Chapter 15 The Chromosomal Basis of Inheritance Power

Chapter 15 The Chromosomal Basis of Inheritance Power. Point® 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 • 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 1800 s • 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 -2 P Generation Yellow-round seeds (YYRR) Y Y R r R Green-wrinkled seeds ( yyrr) y y r Meiosis Fertilization y R Y Gametes r All F 1 plants produce yellow-round seeds (Yy. Rr) F 1 Generation R R y r Y Y LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. y r LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. Meiosis R r Y y r R Y y Metaphase I 1 1 R r Y y r R Y y Anaphase I R r Y y Metaphase II r R Y y 2 2 Y Y Gametes R R 1/ F 2 Generation 4 YR y r r r 1/ 4 Y Y y r 1/ yr 4 Yr y y R R 1/ 4 y. R An F 1 cross-fertilization 3 3 9 : 3 : 1

Fig. 15 -2 a 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 F 1 plants produce yellow-round seeds (Yy. Rr)

Fig. 15 -2 b All F 1 plants produce yellow-round seeds (Yy. Rr) 0. 5 mm F 1 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 Metaphase I Y y 1 1 r R Y y Anaphase I Y y r R Metaphase II R r 2 2 Gametes y Y Y R R 1 4 r 1 YR 3 4 yr Y Y y r y Y r r 14 Yr y y R R 1 4 y. R 3

Fig. 15 -2 c F 2 Generation An F 1 cross-fertilization 3 3 9 : 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 Mendel’s heritable “characters”, and genes are the “traits”. 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: – A generation of many individuals can be bred every two weeks – There are only four pairs of chromosomes – Males are xy and females are xx as in humans Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

• Morgan studied several genes that produced phenotypes that were very common in the fly populations. He called these phenotypes wild. • Alleles on those genes that produced alternatives to the wild type he called mutant. • Once such gene Morgan studied was eye color. 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) – The F 1 generation all had red eyes – The F 2 generation showed the 3: 1 red : white eye ratio, but only males had white eyes • Q: SO, on which chromosome is the gene for eye color located? Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 15 -4 EXPERIMENT P Generation F 1 Generation All offspring had red eyes RESULTS F 2 Generation CONCLUSION P Generation w+ X X w+ X Y w Eggs F 1 Generation w+ Sperm w+ w+ w w+ Eggs F 2 Generation w w+ w Sperm w+ w+ w+ w w w+

Fig. 15 -4 a EXPERIMENT P Generation F 1 Generation All offspring had red eyes

Fig. 15 -4 b RESULTS F 2 Generation

Fig. 15 -4 c CONCLUSION P Generation w+ X X w+ X Y w Eggs F 1 Generation Sperm w+ w+ w+ w w+ Eggs F 2 Generation w w+ Sperm 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

• 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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 15 -5 X Y

The Chromosomal Basis of Sex • In humans and other mammals, there are two varieties of sex chromosomes: a larger X chromosome and a smaller Y chromosome • Only the ends of the Y chromosome have regions that are homologous with the X chromosome • Other animals have different methods of sex determination Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 15 -6 Parents 44 + XX 22 + or Y X Sperm + 22 + X 44 + XX or 44 + XY Egg 44 + XY Zygotes (offspring) (a) The X-Y system 22 + XX 22 + X 76 + ZW 76 + ZZ 32 (Diploid) 16 (Haploid) (b) The X-0 system (c) The Z-W system (d) The haplo-diploid system

Fig. 15 -6 a 44 + XY 44 + XX Parents 22 + or Y X Sperm 44 + XX 22 + X + Egg or 44 + XY Zygotes (offspring) (a) The X-Y system

Fig. 15 -6 b 22 + XX (b) The X-0 system 22 + X

Fig. 15 -6 c 76 + ZW (c) The Z-W system 76 + ZZ

Fig. 15 -6 d 32 (Diploid) (d) The haplo-diploid system 16 (Haploid)

Inheritance of Sex-Linked Genes • The sex chromosomes have genes for many physical traits unrelated to sex, especially the “X” chromosome • A gene located on either sex chromosome is called a sex-linked gene • “SRY” is a sex-linked gene crucial to many traits (pleiotropy) on the “Y” chromosome. However, the “Y” chromosome has been shrinking since Homo sapiens evolved. It has lost 1393 of it’s original 1438 genes. 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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 15 -7 XN XN Sperm Xn Xn. Y (a) Sperm XN Y Eggs XN XNXn XNY XN XN Xn XN Y Xn (b) Sperm Xn Y Eggs XN XNY XN Xn XN Y XN Xn 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 – Hemophilia Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

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 • 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

Fig. 15 -8 X chromosomes Early embryo: Two cell populations in adult cat: Active X Allele for orange fur Allele for black fur Cell division and X chromosome inactivation Active X Inactive X Black fur 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 near each other on the same chromosome tend to be inherited together. • These are called linked genes Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

P Gen: b+ b+ vg+ (wild: gray, long) X b b vg vg (mutant: black, short) ---- F 1 b+ vg+ b vg b+ vg+ b vg F 2 b vg

A dihybrid cross under Mendelian rules yields a 9: 3: 3: 1 phenotypic ratio. So, what about…… Gametes from yellow-round heterozygous parent (Yy. Rr) F 1 Gametes from greenwrinkled homozygous recessive parent ( yyrr) YR yr Yr y. R Yy. Rr yyrr Yyrr yy. Rr yr Parentaltype offspring Recombinant offspring

How Linkage Affects Inheritance • Morgan was one of the first to see that different genes sometimes don’t behave in Mendelian ways. • Ex: When he crossed flies to study the heredity of both body color and wing size, he found that these completely different traits were usually inherited together (they did not “assort independently”). This was because they were _________________!!! Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 15 -9 -1 EXPERIMENT P Generation (homozygous) Double wild (gray body, normal wings) b+ b+ 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+ F 1 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+ F 1 dihybrid (wild type) Double mutant TESTCROSS b+ b vg+ vg Testcross offspring b b vg 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 What would the phenotypic ratio be in the F 2 if Mendelian rules hold up? What would the ratio be if the 2 genes are linked, as Morgan predicted?

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+ F 1 dihybrid (wild type) Double mutant TESTCROSS b+ b vg+ vg Testcross offspring b b vg 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 If genes are located on the same chromosome and parental alleles are always inherited together: 1 : 0 965 : 944 : 206 : 185 RESULTS

Parental and nonparental phenotypes were produced in the cross between a dihybrid and double mutant. • 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 • 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 • A 50% frequency of recombination is observed for any two genes on different chromosomes Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

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 Testcross parents Gray body, normal wings (F 1 dihybrid) Replication of chromosomes Meiosis I Black body, vestigial wings (double mutant) b+ vg+ b vg Replication of chromosomes b+ vg+ b vg b vg b+ vg+ b+ vg Meiosis I and II b vg+ b vg Meiosis II Recombinant chromosomes Eggs Testcross offspring b+ vg+ 965 Wild type (gray-normal) b vg 944 Blackvestigial b+ vg 206 Grayvestigial b vg+ 185 Blacknormal 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 -10 b 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 Parental-type offspring b vg Sperm Recombinant offspring Calculate the Recombination Frequency!

Fig. 15 -10 b 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 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 • 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 • 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 (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 (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 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 • 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 • A trisomic zygote has three copies of a particular chromosome Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

• Polyploidy is a condition in which an organism has more than two complete sets of chromosomes – Triploidy (3 n) is three sets of chromosomes – Tetraploidy (4 n) is four sets of chromosomes • Polyploidy is common in plants, 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 – Duplication repeats a segment – 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 Deletion Duplication A B C E F G H A 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 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

Down Syndrome (Trisomy 21) • Down syndrome is an aneuploid condition that results from three copies of chromosome 21 • It affects about one out of every 700 children born in the United States • The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 15 -16

Fig. 15 -16 a

Fig. 15 -16 b

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 • Monosomy X, called Turner syndrome, produces X 0 females, who are sterile; it is the only known viable monosomy in humans Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

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)

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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 15 -18 Paternal chromosome Normal Igf 2 allele is expressed Maternal chromosome Normal Igf 2 allele is not expressed Wild-type mouse (normal size) (a) Homozygote Mutant Igf 2 allele inherited from mother Normal size mouse (wild type) Mutant Igf 2 allele inherited from father Dwarf mouse (mutant) Normal Igf 2 allele is expressed Mutant Igf 2 allele is not expressed Normal Igf 2 allele is not expressed (b) Heterozygotes

Fig. 15 -18 a Paternal chromosome Normal Igf 2 allele is expressed Maternal chromosome Normal Igf 2 allele is not expressed (a) Homozygote Wild-type mouse (normal size)

Fig. 15 -18 b Mutant Igf 2 allele inherited from mother Normal size mouse (wild type) Mutant Igf 2 allele inherited from father Dwarf mouse (mutant) Normal Igf 2 allele is expressed Mutant Igf 2 allele is not expressed Normal Igf 2 allele is not expressed (b) Heterozygotes

• It appears that imprinting is the result of the methylation (addition of –CH 3) of DNA • Genomic imprinting is thought to affect only a small fraction of mammalian genes • Most imprinted genes are critical for embryonic development Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 15 -UN 3

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 • Extranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the egg • The first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 15 -19

• 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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 15 -UN 4 P generation D gametes Sperm C B A Egg E + c b a d F f The alleles of unlinked genes are either on separate chromosomes (such as d and e) or so far apart on the same chromosome (c and f ) that they assort independently. This F 1 cell has 2 n = 6 chromosomes and is heterozygous for all six genes shown (Aa. Bb. Cc. Dd. Ee. Ff ). Red = maternal; blue = paternal. D Each chromosome has hundreds or thousands of genes. Four (A, B, C, F) are shown on this one. e C B A F e d E cb a f Genes on the same chromosome whose alleles are so close together that they do not assort independently (such as a, b, and c) are said to be linked.

Fig. 15 -UN 5

Fig. 15 -UN 6

Fig. 15 -UN 7

Fig. 15 -UN 8

Fig. 15 -UN 9

You should now be able to: 1. Explain the chromosomal theory of inheritance and its discovery 2. Explain why sex-linked diseases are more common in human males than females 3. Distinguish between sex-linked genes and linked genes 4. Explain how meiosis accounts for recombinant phenotypes 5. Explain how linkage maps are constructed Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

6. Explain how nondisjunction can lead to aneuploidy 7. Define trisomy, triploidy, and polyploidy 8. Distinguish among deletions, duplications, inversions, and translocations 9. Explain genomic imprinting 10. Explain why extranuclear genes are not inherited in a Mendelian fashion Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings