Bio Exam: Final Nov 14 Chromosomal Basis For Mendel’s Independent Assortment If the individual is heterozygous for both genes, gene A and gene B, according to Mendel’s principle of independent assortment, when the gametes are formed, you can get 4 types f gametes: AB, Ab, aB, ab The idea is that the homologous chromosome pairs line up on the metaphase plate independently in metaphase I, and this arrangement is just as likely as the other. Genes on the same chromosome are called Linked Geneso Linked genes don't show independent assortment (dependent assortment) Dependent vs Independent Assortmento Parents: AABB x aabbo F1: AaBb x aabbo Gametes of F1: Independent Assortment AaBb parental Aabb non- parental aaBb non-parental aabb parental Two of the four gametes have the parental phenotype (1:1:1:1 ratio)o Gametes of F1: Dependent Assortment AaBb parental aabb parental Both of the gametes have parental phenotypes (1:1 ratio) Non-Parentals: recombination, new combination of traits that was not in the parents Parental: same phenotype as the parent Sweet Peas P=purple flower p= red flower L= long pollen l= round pollen Find out if the genes are linkedo Homozygous for both parents: PPLL x ppllo Do a test cross with a recessive for both genes F1: PpLl x ppllo If it was independently assorted, we would expect: Purple and Long as one parental 900 Red and long as a non parental 10 Purple and round as a non parental 15 Red and round as another parental 912o Not a 1:1:1:1 ratio but its not linked either because there are non-parentalso Crossing-over generated gametes that were part paternal and part maternal Gamete LP and lp When combine them with ones from test cross, get parentals only In prophase I of meiosis if the maternal and paternal lined up (maternal lp and paternal LP) Produce chromosomes part paternal and part maternal Crossing over accounted for the non-parental summaryo if the genes show IA, 4 phenotypes 1:1:1:1 ratioo If the genes are linked 2 phenotypes 1:1 ratio, all parentalo If neither, its call partial linkage Generate some recombinant gametes by crossing over which creates chromosomes with both paternal and maternal parts and that can result in a recombinant gamete which will result in a recombinant individual when you do a test cross Morgan- scientist the studied fruit flies Had an undergraduate student working for him named Sturtevanto Came up with a way to map genes on a chromosome by determining the frequency of recombinants If you have two chromosomes, one pat and one mat, they have genes A,B, and D on them, when crossing over occurs, the place where it occurs is random The cross over can be anywhere on the chromosomeso He realized that two genes will be more likely switched/exchanged during a cross over if they are far apart versus close together; The cross over can only be in one place on a chromosomeo Said you can map you can map genes on a chromosome by determining the frequency of the recombinants To map genes on a chromosome start with a cross between a double heterozygote The results of the test cross are:- AaBb 20 45- Aabb 18 5- aaBb 22 4- aabb 25 46 Black numbers are close to a 1:1:1:1 ratio= Independent Assortment Green numbers are not independent assortment, so the frequency of recombinance is: the total number of non-parentals (recombinants) the total number of individuals 9100 = 9% = 9 map unit apart1 map unit= 1% recombinantso Summary: to map genes on a chromosome, first you if start with pure strain parents, create a doubleheterozygote, then you test cross the double heterozygote If you get something close to a 1:1:1:1 ratio= IA (not linked genes) If you get an excess of parentals and not many recombinants then that means the genes are linked The map units are the total recombinants divided by the total number of individuals Aneuploidy A chromosomal aberration; When there is one too many or one too few chromosomeso Trisomy: 2N+1 one too many chromosomes 47 chromosomes o Monosomy: 2N-1 one too few chromosomes (45) Arises from chromosomal non-disjunction, in anaphase I, the chromosomes are supposed to separate (disjoin), if they don't separate its called
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