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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 Make sure I have all test corrections & scantrons Ch 9, 11 – 12 Quarter test 1st period – get donut – Thank Michael!! get “pig-in-a-blanket” – Thank Regan!!
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Figure 15.2 The chromosomal basis of Mendel’s laws
Yellow-round seeds (YYRR) Green-wrinkled seeds (yyrr) Meiosis Fertilization Gametes All F1 plants produce yellow-round seeds (YyRr) P Generation F1 Generation Two equally probable arrangements of chromosomes at metaphase I LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT Anaphase I Metaphase II Fertilization among the F1 plants 9 : 3 : 1 1 4 YR yr yR Y R y r F2 Generation 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.) The R and r alleles segregate at anaphase I, yielding two types of daughter cells for this locus. Each gamete gets one long chromosome with either the R or r allele. 2 recombines the R and r alleles at random. 3 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 cells on the left and right Each gamete gets a long and a short chromosome in one of four allele combinations. Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation.
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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
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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 X F2 Generation Expected 3:1 Observed 3:1 Problem!!!!!! Only males had white eyes…..hmmm.
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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 X P Generation F1 F2 Ova (eggs) Sperm X Y W W+ F2 Generation
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Recombinant (nonparental-type)
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. EXPERIMENT Wild type (gray body, normal wings) P Generation (homozygous) b+ b+ vg+ vg+ x Double mutant (black body, vestigial wings) b b vg vg F1 dihybrid (wild type) b+ b vg+ vg TESTCROSS b+vg+ b vg b+ vg b vg+ b+ b vg vg b b vg+ vg 965 (gray-normal) 944 Black- vestigial 206 Gray- 185 normal Sperm Parental-type offspring Recombinant (nonparental-type) RESULTS 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). 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
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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 Gametes from green- wrinkled homozygous recessive parent (yyrr) Gametes from yellow-round heterozygous parent (YyRr) Parental- type offspring Recombinant YyRr yyrr Yyrr yyRr YR yr Yr yR 50% %
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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??
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Announcements Get lab notebooks from table Get test folder from table Grades posted New seats - THURSDAY Genetics problems – Due MONDAY Pre-lab for Meiosis due TOMORROW
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Figure 15.6 Chromosomal basis for recombination of linked genes
Testcross parents Gray body, normal wings (F1 dihybrid) b+ vg+ b vg Replication of chromosomes Meiosis I: Crossing over between b and vg loci produces new allele combinations. Meiosis II: Segregation of chromatids produces recombinant gametes with the new allele Recombinant chromosome b+vg+ b vg b+ vg b vg+ b vg Sperm Meiosis I and II: Even if crossing over occurs, no new allele combinations are produced. Ova Gametes offspring b+ vg+ b vg b+ vg b vg+ 965 Wild type (gray-normal) b vg b vg+ 944 Black- vestigial 206 Gray- 185 normal Recombination frequency = 391 recombinants 2,300 total offspring 100 = 17% Parental-type offspring Recombinant offspring Black body, vestigial wings (double mutant) Sturtevant – developed a genetic linkage map from recombination frequencies
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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? bcn 9% cnvg 9.5% bvg 17% Recombination frequencies 9% 9.5% 17% b cn vg Chromosome 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
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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 Gametes from green- wrinkled homozygous recessive parent (yyrr) Gametes from yellow-round heterozygous parent (YyRr) Parental- type offspring Recombinant YyRr yyrr Yyrr yyRr YR yr Yr yR 50% %
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Figure 15.8 A partial genetic (linkage) map of a Drosophila chromosome
Mutant phenotypes Short aristae Black body Cinnabar eyes Vestigial wings Brown Long aristae (appendages on head) Gray Red Normal Wild-type phenotypes II Y I X IV III 48.5 57.5 67.0 104.5
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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
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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?
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Figure 15.10 The transmission of sex-linked recessive traits
XAXA XaY Xa Y XAXa XAY XAxa XA Ova Sperm XaxA Xaxa 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. 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. 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. (a) (b) (c)
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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
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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
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Figure 15.11 X inactivation and the tortoiseshell (calico) cat
Two cell populations in adult cat: Active X Orange fur Inactive X Early embryo: X chromosomes Allele for black fur Cell division and X chromosome inactivation Black orange fur
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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
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Figure 15.12 Meiotic nondisjunction
Meiosis I Nondisjunction Meiosis II Gametes n + 1 n 1 n – 1 n –1 n Number of chromosomes Nondisjunction of homologous chromosomes in meiosis I Nondisjunction of sister chromatids in meiosis II (a) (b) 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
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Crossing over is NOT exact (Figure 19.18)
This leads to deletions & duplications.
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Figure 15.14 Alterations of chromosome structure
B C D E F G H Deletion Duplication M N O P Q R Inversion Reciprocal translocation (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.
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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
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Figure Down syndrome
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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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