chromosome inheritance

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Figure 15.1 Mendel’s “hereditary factors” were genes Genes are located on chromosomes The location of a gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene

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Chromosome Theory of Inheritance ……. genes have specific loci (positions) on chromosomes Chromosomes undergo segregation and independent assortment © 2011 Pearson Education, Inc.

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Figure 15.2b F 1 Generation All F 1 plants produce yellow-round seeds ( YyRr ). Meiosis Metaphase I Anaphase I Metaphase II R R R R R R R R R R R R r r r r r r r r r r r r Y Y Y Y Y Y Y Y Y Y Y Y y y y y y y y y y y y y Gametes LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. 1 2 2 1 1 / 4 1 / 4 1 / 4 1 / 4 YR yr Yr yR

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Morgan’s Experiments with Fruit Flies (Drosophila melangastor ) Several characteristics make fruit flies a convenient organism for genetic studies………. produce many offspring short generation time (two weeks) have only four pairs of chromosomes © 2011 Pearson Education, Inc. wild type (“normal”) phenotypes are common in the population Traits alternative to the wild type are called mutant phenotypes

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Figure 15.3 Morgan mated female red eyes (wild type) x male white eyes (mutant) red eyes (wild type) white eyes (mutant)

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All offspring had red eyes. P Generation F 1 Generation F 2 Generation F 2 generation showed a 3:1 red : white eye ratio but…. only the males had white eyes ?!?!?

Figure 15.1:

In humans (and other animals) there is a chromosomal basis of sex determination © 2011 Pearson Education, Inc. Females = XX males = XY Every egg (ovum) always carries only an X chromosome A sperm may carry either an X or a Y chromosome

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X The Y chromosome mainly encodes only genes related to sex determination X chromosomes have genes for many characters unrelated to gender……. A gene located on either sex chromosome is called a sex-linked gene Genes on the X chromosome are called X-linked genes (there are very few genes on the Y chromosome ) Y

Figure 15.2b:

F 2 Generation P Generation Eggs Eggs Sperm Sperm X w  X X Y w  w  w  w  w  w  w  w  w  w  w w w w w w F 1 Generation white-eye allele must be located on the X chromosome

Morgan’s Experiments with Fruit Flies (Drosophila melangastor):

Eggs Eggs Eggs Sperm Sperm Sperm (a) (b) (c) X N X N X n Y X N X n X N Y X N X n X n Y X n Y X N Y Y X n X n X n X N X N X N X N X N X n X N Y X N Y X N Y X N Y X n Y X n Y X N X n X N X n X N X n X N X N X n X n For a recessive X-linked trait to be expressed A male needs only one copy of the allele ( hemizygous ) But a female needs two copies of the allele (homozygous) Thus, X-linked recessive disorders are more common in males than females

Figure 15.3:

X Inactivation in Female Mammals In female mammals - one of the two X chromosomes in each cell is randomly inactivated during embryonic development An inactive X chromosome = Barr body © 2011 Pearson Education, Inc. a female heterozygous for a particular gene located on the X chromosome will be a mosaic phenotype

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Figure 15.8 Early embryo development: X chromosomes Allele for orange fur Allele for black fur Mitosis and X chromosome inactivation Active X Inactive X Barr Body Active X Black fur Orange fur

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Each chromosome has hundreds or thousands of genes Law of Independent Assortment – genes assort independently (except for genes located on the SAME chromosome) © 2011 Pearson Education, Inc.

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P Generation (homozygous) Wild type (gray body, normal wings) b  b  vg  vg  b b vg vg Double mutant (black body, vestigial wings) EXPERIMENT

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P Generation (homozygous) Wild type (gray body, normal wings) F 1 dihybrid TESTCROSS b  b  vg  vg  b  b vg  vg b b vg vg b b vg vg Double mutant (black body, vestigial wings) Double mutant EXPERIMENT wild type

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P Generation (homozygous) Wild type (gray body, normal wings) F 1 dihybrid (wild type) Testcross offspring TESTCROSS b  b  vg  vg  b  b vg  vg b b vg vg b b vg vg Double mutant (black body, vestigial wings) Double mutant Eggs Sperm EXPERIMENT RESULTS PREDICTED RATIOS Wild type (gray-normal) Black- vestigial Gray- vestigial Black- normal b  vg  b vg b  vg b vg  b  b vg  vg b b vg vg b  b vg vg b b vg  vg 965 944 206 185 1 1 1 1 1 0 1 0 If genes are located on different chromosomes: If genes are located on the same chromosome And alleles are always inherited together: : : : : : : : : : b vg

X Inactivation in Female Mammals:

Wild type (gray body, normal wings) F 1 dihybrid (wild type) Testcross offspring TESTCROSS b  b vg  vg b b vg vg Double mutant (black body, vestigial wings) Double mutant Eggs Sperm Wild type (gray-normal) Black- vestigial Gray- vestigial Black- normal b  vg  b vg b  vg b vg  b  b vg  vg b b vg vg b  b vg vg b b vg  vg b vg Offspring with a phenotype matching one of the parental phenotypes are called parental types Offspring with non-parental phenotypes (new combinations of traits) are called recombinant types , or recombinants Recombinants

Figure 15.8:

occasionally something breaks the physical connection between genes located on the same chromosome …………resulting in new phenotypes = recombinants… …... © 2011 Pearson Education, Inc. Crossing Over of homologous chromosomes “breaks the physical connection of linked genes” Genes located on the same chromosome tend to be inherited together = linked genes

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Figure 15.10a Testcross parents Replication of chromosomes Gray body, normal wings (F 1 dihybrid) Black body, vestigial wings (double mutant) Replication of chromosomes Meiosis I Meiosis II Meiosis I and II Recombinant chromosomes Eggs b  vg  b vg b  vg  b  vg  b  vg  b  vg b vg  b vg b vg b vg b vg b vg b vg b vg b vg b vg b  vg  b  vg b vg b vg  Sperm b vg Crossing Over

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Figure 15.10b Testcross offspring 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal Sperm Parental-type offspring Recombinant offspring Recombination frequency 391 recombinants 2,300 total offspring  100  17%  b  vg  b vg  b  vg b vg b vg b vg b vg b vg b vg Eggs Recombinant chromosomes b  vg  b vg b  vg b vg 

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Chromosome Recombination frequencies 9% 9.5% 17% b cn vg genetic map - genetic loci on a chromosome farther apart two genes are = higher the probability that a crossover will occur between them and a higher recombination frequency

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Recombinant chromosomes (from crossing over) puts alleles together in new combinations that are passed on in gametes Random fertilization increases even further the number of variant combinations produced This abundance of genetic variation is the raw material upon which natural selection works © 2011 Pearson Education, Inc.

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Meiosis I Meiosis II Non-disjunction Non- disjunction Nondisjunction - homologous chromosomes do not separate normally during meiosis Alterations of chromosome number or structure cause abortion or genetic developmental disorders

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Meiosis I Meiosis II Nondisjunction Non- disjunction Gametes Number of chromosomes Nondisjunction of homologous chromosomes in meiosis I Nondisjunction of sister chromatids in meiosis II n  1 n  1 n  1 n  1 n  1 n  1 n n Figure 15.13-3

Figure 15.10a:

Abnormal egg cell with extra chromosome Normal sperm cell n  1 n (normal) Abnormal zygote with extra chromosome 2 n  1 Figure 8.21

Figure 15.10b:

Down syndrome - three copies of chromosome 21 Down Syndrome (Trisomy 21)

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Alterations of Chromosome Structure Breakage of a chromosome can lead to four types of changes in chromosome structure……… Deletion Duplication Inversion Translocation © 2011 Pearson Education, Inc.

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Figure 15.14 Deletion Duplication Inversion Translocation A deletion removes a chromosomal segment. A duplication repeats a segment. An inversion reverses a segment within a chromosome. A translocation moves a segment from one chromosome to a nonhomologous chromosome. A B C D E F G H A B C E F G H A B C D E F G H A B C D E F G H B C A B C D E F G H A D C B E F G H A B C D E F G H M N O P Q R G M N O C H F E D A B P Q R

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