Chapter 14 Mendel and the Gene Idea

Transcription

1 Gregor Mendel Chater Mendel and the Gene Idea Mendel and the Black Box Mendel and the Black Box Gregor Mendel monk who worked with ea lants Strong training in the sciences and mathema<cs Good exerimental design and me<culous records First erson to comrehend some of the most asic rinciles of gene<cs. ulished aer in 866 Said arents ass on discrete heritale factors Oosed two theories of the <me ) angenesis Hiocrates theory Acquired traits of the adult migrate to gametes ) Blending Hyothesis Early 9 th century Heritale traits from each arent lend in young

2 The Exerimental Sujects Worked with ea lants Easily trace 7 traits with dis<nct forms each Dominant Recessive Easily do oth self fer<liza<on and crossfer<liza<on Allowed him to kee track of arentage The Exerimental Sujects Started with lants that had a given set of traits Cross red them Then oserved which of those traits showed u in succeeding genera<ons. Ket going un<l he had true reeding varie<es All iden<cal to the arent lant For all offsring Cross ollina>on arent X generation (yellow seeds) (green seeds). Before fertilization occurs, eel ack the closed etals of a ea lant (in this case, one that came from a line that yielded yellow eas). Then ull out the ollenearing stamens with tweezers so that self-fertilization is no longer ossile.. Next, gather ollen from another lant y daing its anthers with a aintrush. crossollination henotyes and Genotyes henotye any hysical feature, odily characteris<c, or ehavior of an organism In Mendel s lants urle flowers white flowers Others 3. Finally, ru these ollen grains onto the stigma of the first lant. The results of the cross-ollination can e oserved when the fertilized eggs mature into seeds in the ovary, meaning eas in a od. The resulting seeds are yellow in this case ecause yellow is dominant over green. offsring (yellow seeds) Figure.3

3 Genes Genes One of Mendel s central insights The asic units of gene<cs are material elements In his ea lants, came in airs Heritale traits We know Genes code roteins These roteins dictate hysical outcomes, or features Cause the differences among individuals Therefore, determine henotye henotyes and Genotyes Genotye The gene<c makeu that determines a henotye The secific genes resent on the chromosomes Mendel realized that the henotyes in his lants were eing controlled y their genotyes. Alleles Alleles Alleles The alterna<ve forms of genes airs of alleles in the case of the eas Homologous chromosomes contain the same sets of genes The two different versions of a gene are alleles of that gene or trait One allele from one arent, the other allele from the other arent 3

4 Dominant/Recessive Dominant/Recessive alleles When one allele is exressed (seen hysically) over the other Dominant allele The allele that is seen in the henotye Recessive allele The allele that is not seen in the henotye Homozygous/Heterozygous Homozygous Both alleles are the same May e homozygous dominant or homozygous recessive Heterozygous Alleles differ Dominant/Recessive henotyes and Genotyes Allele for Blue Eyes (recessive) Allele for Brown Eyes (dominant) Individual A: heterozygous Individual B: homozygous dominant Individual C: homozygous recessive

5 Star>ng the Exeriments Three Genotyes Yield Two henotyes Mendel realized that it was ossile for organisms to have iden<cal henotyes for all his ea lants to have yellow seeds, for examle and yet to have differing underlying genotyes Y Y Y y Y y y y Three genotyes yield... two henotyes. yellow green Figure.7 Genes Retain Their Character Another of Mendel s insights Genes retain their character through many genera<ons rather than eing lended together. Genes that coded for green ea color were retained in their exis<ng form over many genera<ons. unnei Square unnec square Visually shows the Law of Segrega<on rincile of indeendent assortment Used to calculate ossile gametes roduced y arents genera<on (arent) ossile outcomes of comina<ons of those gametes F genera<on (filial) 5

6 unnei Square Monohyrid cross Comina<on of two individuals of genera<on Track inheritance of a single trait Mono = Based on four Mendelian hyotheses Different forms of a gene exist (alleles) Each organism carries two coies (diloid) Gametes only carry one coy (haloid) Due to the Law of segrega<on Forms may differ in exression (dominant or recessive) genera<on X genera<on F genera<on X F genera<on F genera<on Solving a unnei Square Determine genera<on genotyes Diloid Examle Brown eyes = BB or B Determine gametes Haloid Examle B and B, or B and Cross gametes in a square to yield offsring Diloid Summarize Count the genotyes and henotyes ased uon the genotyes F unnei Square (a) generation crosses unnei Square female male generation. Female gametes are eing rovided y a lant that has the dominant, yellow alleles (); male gametes are eing rovided y a lant that has the recessive, green alleles (). Y Y y y. The cells of the ea lants that give rise to gametes start to go through meiosis. Y Y female gametes y y y male gametes y 3. The two alleles for ea color, which lie on searate homologous chromosomes, searate in meiosis, yielding gametes that each ear a single allele for seed color. In the female, each gamete ears a Y allele; in the male, each ears a y allele. Y ossile outcomes in Y fertilization F generation. The unnett square shows the ossile cominations that can result when the male and female gametes come together in the moment of fertilization. (If you have troule reading the unnett square, see Figure.5). The single ossile outcome in this fertilization is a mixed genotye,. 5. Because Y (yellow) is dominant over y (green), the result is that all the offsring in the F generation are yellow ecause they all contain a Y allele. () How to read a unnett square female gametes female gametes male gametes male gametes. A gamete from the male comines. A gamete from the male comines with a gamete from the female to with a gamete from the female to roduce an offsring of genotye roduce an offsring of genotye (and white color). (and urle color). Figure.5 6

7 Rules of roaility Rule of mul<lica<on roduct of each event Two alleles for each trait 50/50 (or /) chance of inheri<ng one Like a coin toss heads or tails Mul<ly the roaility of one allele from one arent y the roaility of one allele from the other arent / X / = / or 5% roaility of a ar<cular genotye in offsring (F) of a monohyrid cross F genotyes B female Formation of eggs B Formation of serm B B male B B B B F genotyes 7