Biol 1408 : Chapter 9 Patterns of Inheritance

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Biol 08 : Chapter Patterns of Inheritance We previously discussed mono - hybrid crosses The Experiment P generation (true-breeding parents) Purple

of plants have white flowers Mono Hybrid Crosses A cross between true breeding plants that differ in only one character trait Examples such as

1 Biol 08 : Chapter Patterns of Inheritance We previously discussed mono - hybrid crosses The Experiment P generation (true-breeding parents) Purple flowers White flowers F generation All plants have purple flowers Fertilization among F plants (F F ) F 2 generation of plants have purple flowers of plants have white flowers Mono Hybrid Crosses A cross between true breeding plants that differ in only one character trait Examples such as flower color, plant height, pod color, pod shape, Outcome of F2 generation was : with respect to phenotype With respect to genotype, the outcome is :2:

2 .5 Dihybrid Crosses A dihybrid cross is a mating of parental varieties that differ in two characters. Mendel performed the following dihybrid cross Parent : Round seeds ( st character state) that were yellow colored (2 nd character state) Parent 2 : Wrinkled seeds ( variation on st character state) that were green colored (variation on 2 nd character state) (from earlier discussion, round and yellow are dominant traits) 202 Pearson Education, Inc..5 Dihybrid Crosses First, how do we represent an organism with two character state? A one character state with respect to color (yellow) is represented as YY (homozygous dominant), Yy (heterozygous), or yy (homozygous recessive). Two character states, such as Round and Yellow seeds, can thus be represented as RR YY ( homozygous dominant for both traits) Rr Yy (heterozygous for both traits) rr yy (homozygous recessive for both traits) 202 Pearson Education, Inc..5 Dihybrid Crosses Mendel reasoned that if seed shape and seed color were traits related to each other, they should sort out evenly in the gametes. P generation RY ry Since the parents are double homozygous, the gametes don t immediately tell the sto of what happens. What will a cross between these two plant above give us? 202 Pearson Education, Inc. 2

3 .5 Dihybrid Crosses P generation RY ry F generation The F generation is thus 00 % double heterozygous. And since R ( round) and Y (yellow) are dominant characteristics, all seeds from the F generation plants will yield round, yellow seeds..5 Dihybrid Crosses P generation RY ry F generation What if we now do a cross between F plants? Since Mendel thought that parental traits are always related to each other, so they should sort out in the gametes together..5 Dihybrid Crosses P generation RY ry F generation In other words, if the traits were dependent on each other, they should sort out dependently into the gametes ( called dependent assortment) Thus the R and Y trait, coming from the one parents would always travel together, and the r and y trait would do the same as well

.5 Dihybrid F cross assuming parental traits are related The gametes would contain the parental traits if they are related by parental origin (dependent assortment) The expected outcomes would thus

4 .5 Dihybrid F cross assuming parental traits are related The gametes would contain the parental traits if they are related by parental origin (dependent assortment) The expected outcomes would thus be : in phenotypes ( showing round and yellow seeds, having wrinkled green seeds) gametes RY Male Female ry.5 Dihybrid F cross assuming parental traits are related F generation Sperm 2 2 F 2 generation 2 Eggs 2 Data did not support this hypothesis of dependent assortment of parental traits and resulted in rejection of this hypothesis..5 The law of independent assortment is Mendel observed the following when performed this dihybrid cross : P generation: round yellow seeds wrinkled green seeds F generation: all plants with round yellow seeds F 2 generation: /6 had round yellow seeds /6 had wrinkled yellow seeds /6 had round green seeds /6 had wrinkled green seeds Two new phenotypes not observed in either P or F generation 202 Pearson Education, Inc.

5 .5 Dihybrid Crosses P generation RY ry F generation The only way this could have happened if the traits sorted out independently of each other in the gametes Instead of two possibilities in the gametes, possibilities can occur..5 Dihybrid Crosses P generation RY ry F generation Possible gametes Ry ry The traits thus move independent of each other into the gametes, generating possible gamete formation Called independent assortment of the traits A punnett square will show all the possible combinations..5 Dihybrid Cross among F plants with independent sorting out of traits in gametes F generation Sperm ry Ry RY RrYY Ry Eggs ry Ry RrYY rryy rryy Ry RRyy Ry rryy Ry ry Yellow round Green round Yellow wrinkled Green wrinkled 5

6 .5 The law of independent assortment is Mendel observed that the F 2 offspring a ::: phenotypic ratio. had new non-parental combinations of traits showing double dominant trait, heterozygous in st trait and homozygous in 2 nd trait, homozygous in st trait and heterozygous in 2 nd trait, double recessive Mendel suggested that the inheritance of one character has no effect on the inheritance of another, suggested that the dihybrid cross is the equivalent to two independent monohybrid crosses ((:) x (:)) He called this the law of independent assortment. 202 Pearson Education, Inc..5 Dihybrid Cross among F plants with independent sorting out of traits in gametes Genotypes F generation of RR YY 2 of RR Yy 2 of Rr YY of Rr Yy of RR yy 2 of Rr yy of rr YY 2 of rr Yy of rr yy Eggs ry Ry Sperm ry Ry RY RrYY Ry RrYY rryy rryy Ry RRyy Ry rryy Ry ry Yellow round Green round Yellow wrinkled Green wrinkled.5 The law of independent assortment is The following figure demonstrates the law of independent assortment as it applies to two characters in Labrador retrievers: black versus chocolate color, versus progressive retinal atrophy (blindness). Black and are dominant traits 202 Pearson Education, Inc. 6

7 Phenotype Genotype BBNN What is the result if we cross these two animals? B_nn bbn_ bbnn Phenotype Genotype BBNN F BbNn 00 % Double heterozygous BBNN gives gametes of BN bbnn gives gametes of bn bbn_ bbnn.5 Dihybrid Cross among F Labradors with independent sorting out of traits in gametes Genotypes F generation BbNn of BB NN 2 of BB Nn 2 of Bb NN of Bb Nn BN Sperm BN Bn bn bn BBNN BBNn BbNN BbNn 2 of Bb nn of BB nn of bb NN 2 of bb Nn Bn Eggs bn BBNn BBnn BbNn Bbnn BbNN BbNn bbnn bbnn of 6 Black Normal of 6 Black blind of bb nn bn BbNn Bbnn bbnn bbnn of 6 Brown Normal of 6 Brown 7

8 .5 The law of independent assortment is The result of a cross between two F animals Mating of double heterozygotes (black coat, ) BbNn BbNn Phenotypic ratio of the offspring.5 The law of independent assortment is Phenotypic ratio of the offspring have at least one copy of each dominant trait have at least one copy of the dominant allele of the st trait and no dominant allele of the 2 nd trait have at least one copy of the dominant allele of the 2 nd trait and no dominant allele of the st trait has only recessive alleles of both traits 8

Lecture 3 Monohybrid and Dihybrid Crosses

THE MONOHYBRID CROSS Lecture 3 Monohybrid and Dihybrid Crosses FOLLOWING THE INHERITANCE OF ONE TRAIT Monohybrid cross Parents differ by a single trait. Crossing two pea plants that differ in stem size,