BIOLOGY. The Chromosomal Basis of Inheritance CAMPBELL. Reece Urry Cain Wasserman Minorsky Jackson

Transcription

1 CAMPBELL BIOLOGY TENTH EDITION Reece Urry Cain Wasserman Minorsky Jackson 15 The Chromosomal Basis of Inheritance Lecture Presentation by Nicole Tunbridge and Kathleen Fitzpatrick

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

3 Mendelian inheritance has its physical basis in the behavior of chromosomes Mitosis and meiosis were first described in the late 1800s - Chromosomes and genes are both present in pairs in diploid cells. Homologous chromosomes separate and alleles segregate during meiosis. Fertilization restores the paired condition for both chromosomes and genes.

4 Chromosome Theory of Inheritance Around 1902 a chromosome theory of inheritance began to take form: Genes occupy specific loci on chromosomes. Chromosomes undergo segregation during meiosis. Chromosomes undergo independent assortment during meiosis. The behavior of homologous chromosomes during meiosis can account for the segregation of the alleles at each genetic locus to different gametes. The behavior of nonhomologous chromosomes can account for the independent assortment of alleles for two or more genes located on different chromosomes. Mendelian genes have specific loci (positions) on chromosomes The behavior of chromosomes during meiosis was said to account for Mendel s laws of segregation and independent assortment

5 Law of Segregation: Chromosomes during F1 Meiosis The two alleles for each gene separate during gamete formation. R and r alleles segregate at anaphase I, yielding two types of daughter cells. Each gamete gets either the R or r allele. During fertilization, the R and r alleles recombine randomly. Figure15.2

6 Law of Independent Assortment: Chromosomes during F1 Meiosis Alleles on nonhomologous chromosomes assort independently during gamete formation. 1. Alleles at both loci segregate during anaphase I giving rise to 4 different daughter cells, depending on how they arranged during metaphase I. 2. Each gamete gets a chromosome in one of 4 allele combinations. 3. Fertilization results in a F2 9:3:3:1 phenotypic ratio. Fig.15.2

7 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

8 Morgan used fruit flies for his experiments Several characteristics make fruit flies a convenient organism for genetic studies: They breed at a high rate and have more offspring A generation can be bred every two weeks They have only four pairs of chromosomes Still, Morgan spent a year looking for variant individuals among the flies he was breeding! Finally he discovered a single male fly with white eyes instead of the usual red. Figure 15.3

9 Wild and Mutant The normal character phenotype is called the wild type. For a given character in flies, the gene s symbol is chosen from the first mutant discovered. The allele for white eyes in Drosophila is symbolized by w. A + superscript identifies the wild-type (red-eye) allele (w+). Alternative traits are called mutant phenotypes because they are due to alleles that originate as mutations in the wild-type allele.

10 Correlating Behavior of a Gene s Alleles with Behavior of a Chromosome Pair Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) 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 (All the F2 females and half the F2 males had red eyes) Morgan concluded that a fly s eye color was linked to its sex Morgan determined that the white-eyed mutant allele must be located on the X chromosome Morgan s finding supported the chromosome theory of inheritance

11 Morgan s Experiment Figure Mate w+ female with w male. 2. All F1 offspring had red eyes, suggesting that the red eye allele is dominant. 3. Breed w+ F1 female to w+ F1male. 4. All F1 females had w+, only male flies had w (3:1 ratio) Conclusion: the gene with the white-eyed mutation is on the X chromosome, with no corresponding allele present on the Y chromosome. Females (XX) may have two redeyed alleles and have red eyes or may be heterozygous and have red eyes. Males (XY) have only a single allele. They will have red eyes if they have a red-eyed allele or white eyes if they have a white-eyed allele.

12 Sex-linked genes exhibit unique patterns of inheritance In humans and some other animals, there is a chromosomal basis of sex determination

13 The Chromosomal Basis of Sex In humans and other mammals, there are two Figure 15.5 varieties of sex chromosomes, X and Y. An individual who inherits two X chromosomes usually develops as a female. An individual who inherits an X and a Y chromosome usually develops as a male. Short segments at either end of the Y chromosome are the only regions that are homologous with the corresponding regions of the X. The SRY gene on the Y chromosome codes for a protein that directs the development of male anatomical features These homologous regions allow the X and Y chromosomes in males to pair and behave like homologous chromosomes during meiosis in the testes.

14 Each conception has about a fifty-fifty chance of producing a particular sex In both testes (XY) and ovaries (XX), the two sex chromosomes segregate during meiosis, and each gamete receives one. Each ovum receives an X chromosome. Half the sperm cells receive an X chromosome, and half receive a Y chromosome. If a sperm cell bearing an X chromosome fertilizes an ovum, the resulting zygote is female (XX). If a sperm cell bearing a Y chromosome fertilizes an ovum, the resulting zygote is male (XY). Therefore, each conception has about a fiftyfifty chance of producing a particular sex.

15 Figure 15.6 Chromosomal Systems of Sex Determination Animals have different methods of sex determination X 44 + XY or Sperm 44 + XX 22 + Y Parents or 44 + XX 22 + X Egg 44 + XY Zygotes (offspring) (a) The X-Y system The X-0 system is found in some insects. Females are XX and males are X. In birds, some fishes, and some insects, females are ZW and males are ZZ. In bees and ants, females are diploid and males are haploid. (c) The Z-W system 76 + ZW 76 + ZZ 22 + XX 22 + X 32 (Diploid) 16 (Haploid) (b) The X-0 system (d) The haplo-diploid system

16 Anatomical signs of sex first appear when the embryo is about two months old Before that, the gonads can develop into either testes or ovaries. The SRY (sex-determining region of the Y chromosome) gene on the Y chromosome required for the development of testes. In individuals with the SRY gene, the generic embryonic gonads develop into testes. The SRY gene codes for a protein that regulates many other genes, triggering a cascade of biochemical, physiological, and anatomical features. In individuals lacking the SRY gene, the generic embryonic gonads develop into ovaries.

17 The sex chromosomes Researchers have sequenced the Y chromosome and identified 78 genes coding for about 25 proteins. Half of the genes are expressed only in the testes, and some are required for normal testicular function. Some genes on the Y chromosome are necessary for the production of functional sperm. In the absence of these genes, an XY individual is male but does not produce normal sperm. In addition to their role in determining sex, the sex chromosomes, especially the X chromosome, have genes for many characters unrelated to sex.

18 Inheritance of Sex-Linked Genes A gene located on either sex chromosome is called a sex-linked gene. Genes on the Y chromosome are called Y-linked genes; there are few of these Genes on the X chromosome are called X-linked genes In humans, the term sex-linked gene refers to a gene on the X chromosome. X chromosome have genes for many characters unrelated to sex, whereas the Y chromosome mainly encodes genes related to sex determination Human sex-linked genes follow the same pattern of inheritance as Morgan s white-eye locus in Drosophila. Fathers pass sex-linked alleles to all their daughters but none of their sons. Mothers pass sex-linked alleles to both sons and daughters. If a sex-linked trait is due to a recessive allele, a female will express this phenotype only if she is homozygous. Heterozygous females are carriers for the recessive trait. Because males have only one X chromosome (hemizygous), any male who receives the recessive allele from his mother will express the recessive trait.

19 Disorders caused by recessive alleles on the X chromosome in humans Color blindness (mostly X-linked) Duchenne muscular dystrophy- a lethal muscular disorder Affected individuals rarely live past their early 20s. This disorder is due to the absence of an X-linked gene for a key muscle protein called dystrophin. The disease is characterized by a progressive weakening of the muscles and a loss of coordination. Hemophilia- absence of one or more proteins required for blood clotting. These proteins normally slow and then stop bleeding. Individuals with hemophilia have prolonged bleeding because a firm clot forms slowly. Bleeding in muscles and joints can be painful and can lead to serious damage. Today, people with hemophilia can be treated with intravenous injections of the missing protein.

20 Sex-linked recessive traits Figure 15.7 a) A color-blind father (X n Y) will pass the mutant allele to all daughters, but not sons. If the mother is a normal dominant homozygote (X N X N ) the daughters will have a normal phenotype, but be carriers of the N mutation (X N X n ). b) If one of the female carriers (X N X n ) mate with a normal male (X N Y) there is a 50% chance that each daughter will be a carrier (X N X n ) and a 50% chance that a son will have the disease (X n Y). c) If one of the female carriers (X N X n ) mate with a diseased male (X n Y) there is a 50% chance that each child ( or ) will have the disease. All normal daughters will be carriers (X N X n ) and normal sons will not carry the recessive allele (X N Y).

21 Q-Do females (XX) express twice as many genes as males (XY)? Answer - NO One of the X chromosomes condense in every cell during female embryo development and becomes a Barr body Most of the genes on the Barr-body chromosome are not expressed. Selection of which X chromosome will form the Barr body occurs randomly and independently in embryonic cells at the time of X inactivation. As a consequence, females consist of a mosaic of two types of cells, some with an active X chromosome from their fathers and others with an active X chromosome from their mothers. After an X chromosome is inactivated in a particular cell, all mitotic descendants of that cell will have the same inactive X. If a female is heterozygous for a sex-linked trait, approximately half her cells will express one allele, and the other half will express the other allele.

22 X Inactivation in the tortoiseshell cat The tortoiseshell gene is located on the X chromosome. The tortoiseshell phenotype requires both the orange fur and black fur alleles. Only females can have both alleles (XX). Females heterozygous for the tortoiseshell gene have orange patches of fur where the orange allele is active as well as patches of black fur where the black allele is active. Figure 15.8 X inactivation and the tortoiseshell cat

23 What causes X chromosome inactivation? 1. CH 4 groups are added to DNA nucleotides. 2. XIST (X-inactive specific transcript) This gene is active only on the Barr-body chromosome and produces multiple copies of an RNA molecule that attach to the X chromosome on which they were made. This initiates X inactivation. The mechanism that connects XIST RNA and DNA methylation is unknown.

24 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 The results of crosses with linked genes differ from those expected according to the law of independent assortment. Morgan observed this linkage and its deviations when he followed the inheritance of characters for body color and wing size in Drosophila.

25 Testcross: body color and wing size or The wild-type body color is gray (b+), and the mutant is black (b). The wild-type wing size is normal (vg+), and the mutant has vestigial wings (vg). The mutant alleles are recessive to the wild-type alleles. Neither gene is on a sex chromosome. Morgan crossed F1 heterozygous females (b+bvg+vg) with homozygous recessive males (bbvgvg). According to independent assortment, this should produce four phenotypes in a 1:1:1:1 ratio. Morgan observed that most F1 offspring resembled parents

26 1. Mate true-breeding wild type (b + b + vg + vg + ) with recessive (bbvgvg) to obtain heterozygous F1. All of the F1 heterozygotes have wild type appearance. 2. Testcross: Mate dihybrid F1 females (b + bvg + vg) with black vestigialwinged males (bbvgvg). 3. Results: Most offspring are gray body /large wings or back body/small wings. 4. Conclusion: Genes are located on same chromosome and are inherited together (usually). Morgan s Experiment Figure 15.9 Some nonparentals??

27 Genetic Recombination Genetic recombination- The production of offspring with combinations of traits that differ from those found in either parent. Genetic recombination can result from independent assortment of genes located on nonhomologous chromosomes. 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 Mendel also observed that combinations of traits in some offspring differed from either parent

28 Dihybrid Cross: YyRr x yyrr Testcross: X F1 Mendel s dihybrid cross experiments produced offspring that had a combination of traits that did not match either parent in the P generation. If the P generation consists of a yellow-round seed parent (YYRR) crossed with a green-wrinkled seed parent (yyrr), all the F1 plants have yellow-round seeds (YyRr). A cross between an F1 plant and a homozygous recessive plant (a testcross) produces four phenotypes. Half are the parental types, with phenotypes that match the original P parents, with either yellow-round seeds or green-wrinkled seeds. Half are recombinant types or recombinants, new combinations of parental traits, with yellow-wrinkled or green-round seeds.

29 Independent Assortment of alleles results in genetic recombination A 50% frequency of recombination is observed for any two genes located on different (nonhomologous) chromosomes. The physical basis of recombination between unlinked genes is the random orientation of homologous chromosomes at metaphase I of meiosis, which leads to the independent assortment of alleles. The F1 parent (YyRr) produces gametes with four different combinations of alleles: YR, Yr, yr, and yr. The orientation of the tetrad containing the seed-color gene has no bearing on the orientation of the tetrad with the seed-shape gene.

30 Recombination of Linked Genes is a result of Crossing Over Linked genes (genes located on the same chromosome) tend to move together through meiosis and fertilization. Under normal Mendelian genetic rules we would not expect linked genes to recombine into assortments of alleles not found in the parents. 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

31 Crossing Over produced recombinant offspring in Morgan s testcross experiments 1. During Meiosis I crossing over between b and vg loci produces new allele combinations in some (not most) egg producing cells. Most eggs will harbor the maternal type chromosomes. 2. During Meiosis II separation of the chromatids produces recombinant gametes (eggs) with the new allele combinations. Note that no new allele combinations are produced in the bbvgvg sperm during Meiosis I and II. 3. Fertilization of the eggs by the bbvgvg sperm will give rise to the recombinant offspring.

32 Testcross parents Gray body, normal wings (F 1 dihybrid) b + vg + Black body, vestigial wings (double mutant) b vg Replication of chromosomes b vg b + vg + b vg Replication of chromosomes b vg Meiosis I b + vg + b vg b vg b vg b vg b vg b + vg + b + vg b vg + Meiosis I and II Meiosis II b vg Recombinant chromosomes Eggs b + vg + b vg b + vg b vg + Testcross offspring 965 Wild type (gray-normal) 944 Blackvestigial 206 Grayvestigial 185 Blacknormal 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 2,300 total offspring 100 = 17% Figure 15.10

33 New Combinations of Alleles: Variation for Normal Selection Recombinant chromosomes bring alleles together in new combinations in gametes Random fertilization increases even further the number of variant combinations that can be produced This abundance of genetic variation is the raw material upon which natural selection works 2011 Pearson Education, Inc.

34 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 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

35 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 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

36 The percentage of recombinant offspring, the recombination frequency, is related to the distance between linked genes. The farther apart two genes are, the higher the probability that a crossover will occur between them and, therefore, the higher the recombination frequency The greater the distance between two genes, the more points there are between them where crossing over can occur A genetic map based on recombination frequencies is called a linkage map

37 Linkage Maps Figure Figure Recombination frequencies from fruit fly crosses were used to map the relative positions of the body color (b), wing size (vg), and eye color (cn) genes along chromosomes. By combining linkage maps with other methods like chromosomal banding, geneticists can develop cytogenetic maps of chromosomes. These maps indicate the positions of genes with respect to chromosomal features. Recent techniques show the physical distances between gene loci in DNA nucleotides.

38 Alterations of chromosome number or structure cause some genetic disorders Small-scale random mutations are the source of all new alleles and lead to new phenotypic traits. Physical and chemical disturbances can also damage chromosomes in major ways. Errors during meiosis can alter the number of chromosomes in a cell. Plants tolerate genetic defects to a greater extent than do animals. Large-scale chromosomal alterations often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders

39 Errors during meiosis can alter the number of chromosomes in a cell Nondisjunction occurs when problems with the meiotic spindle cause errors in daughter cells. a) Nondisjunction may occur if tetrad chromosomes do not separate properly during meiosis I. b) Alternatively, sister chromatids may fail to separate during meiosis II. As a consequence of nondisjunction, one gamete receives two of the same type of chromosome, and another gamete receives no copy. Figure 15.13

40 Offspring produced by nondisjunction Offspring resulting from the fertilization of a normal gamete with one produced by nondisjunction have an abnormal chromosome number, a condition known as aneuploidy. Trisomic cells have three copies of a particular chromosome and have 2n + 1 total chromosomes. Down Syndrome is caused by trisomy Monosomic cells have only one copy of a particular chromosome and have 2n 1 chromosomes. If the organism survives, aneuploidy typically leads to a distinct phenotype. Tetrasomic cells have two copies of a particular chromosome and have 2n + 2 total chromosomes. Nullisomicsomic cells have no copies of a particular chromosome and have 2n -2 total chromosomes.

41 Polyploidy Polyploid organisms have more than two complete sets of chromosomes in all somatic cells. Triploidy-3 sets (3n) of chromosomes Tetraploidy-4 sets (4n) of chromosomes Tetraploid mammals of burrowing rodent has twice chromosome as those of closely related species. Polyploidy is relatively common among plants and much less common among animals, although it is known to occur in fishes and amphibians. The spontaneous origin of polyploid individuals plays an important role in the evolution of plants. Many crop plants are polyploid. For example, bananas are triploid (3n) and wheat is hexaploid (6n). Polyploids are more nearly normal in phenotype than aneuploids. One extra or missing chromosome apparently upsets the genetic balance during development more than does an entire extra set of chromosomes.

42 Figure (a) Deletion A B C D E F G H Alterations of Chromosome Structure A B C E F G H A deletion removes a chromosomal segment. (b) Duplication A B C D E F G H A duplication repeats a segment. A B C B C D E F G H (c) Inversion A B C D E F G H (d) Translocation An inversion reverses a segment within a chromosome. A D C B E F G H A B C D E F G H M N O P Q R A translocation moves a segment from one chromosome to a nonhomologous chromosome. M N O C D E F G H A B P Q R Breakage of a chromosome can lead to four types of changes in chromosome structure.

43 Human Disorders Due to Chromosomal Alterations Alterations of chromosome number and structure are associated with some serious disorders Most result in miscarriage of the fetus 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

44 Down Syndrome -Trisomy 21 Down syndrome is an aneuploid condition that results from three copies of chromosome 21 Although chromosome 21 is the smallest human chromosome, trisomy 21 severely alters an individual s phenotype in specific ways. Most cases of Down syndrome result from nondisjunction during gamete production in one parent. The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained. Trisomy 21 may be linked to some age-dependent abnormality in a meiosis I checkpoint that normally delays anaphase until all the kinetochores are attached to the spindle. It affects about one out of every 700 children born in the United States

45 Down s Syndrome Figure 15.15

46 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 These individuals have male sex organs but abnormally small testes and are sterile. Although the extra X is inactivated, some breast enlargement and other female characteristics are common. Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans X0 individuals are phenotypically female but are sterile because their sex organs do not mature. When given estrogen replacement therapy, girls with Turner syndrome develop secondary sex characteristics.

47 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

48 Chromosomal Translocation and CML Figure Chromosomal translocations have been implicated in certain cancers, including chronic myelogenous leukemia (CML). CML occurs when a large fragment of chromosome 22 switches places with a small fragment from the tip of chromosome 9. The resulting short, easily recognized chromosome 22 is called the Philadelphia chromosome.

49 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 In both cases, the sex of the parent contributing the allele is a factor in the pattern of inheritance The genes involved are not necessarily sex-linked and may or may not lie on the X chromosome

50 Genomic Imprinting Variation in phenotype depending on whether an allele is inherited from the male or female parent is called genomic imprinting. Genomic imprinting occurs during the formation of gametes and results in the silencing of the imprinted genes. Because different genes are imprinted in sperm and ova, some genes in a zygote are maternally imprinted and others are paternally imprinted. For a maternally imprinted gene, only the paternal allele is expressed. For a paternally imprinted gene, only the maternal allele is expressed. The maternal and paternal imprints are transmitted to all body cells during development. Although only a few genes are imprinted most of these genes are critical for embryonic development. The gene for insulin-like growth factor 2 (Igf2) was one of the first imprinted genes to be identified. Although the growth factor is required for normal prenatal growth, only the paternal allele is expressed.

51 Only the paternal Igf2 allele is expressed Evidence that the Igf2 allele is imprinted initially came from crosses between wild-type mice and dwarf mice homozygous for a recessive mutation in the Igf2 gene. The phenotypes of heterozygous offspring differ, depending on whether the mutant allele comes from the mother or the father. The Igf2 allele is imprinted in eggs, turning off expression of the imprinted allele. In sperm, the Igf2 allele is not imprinted and functions normally. Figure 15.17

52 It appears that imprinting is the result of the methylation (addition of CH 3 ) of cysteine nucleotides Genomic imprinting is thought to affect only a small fraction of mammalian genes Most imprinted genes are critical for embryonic development

53 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 These organelles reproduce themselves Extranuclear genes are inherited maternally because the zygote s cytoplasm comes from the egg Because a zygote inherits all its mitochondria from the ovum, all mitochondrial genes in most animals and plants demonstrate maternal inheritance. The products of mitochondrial genes make up the protein complexes of the electron transport chain and ATP synthase. 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

54 Some rare human disorders are produced by mutations to mitochondrial DNA Tissues that require large energy supplies (the nervous system and muscles) may suffer energy deprivation from these defects. For example, a person with mitochondrial myopathy suffers weakness, intolerance of exercise, and muscle deterioration. Another mitochondrial disorder is Leber s hereditary optic neuropathy, which can produce sudden blindness in young adults. Other mitochondrial mutations may contribute to diabetes, heart disease, and other diseases of aging, such as Alzheimer s disease. Over a lifetime, new mutations gradually accumulate in mitochondrial DNA and contribute to the aging process.

55 Figure The first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant