Transcript Genetics
GENERAL GENETICS (PRACTICAL SESSION) BIO221 LECTURER ALSHEHRI, DOKHNAH SAEED [email protected] Alshehri, Dikhnah Saeed TH 7 THE LAB THE DIHYBRID CROSS STUDYING THE INHERITANCE OF TWO CHARACTERS SIMULTANEOUSLY Alshehri, Dikhnah Saeed © 2007 Paul Billiet ODWS OUTLINES: • What is the di-hybrid cross?. • Mendel’s Peas. • Some practical exercise And class activates. Alshehri, Dikhnah Saeed MENDEL’S PEAS Character Seed shape Seed colour Alshehri, Dikhnah Saeed © 2007 Paul Billiet ODWS Trait Allele Round R Wrinkled r Yellow Y Green y COMBINATIONS Genotype RRYY RRYy RrYY RrYy RRyy Rryy rrYY rrYy rryy Alshehri, Dikhnah Saeed © 2007 Paul Billiet ODWS Phenotype Round Yellow Round Yellow Round Yellow Round Yellow Round Green Round Green Wrinkled Yellow Wrinkled Yellow Wrinkled Green IS THE INHERITANCE OF ONE CHARACTER AFFECTED BY THE INHERITANCE OF ANOTHER? P Phenotypes Round Yellow F1 Phenotypes x Wrinkled Green All Round Yellow (Pure Bred) (Selfed) F2 Phenotypes Seed colour Yellow Green TOTAL Seed Round 315 108 423 shape Wrinkled 101 32 133 TOTAL 416 140 556 Alshehri, Dikhnah Saeed RATIO © 2007 Paul Billiet ODWS RATIO A DIHYBRID CROSS CAN BE TREATED AS TWO SEPARATE MONOHYBRID CROSSES THE EXPECTED PROBABILITY OF EACH TYPE OF SEED CAN BE CALCULATED: • PROBABILITY OF AN F2 SEED BEING ROUND = 75% OR ¾ • PROBABILITY OF AN F2 SEED BEING WRINKLED = • PROBABILITY OF AN F2 SEED BEING YELLOW = • PROBABILITY OF AN F2 SEED BEING GREEN = Alshehri, Dikhnah Saeed © 2007 Paul Billiet ODWS A dihybrid cross can be treated as two separate monohybrid crosses The expected probability of each type of seed can be calculated: Probability of an F2 seed being round = 75% or ¾ Probability of an F2 seed being wrinkled = 25% or ¼ Probability of an F2 seed being yellow = 75% or ¾ Probability of an F2 seed being green = 25% or ¼ Alshehri, Dikhnah Saeed © 2007 Paul Billiet ODWS Therefore Probability of an F2 seed being round and yellow = ¾ x ¾ = 9/16 = 56.25% Probability of an F2 seed being round and green = Probability of an F2 seed being wrinkled and yellow = Probability of an F2 seed being wrinkled and green Alshehri, Dikhnah Saeed = © 2007 Paul Billiet ODWS THEREFORE • Probability of an F2 seed being round and yellow = ¾ x ¾ = 9/16 = 56.25% • Probability of an f2 seed being round and green = ¾ x ¼ = 3/16 = 18.75% • Probability of an f2 seed being wrinkled and yellow = ¼ x ¾ = 3/16 = 18.75% • Probability of an f2 seed being wrinkled and green = ¼ x ¼ = 1/16 = 6.25% Alshehri, Dikhnah Saeed © 2007 Paul Billiet ODWS Predicting how many seeds we could expect to get in a sample 556 x 9/16 round yellow We could expect What Mendel observed 313 315 556 x 3/16 round green 108 556 x 3/16 wrinkled yellow 101 556 x 1/16 wrinkled green 32 Alshehri, Dikhnah Saeed © 2007 Paul Billiet ODWS PREDICTING HOW MANY SEEDS WE COULD EXPECT TO GET IN A SAMPLE We could expect What Mendel observed 556 x 9/16 round yellow 313 315 556 x 3/16 round green 104 108 556 x 3/16 wrinkled yellow 104 101 556 x 1/16 wrinkled green 35 32 Alshehri, Dikhnah Saeed © 2007 Paul Billiet ODWS The law of independent assortment • It appears that the inheritance of seed shape has no influence over the inheritance of seed colour • The two characters are inherited INDEPENDENTLY • The pairs of alleles that control these two characters assort themselves independently Alshehri, Dikhnah Saeed © 2007 Paul Billiet ODWS MENDEL & MEIOSIS • The pairs of chromosomes could orientate in different ways at anaphase 1 Alshehri, Dikhnah Saeed © 2007 Paul Billiet ODWS DIHYBRID CROSS GENETIC DIAGRAM P Phenotypes Round Yellow seed Genotypes Gametes X Wrinkled Green seed RRYY rryy meiosis meiosis RY ry (Pure bred) fertilisation F1 Phenotypes Genotypes Alshehri, Dikhnah Saeed Proportions © 2007 Paul Billiet ODWS RrYy Round Yellow 100% (Selfed) DIHYBRID CROSS GENETIC DIAGRAM F1 Phenotypes RrYy Genotypes Round Yellow Proportions 100% (Selfed) meiosis Gametes Alshehri, Dikhnah Saeed © 2007 Paul Billiet ODWS Y y R RY Ry r rY ry DIHYBRID CROSS GENETIC DIAGRAM Gametes Y y R RY Ry r rY ry fertilisation F2 Genotypes Alshehri, Dikhnah Saeed © 2007 Paul Billiet ODWS RY Ry rY ry RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy DIHYBRID CROSS PROPORTIONS Phenotypes Round Yellow Round Green Wrinkled Yellow Wrinkled Green Alshehri, Dikhnah Saeed © 2007 Paul Billiet ODWS Proportions 9/16 or 56.25% Dihybrid cross proportions Phenotypes Round Yellow Proportions 9/16 or 56.25% Round Green 3/16 or 18.75% Wrinkled Yellow 3/16 or 18.75% Wrinkled Green 1/16 or 6.25% Alshehri, Dikhnah Saeed © 2007 Paul Billiet ODWS Dihybrid test cross • In monohybrid crosses, to know if a dominant trait is homozygous (RR) or heterozygous (rr) it is necessary to carry out a test cross • This is done with a homozygous recessive (rr) individual • The same is true for a dihybrid cross where the test cross is made with an individual which is homozygous recessive for both characters (rryy) Alshehri, Dikhnah Saeed © 2007 Paul Billiet ODWS DIHYBRID TEST CROSS Phenotypes Genotypes Round Yellow RrYy Gametes RY, Ry, rY, ry Genotypes ry Phenotypes X Wrinkled Green rryy ry RY Ry rY ry RrYy Rryy rrYy rryy Round Round Wrinkled Wrinkled Yellow Green Yellow Green Alshehri, Dikhnah Saeed Proportions 1/4 or 25% 1/4 or 25% 1/4 or 25% 1/4 or 25% © 2007 Paul Billiet ODWS CLASS ACTIVITY: • Set up a punnett square using the following information: • ·· Dominate allele for black fur in guinea pigs = B • ·· Recessive allele for white fur in guinea pigs =b • ·· Dominate allele for rough fur in guinea pigs = R • ·· Recessive allele for smooth fur in guinea pigs = r • ·· Cross a heterozygous parent (bbrr) with a heterozygous parent (bbrr) Alshehri, Dikhnah Saeed • Using the punnett square produced in the previous question: • A. What is the probability of producing guinea pigs with black, rough fur? What is/are the possible genotype(s)? • B. What is the probability of producing guinea pigs with black, smooth fur? • What is/are the possible genotype(s)? • C. What is the probability of producing guinea pigs with white, rough fur? What is/are the possible genotype(s)? • D. What is the probability of producing guinea pigs with white, smooth fur? What is/are the possible genotype(s)? Alshehri, Dikhnah Saeed EXERCISE 1 • A brown-eyed, long-winged fly mated with a redeyed, long-winged fly. The progeny were: • 51 long, red • 53 long, brown 18 short, red 16 short, brown • What are the genotypes of the parents? Alshehri, Dikhnah Saeed EXERCISE 2 • Two rough black guinea-pigs bred together have two offsprings; one of them roughly white and the other smooth black. If these same parents were further bred together, what offspring would you expect for them? Alshehri, Dikhnah Saeed EXERCISE 3 • 1. In guinea-pigs, black fur (B) is dominant over albino (b) and course hair (R) is dominant over smooth hair ( r ). • A. What are the genotypes and phenotypes of a cross between homozygous black animal, course hair and an albino animal, smooth hair. • B. What are the genotypes and phenotypes of the F2 generation if two of the offspring from the F1 generation crossed. Alshehri, Dikhnah Saeed Alshehri, Dikhnah Saeed