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CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE Section A: Relating Mendelism to Chromosomes 1.Mendelian inheritance has its physical basis in the behavior.

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Presentation on theme: "CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE Section A: Relating Mendelism to Chromosomes 1.Mendelian inheritance has its physical basis in the behavior."— Presentation transcript:

1 CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE Section A: Relating Mendelism to Chromosomes 1.Mendelian inheritance has its physical basis in the behavior of chromosomes during sexual life cycles 2. Morgan traced a gene to a specific chromosome 3. Linked genes tend to be inherited together because they are located on the same chromosome 4. Independent assortment of chromosomes and crossing over produce genetic recombinants 5. Geneticists use recombination data to map a chromosome’s genetic loci 生物醫學暨環境生物學系 張學偉 助理教授

2 Mendel’s hereditary factors are the genes located on chromosomes.  chromosomes theory of inheritance. Introduction Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 1. Mendelian inheritance has its physical basis in the behavior of chromosomes during sexual life cycles

3 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Chromosomal basis of Mendel’s laws. Fig. 15.1

4 Thomas Hunt Morgan was the first to associate a specific gene with a specific chromosome. 2. Morgan traced a gene to a specific chromosome Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Morgan choice an experimental animal, Drosophila melanogaster fruit fly species that eats fungi on fruit prolific breeders generation time of two weeks. Fruit flies have three pairs of autosomes and a pair of sex chromosomes (XX in females, XY in males).  3+1

5 The normal character phenotype is the wild type (red). Alternative traits are mutant phenotypes. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig Genes located on a sex chromosome are called sex-linked genes. femalemale

6 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig Morgan concluded that a fly’s eye color was linked to its sex (x).

7 Results of crosses with linked genes deviate from those expected according to independent assortment. 3. Linked genes tend to be inherited together because they are located on the same chromosome Morgan reasoned that body color and wing shape are usually inherited together because their genes are on the same chromosome.

8 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig (b + vg + Or b vg + ) (b vg)

9  result from: (1) independent assortment of genes located on nonhomologous chromosomes (2) or crossing over of genes located on homologous chromosomes. 4. Independent assortment of chromosomes and crossing over produce genetic recombinants Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings The production of offspring with new combinations of traits inherited from two parents is genetic recombination.

10 The physical basis of recombination between unlinked genes is the random orientation of homologous chromosomes at metaphase I of meiosis.  leads to the independent assortment of alleles. linked genes, genes located on the same chromosome, tend to move together through meiosis and fertilization.  [Don’t assort independently] Under normal Mendelian genetic rules, we would not expect linked genes to recombine into assortments of alleles not found in the parents. But in fact, recombination between linked genes does occur. (see Morgan)

11 This switched alleles between homologous chromosomes. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 15.5a A 50% frequency of recombination is observed for any two genes located on different chromosomes.

12 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 15.5b

13 The results of Morgan’s testcross for body color and wing shape did not conform to either independent assortment or complete linkage. Most of the offspring had parental phenotypes, suggesting linkage between the genes. However, 17% of the flies were recombinants, suggesting incomplete linkage. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

14 One of Morgan’s students, Alfred Sturtevant, used crossing over of linked genes to develop a method for constructing a chromosome map. This map is an ordered list of the genetic loci along a particular chromosome. 5. Geneticists can use recombination data to map a chromosome’s genetic loci Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

15 Sturtevant hypothesized : that the frequency of recombinant offspring reflected the distances between genes on a chromosome. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings The farther apart two genes are, the higher the probability that a crossover will occur between them and therefore a higher recombination frequency. Sturtevant used recombination frequencies from fruit fly crosses to map the relative position of genes along chromosomes, a linkage map.

16 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig Using recombination frequencies to construct a genetic map One map unit (sometimes called a centimorgan) is equivalent to a 1% recombination frequency.

17 Some genes on a chromosome are so far apart, the frequency of recombination reaches is its maximum value of 50% and the genes act as if found on separate chromosomes and are inherited independently. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

18 Fig More than one gene can affect a given phenotypic characteristic (eye color) Partial genetic map

19 Map units (linkage map) indicate relative distance and order, not precise locations of genes.  The frequency of crossing over is not actually uniform over the length of a chromosome. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Combined with other methods like chromosomal banding, geneticists can develop cytological maps.  the positions of genes with respect to chromosomal features. More recent techniques show the absolute distances between gene loci in DNA nucleotides.

20 CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE Section B: Sex Chromosomes 1.The chromosomal basis of sex varies with the organism 2. Sex-linked genes have unique patterns of inheritance In human and other mammals, there are two varieties of sex chromosomes, X and Y. 1. The chromosomal basis of sex varies with the organism

21 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig Avoid confusion with XY system Male develop from unfertilized eggs (they have no father) In humans, the anatomical signs of sex first appear when the embryo is about two months old.

22 In individuals  with the SRY gene (sex determining region of the Y chromosome), the generic embryonic gonads are modified into testes.  lacking the SRY gene, the generic embryonic gonads develop into ovaries. Activity of the SRY gene triggers a cascade of biochemical, physiological, and anatomical features because it regulates many other genes. In addition, other genes on the Y chromosome are necessary for the production of functional sperm. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

23 the sex chromosomes, especially the X chromosome, have genes for many characters unrelated to sex. E.g., white-eye locus in Drosophila. 2. Sex-linked genes have unique patterns of inheritance Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 15.9

24 Because males have only one X chromosome (hemizygous), any male receiving the recessive allele from his mother will express the trait. males are far more likely to inherit sex-linked recessive disorders than are females. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

25 human sex-linked disorders Duchenne muscular dystrophy. due to the absence of an X-linked gene for a key muscle protein, called dystrophin. characterized by a progressive weakening of the muscles and loss of coordination. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Hemophilia is a sex-linked recessive trait defined by the absence of one or more clotting factors. Individuals can be treated with intravenous injections of the missing protein.

26 2. Sex-linked genes have unique patterns of inheritance Although female mammals inherit two X chromosomes, only one X chromosome is active and one X chromosome condenses into a compact object, a Barr body  After Barr body formation, all descendent cells have the same inactive X. The condensed Barr body chromosome is reactivated in ovarian cells that produce ova.  involves the attachment of methyl (CH 3 ) groups to cytosine nucleotides on the X chromosome (inactivates most of its genes).

27 Fig Barr body occurs randomly and independently in embryonic cells at the time of X inactivation. As a consequence, females consist of a mosaic of cells, some with an active paternal X, others with an active maternal X. If a female human is heterozygous for a sex-linked trait, will have patches of normal skin and skin patches lacking sweat glands.

28 CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE Section C: Errors and Exceptions in Chromosomal Inheritance 1.Alterations of chromosome number or structure cause some genetic disorders 2. The phenotypic effects of some mammalian genes depend on whether they are inherited from the mother or the father (imprinting) 3. Extranuclear genes exhibit a non-Mendelian pattern of inheritance

29 1. Alterations of chromosome number or structure cause some genetic disorders Fig Meiotic nondisjunction Nondisjunction problems with the meiotic spindle

30 after nondisjunction cells will have an abnormal chromosome number or aneuploidy.  Trisomic = 2n + 1; Monosomic = 2n - 1. Although the frequency of aneuploid zygotes may be quite high in humans, most of these alterations are so disastrous that the embryos are spontaneously aborted long before birth. Organisms with more than two complete sets of chromosomes  polypoidy. (nondisjunction) e.g, triploid (3n). tetraploid (4n) e.g., both fishes and amphibians have polyploid species. Polyploidy is relatively common among plants and much less common among animals. Polyploids are more nearly normal in phenotype than aneuploids.

31 Down syndrome: three copies of chromosome 21  aneuploid (nondisjunction) Fig The frequency of Down syndrome correlates with the age of the mother.  age-dependent abnormality

32 Fig Breakage of a chromosome can lead to four types of changes in chromosome structure. common in meiosis Typically harmful chronic myelogenous leukemia (CML).[ch9/ch22]

33 For most genes it is a reasonable assumption that a specific allele will have the same effect regardless of whether it was inherited from the mother or father. 2. The phenotypic effects of some mammalian genes depend on whether they were inherited from the mother or the father (imprinting) However, for some traits in mammals, it does depend on which parent passed along the alleles for those traits.  The imprinting genes involved are not sex linked and may or may not lie on the X chromosome.

34 . a deletion of a specific segment of chromosome 15 Prader-Willi syndrome  abnormal chromosome from father. Angelman syndrome  from the mother. In genomic imprinting process, a gene on one homologous chromosome is silenced, while the other is expressed. cp: barr body is for x chromosome inactivation.  same alleles may have different effects on offspring, depending on whether they arrive in the zygote via ovum or sperm.  The imprinting status of a given gene

35 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig

36 In many cases, genomic imprinting occurs when methyl groups are added to cytosine nucleotides on one of the alleles. Fragile X syndrome, which leads to various degrees of mental retardation, also appears to be subject to genomic imprinting.  (inherited from the mother) This disorder is named for an abnormal X chromosome with extra CGG repeat.

37 Not all of a eukaryote cell’s genes are located in the nucleus. 3. Extranuclear genes exhibit a non- Mendelian pattern of inheritance  phenotype of the offspring was determined only by the maternal parent (maternal inheritance). Extranuclear genes are found on small circles of DNA in mitochondria and chloroplasts. These organelles reproduce themselves. Their cytoplasmic genes do not display Mendelian inheritance. They are not distributed to offspring during meiosis.

38 Mendel didn’t include: Sex-linked traits Major chromosomal aberrations imprinting & extranuclear genes summary


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