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Chapter 15: The Chromosomal Basis of Inheritance

This chapter explores the chromosomal basis of inheritance, including the concepts of meiosis, Mendel's laws, and gene linkage. The experiments conducted by Thomas Hunt Morgan on fruit flies are also discussed.

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Chapter 15: The Chromosomal Basis of Inheritance

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  1. Chapter 15: The Chromosomal Basis of Inheritance • Let’s review • Ch 13 - Meiosis makes gametes – sperm & egg • Ch 14 – Mendel studied peas • gametes pass on traits • unknown what was in the gametes

  2. P Generation Yellow-round seeds (YYRR) Green-wrinkled seeds (yyrr) Starting with two true-breeding pea plants, we follow two genes through the F1 and F2 generations. The two genes specify seed color (allele Y for yellow and allele y for green) and seed shape (allele R for round and allele r for wrinkled). These two genes are on different chromosomes. (Peas have seven chromosome pairs, but only two pairs are illustrated here.) y Y r R R Y r y Meiosis Fertilization y r Y R Gametes All F1 plants produce yellow-round seeds (YyRr) R R y F1 Generation y r r Y Y Meiosis LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT r R r R Two equally probable arrangements of chromosomes at metaphase I y Y Y y 1 Alleles at both loci segregate in anaphase I, yielding four types of daughter cells depending on the chromosome arrangement at metaphase I. Compare the arrangement of the R and r alleles in the cellson the left and right 1 3 The R and r alleles segregate at anaphase I, yielding two types of daughter cells for this locus. r R R r Anaphase I y Y y Y r r R R Metaphase II 2 Each gamete gets a long and a short chromosome in one of four allele combinations. 2 Each gamete gets one long chromosome with either the R or r allele. Y y y y Y Y Y y Y Y Y y Gametes r R r R R r r R 1 4 1 4 1 4 1 4 YR yr yr yR F2 Generation Fertilization among the F1 plants Fertilization recombines the R and r alleles at random. Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation. 3 9 : 3 : 3 : 1 Figure 15.2 The chromosomal basis of Mendel’s laws

  3. Chapter 15: The Chromosomal Basis of Inheritance • How was it determined that chromosomes carry genes? • Thomas Hunt Morgan • 1st to trace a specific gene to a specific chromosome • Noticed a fly with white eyes (wild-type is red) • Wild-type – phenotype most common in the natural population (+) • Mutants – alternative trait to the wild-type

  4. P Generation X F1 Generation F2 Generation 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? Expected 3:1 Observed 3:1 Problem!!!!!! Only males had white eyes…..hmmm.

  5. P Generation X W+ W X X P Generation X X Y W+ F1 Generation W Ova (eggs) Sperm W+ W+ F1 Generation W+ W W+ Ova (eggs) Sperm F2 Generation W+ W+ W+ W+ W W F2 Generation 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?

  6. RESULTS EXPERIMENT Morgan first mated true-breeding wild-type flies with black, vestigial-winged flies to produce heterozygous F1 dihybrids, all of which are wild-type in appearance. He then mated wild-type F1 dihybrid females with black, vestigial-winged males, producing 2,300 F2 offspring, which he “scored” (classified according to phenotype). P Generation (homozygous) Double mutant (black body, vestigial wings) x Wild type (gray body, normal wings) b b vg vg b+ b+ vg+ vg+ Double mutant (black body, vestigial wings) F1 dihybrid (wild type) (gray body, normal wings) TESTCROSS x b b vg vg b+ b vg+ vg b+ vg b+vg+ b vg b vg+ 185 Black- normal 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial b vg Sperm b+ b vg+ vg b b vg vg b+ b vg vg b b vg+ vg Recombinant (nonparental-type) offspring Parental-type offspring Chapter 15: The Chromosomal Basis of Inheritance • How was it determined that chromosomes carry genes? • Morgan’s next cross showed that linked genes are inherited together. • Noticed a disproportionately • large number with same • phenotype as parents • - Deduced 2 genes must be • on the same chromosome • Crossing over accounts for • the recombinant phenotypes

  7. Gametes from yellow-round heterozygous parent (YyRr) yR YR Yr yr Gametes from green- wrinkled homozygous recessive parent (yyrr) yr YyRr yyrr Yyrr yyRr Parental- type offspring Recombinant offspring Chapter 15: The Chromosomal Basis of Inheritance • How was it determined that chromosomes carry genes? • Morgan’s next cross showed that linked genes are inherited together. • What if the genes were unlinked…meaning independent assortment? P generation: YyRr x yyrr 50% 50%

  8. Chapter 15: The Chromosomal Basis of Inheritance • How was it determined that chromosomes carry genes? • Morgan’s next cross showed that linked genes are inherited together. • What if the genes were unlinked…meaning independent assortment? • How often will recombination occur…frequency??

  9. Black body,vestigial wings(double mutant) b+ vg+ b vg Testcross parents Gray body, normal wings (F1 dihybrid)  b vg b vg Replication of chromosomes Replication of chromosomes vg vg b b+ b b+ vg+ vg vg vg b b Meiosis I: Crossing over between b and vg loci produces new allele combinations. b vg b vg Meiosis I and II: Even if crossing over occurs, no new allele combinations are produced. Meiosis II: Segregation of chromatids produces recombinant gametes with the new allele combinations. Recombinant chromosome Gametes Ova Sperm b+vg+ b vg b vg+ b+ vg b vg Ova b+  vg+ b+   vg bvg+ bvg Testcross offspring 944 Black- vestigial 965 Wild type (gray-normal) 206 Gray- vestigial 185 Black- normal Sperm Recombination frequency 391 recombinants  = 100 = 17% b+  vg+ b vg+ b+vg+ b vg+ 2,300 total offspring b vg b vg b vg b vg b vg Parental-type offspring Recombinant offspring Figure 15.6 Chromosomal basis for recombination of linked genes Sturtevant – developed a genetic linkage map from recombination frequencies

  10. Recombination frequencies 9.5% 9% 17% vg b cn Chromosome Chapter 15: The Chromosomal Basis of Inheritance • How was it determined that chromosomes carry genes? • Morgan’s next cross showed that linked genes are inherited together. • What if the genes were unlinked…meaning independent assortment? • How often will recombination occur…frequency?? • How can a genetic map be created from recombination frequencies? bcn 9% cnvg 9.5% bvg 17% • 1% RF = 1 map unit (m.u.) • Some linked genes are so far apart that crossovers occur very often. • 50% RF is MAX • Recall 50% is seen with unlinked genes

  11. Gametes from yellow-round heterozygous parent (YyRr) yR YR Yr yr Gametes from green- wrinkled homozygous recessive parent (yyrr) yr YyRr yyrr Yyrr yyRr Parental- type offspring Recombinant offspring Chapter 15: The Chromosomal Basis of Inheritance • How was it determined that chromosomes carry genes? • Morgan’s next cross showed that linked genes are inherited together. • What if the genes were unlinked…meaning independent assortment? P generation: YyRr x yyrr 50% 50%

  12. I IV X Y III II Mutant phenotypes Short aristae Black body Cinnabar eyes Vestigial wings Brown eyes 0 48.5 57.5 67.0 104.5 Long aristae (appendages on head) Red eyes Gray body Normal wings Red eyes Wild-type phenotypes Figure 15.8 A partial genetic (linkage) map of a Drosophila chromosome

  13. Chapter 15: The Chromosomal Basis of Inheritance • How was it determined that chromosomes carry genes? • Morgan’s next cross showed that linked genes are inherited together. • What if the genes were unlinked…meaning independent assortment? • How often will recombination occur…frequency?? • How can a genetic map be created from recombination frequencies? • What determines male or female in utero? • SRY – sex-determining region of Y • w/ SRY – gonads develop into testes • w/o SRY – gonads develop into ovaries • X – has genes not associated w/ sex characteristics • Sex-linked is usually X-linked • Fathers pass X-linked alleles to daughters (XX) • Moms pass X-linked alleles to sons or daughters • If X-linked allele is recessive • ♀ shows phenotype when homozygous • ♂ shows phenotype when hemizygous – more males affected

  14. Chapter 15: The Chromosomal Basis of Inheritance • How was it determined that chromosomes carry genes? • Morgan’s next cross showed that linked genes are inherited together. • What if the genes were unlinked…meaning independent assortment? • How often will recombination occur…frequency?? • How can a genetic map be created from recombination frequencies? • What determines male or female in utero? • How are sex-linked alleles transmitted?

  15. XaY XAXA (a) A father with the disorder will transmit the mutant allele to all daughters but to no sons. When the mother is a dominant homozygote, the daughters will have the normal phenotype but will be carriers of the mutation. Sperm Xa Y XAXa XAY Ova XA XAxa XAY XA  XAXa XAY If a carrier mates with a male of normal phenotype, there is a 50% chance that each daughter will be a carrier like her mother, and a 50% chance that each son will have the disorder. (b) Sperm XA Y XAXA XAY Ova XA XaxA XaY Xa  XAXa XaY If a carrier mates with a male who has the disorder, there is a 50% chance that each child born to them will have the disorder, regardless of sex. Daughters who do not have the disorder will be carriers, where as males without the disorder will be completely free of the recessive allele. (c) Sperm Xa Y Ova XAY XA XAXa Xaxa Xa XaY Figure 15.10 The transmission of sex-linked recessive traits

  16. Chapter 15: The Chromosomal Basis of Inheritance • How was it determined that chromosomes carry genes? • Morgan’s next cross showed that linked genes are inherited together. • What if the genes were unlinked…meaning independent assortment? • How often will recombination occur…frequency?? • How can a genetic map be created from recombination frequencies? • What determines male or female in utero? • How are sex-linked alleles transmitted? • What are some sex-linked alleles in humans? • Duchenne’s muscular dystrophy • dystrophin – key muscle protein is absent • Progressive weakening of muscles & loss of coordination • 1 in 3500 ♂ - rarely live past early 20s • Hemophilia • Sex-linked recessive • Protein needed for blood clotting • Color blindness

  17. Chapter 15: The Chromosomal Basis of Inheritance • How was it determined that chromosomes carry genes? • Morgan’s next cross showed that linked genes are inherited together. • What if the genes were unlinked…meaning independent assortment? • How often will recombination occur…frequency?? • How can a genetic map be created from recombination frequencies? • What determines male or female in utero? • How are sex-linked alleles transmitted? • What are some sex-linked alleles in humans? • What are Barr bodies? • 1 of the 2 Xs becomes almost completely inactive during • embryonic development • Inactive X in each ♀ ♀ cell condenses into a Barr body • Most genes on the Barr body are not expressed • Barr body chromosomes are reactivated in ovary cells that • give rise to ova • Calico cats

  18. Two cell populations in adult cat: Active X Early embryo: Orange fur X chromosomes Cell division and X chromosome inactivation Inactive X Inactive X Black fur Allele for orange fur Allele for black fur Active X Figure 15.11 X inactivation and the tortoiseshell (calico) cat

  19. Chapter 15: The Chromosomal Basis of Inheritance • How was it determined that chromosomes carry genes? • Morgan’s next cross showed that linked genes are inherited together. • What if the genes were unlinked…meaning independent assortment? • How often will recombination occur…frequency?? • How can a genetic map be created from recombination frequencies? • What determines male or female in utero? • How are sex-linked alleles transmitted? • What are some sex-linked alleles in humans? • What are Barr bodies? • What are some chromosomal errors & exceptions? • Nondisjunction • Homologous chromosomes fail to separate during meiosis • Chromosomal rearrangements

  20. Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n + 1 n 1 n + 1 n –1 n + 1 n – 1 n n Number of chromosomes (b) (a) Nondisjunction of sister chromatids in meiosis II Nondisjunction of homologous chromosomes in meiosis I Figure 15.12 Meiotic nondisjunction Aneuploidy – an offspring that has an abnormal # of chromosomes (formed from a nondisjunction gamete) Trisomic – 2n + 1, Monosomic – 2n – 1 Polyploidy – more than 2 complete chromosome SETS: 3n – triploid, 4n - tetraploid

  21. Crossing over is NOT exact (Figure 19.18) This leads to deletions & duplications.

  22. A F H B C G C D F G B A E H E Deletion (a) A deletion removes a chromosomal segment. C E B C D A C D F G F H B A E H B G Duplication (b) A duplication repeats a segment. B A D A C D E F G H G C B E F H Inversion (c) An inversion reverses a segment within a chromosome. (d) A translocation moves a segment fromone chromosome to another, nonhomologous one. In a reciprocal   translocation, the most common type, nonhomologous chromosomes exchange fragments. Nonreciprocal translocations also occur, in which a chromosome transfers a fragment without receiving a fragment in return. A B M C D G E F G H N O C D E F H Reciprocal translocation M N A O P Q R B P Q R Figure 15.14 Alterations of chromosome structure

  23. Chapter 15: The Chromosomal Basis of Inheritance • How was it determined that chromosomes carry genes? • Morgan’s next cross showed that linked genes are inherited together. • What if the genes were unlinked…meaning independent assortment? • How often will recombination occur…frequency?? • How can a genetic map be created from recombination frequencies? • What determines male or female in utero? • How are sex-linked alleles transmitted? • What are some sex-linked alleles in humans? • What are Barr bodies? • What are some chromosomal errors & exceptions? • What are some human disorders due to chromosomal alterations? • Down syndrome • Trisomy 21 aka nondisjunction of 21st chromosome • Each cell has 47 chromosomes

  24. Figure 15.15 Down syndrome

  25. Chapter 15: The Chromosomal Basis of Inheritance • How was it determined that chromosomes carry genes? • Morgan’s next cross showed that linked genes are inherited together. • What if the genes were unlinked…meaning independent assortment? • How often will recombination occur…frequency?? • How can a genetic map be created from recombination frequencies? • What determines male or female in utero? • How are sex-linked alleles transmitted? • What are some sex-linked alleles in humans? • What are Barr bodies? • What are some chromosomal errors & exceptions? • What are some human disorders due to chromosomal alterations? • Down syndrome • Nondisjunction of sex chromosomes • Klinefelter syndrome – XXY – 1 in 2000 ♂ • XYY syndrome – 1 in 1000 ♂ • XXX - 1 in 1000 ♂ • Turner syndrome - XO – monosomy X – 1 in 5000 ♂

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