by ther genes blck sex-determined genes frm expressing in an rganism Reasn: Parents f ne gender may nt express the same traits as children f the ppsite gender. Hwever, parents still need t pass all necessary genes t their child regardless f age. Pattern #7: Plygenic Inheritance Plygenic (Ply=many, genic=genes) is when a trait is cntrlled by mre than ne gene. Each gene may have 2 r mre than 2 alleles Plygenic inheritance resembles incmplete dminance. Rather than ne allele being dminant, all the alleles in all the genes cntribute an equal prtin twards the phentype Instead f Punnett squares, yu write each gene in a list. Each gene is given a different letter Each trait is given a different frm f the letter A, B, C, D culd all be the genes Uppercase and lwercase, r superscripts, culd be alleles A Y A R B Y B Y C R C R D Y D R Y=Yellw crn, R=Red Crn. This gentype has 4 Yellw alleles and 4 Red alleles amng all the genes. Lk fr keys n plygenic inheritance as well. Simple Inheritance in Humans We ve talked abut many f these already Earlbes Widw s Peak Hitchhiker s Thumb Mid-digital Hair We will nw g thrugh each f these patterns f inheritance and see hw they apply t humans Incmplete Dminance in Humans Incmplete dminance is when the heterzygus phentype is a mix f the tw hmzygte phentypes. Fr humans, an example f this is Hair style Alleles: H=Straight hair, H = Curly Hair Gentypes HH=Straight Hair HH =Wavy Hair H H =Curly Hair

7 Cdminance Cdminance is when heterzygte rganisms have a phentype that equally shws bth traits. An example f this fr humans is sickle-cell anemia, when bld cells frm sickle shapes instead f rund shapes. Alleles: R=Rund, S=Sickle Cell Gentypes RR=All rund bld cells RS=1/2 rund bld cells, ½ sickle bld cells SS=All sickle bld cells Multiple Alleles in Humans Multiple alleles are when there are 3 r mre pssible alleles in a gene. An imprtant trait fr humans that expresses multiple allele inheritance is bld type Yur immune system must recgnize the difference between freign substances and yur wn bld T d this, yur bld has specific prteins called antigens n its plasma membrane. Yur immune system recgnizes these prteins and knws that the bld cell belngs t yu and isn t an intruder The different antigens are labeled A and B Alleles: I A =A-Type Bld; I B =B-Type Bld; i=neither type There are 4 pssible phentypes f bld, arising frm 6 pssible gentypes Gentypes Antigens Present Phentype (Bld Type) I A I A (AA); I A i (AO) A-Type nly I B I B (BB); I B i (BO) B-Type nly I A I B (AB) Bth A and B Types ii (OO) Neither A r B Types It is imprtant fr yu t knw what yur bld type is BEFORE yu get a bld transfusin If yu get bld with a different prtein than what yur immune system is used t, it will attack the bld This results in bld clts and, usually, is deadly (Because O type bld has NO prteins n it, yur cells wn t recgnize the WRONG prteins.) If yu have this bld type Yu can receive these bld types Type A Type B Type AB Type O

8 Sex-Linked Traits in Humans Females have tw X chrmsmes. Mthers can nly dnate an X chrmsme t their ffspring Males have bth an X and Y chrmsme. Whichever chrmsme a father dnates determines his ffspring s gender A male has an easier time f having a phentype frm a recessive sex-linked trait than a female Females need t receive tw recessive alleles t have the recessive trait. Males can nly receive ne allele. A father cannt pass a sex-linked Y-chrmsme trait t his daughter OR an X- chrmsme trait t his sn. A sex-determined trait is a gene that will nly be expressed depending n yur gender Bth men and wmen prduce teststerne and estrgen. Arund puberty, the bdy begins t prduce higher amunts f ne r the ther Yur gender determines which structures yur bdy will frm (testes r varies), and these structures prduce high quantities f teststerne and estrgen, respectively Therefre, even thugh yu have the gene t prduce bth hrmnes, yur gender decides which yu will prduce mre f. Plygenic Inheritance Plygenic Inheritance is when a trait is cntrlled by multiple genes. In humans, skin clr is cntrlled by between fur and six different genes. Each f these genes will have tw alleles Alleles fr sme genes: Yellw and Red Alleles fr ther genes: Light t Dark Yur skin clr is determined by cmbining the all f the alleles in all f yur genes Gene labels: A, B, C, D, E, F Superscript Labels: Y=Yellw, R=Red, L=Light, D=Dark Pssible phentypes: White, Yellw, Brwn, Red, Olive, Dark, r a cmbinatin f all these Examples f phentypes A R A L B L B L C L C Y D D D L E L E R F Y F L =2 Yellw, 2 Red, 7, Light, 1 Dark A R A L B D B D C R C R D L D R E L E D F R F R =0 Yellw, 6 Red, 3 Light, 3 Dark