The Chromosomal Basis of Inheritance

The Chromosomal Basis of Inheritance
Chapter 15 The Chromosomal Basis of Inheritance

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 Figure 15.1 Where are Mendel’s hereditary factors located in the cell?

Mitosis and meiosis were first described in the late 1800s
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

Figure 15.2 The chromosomal basis of Mendel’s laws
P Generation Yellow-round seeds (YYRR) Green-wrinkled seeds ( yyrr) Y y  r Y R R r y Meiosis Fertilization R Y y r Gametes All F1 plants produce yellow-round seeds (YyRr) F1 Generation R R y y r r Y Y LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. Meiosis LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. R r r R Metaphase I Y y Y y 1 1 R r r R Anaphase I Y y Y y Figure 15.2 The chromosomal basis of Mendel’s laws R r Metaphase II r R 2 2 Y y Y y y Y Y y Y Y y y Gametes R R r r r r R R 1/4 YR 1/4 yr 1/4 Yr 1/4 yR F2 Generation An F1  F1 cross-fertilization 3 3 9 : 3 : 3 : 1

yellow-round seeds (YyRr)
Fig. 15-2a Yellow-round seeds (YYRR) Green-wrinkled seeds ( yyrr) P Generation y Y r  R R r Y y Meiosis Fertilization y r R Y Figure 15.2 The chromosomal basis of Mendel’s laws Gametes All F1 plants produce yellow-round seeds (YyRr)

0.5 mm All F1 plants produce yellow-round seeds (YyRr) F1 Generation
Fig. 15-2b All F1 plants produce yellow-round seeds (YyRr) 0.5 mm F1 Generation R R y y r r Y Y LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. Meiosis LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. R r r R Metaphase I Y y Y y 1 1 R r r R Anaphase I Y y Y y R r Metaphase II Figure 15.2 The chromosomal basis of Mendel’s laws r R 2 2 Y y Y y y Y Y Y Y y y y Gametes R R r r r r R R 1 4 YR 1 4 yr 1 4 Yr 1 4 yR 3 3

An F1  F1 cross-fertilization
Fig. 15-2c F2 Generation An F1  F1 cross-fertilization 3 3 9 : 3 : 3 : 1 Figure 15.2 The chromosomal basis of Mendel’s laws

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

Traits alternative to the wild type are called mutant phenotypes
Morgan noted wild type, or normal, phenotypes that were common in the fly populations Traits alternative to the wild type are called mutant phenotypes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 15-3 Figure 15.3 Morgan’s first mutant

Morgan’s finding supported the chromosome theory of inheritance
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

EXPERIMENT RESULTS CONCLUSION Fig. 15-4 + w w + w w + +
 Generation F1 All offspring had red eyes Generation RESULTS F2 Generation CONCLUSION P + X w X w Generation X  Y + w w Sperm Eggs F1 + + 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? + w w Generation w w + w Sperm Eggs + + + w w F2 w Generation + w w w w + w

EXPERIMENT P  Generation F1 All offspring Generation had red eyes
Fig. 15-4a EXPERIMENT P  Generation F1 All offspring had red eyes Generation 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?

RESULTS F2 Generation Fig. 15-4b
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?

CONCLUSION + P X X Generation  X Y + Sperm Eggs + + F1 + Generation +
Fig. 15-4c CONCLUSION + P w w X X Generation  X Y + w w Sperm Eggs + + F1 w w + Generation w w + w 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? Sperm Eggs + + w w + F2 w Generation 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 X chromosome and a smaller Y chromosome Only the ends of the Y chromosome have regions that are homologous with the X chromosome The SRY gene on the Y chromosome codes for the development of testes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 15-5 X Y Figure 15.5 Human sex chromosomes

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

Figure 15.6 Some chromosomal systems of sex determination
44 + XY 44 + XX Parents 22 + X 22 + Y 22 + X or + Sperm Egg 44 + XX 44 + XY or Zygotes (offspring) (a) The X-Y system 22 + XX 22 + X (b) The X-0 system 76 + ZW 76 + ZZ Figure 15.6 Some chromosomal systems of sex determination (c) The Z-W system 32 (Diploid) 16 (Haploid) (d) The haplo-diploid system

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

(a) (b) (c) Sperm Sperm Sperm Eggs Eggs Eggs XNXN  XnY XNXn  XNY
Fig. 15-7 XNXN  XnY XNXn  XNY XNXn  XnY Sperm Xn Y Sperm XN Y Sperm Xn Y Eggs XN XNXn XNY Eggs XN XNXN XNY Eggs XN XNXn XNY XN XNXn XNY Xn XnXN XnY Xn XnXn XnY Figure 15.7 The transmission of sex-linked recessive traits (a) (b) (c)

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

X chromosomes Allele for orange fur Early embryo: Allele for black fur
Fig. 15-8 X chromosomes Allele for orange fur Early embryo: Allele for black fur Cell division and X chromosome inactivation Two cell populations in adult cat: Active X Inactive X Active X Black fur Orange fur Figure 15.8 X inactivation and the tortoiseshell cat

Each chromosome has hundreds or thousands of genes
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

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

b+ vg+ b vg  Parents in testcross b vg b vg b+ vg+ b vg Most or
Fig. 15-UN1 b+ vg+ b vg  Parents in testcross b vg b vg b+ vg+ b vg Most offspring or b vg b vg

EXPERIMENT Fig. 15-9-1 P Generation (homozygous) b+ b+ vg+ vg+
Wild type (gray body, normal wings) Double mutant (black body, vestigial wings)  b+ b+ vg+ vg+ b b vg vg Figure 15.9 How does linkage between two genes affect inheritance of characters?

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 b b vg vg Figure 15.9 How does linkage between two genes affect inheritance of characters?

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 b b vg vg Testcross offspring Eggs b+ vg+ b vg b+ vg b vg+ Wild type (gray-normal) Black- vestigial Gray- vestigial Black- normal b vg Figure 15.9 How does linkage between two genes affect inheritance of characters? Sperm b+ b vg+ vg b b vg vg b+ b vg vg b b vg+ vg

EXPERIMENT RESULTS Fig. 15-9-4 P Generation (homozygous) b+ b+ vg+ vg+
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 b b vg vg Testcross offspring Eggs b+ vg+ b vg b+ vg b vg+ Wild type (gray-normal) Black- vestigial Gray- vestigial Black- normal b vg Figure 15.9 How does linkage between two genes affect inheritance of characters? Sperm b+ b vg+ vg b b vg vg b+ b vg vg b b vg+ vg 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 : : RESULTS 965 : 944 : 206 : 185

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
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

YR yr Yr yR yr YyRr yyrr Yyrr yyRr Gametes from yellow-round
Fig. 15-UN2 Gametes from yellow-round heterozygous parent (YyRr) YR yr Yr yR Gametes from green- wrinkled homozygous recessive parent ( yyrr) yr YyRr yyrr Yyrr yyRr Parental- type 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

Black body, vestigial wings
Fig Testcross parents Gray body, normal wings (F1 dihybrid) Black body, vestigial wings (double mutant) b+ vg+ b vg b vg b vg Replication of chromosomes Replication of chromosomes b+ vg+ b vg b+ vg+ b vg b vg b vg b vg b vg Meiosis I b+ vg+ Meiosis I and II b+ vg b vg+ b vg Meiosis II Recombinant chromosomes Figure Chromosomal basis for recombination of linked genes b+ vg+ b vg b+ vg b vg+ Eggs Testcross offspring 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal b vg b+ vg+ b vg b+ vg b vg+ b vg b vg b vg b vg Sperm Parental-type offspring Recombinant offspring Recombination frequency 391 recombinants =  100 = 17% 2,300 total offspring

Black body, vestigial wings
Fig a Testcross parents Gray body, normal wings (F1 dihybrid) Black body, vestigial wings (double mutant) b+ vg+ b vg b vg b vg Replication of chromosomes Replication of chromosomes b+ vg+ b vg b+ vg+ b vg b vg b vg b vg b vg Meiosis I b+ vg+ Meiosis I and II b+ vg b vg+ Figure Chromosomal basis for recombination of linked genes b vg Meiosis II Recombinant chromosomes b+ vg+ b vg b+ vg b vg+ b vg Eggs Sperm

965 944 Black- vestigial 206 Gray- vestigial 185 Black- normal
Fig b Recombinant chromosomes b+ vg+ b vg b+ vg b vg+ Eggs Testcross offspring 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal b vg b+ vg+ b vg b+ vg b vg+ Figure Chromosomal basis for recombination of linked genes b vg b vg b vg b vg Sperm Parental-type offspring Recombinant offspring Recombination frequency 391 recombinants =  100 = 17% 2,300 total offspring

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

RESULTS Recombination frequencies 9% 9.5% Chromosome 17% b cn vg
Fig RESULTS Recombination frequencies 9% 9.5% Chromosome 17% Figure Constructing a linkage map 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

Mutant phenotypes Short aristae Black body Cinnabar eyes Vestigial
Fig Mutant phenotypes Short aristae Black body Cinnabar eyes Vestigial wings Brown eyes 48.5 57.5 67.0 104.5 Figure A partial genetic (linkage) map of a Drosophila chromosome Long aristae (appendages on head) Gray body Red eyes Normal wings Red eyes Wild-type phenotypes

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

Meiosis I (a) Nondisjunction of homologous
Fig Meiosis I Nondisjunction Figure Meiotic nondisjunction For the Cell Biology Video Nondisjunction in Mitosis, go to Animation and Video Files. (a) Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II

Meiosis I Meiosis II (a) Nondisjunction of homologous
Fig Meiosis I Nondisjunction Meiosis II Nondisjunction Figure Meiotic nondisjunction (a) Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II

Meiosis I Meiosis II Gametes (a) Nondisjunction of homologous
Fig Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes Figure Meiotic nondisjunction n + 1 n + 1 n – 1 n – 1 n + 1 n – 1 n n Number of chromosomes (a) Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II

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 common in plants, but not animals
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, but not animals Polyploids are more normal in appearance than aneuploids Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Red Vizcacha Rat (Tympanoctomys barrerae)
Fig Figure A tetraploid mammal Red Vizcacha Rat (Tympanoctomys barrerae) 4n = 102

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

Reciprocal translocation
Fig A B C D E F G H A B C E F G H Deletion (a) A B C D E F G H A B C B C D E F G H Duplication (b) A B C D E F G H A D C B E F G H (c) Inversion Figure Alterations of chromosome structure A B C D E F G H M N O C D E F G H (d) Reciprocal translocation M N O P Q R A B P Q R

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

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 Figure Down syndrome

Fig a Figure Down syndrome

Fig b Figure Down syndrome

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 X0 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

Translocated chromosome 22 (Philadelphia chromosome)
Fig Reciprocal translocation Normal chromosome 9 Translocated chromosome 9 Translocated chromosome 22 (Philadelphia chromosome) Normal chromosome 22 Figure Translocation associated with chronic myelogenous leukemia (CML)

There are two normal exceptions to Mendelian genetics
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

Such variation in phenotype is called 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

Normal Igf2 allele is expressed Paternal chromosome Maternal
Fig Normal Igf2 allele is expressed Paternal chromosome Maternal chromosome Normal Igf2 allele is not expressed Wild-type mouse (normal size) (a) Homozygote Mutant Igf2 allele inherited from mother Mutant Igf2 allele inherited from father Normal size mouse (wild type) Dwarf mouse (mutant) Figure Genomic imprinting of the mouse Igf2 gene Normal Igf2 allele is expressed Mutant Igf2 allele is expressed Mutant Igf2 allele is not expressed Normal Igf2 allele is not expressed (b) Heterozygotes

Normal Igf2 allele is expressed Paternal chromosome Maternal
Fig a Normal Igf2 allele is expressed Paternal chromosome Maternal chromosome Normal Igf2 allele is not expressed Wild-type mouse (normal size) Figure Genomic imprinting of the mouse Igf2 gene (a) Homozygote

Mutant Igf2 allele Mutant Igf2 allele inherited from mother
Fig b Mutant Igf2 allele inherited from mother Mutant Igf2 allele inherited from father Normal size mouse (wild type) Dwarf mouse (mutant) Normal Igf2 allele is expressed Mutant Igf2 allele is expressed Figure Genomic imprinting of the mouse Igf2 gene Mutant Igf2 allele is not expressed Normal Igf2 allele is not expressed (b) Heterozygotes

Most imprinted genes are critical for embryonic development
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 Most imprinted genes are critical for embryonic development Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 15-UN3

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 Figure Variegated leaves from Croton dioicus

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

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

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

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