Name: Key: E = brown eye color (note that blue eye color is still represented by the letter e, but a lower case one...this is very important)

Transcription

1 MONOHYBRID CROSS v2 Name: Introduction A gamete is te egg or sperm cell tat is produced by meiosis. A gamete contains te aploid number of cromosomes (in a uman tis number is 23). In eac of tese cromosomes are a number of different genes, eac of wic confer a different trait to an organism. In genetics, we represent eac copy of a gene for a trait wit a letter from te alpabet. For example, eac of te gametes mentioned above would ave one copy of a gene for eye color in one of its cromosomes. Lets say tis copy of te eye color gene is a brown eye color gene. We call tis single copy of te gene an allele (u-leel) and we can represent tis allele wit te letter E. After te aploid egg is fertilized wit a aploid sperm, owever, te result is a zygote, wic is diploid. Tis zygote will now ave not one copy of te eye color gene, but two, one from eac gamete. If bot te egg and sperm ave contributed brown eye color alleles, ten we would represent tis situation wit two letters, EE. Wen we are referring to te particular genes an organism as for a certain trait, we are referring to its genotype. Te genotype of our example organism is EE. Wen we are referring to te pysical trait tat results from tis set of genes an organism as, we are referring to its penotype. Te penotype of our example organism is Brown Eyes. From te example given above, it is easy to see ow two brown eyed parents can give birt to brown eyed cildren. But ow do tese same parents give birt to blue eyed cildren? Wat color eyes will te cild of a brown eyed parent and a blue eyed parent ave? Read on to find out... Problem Set-up First you will see ow to set up a cross between a blue eyed and a brown eyed parent. Tis cross is called a monoybrid cross because it involves only one trait, eye color (later on we will consider more complicated diybrid crosses wic sow te ineritance of two traits). In setting up a problem you sould always ave a key. A key represents te genes you are dealing wit, i.e. a brown eyed gene or a blue eyed gene. Any letter may be used but te dominant must be expressed wit a capital letter, suc as A, D, or Z, and te recessive wit te same letter but lower case, suc as a, d, or z. You will understand wat dominant and recessive mean after te example, so keep going for now. Example 1 A cross between a brown eyed male and a blue eyed female, given tat brown eye color is dominant to blue eye color. Key: E = brown eye color (note tat blue eye color is still represented by te letter e, e = blue eye color but a lower case one...tis is very important) Our male as a genotype of EE, wile our female as a genotype of ee. Monoybrid Cross page 1

2 Te male's gametes (sperm) owever, are represented simply by E, since tey eac only ave one copy of te eye color gene. Likewise te female's gametes (eggs) are represented by e. Male genotype: EE Male gametes: E and E Female genotype: ee Female gametes: e and e Now make a Punnett Square (PUH-net) to sow te results of tis cross: 1) E E (put one parent's gametes ere) (put oter parent's gametes ere) e e 2) Now fill in te small squares by moving eac gamete allele into te appropriate box, as sown. E E e Ee Ee Eac small square represents te genotype of an offspring. In tis case tey are all alike (Ee) but tis is not always te case. e Ee Ee **Now in tis case, all offspring will ave te genotype Ee, meaning a copy of bot te blue and brown eye color genes. Wat will te eye color of an Ee individual be? BROWN, because, as you were told at te beginning of te example, te brown eye color allele is DOMINANT to te blue eye color allele. Te blue eye color allele is RECESSIVE to te brown allele. Wen bot alleles are present, te dominant allele is te one tat is expressed or seen. It is important to realize tat eac parent contributes one gamete to te zygote. In tis case, te gametes contain alleles E and e. Te zygotes ten contain te alleles Ee. 3) Can you fill in tis practice Punnett Square? B B B b Note: Wen a dominant allele is present, it always is written before te recessive allele. ex) Write Ee, not ee. Answer on next page -- Don't look until you are finised! Monoybrid Cross page 2

3 Answer: B B B BB BB Important Genetics Vocabulary b Bb Bb In doing a cross we refer to te original cross as te P (parent) generation (EE x ee). Te results of tis cross (te cildren) are all Ee and are known as te F 1 (filial) generation. Te offspring of te F 1 generation are referred to as te F 2 generation. Wat do you tink te offspring of te F 2 generation are called? Wen bot alleles are alike for one trait, EE or ee, te genotype is described as omozygous. Wen te alleles of te genotype are different, Ee, te genotype is described as eterozygous. (How to remember? Homo- refers to "same" and etero- refers to "different.") Wen assigning letters, te dominant trait is referred to wit a capital letter, E, and a recessive trait wit te small letter, e. In referring to te genotype EE, we would call it omozygous dominant; ee would be omozygous recessive; and Ee would be te eterozygous genotype. Also recall tat in talking about traits we frequently refer to penotypes (wat a trait pysically looks like) in addition to te genotypes (specific genetic makeup suc as EE, Ee, ee). Going back to te cross we just made, we can make te following assignments: In pea plants tall eigt is dominant over sort eigt (and sort is recessive to tall). Key: H = tall = sort So, (to assign gene symbols, you may select any letter if none is given) HH = Tall (omozygous dominant) H = Tall (eterozygous) = Sort (omozygous recessive) Question 1: Wat is te penotype for a plant of genotype H? Question 2: Can you ave a eterozygous sort plant? Wy or wy not? (see answer seet at end of packet) Sample Cross #1: Write out te penotypes for a cross between a omozygous tall male plant and a omozygous sort female plant. Key: = Tall P generation Tall x Sort = Sort (fill in genotypes) x (fill in gametes),, Now fill in te Punnett square using te parent genotypes you figured out above. (ceck answer seet at end of packet wen you are done) Monoybrid Cross page 3

4 Sample Cross #1 (continued) If you understand te cross from te previous page, your results sould ave come out wit 4 out of 4 (4/4) or 100% eterozygous tall plants, suc as H. Tese offspring are te F 1 generation (te first filial generation). We would describe tem as: Genotype: 100% H Penotype: 100% tall Now, take te F 1 generation and cross it wit itself. Tis will give you te F 2 generation. H x H Tis time, te gametes tat are produced by meiosis ave te alleles H and from te male and H and from te female. Arrange tese on te Punnett Square: H x H (F 1 genotypes) H and H and (F 1 gametes) H Te important ting to realize ere is tat a parent can contribute eiter an H or an, but NOT bot. And H HH H since eiter one or te oter must be given, tere is a 1/2 cance of H and an 1/2 cance of being given. H In a pysical sense, an H female will ave an H in 1/2 of er eggs and an in te oter 1/2 of er eggs (te coice of wic egg gets fertilized is random). Likewise, 1/2 te sperm from an H male will ave an H allele and te oter 1/2 of is sperm will ave an. Again, wen te F1 generation is crossed wit itself, te result is te F2 generation. Te results of tis cross are: F 2 GENOTYPE 1/4 HH (omozygous dominant) Tall 1/2 H (eterozygous) Tall F 2 PHENOTYPE 75% Tall 1/4 (omozygous recessive) Sort 25% Sort Monoybrid Cross page 4

5 Sample Cross #2: Cross a eterozygous tall plant wit a omozygous sort plant. Key: = Tall P generation Tall x Sort = Sort (fill in genotypes) x (fill in gametes),, (fill in Punnett Square) Question 3: Wat % of te offspring are tall? Question 4: Wat % of te offspring ave omozygous genotypes? (ceck answer seet for answers) Monoybrid Cross page 5

6 Monoybrid Cross ANSWER SHEET Question 1: Te penotype is Tall. Question 2: No, because te presence of a dominant allele blocks te expression of. Sample Cross #1 Key: _H_ = Tall P generation Tall x Sort = Sort (genotypes) _HH_ x (gametes) _H_, _H, H H H H H H Sample Cross #2 (problem on page 5) Key: _H_ = Tall P generation Tall x Sort = Sort (genotypes) _H_ x (gametes) _H_,, H H H Question 3: 50% Tall plants (1/2 H) Question 4: 50% (1/2 ) Monoybrid Cross page 6