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Chapter 15 The Chromosomal Basis of Inheritance PowerPoint® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Overview: Locating Genes Along Chromosomes Crash Course Genetics • Mendel’s “hereditary factors” were genes, though this wasn’t known at the time • Today we can show that genes are located on chromosomes • The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 15-1 Concept 15.1: Mendelian inheritance has its physical basis in the behavior of chromosomes • Mitosis and meiosis were first described in the late 1800s • The chromosome theory of inheritance states: – Mendelian genes have specific loci (positions) on chromosomes – Chromosomes undergo segregation and independent assortment • The behavior of chromosomes during meiosis was said to account for Mendel’s laws of segregation and independent assortment Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 15-2a Green-wrinkled seeds ( yyrr) Yellow-round seeds (YYRR) P Generation Y Y R R r y y r Meiosis Fertilization Gametes R Y y r All F1 plants produce yellow-round seeds (YyRr) Fig. 15-2b All F1 plants produce yellow-round seeds (YyRr) 0.5 mm F1 Generation R R y r Y LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. y r Y LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. Meiosis r R Y y r R Metaphase I Y y 1 1 r R r R Y y Anaphase I Y y r R Metaphase II R r 2 2 Gametes y Y Y R R 1 4 YR r 1 3 4 yr Y Y y r y Y y Y r r 14 Yr y y R R 14 yR 3 Fig. 15-2c F2 Generation An F1 F1 cross-fertilization 3 3 9 :3 :3 :1 Morgan’s Experimental Evidence: Scientific Inquiry • The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan, an embryologist • Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Morgan’s Choice of Experimental Organism • Several characteristics make fruit flies a convenient organism for genetic studies: – They breed at a high rate – A generation can be bred every two weeks – They have only four pairs of chromosomes • Morgan noted wild type, or normal, phenotypes that were common in the fly populations • Traits alternative to the wild type are called mutant phenotypes • He crossed these organisms for 2 years in order to see naturally occurring mutants. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 15-3 Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair • In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) – The F1 generation all had red eyes – The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes • Morgan determined that the white-eyed mutant allele must be located on the X chromosome • Morgan’s finding supported the chromosome theory of inheritance Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Where do genes come from? • Morgan’s experiments suggested that a specific gene is carried on a specific chromosome • ALSO, genes located on a sex chromosome exhibit unique inheritance patterns Fig. 15-4a EXPERIMENT P Generation F1 Generation All offspring had red eyes Fig. 15-4b RESULTS F2 Generation Fig. 15-4c CONCLUSION P Generation w+ X X w+ X Y w Eggs F1 Generation Sperm w+ w+ w+ w w+ Eggs F2 Generation w w+ Sperm w+ w+ 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 The Chromosomal Basis of Sex • In humans and other mammals, there are two varieties of sex chromosomes: a larger and far superior X chromosome and a smaller and weaker Y chromosome • Only the ends of the Y chromosome have regions that are homologous with the X chromosome (these areas will pair during meiosis- increase the variation) • The SRY (sex determining region on the Y) gene on the Y chromosome codes for the development of testes • Remember: female is the default setting for us, so in the absence of a Y cs, we survive and do just fine (almost) • Without an X, we die Fig. 15-5 X Y • 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 Fig. 15-6a 44 + XY 44 + XX Parents 22 + 22 + or Y X Sperm 44 + XX 22 + X + Egg or 44 + XY Zygotes (offspring) (a) The X-Y system Fig. 15-6b 22 + XX 22 + X (b) The X-0 system Only one type of sex cs, the females are xx And males are xo; so sex of offspring is determined by whether the sperm has two X or one cs. Fig. 15-6c 76 + ZW 76 + ZZ (c) The Z-W system The sex cs present in the EGG determine the sex; females Are ZW, males ZZ Fig. 15-6d 32 (Diploid) 16 (Haploid) (d) The haplo-diploid system There are no sex cs in most species of bees and ants. Females develop from fertilized eggs (2n); males develop from Unfertilized eggs and are haploid (n) 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 and more intelligent 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 (why?) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • If a trait is sex linked you must diagram it in a pedigree as attached to the sex chromosomes. This way we know it is “influenced” by the sex of the individual Fig. 15-7 XNXN Sperm Xn Xn Y (a) Sperm XN Y Eggs XN XNXn XNY XN XNXn XNY Xn (b) Sperm Xn Y Eggs XN XNXN XNY XNXn XNY XNXn Xn Y Y Eggs XN XNXn XNY Xn XN Xn Y Xn (c) Xn Xn Xn Y • Some disorders caused by recessive alleles on the X chromosome in humans: – Color blindness – Duchenne muscular dystrophy- weakening of the muscles and loss of coordination; missing a muscle protein called dystrophin – Hemophilia- proteins for clotting factors are missing X Inactivation in Female Mammals • In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development • The inactive X condenses into a Barr body (not the same as polar bodies) • If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Barr body chromosomes are reactivated in cells that give rise to gametes, so the female gamete will have an active X • So, we (girls) are a MOSAIC; some of our cells have the X from dad active, others have from mom • Once an X is inactivated in a cell, through mitosis each cell will have a copy of the inactive X • Women can show this in sweat glands, some work, other “patches” don’t work X inactivation and epigenetics Bozeman: X inactivation • Inactivation involves modification of DNA (chapter 18) or a gene called XIST (x-inactive specific transcript) that is only on the barr body cs.; • it is thought that multiple RNA copies of this gene attach only to the X cs that is destined to be inactivated, ultimately covering it Fig. 15-8 X chromosomes Early embryo: Allele for orange fur Allele for black fur Cell division and X chromosome inactivation Two cell populations in adult cat: Active X Active X Inactive X Black fur What can you tell Me about this cat? Orange fur 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 Recombination frequency= # of recombinants x 100= % # of total offspring 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 Fig. 15-UN1 b vg b+ vg+ Parents in testcross Most offspring b vg b vg b+ vg+ b vg or b vg b vg Fig. 15-9-1 EXPERIMENT P Generation (homozygous) Wild type (gray body, normal wings) b+ b+ vg+ vg+ Double mutant (black body, vestigial wings) b b vg vg Fig. 15-9-2 EXPERIMENT P Generation (homozygous) Wild type (gray body, normal wings) b b vg vg b+ b+ vg+ vg+ F1 dihybrid (wild type) b+ b vg+ vg Double mutant (black body, vestigial wings) TESTCROSS Double mutant b b vg vg Fig. 15-9-3 EXPERIMENT P Generation (homozygous) 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 Testcross offspring b b vg vg b vg b+ vg b vg+ Wild type (gray-normal) Blackvestigial Grayvestigial Blacknormal b+ b vg+ vg b b vg vg b+ b vg vg b b vg+ vg Eggs b+ vg+ b vg Sperm Fig. 15-9-4 EXPERIMENT P Generation (homozygous) 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 Testcross offspring b b vg vg b vg b+ vg b vg+ Wild type (gray-normal) Blackvestigial Grayvestigial Blacknormal b+ b vg+ vg b b vg vg b+ b vg vg b b vg+ vg Eggs b+ vg+ b vg Sperm 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 : 0 : 0 965 : 944 : 206 : 185 RESULTS • 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 Genetic Recombination and Linkage Bozeman: gene recombination and gene mapping • The genetic findings of Mendel and Morgan relate to the chromosomal basis of recombination Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Recombination of Unlinked Genes: Independent Assortment of Chromosomes • Mendel observed that combinations of traits in some offspring differ from either parent • Offspring with a phenotype matching one of the parental phenotypes are called parental types • Offspring with nonparental phenotypes (new combinations of traits) are called recombinant types, or recombinants Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • A 50% frequency of recombination is observed for any two genes on different chromosomes • (from a test cross between parents a parent hetero for two traits with a parent homo recessive for two traits --- like AaBb x aabb Fig. 15-UN2 Gametes from yellow-round heterozygous parent (YyRr) Gametes from greenwrinkled homozygous recessive parent ( yyrr) YR yr Yr yR YyRr yyrr Yyrr yyRr yr Parentaltype offspring Recombinant offspring Recombination of Linked Genes: Crossing Over • Morgan discovered that genes can be linked, but the linkage was incomplete, as evident from recombinant phenotypes • Morgan proposed that some process must sometimes break the physical connection between genes on the same chromosome • That mechanism was the crossing over of homologous chromosomes Animation: Crossing Over Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 15-10 Gray body, normal wings (F1 dihybrid) Testcross parents Replication of chromosomes Meiosis I Black body, vestigial wings (double mutant) b+ vg+ b vg b vg b vg Replication of chromosomes b+ vg+ b vg b+ vg+ b vg b vg b vg b vg b vg b+ vg+ Meiosis I and II b+ vg b vg+ b vg Meiosis II Recombinant chromosomes Eggs Testcross offspring b+ vg+ b vg b+ vg b vg+ 965 944 206 185 Wild type (gray-normal) Blackvestigial Grayvestigial Blacknormal b+ vg+ b vg b+ vg b vg+ b vg b vg b vg b vg Parental-type offspring Recombinant offspring 391 recombinants Recombination 100 = 17% = frequency 2,300 total offspring b vg Sperm Fig. 15-10a Testcross parents Black body, vestigial wings (double mutant) Gray body, normal wings (F1 dihybrid) Replication of chromosomes Meiosis I 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+ Meiosis I and II vg b vg+ b vg Meiosis II Recombinant chromosomes b+ vg+ b vg Eggs b+ vg b vg+ b vg Sperm Replication of chromosomes Fig. 15-10b Recombinant chromosomes Eggs Testcross offspring b+ vg+ b vg b+ vg b vg+ 944 Wild type Black(gray-normal) vestigial 206 Grayvestigial 185 Blacknormal 965 b+ vg+ b vg b+ vg b vg+ b vg b vg b vg b vg Parental-type offspring Recombinant offspring 391 recombinants Recombination 100 = 17% = frequency 2,300 total offspring b vg Sperm Mapping the Distance Between Genes Using Recombination Data: Scientific Inquiry • Alfred Sturtevant, one of Morgan’s students, constructed a genetic map, an ordered list of the genetic loci along a particular chromosome • Sturtevant predicted that the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • A linkage map is a genetic map of a chromosome based on recombination frequencies • Distances between genes can be expressed as map units; one map unit, or centimorgan, represents a 1% recombination frequency (example I showed you earlier) • Map units indicate relative distance and order, not precise locations of genes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 15-11 RESULTS Recombination frequencies 9% Chromosome 9.5% 17% b cn vg • Genes that are far apart on the same chromosome can have a recombination frequency near 50% • Such genes are physically linked, but genetically unlinked, and behave as if found on different chromosomes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Sturtevant used recombination frequencies to make linkage maps of fruit fly genes • Using methods like chromosomal banding, geneticists can develop cytogenetic maps of chromosomes • Cytogenetic maps indicate the positions of genes with respect to chromosomal features Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 15-12 Short aristae 0 Long aristae (appendages on head) Mutant phenotypes Black body 48.5 Gray body Cinnabar Vestigial eyes wings 57.5 Red eyes 67.0 Normal wings Wild-type phenotypes Brown eyes 104.5 Red eyes Concept 15.4: Alterations of chromosome number or structure cause some genetic disorders • Large-scale chromosomal alterations often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Abnormal Chromosome Number • In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis – (anaphase I or II) • As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 15-13-1 Meiosis I Nondisjunction One homologous pair Doesn’t separate during Anaphase 1 Therefore all gametes Are affected (a) Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II Fig. 15-13-2 Meiosis I Nondisjunction Meiosis II Nondisjunction The homologous pair Separated (tetrad) into Individual chromatids And the sister chromatid Separated It has 2 copies of a cs that codes for the same information Sister chromatid Separated But it is missing all The genes that were coded For by the homologous cs That all went into the other gamete (a) Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II Fig. 15-13-3 Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n+1 n+1 n–1 n–1 n+1 n–1 n Number of chromosomes (a) Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II n • Aneuploidy results from the fertilization of gametes in which nondisjunction occurred – So the organism will either have more copies or fewer than necessary • Offspring with this condition have an abnormal number of a particular chromosome Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • A monosomic zygote has only one copy of a particular chromosome – usually lethal (only 1 viable form in humans- Turner’s syndrome) • A trisomic zygote has three copies of a particular chromosome Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Down syndrome (trisomy 21): The result of an extra copy of chromosome 21. People with Down syndrome are 47, 21+. Down syndrome affects 1:700 children and alters the child's phenotype either moderately or severely: • characteristic facial features, short stature; heart defects • susceptibility to respiratory disease, shorter lifespan • prone to developing early Alzheimer's and leukemia • often sexually underdeveloped and sterile, usually some degree of mental retardation. • Down Syndrome is correlated with age of mother but can also be the result of nondisjunction of the father's chromosome 21. • Fig. 15-16 • Patau syndrome (trisomy 13): serious eye, brain, circulatory defects as well as cleft palate. 1:5000 live births. Children rarely live more than a few months. • Edward's syndrome (trisomy 18): almost every organ system affected 1:10,000 live births. Children with full Trisomy 18 generally do not live more than a few months • 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 (bananas and wheat), but not animals • Polyploids are more normal in appearance than aneuploids Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 15-14 Alterations of Chromosome Structure • Breakage of a chromosome can lead to four types of changes in chromosome structure: – Deletion removes a chromosomal segment (more likely to occur during meiosis) – Duplication repeats a segment (more likely to occur during meiosis) – Inversion reverses a segment within a chromosome – Translocation moves a segment from one chromosome to another Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 15-15 (a) (b) (c) (d) 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 A B C E F G H A B C B C D E Inversion A D C B E R F G H M N O C D E Reciprocal translocation M N O P Q F G H A B P Q R F G H Human Disorders Due to Chromosomal Alterations • Alterations of chromosome number and structure are associated with some serious disorders • Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond • These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Aneuploidy of Sex Chromosomes • Nondisjunction of sex chromosomes produces a variety of aneuploid conditions • Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY individuals – Individuals are somewhat taller than average – often have below normal intelligence. – At one time (~1970s), it was thought that these men were likely to be criminally aggressive, but this hypothesis has been disproven over time. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Monosomy X, called Turner syndrome, produces X0 females • 1:5000 live births; the only viable monosomy in humans - women with Turner's have only 45 chromosomes!!! XO individuals are genetically female, however, they do not mature sexually during puberty and are sterile. Short stature and normal intelligence. (98% of these fetuses die before birth) Disorders Caused by Structurally Altered Chromosomes • The syndrome cri du chat (“cry of the cat”), results from a specific deletion in chromosome 5 • A child born with this syndrome is mentally retarded and has a catlike cry; individuals usually die in infancy or early childhood • Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 15-17 Normal chromosome 9 Normal chromosome 22 Reciprocal translocation Translocated chromosome 9 Translocated chromosome 22 (Philadelphia chromosome) Fragile X: the most common form of mental retardation. The X chromosome of some people is unusually fragile at one tip - seen "hanging by a thread" under a microscope. Most people have 29 "repeats" at this end of their X-chromosome, those with Fragile X have over 700 repeats due to duplications. Affects 1:1500 males, 1:2500 females. Translocation: a fragment of a chromosome is moved ("trans-located") from one chromosome to another - joins a non-homologous chromosome. The balance of genes is still normal (nothing has been gained or lost) but can alter phenotype as it places genes in a new environment. Can also cause difficulties in egg or sperm development and normal development of a zygote. Acute Myelogenous Leukemia is caused by this translocation: Concept 15.5: Some inheritance patterns are exceptions to the standard chromosome theory • There are two normal exceptions to Mendelian genetics • One exception involves genes located in the nucleus, and the other exception involves genes located outside the nucleus Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Genomic Imprinting • For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits • Such variation in phenotype is called genomic imprinting • Genomic imprinting involves the silencing of certain genes that are “stamped” with an imprint during gamete production (most are on the autosomes and not the sex cs.) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Normally, most genes get a copy of an allele from both parents. • With imprinted genes there is only ONE working copy of the gene, one is turned off during gamete formation – Usually by the addition of a methyl group Once it has been silenced, it will remain silent until they become reprogrammed right before gamete formation • But that doesn’t always happen, sometimes the imprinted genes don’t ever reset and begin their life with epigenetic tags in place What happens if imprinting goes wrong? You can have two active alleles (when you only need one) or you have two inactive alleles this can lead to cancer, developmental abnormalities and other problems Prader-Willi and Angelman’s Syndrome • Both on chromosomes 15 • Both have the same imprinted area on the cs – Some of the genes on this cs are inactivated in the egg, and atleast one of them is inactive in the sperm Someone who inherits this disorder on this cs gets this because of the inactive allele from either parent Prader-Willi • Symptoms include learning difficulties, short stature, and compulsive eating. MOST individuals are missing gene activity that normally comes from dad. Happens when dad's copy is missing, or when there are two maternal copies. Angelman’s • Symptoms include learning difficulties, speech problems, seizures, jerky movements, and an unusually happy disposition. • Individuals are missing gene activity that normally comes from mom. • Happens when mom's copy is defective or missing, or when there are two paternal copies So, YOU ALL ARE ASKING, what the heck is epigenetics and why do I care • “The development and maintenance of an organism is orchestrated by a set of chemical reactions that switch parts of the genome off and on at strategic times and locations. Epigenetics is the study of these reactions and the factors that influence them.” • Source: • http://learn.genetics.utah.edu/content/epigenetics/ • This is important because it makes you the wonderful being that you are! • Neil Tyson DeGrasse • https://www.youtube.com/watch?v=7WEHoCA1hp o • It appears that imprinting is the result of the methylation (addition of –CH3) of DNA • Genomic imprinting is thought to affect only a small fraction of mammalian genes (about 1%) • Most imprinted genes are critical for embryonic development Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings EVERYTHING is about EVOLUTION • Genetic Conflict Theory: some organisms have litters of babies that can be fathered by different males; so the dad would “want” its own offspring to have a better chance to survive (and taking more of the nutrients offered by mother from other litter mates); dad would prefer his offspring to be larger • OR • It would be better for all the babies to have the maternal imprint to ensure survival of ALL the babies, mom would prefer smaller babies in order to be able to support them Ligers and Tigons and Mules The differences seen between the different combinations is determined by which parental genes are being imprinted Inheritance of Organelle Genes • Extranuclear genes (or cytoplasmic genes) are genes found in organelles in the cytoplasm • Mitochondria, chloroplasts, and other plant plastids carry small circular DNA molecules Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Extranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the egg • Remember: most of the paternal mitochondria are located behind the head of the sperm, and fall off • The first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant Fig. 15-19 Variegations are Due to mutations in Plastids genes that Control pigmentation These pigment genes Are distributed Randomly to daughter Cells; the plant Pattern is determined By the ratio of wild-type To mutant plastids In the tissue • Some defects in mitochondrial genes prevent cells from making enough ATP and result in diseases that affect the muscular and nervous systems – For example, mitochondrial myopathy and Leber’s hereditary optic neuropathy – Blame it on your mother Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings