10.4 Genes of Different Chromosomes Are Inherited Independently A. Dihybrid Crosses Track the Inheritance of Two Genes at Once P generation true-breeding for 2 traits F1 crossed to yield F2 Four phenotypes in ratio 9:3:3:1 All possible combinations found Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. RR YY rr yy

F1 generation rr yy Female gametes r y r y R Y Rr Yy Rr Yy Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. F1 generation rr yy Female gametes r y r y R Y Rr Yy Rr Yy Male gametes R Y RR YY Rr Yy Rr Yy

F2 generation Rr Yy Female gametes R Y R y r Y r y R Y RR Y Y RR Yy Rr Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. F2 generation Rr Yy Female gametes R Y R y r Y r y R Y RR Y Y RR Yy Rr YY Rr Yy Phenotypic ratio 9:3:3:1 R y 9 Smooth, yellow RR Yy RR yy Rr Yy Rr yy 3 Smooth, green Male gametes 3 Wrinkled, yellow 1 Wrinkled, green Rr Yy r Y Rr YY Rr Yy rr YY rr Yy r y Rr Yy Rr yy rr Yy rr yy

10.4 Genes of Different Chromosomes Are Inherited Independently B. Meiosis Explains Mendel’s Law of Independent Assortment During gamete formation, segregation of alleles for one gene does not influence alleles for another gene Provided genes are on different chromosomes

Figure 10.9 Mendel’s Law of Independent Assortment. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. R R Y Y MEIOSIS II R R R Y Y R Y Y r y MEIOSIS I r y r y r y y MEIOSIS II y Alternative 1 r y R R r r r Haploid gametes y Y Metaphase I Metaphase II Y R y R Diploid cell R y MEIOSIS II R y R y y MEIOSIS I R r Y r Y y r r Y Alternative 2 MEIOSIS II Y r Y r Y Figure 10.9 Mendel’s Law of Independent Assortment.

10.4 Genes of Different Chromosomes Are Inherited Independently B. Meiosis Explains Mendel’s Law of Independent Assortment Punnet squares for more than 2 genes cumbersome Product rule Chance that two independent events will both occur equals the product of the individual chances that each event will occur

Probability that off spring is Rr Yy Tt = 1/8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. R r Y y T t R RR Rr Y YY Yy T TT Tt r Rr rr y Yy yy t Tt tt 1/2 Rr × 1/2 Yy × 1/2 Tt Probability that off spring is Rr Yy Tt = 1/8 Figure 10.10 The Product Rule.

Explain how meiosis contributes to independent assortment of alleles. 10.4 Mastering Concepts Explain how meiosis contributes to independent assortment of alleles.

10.5 Genes on the Same Chromosome Many Be Inherited Together A. Genes on the Same Chromosome Are Linked Unlike genes on different chromosomes, they do not assort independently during meiosis First noticed in early 1900s

Figure 10.11 Gene Linkage Changes the Results of a Dihybrid Cross. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. Pp Ll self-cross, genes not linked b. Pp Ll self-cross, genes linked P p P p × × P p P p L l L l L l L l Male Female Male Female Crimson flowers, long pollen Crimson flowers, Long pollen Crimson flowers, long pollen Crimson flowers, long pollen Female gametes Female gametes P L P l P L P l P L p l P L PP LL PP Ll Pp LL Pp Ll 9 Crimson flowers, long pollen P L PP LL Pp Ll 3 Crimson flowers, long pollen P l PP Ll PP ll Pp Ll Pp ll 3 Crimson flowers, round pollen Male gametes p l Pp Ll pp ll 1 Red flowers, round pollen Male gametes 3 Red flowers, long pollen p L Pp LL Pp Ll pp LL pp Ll 1 Red flowers, round pollen p l Pp Ll Pp ll pp Ll pp ll Figure 10.11 Gene Linkage Changes the Results of a Dihybrid Cross.

10.5 Genes on the Same Chromosome May Be Inherited Together A. Genes on the Same Chromosome Are Linked Thomas Hunt Morgan Drosophila melanogaster – fruit fly Linkage groups are on the same chromosome

10.5 Genes on the Same Chromosome Many Be Inherited Together A. Genes on the Same Chromosome Are Linked Sometimes did see combinations not found in parents Due to crossing over Parental chromatid- no crossing over Recombinant chromatid

Homologous chromosomes cross over in prophase I Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gametes P P p L L L Parental MEIOSIS II p L P p P p L L l MEIOSIS I Homologs part Recombinant p l l P p l Homologous chromosomes cross over in prophase I Parental MEIOSIS II P l Recombinant Figure 10.12 Crossing Over.

10.5 Genes on the Same Chromosome Many Be Inherited Together B. Studies of Linked Genes Have Yielded Chromosome Maps Some crosses had a higher proportion of recombinant offspring Alfred Sturtevant, Morgan’s undergraduate assistant, proposed linkage maps Genetic markers used now

Figure 10.13 Breaking Linkage. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. A a Genes A and B are far apart; crossing over is more likely to separate these alleles. 0 1.5 33.0 36.1 54.5 B b Genes B and C are close together; crossing over is less likely to separate these alleles. C c y w v m r a. b. Figure 10.13 Breaking Linkage.

Clicker Question What is the order of genes on a chromosome if g and m are 4 units apart, k and h are 6 units apart and m and h are 2 units apart? ghmk gmkh hkmg 0 2.0 4.0 8.0 g h m k Correct answer is A