The Chromosomal Basis of Inheritance Chapter 15 The Chromosomal Basis of Inheritance

EXPERIMENT RESULTS CONCLUSION Fig. 15-4 + w w + w w + +  Generation F1 All offspring had red eyes Generation RESULTS F2 Generation CONCLUSION P + X w X w Generation X  Y + w w Sperm Eggs F1 + + Figure 15.4 In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have? + w w Generation w w + w Sperm Eggs + + + w w F2 w Generation + w w w w + w

CONCLUSION + P X X Generation  X Y + Sperm Eggs + + F1 + Generation + Fig. 15-4c CONCLUSION + P w w X X Generation  X Y + w w Sperm Eggs + + F1 w w + Generation w w + w Figure 15.4 In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have? Sperm Eggs + + w w + F2 w Generation w w w + w

Concept 15.2: Sex-linked genes exhibit unique patterns of inheritance In humans and some other animals, there is a chromosomal basis of sex determination Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 15-5 X Y Figure 15.5 Human sex chromosomes

Females are XX, and males are XY Each ovum contains an X chromosome, while a sperm may contain either an X or a Y chromosome Other animals have different methods of sex determination Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Figure 15.6 Some chromosomal systems of sex determination 44 + XY 44 + XX Parents 22 + X 22 + Y 22 + X or + Sperm Egg 44 + XX 44 + XY or Zygotes (offspring) (a) The X-Y system 22 + XX 22 + X (b) The X-0 system 76 + ZW 76 + ZZ Figure 15.6 Some chromosomal systems of sex determination (c) The Z-W system 32 (Diploid) 16 (Haploid) (d) The haplo-diploid system

Inheritance of Sex-Linked Genes The sex chromosomes have genes for many characters unrelated to sex A gene located on either sex chromosome is called a sex-linked gene In humans, sex-linked usually refers to a gene on the larger X chromosome Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Sex-linked genes follow specific patterns of inheritance For a recessive sex-linked trait to be expressed A female needs two copies of the allele A male needs only one copy of the allele Sex-linked recessive disorders are much more common in males than in females Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

(a) (b) (c) Sperm Sperm Sperm Eggs Eggs Eggs XNXN  XnY XNXn  XNY Fig. 15-7 XNXN  XnY XNXn  XNY XNXn  XnY Sperm Xn Y Sperm XN Y Sperm Xn Y Eggs XN XNXn XNY Eggs XN XNXN XNY Eggs XN XNXn XNY XN XNXn XNY Xn XnXN XnY Xn XnXn XnY Figure 15.7 The transmission of sex-linked recessive traits (a) (b) (c)

Each chromosome has hundreds or thousands of genes Concept 15.3: Linked genes tend to be inherited together because they are located near each other on the same chromosome Each chromosome has hundreds or thousands of genes Genes located on the same chromosome that tend to be inherited together are called linked genes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

How Linkage Affects Inheritance Morgan did other experiments with fruit flies to see how linkage affects inheritance of two characters Morgan crossed flies that differed in traits of body color and wing size Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

b+ vg+ b vg  Parents in testcross b vg b vg b+ vg+ b vg Most or Fig. 15-UN1 b+ vg+ b vg  Parents in testcross b vg b vg b+ vg+ b vg Most offspring or b vg b vg

EXPERIMENT RESULTS Fig. 15-9-4 P Generation (homozygous) b+ b+ vg+ vg+ Wild type (gray body, normal wings) Double mutant (black body, vestigial wings)  b+ b+ vg+ vg+ b b vg vg F1 dihybrid (wild type) Double mutant TESTCROSS  b+ b vg+ vg b b vg vg Testcross offspring Eggs b+ vg+ b vg b+ vg b vg+ Wild type (gray-normal) Black- vestigial Gray- vestigial Black- normal b vg Figure 15.9 How does linkage between two genes affect inheritance of characters? Sperm b+ b vg+ vg b b vg vg b+ b vg vg b b vg+ vg PREDICTED RATIOS If genes are located on different chromosomes: 1 : 1 : 1 : 1 If genes are located on the same chromosome and parental alleles are always inherited together: 1 : 1 : : RESULTS 965 : 944 : 206 : 185

Morgan found that body color and wing size are usually inherited together in specific combinations (parental phenotypes) He noted that these genes do not assort independently, and reasoned that they were on the same chromosome Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

However, nonparental phenotypes were also produced Understanding this result involves exploring genetic recombination, the production of offspring with combinations of traits differing from either parent Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Black body, vestigial wings Fig. 15-10a Testcross parents Gray body, normal wings (F1 dihybrid) Black body, vestigial wings (double mutant) b+ vg+ b vg b vg b vg Replication of chromo- somes Replication of chromo- somes b+ vg+ b vg b+ vg+ b vg b vg b vg b vg b vg Meiosis I b+ vg+ Meiosis I and II b+ vg b vg+ Figure 15.10 Chromosomal basis for recombination of linked genes b vg Meiosis II Recombinant chromosomes b+ vg+ b vg b+ vg b vg+ b vg Eggs Sperm

965 944 Black- vestigial 206 Gray- vestigial 185 Black- normal Fig. 15-10b Recombinant chromosomes b+ vg+ b vg b+ vg b vg+ Eggs Testcross offspring 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal b vg b+ vg+ b vg b+ vg b vg+ Figure 15.10 Chromosomal basis for recombination of linked genes b vg b vg b vg b vg Sperm Parental-type offspring Recombinant offspring Recombination frequency 391 recombinants =  100 = 17% 2,300 total offspring

A monosomic zygote has only one copy of a particular chromosome A trisomic zygote has three copies of a particular chromosome Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Polyploidy is common in plants, but not animals Polyploidy is a condition in which an organism has more than two complete sets of chromosomes Triploidy (3n) is three sets of chromosomes Tetraploidy (4n) is four sets of chromosomes Polyploidy is common in plants, but not animals Polyploids are more normal in appearance than aneuploids Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Reciprocal translocation Fig. 15-15 A B C D E F G H A B C E F G H Deletion (a) A B C D E F G H A B C B C D E F G H Duplication (b) A B C D E F G H A D C B E F G H (c) Inversion Figure 15.15 Alterations of chromosome structure A B C D E F G H M N O C D E F G H (d) Reciprocal translocation M N O P Q R A B P Q R