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Transcript
Mendelian genetic problems • In a cross between a black and a white guinea pig, all members of the F1 generation are black. The F2 generation is made up of approximately ¾ black and ¼ white guinea pigs. Diagram this cross an show the genotypes and phenotypes. – BB- black bb- white Bb=black F1 B B b b Bb Bb Bb Bb 100% Bb, 100% black F2 B b B b BB Bb Bb bb ¼ BB. ¼ bb and ½ Bb ¾ Black and ¼ white Mendelian genetic problems • Albinism in humans is inherited as a s simple recessive trait. Determine the genotypes of the parents in the following families: – Two non albino parents have five children, four normal and 1 albino = Aa x Aa – A normal male and an albino female have 6 normal children = mom (aa), dad (AA or Aa?) Mendelian genetic problems • In a problem involving albinism which of Mendel’s postulate are demonstrated? – The first three postulates can apply, however it is a clear example of segregation. Segregation vs. Independent Assortment • Segregation • Diploid germ-line cells of sexually reproducing species contain two copies of almost every chromosomal gene. • The two copies are located on members of a homologous chromosome pair. • During meiosis, the two copies separate, so that a gamete receives only one copy of each gene. Segregation vs. Independent Assortment • Independent assortment • When the alleles of two different genes separate during meiosis. • They do so independently of one another. • Unless the genes are located on the same chromosome (linked). Mendelian genetic problems • Diagram the cross in problem 6 through the F2 generation using Punnett square and forked-line method. What happens to F2? YyRr x YyRr Possible gametes: YR • YR YR YYRR • Yr Yr YYRr • yR yR YyRR • yr yr YyRr 9/16 yellow and round 3/16 yellow and wrinkled Yr yR yr YYRr YyRR YyRr YYrr YyRr Yyrr YyRr yyRR yyRr Yyrr yyRr yyrr 3/16 green and round 1/16 green and wrinkled The Forked-line Method What is it? • Another way to predict genotype and phenotype ratios in dihybrid problems. • You don’t have to write a Punnett Square. • It requires you to know the basic ratios that arise from monohybrid crosses. • In a dihybrid cross, the two traits sort INDEPENDENTLY of one another. Use the following dihybrid cross: PpYy x PpYy Use of two simple concepts: 1. The traits (flower color and seed color) sort out independently of each other. 2. There are essentially only three different ratios that can result in a monohybrid cross. 1 homozyg x homozyg: PP x PP -----------> 100% PP or pp x pp -–-----------------------------------------> 100% pp 2 heterozyg x homozyg: Pp x PP -----------> ½ Pp, ½ PP or Pp x pp ---------------------------------------------> ½ Pp, ½ PP 3 heterozyg x heterozyg: Pp x Pp: ¼ PP; ½ Pp; ¼ pp PpYy x PpYy so to solve this dihybrid, separate the two traits (since they sort independently): Pp x Pp will give: ¼ PP ½ Pp ¼ pp PpYy x PpYy so to solve this dihybrid, separate the two traits (since they sort independently): Pp x Pp will give: similarly, Yy x Yy will give: ¼ YY ½ Yy ¼ yy ¼ PP ½ Pp ¼ pp PpYy x PpYy so to solve this dihybrid, separate the two traits (since they sort independently): Yy x Yy will give: Pp x Pp will give: ¼ YY ½ Yy ¼ yy ¼ YY ¼ PP ½ Pp ¼ pp multiply fractions 1/16 PPYY ½ Yy 1/8 PPYY ¼ yy 1/16 PPYY PpYy x PpYy so to solve this dihybrid, separate the two traits (since they sort independently): Pp x Pp will give: ¼ YY ¼ PP ½ Pp ¼ pp multiply fractions 1/16 PPYY ½ Yy 1/8 PPYy ¼ yy 1/16 PPyy ¼ YY 1/8 PpYY ½ Yy 1/4 PpYy ¼ yy 1/8 Ppyy ¼ YY 1/16 ppYY ½ Yy 1/8 ppYy ¼ yy 1/16 ppyy PpYy x PpYy so to solve this dihybrid, separate the two traits (since they sort independently): convert all to 16ths for consistency Pp x Pp will give: 1/16 PPYY 1/16 ½ Yy 1/8 PPYy 2/16 ¼ yy 1/16 PPyy 1/16 ¼ YY 1/8 PpYY 2/16 ½ Yy 1/4 PpYy 4/16 ¼ yy 1/8 Ppyy 2/16 ¼ YY 1/16 ppYY 1/16 ½ Yy 1/8 ppYy 2/16 ¼ yy 1/16 ppyy 1/16 ¼ YY ¼ PP ½ Pp ¼ pp multiply fractions Remember to identify the phenotype ratios as well. Mendelian genetic problems • Are any of the crosses in problem 3-2 a test cross? – Yes, c is a test cross. – Test cross = An unknown genotype is crossed with a homozygous recessive individual. Testcross • Can you determine the genotype of the F1 generation if F2 consists of 4 purple plants? P = purple (dominant) p = white (recessive) Mendelian genetic problems • Which of Mendel’s postulates can be demonstrated in the Problem 3-2 vs. problem 2? – Independent assortment Mendelian genetic problems • Correlate Mendel’s four postulates with what is known about homologous chromosomes, genes, alleles and the process of meiosis. – This answer requires knowledge of meiosis. I will ask you to answer this question after meiosis lecture.
