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Chapter 15: The Chromosomal Basis of Inheritance Students ½ Learning log – due Tuesday (10/6) Genetics Problems – both chapters – due 10/15 Cell phones in bin….off or muted…please & thank you Figure 15.2 The chromosomal basis of Mendel’s laws P Generation Starting with two true-breeding pea plants, we follow two genes through the F 1 and F2 generations. The two genes specify seed color (allele Y for yellow and allele y for Y 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.) Yellow-round seeds (YYRR) Green-wrinkled seeds (yyrr) Y R r R y r y Meiosis Fertilization y R Y Gametes r All F1 plants produce yellow-round seeds (YyRr) R R y F1 Generation y r r Y Y Meiosis LAW OF SEGREGATION r R Y 1 The R and r alleles segregate R at anaphase I, yielding two types of daughter cells for this locus. Y y R Y y LAW OF INDEPENDENT ASSORTMENT r Anaphase I y Y r 1 Alleles at both loci segregate in anaphase I, yielding four types of daughter cells R depending on the chromosome arrangement at metaphase I. Compare the arrangement of the R and r alleles in the cells y on the left and right r R y y Metaphase II Y y Y Y Y Gametes r r R 2 Each gamete gets one long chromosome with either the R or r allele. Two equally probable arrangements of chromosomes at metaphase I R R r 1 YR 4 Y r r 1 yr 4 F2 Generation 3 Fertilization recombines the R and r alleles at random. Y Y r 1 Yr 4 2 Each gamete gets a long and a short chromosome in one of four allele combinations. y y R R 1 yR 4 Fertilization among the F1 plants 9 :3 :3 :1 3 Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation. 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 Chapter 15 Essential Questions LO 3.9 The student is able to construct an explanation, using visual representations or narratives, as to how DNA in chromosomes is transmitted to the next generation via mitosis, or meiosis followed by fertilization. LO 3.15 The student is able to explain deviations from Mendel’s model of the inheritance of traits. LO 3.16 The student is able to explain how the inheritance patterns of many traits cannot be accounted for by Mendelian genetics. LO 3.17 The student is able to describe representations of an appropriate example of inheritance patterns that cannot be explained by Mendel’s model of the inheritance of traits. Chapter 15: The Chromosomal Basis of Inheritance 1. 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 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? P Generation X F1 Generation F2 Generation Expected 3:1 Observed 3:1 Problem!!!!!! Only males had white eyes…..hmmm. 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? P Generation F1 Generation X P Generation W+ X X X X Y W+ W+ W W+ W Ova (eggs) F1 Generation Sperm W+ W F2 Generation W+ Ova (eggs) F2 Generation Sperm W+ W W+ W+ W+ W W W+ Chapter 15: The Chromosomal Basis of Inheritance 1. How was it determined that chromosomes carry genes? 2. Morgan’s next cross showed that linked genes are inherited together. 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) b b vg vg F1 dihybrid (wild type) (gray body, normal wings) b+ b vg+ Double mutant (black body, vestigial wings) x Wild type (gray body, normal wings) b+ b+ vg+ vg+ Double mutant (black body, vestigial wings) TESTCROSS x b b vg vg vg RESULTS b+vg+ b vg 965 944 Wild type Black(gray-normal) vestigial b+ vg b vg+ 206 Grayvestigial 185 Blacknormal b vg Sperm b+ b vg+ vg b b vg vg b+ b vg vgb b vg+ vg Parental-type offspring Recombinant (nonparental-type) offspring - 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 Chapter 15: The Chromosomal Basis of Inheritance 1. How was it determined that chromosomes carry genes? 2. Morgan’s next cross showed that linked genes are inherited together. 3. What if the genes were unlinked…meaning independent assortment? P generation: YyRr x yyrr Gametes from yellow-round heterozygous parent (YyRr) YR Gametes from greenwrinkled homozygous recessive parent (yyrr) yr Yr yR Yyrr yyRr yr YyRr Parentaltype offspring 50% yyrr Recombinant offspring 50% Chapter 15: The Chromosomal Basis of Inheritance 1. 2. 3. 4. 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?? Figure 15.6 Chromosomal basis for recombination of linked genes Testcross parents b+ vg+ Gray body, normal wings b vg (F1 dihybrid) Replication of chromosomes b+ Meiosis I: Crossing over between b and vg loci produces new allele combinations. Black body, vestigial wings b vg (double mutant) Replication of chromosomes vg b vg b+ vg+ vg b b vg vg b b vg b vg Meiosis II: Segregation of chromatids produces recombinant gametes with the new allele combinations. Gametes b vg Meiosis I and II: Even if crossing over occurs, no new allele combinations are produced. Recombinant chromosome Ova Sperm b+vg+ b vg b+ vg b vg+ b vg b+ vg+ Testcross offspring Sperm b vg 965 Wild type (gray-normal) b vg 944 Blackvestigial b+ vg b vg+ 206 Grayvestigial 185 Blacknormal Ova b+ vg+ b vg b+ vg Recombination 391 recombinants = b vg+ frequency 2,300 total offspring b vg b vg b vg b vg Parental-type offspring 100 = 17% Recombinant offspring Sturtevant – developed a genetic linkage map from recombination frequencies Chapter 15: The Chromosomal Basis of Inheritance 1. 2. 3. 4. 5. 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? bcn 9% cnvg 9.5% bvg 17% Recombination frequencies 9.5% 9% 17% Chromosome b cn vg -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 Chapter 15: The Chromosomal Basis of Inheritance 1. How was it determined that chromosomes carry genes? 2. Morgan’s next cross showed that linked genes are inherited together. 3. What if the genes were unlinked…meaning independent assortment? P generation: YyRr x yyrr Gametes from yellow-round heterozygous parent (YyRr) YR Gametes from greenwrinkled homozygous recessive parent (yyrr) yr Yr yR Yyrr yyRr yr YyRr Parentaltype offspring 50% yyrr Recombinant offspring 50% Figure 15.8 A partial genetic (linkage) map of a Drosophila chromosome I Y II X IV III Mutant phenotypes Short aristae Black body 0 Long aristae (appendages on head) Cinnabar eyes Vestigial wings 48.5 57.5 67.0 Gray body Red eyes Normal wings Wild-type phenotypes Brown eyes 104.5 Red eyes Students -Get a folder -From student table closest to the door -Cross out current name -Add your name -Learning logs mid-point check is tomorrow (Ch 13 & 14) -FCA – Career Center -Meets every other Wednesday – Rm 112 7:25 am -Fields of Faith -Reagan Stadium -7PM 10/14 -Phones in bin…muted off…please & thank you Chapter 15: The Chromosomal Basis of Inheritance 1. 2. 3. 4. 5. 6. 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 Chapter 15: The Chromosomal Basis of Inheritance 1. 2. 3. 4. 5. 6. 7. 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? Figure 15.10 The transmission of sex-linked recessive traits 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. Ova Xa (c) 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. Y XAXa XAY XA XAxa XAY XA Ova XaY XA XAXa (b) 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. XAY Y XA XAXA XAY Xa a XaxA X Y XAXa Sperm Sperm XaY Sperm Xa Ova Y XA XAXa XAY Xa Xaxa XaY Students -Get test folder -Grades posted – highlights are UNattractive -Pull out Learning logs for check -Phones in the bin…muted or off…please & thank you Chapter 15: The Chromosomal Basis of Inheritance 1. 2. 3. 4. 5. 6. 7. 8. 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? - Sex-linked recessive - 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 - Protein needed for blood clotting - Color blindness Chapter 15: The Chromosomal Basis of Inheritance 1. 2. 3. 4. 5. 6. 7. 8. 9. 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 (females) becomes almost completely inactive during embryonic development in mammals - 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 Figure 15.11 X inactivation and the tortoiseshell (calico) cat Two cell populations in adult cat: Active X Early embryo: X chromosomes Cell division Inactive X and X chromosome Inactive X inactivation Allele for orange fur Allele for black fur Orange fur Black fur Active X Students -New seats tomorrow -Genetics Problems -Write allele info: B - brown, b – black -Write parents gentoype for crosses -Bozeman videos -28 – Meiosis -29 & 30 – Genetics -33 & 34 – Genotypes & Phenotypes, Variation -Have a great day!! -Phones in bin….muted or off…please & thank you! Chapter 15: The Chromosomal Basis of Inheritance 1. How was it determined that chromosomes carry genes? 2. Morgan’s next cross showed that linked genes are inherited together. 3. What if the genes were unlinked…meaning independent assortment? 4. How often will recombination occur…frequency?? 5. How can a genetic map be created from recombination frequencies? 6. What determines male or female in utero? 7. How are sex-linked alleles transmitted? 8. What are some sex-linked alleles in humans? 9. What are Barr bodies? 10. What are some chromosomal errors & exceptions? - Nondisjunction - Homologous chromosomes fail to separate during meiosis - Chromosomal rearrangements Figure 15.12 Meiotic nondisjunction Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n+1 n+1 n1 n+1 n –1 n–1 Number of chromosomes (a) Nondisjunction of homologous chromosomes in meiosis I n n (b) Nondisjunction of sister chromatids in meiosis II 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 Crossing over is NOT exact (Figure 19.18) This leads to deletions & duplications. Figure 15.14 Alterations of chromosome structure (a) A deletion removes a chromosomal segment. (b) A duplication repeats a segment. (c) An inversion reverses a segment within a chromosome. (d) A translocation moves a segment from one 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 C D E F G H A B C D E F G H A B C D E F G H A B C D E F G H Deletion Duplication Inversion A B C E F G H A B C B C D E A D C B E F G H M N O C D E Reciprocal translocation M N O P Q R A B P Q F G H R F G H Chapter 15: The Chromosomal Basis of Inheritance 1. How was it determined that chromosomes carry genes? 2. Morgan’s next cross showed that linked genes are inherited together. 3. What if the genes were unlinked…meaning independent assortment? 4. How often will recombination occur…frequency?? 5. How can a genetic map be created from recombination frequencies? 6. What determines male or female in utero? 7. How are sex-linked alleles transmitted? 8. What are some sex-linked alleles in humans? 9. What are Barr bodies? 10. What are some chromosomal errors & exceptions? 11. What are some human disorders due to chromosomal alterations? - Down syndrome - Trisomy 21 aka nondisjunction of 21st chromosome - Each cell has 47 chromosomes Figure 15.15 Down syndrome Chapter 15: The Chromosomal Basis of Inheritance 1. How was it determined that chromosomes carry genes? 2. Morgan’s next cross showed that linked genes are inherited together. 3. What if the genes were unlinked…meaning independent assortment? 4. How often will recombination occur…frequency?? 5. How can a genetic map be created from recombination frequencies? 6. What determines male or female in utero? 7. How are sex-linked alleles transmitted? 8. What are some sex-linked alleles in humans? 9. What are Barr bodies? 10. What are some chromosomal errors & exceptions? 11. 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 ♀ Chapter 15: The Chromosomal Basis of Inheritance 1. How was it determined that chromosomes carry genes? 2. Morgan’s next cross showed that linked genes are inherited together. 3. What if the genes were unlinked…meaning independent assortment? 4. How often will recombination occur…frequency?? 5. How can a genetic map be created from recombination frequencies? 6. What determines male or female in utero? 7. How are sex-linked alleles transmitted? 8. What are some sex-linked alleles in humans? 9. What are Barr bodies? 10. What are some chromosomal errors & exceptions? 11. What are some human disorders due to chromosomal alterations? 12. How are extra-chromosomal genes inherited? - Maternally – from egg – only sperm nucleus enters egg during fertilization - All organelles including mitochondria