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Chapter 15

Chapter 15. The Chromosomal Basis of Inheritance. Mendel and Meiosis. Mendel began working with peas in 1857 Cytologists worked out the process of mitosis in 1875 and meiosis in the 1890s Several researchers proposed in the early 1900s that genes are located on chromosomes

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Chapter 15

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  1. Chapter 15 The Chromosomal Basis of Inheritance

  2. Mendel and Meiosis • Mendel began working with peas in 1857 • Cytologists worked out the process of mitosis in 1875 and meiosis in the 1890s • Several researchers proposed in the early 1900s that genes are located on chromosomes • The behavior of chromosomes during meiosis was said to account for Mendel’s laws of segregation and independent assortment

  3. The Chromosome Theory of Inheritance • The chromosome theory of inheritance states that • Mendelian genes have specific loci on chromosomes • Chromosomes undergo segregation and independent assortment Figure 15.2

  4. Morgan’s Experimental Evidence: Scientific Inquiry • Morgan worked with fruit flies, Drosophila melanogaster • Observed wild type, or normal, phenotypes that were common in the fly populations • Traits alternative to the wild type • Are called mutant phenotypes Figure 15.3

  5. 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 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-eye mutant allele must be located on the X chromosome Figure 15.4

  6. The Significance of Morgan’s Experiment • Morgan’s discovery that transmission of the X chromosome in fruit flies correlates with inheritance of the eye-color trait • Was the first solid evidence indicating that a specific gene is associated with a specific chromosome

  7. Linked Genes • Linked genes tend to be inherited together because they are located near each other on the same chromosome • Autosomal genes reside on the autosomal chromosomes (pairs 1-22) • Sex-linked genes are found on the sex chromosomes (pair 23, usually on the X)

  8. How Linkage Affects Inheritance: Scientific Inquiry • With further experimentation, Morgan discovered that genes can be linked • But due to the appearance of recombinant phenotypes, the linkage appeared incomplete • Morgan determined that • Genes that are close together on the same chromosome are linked and do not assort independently • Genes on separate chromosomes or genes that are far apart on the same chromosome assort independently

  9. Recombination of Linked Genes:Crossing Over • Morgan proposed that • Some process must occasionally break the physical connection between genes on the same chromosome • Crossing over of homologous chromosomes during Meiosis I is the mechanism

  10. 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 Unlinked Genes: Independent Assortment of Chromosomes • When Mendel followed the inheritance of two characters • He observed that some offspring have combinations of traits that do not match either parent in the P generation • Independent assortment of homologous chromosomes during meiosis I is the mechanism

  11. Recombination Frequencies • Recombinant offspring • Are those that show new combinations of the parental traits • When 50% of all offspring are recombinants • Geneticists say that there is a 50% frequency of recombination • Linked genes • Exhibit recombination frequencies less than 50%

  12. I IV X Y III II Mutant phenotypes Short aristae Black body Cinnabar eyes Vestigial wings Brown eyes 0 48.5 57.5 67.0 104.5 Long aristae (appendages on head) Red eyes Gray body Normal wings Red eyes Wild-type phenotypes Figure 15.8 Linkage Mapping:Using Recombination Data • A linkage map • Is the actual map of a chromosome based on recombination frequencies • The farther apart genes are on a chromosome • The more likely they are to be separated during crossing over

  13. The Chromosomal Basis of Sex • An organism’s sex • Is an inherited phenotypic character determined by the presence or absence of certain chromosomes • In humans and other mammals • There are two varieties of sex chromosomes, X and Y • Males have one X and one Y (XY) • Females have 2 X (XX) • Different systems of sex determination • Are found in other organisms

  14. Inheritance of Sex-Linked Genes • The sex chromosomes • Also have genes for many characters unrelated to sex • A gene located on either sex chromosome • Is called a sex-linked gene • Sex-linked genes • Follow specific patterns of inheritance

  15. X linked disorders • Some recessive alleles found on the X chromosome in humans cause certain types of disorders • Color blindness • Hemophilia • Because the male has only one X chromosome, these recessive traits are more prevalent in males

  16. Figure 15.10 The transmission of sex-linked recessive traits  XaY XAXA (a) A father with the disorder will transmit the mutant allele to all daughters but to no sons. When the mother is a dominant homozygote, the daughters will have the normal phenotype but will be carriers of the mutation. Sperm Xa Y XAXa XAY Ova XA XAYa XAY XA  XAXa XAY If a carrier mates with a male of normal phenotype, there is a 50% chance that each daughter will be a carrier like her mother, and a 50% chance that each son will have the disorder. (b) Sperm XA Y XAXA XAY Ova XA XaY XaYA Xa  XAXa XaY If a carrier mates with a male who has the disorder, there is a 50% chance that each child born to them will have the disorder, regardless of sex. Daughters who do not have the disorder will be carriers, where as males without the disorder will be completely free of the recessive allele. (c) Sperm Xa Y Ova XAY XA XAXa Xa XaYa XaY

  17. Predicting the Outcome of Crosses Problem 1: Red–green color blindness is caused by a sex–linked recessive allele. A color–blind man marries a woman with normal vision whose father was color–blind. What is the probability that they will have a color–blind daughter? What is the probability that their first son will be color–blind? XC = colorblindness X = normal vision 1. Determine the gametes involved Color-blind man Woman with normal vision, father was color-blind

  18. 2. Set up the Punnett Square XCY x XCX 3. Determine if the question is asking about all children, only males, or only females What is the probability that they will have a color–blind daughter? What is the probability that their first son will be color–blind?

  19. Alterations of ChromosomeNumber or Structure • Alterations of chromosome number or structure cause some genetic disorders • Large-scale chromosomal alterations • Often lead to spontaneous abortions or cause a variety of developmental disorders

  20. Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n  1 n + 1 n + 1 n –1 n + 1 n – 1 n n Number of chromosomes (a) (b) Nondisjunction of homologous chromosomes in meiosis I Nondisjunction of sister chromatids in meiosis II Figure 15.12a, b Abnormal Chromosome Number • When nondisjunction occurs • Pairs of homologous chromosomes do not separate normally during meiosis • Gametes contain two copies or no copies of a particular chromosome

  21. Aneuploidy and Polyploidy • Aneuploidy • Is a condition in which offspring have an abnormal number of a particular chromosome (2n+1, 2n-1) • Results from the fertilization of gametes in which nondisjunction occurred • Polyploidy • Is a condition in which there are more than two complete sets of chromosomes in an organism (3n, 4n) • May be produced by the fertilization of an abnormal diploid egg produced by nondisjunction of all its chromosomes

  22. Figure 15.15 Aneuploidy of an Autosome:Down Syndrome • Any missing or extra autosome is usually fatal, but chromosome #21 is very short • an extra chromosome 21 is also called trisomy 21 • These individuals have Down Syndrome

  23. Aneuploidy of Sex Chromosomes • The X chromosome is required for life – a missing Y is viable (XO), a missing X is not viable (YO) • The presence of a Y chromosome codes for a male (XXY, XYY), the absence of a Y chromosome codes for a female (XXX) • Individuals can survive with too many or too few sex chromosomes but will have abnormalities:

  24. Alterations of Chromosome Structure • Breakage of a chromosome can lead to four types of changes in chromosome structure • Deletion • Duplication • Inversion • Translocation

  25. A F H B C G C D F G B A E H E Deletion (a) A deletion removes a chromosomal segment. C E B C D A C D F G F H B A E H B G Duplication (b) A duplication repeats a segment. B A D A C D E F G H G C B E F H Inversion (c) An inversion reverses a segment within a chromosome. (d) A translocation moves a segment fromone chromosome to another, nonhomologous one. In a reciprocal   translocation, the most common type, nonhomologous chromosomes exchange fragments. Nonreciprocal translocations also occur, in which a chromosome transfers a fragment without receiving a fragment in return. A B M C D G E F G H N O C D E F H Reciprocal translocation M N A O P Q R B P Q R Figure 15.14 Alterations of chromosome structure

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