Chapter 15

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The Chromosomal Basis of Inheritance
updated 2 years ago by Tanner_Wayne_Adams
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1

law of segregation

two alleles for a given character separate during gamete formation, and go to different gametes

2

law of independent assortment

alleles for different characters sort independently of each other during gamete formation

3

Gregor Mendel's "hereditary factors" and year introduced

(1860) Genes are segments of DNA located along chromosomes

4

Cytologists worked out the process of mitosis (year) and meiosis (year)

1875;1890s

5

chromosome theory of inheritance (year)(who)(definition)

  • (1902) Walter S. Sutton, Theodor Boveri
  • Genes occupy specific loci on chromosomes
  • The chromosomes undergo segregation and independent assortment.
6

Thomas Hunt Morgan

Provided the first solid evidence of a specific gene associating with a specific chromosome.

  • Drosophila Melanogaster
  • (eye color)
  • (body color, wing shape)
7

Morgans choice of experimental organism (spelled correctly) (# of chromosomes) (how often a new generation is bred)

Drosophila Melanogaster; 4; every 2 weeks

8

Wild type

the phenotype for most organisms

9

Morgans experimental conclusions

  1. Fly's eye color linked to it's sex
  2. Gene involved in white-eyed mutant was located on the X chromosome only
10

Which one of Mendel's laws relates to the inheritance of alleles for a single character?

law of segregation

11

Which one of Mendel's laws relates to the inheritance of alleles for two characters in a dihybrid cross?

law of independent assortment

12

What is the physical basis for each of Mendel's laws in meiosis?

the law of segregation: separation of homologs in anaphase I

the law of independent assortment: alternative arrangements of different homologous chromosome pairs in metaphase I

13

When does the anatomical signs of sex begin to emerge in humans?

when the embryo is about 2 months old

14

SRY

  • Gene on the Y chromosome required for the development of testes.
  • In the absence of SRY, the gonads develop into ovaries.
  • SRY codes for a protein that regulates other genes.
15

Y-linked genes

78 genes code for about 25 proteins

16

X-linked genes (human)

1,100 genes

17

X-linked recessive disorders

  1. color blindness
  2. Duchenne muscular dystrophy
  3. Hemophilia
18

hemizygous

term used in describing a males X-linked gene due to only one locus (XnY)

19

Duchenne muscular dystrophy

  • 1/3500 males (US)
  • progressive weakening of the muscles and loss of coordination
  • absent muscle protein called dystrophin (gene for this protein located on X-chromosome)
  • rarely live past early 20's
20

Hemophilia

  • absence of one or more of the proteins required for blood clotting
  • bleeding prolonged because clot is slow to form
  • treated with intravenous injections of the protein that is missing
21

XIST

  • Inactivation of an X chromosome involves modification of the DNA and proteins bound to it, called histones, including attachment of methyl groups (--CH3) to DNA nucleotides
  • Two regions, one on each X chromosome, associate briefly with each other in each cell
  • Gene called XIST (X-inactive specific transcript) becomes active only on the chromosome that will become the Barr body
22

Barr body

A compact object of condensed inactive X found on the inside of the nuclear envelope.

23

Mosaicism

When an organism has cells with more than one genotype

24

Tortoiseshell cat

  • Tortoiseshell gene is on the X chromosome
  • Tortoiseshell phenotype requires the presence of two different alleles, one for orange fur and one for black fur.
  • Only females can have both alleles, because only they have two X chromosomes.
  • Orange patches are formed by populations of cells in which the X chromosome with the orange allele is active; black patches have cells in which the X chromosome with black allele is active.
25

linked genes

genes located near each other on the same chromosome tend to be inherited together

26

genetic recombination

the production of offspring with combinations of traits that differ from those found in either P generation parent.

27

parental types

an offspring whose phenotype matches one of the true-breeding parental phenotypes

28

recombinant types

an offspring whose phenotype differs from that of the true-breeding parental phenotypes

29

50% or greater frequency of occurrence of parental types indicates two genes are on the same chromosome.

...

30

crossing over

end portions of two nonsister chromatids trade places; recombination of linked genes

31

genetic map (who)(what)

Alfred H. Sturtevant; an ordered list of the genetic loci along a particular chromosome

32

recombination frequency

the percentage of recombinant offspring; depends on the distance between genes on a chromosome

33

the farther apart two genes are, the higher the probability that a crossover will occur between them and therefor the higher the recombination frequency

...

34

linkage map

a genetic map based on recombination frequencies

35

cinnabar

one of many Drosophila genes affecting eye color

36

map units

the distances between genes; one map unit is equivalent to a 1% recombination frequency

37

cytogenetic maps

locate genes with respect to chromosomal features, such as stained bands, that can be seen in the microscope

38

nondisjunction

the members of a pair of homologous chromosomes do not move apart properly during meiosis I or sister chromatids fail to separate during meiosis II

39

aneuploidy

an aberrant gamete that unites with a normal gamete at fertilization

40

monosomic

2n-1; missing chromosome in zygote resulting from fertilization of normal gamete with one that has no copy of a particular chromosome

41

trisomic

2n+1; extra chromosome in zygote resulting from fertilization of normal gamete with one that has an extra copy of a particular chromosome

42

Monosomy and trisomy are estimated to occur in between 10 and 25% of human conceptions, and is the main reason for pregnancy loss

...

43

polyploidy

more than two complete chromosome sets in all somatic cells

44

triploidy

3n; three chromosome sets; may arise by the fertilization of an abnormal diploid egg produced by nondisjunction of all its chromosomes

45

tetraploidy

4n; four chromosome sets; may arise from the failure of a 2n zygote to divide after replicating its chromosomes

46

examples of polyploid

bananas (triploid, 3n); wheat (hexaploid, 6n); strawberries (octoploid, 8n)

47

polyploids appear more normal than aneuploids. One extra (or missing) chromosome apparently disrupts genetic balance more than does an entire extra set of chromosomes.

...

48

2 ways that lead to changes in chromosome structure

  1. errors in meiosis
  2. radiation can cause breakage of chromosomes
49

4 types of alterations of chromosome structure

  1. deletion
  2. duplication
  3. inversion
  4. translocation
50

deletion

chromosomal fragment is lost

51

duplication

the "deleted" fragment may become attached as an extra segment to a sister chromatid

52

inversion

a chromosomal fragment may also reattach to the original chromosome but in the reverse orientation

53

reciprocal translocation

most common; nonhomologous chromosomes exchange fragments

54

nonreciprocal translocation

less common; a chromosome transfers a fragment but receives none in return

55

translocations and inversions can alter phenotype because a gene's expression can be influenced by its location among neighboring genes

...

56

syndrome

a set of traits characteristic of the type of aneuploidy

57

down syndrome

  • trisomy 21 (extra chromosome 21)
  • 1/830
  • characteristic facial features, short stature, correctable heart defects, developmental delays
  • increased chance of leukemia & Alzheimers
  • lower rate of high blood pressure, atherosclerosis, stroke & many types of solid tumors
  • life span: shorter than normal
  • all males and half females are sexually underdeveloped & sterile
  • frequency increases with mothers age (0.04% under 30)(0.92% at 40)
  • prenatal screening available
58

Prenatally and Postnatally Diagnosed Conditions Awareness Act (when)(what)

2008; law stipulates that medical practitioners give accurate, up-to-date information about any prenatal or postnatal diagnosis received by parents and that they connect parents with appropriate support services

59

aneuploidy of sex chromosomes

  • XXY (Klinefelter syndrome)
  • XYY
  • Trisomy X (XXX)
  • Monosomy X (Turners syndrome)
60

XXY

  • Klinefelter syndrome
  • 1/500-1000 male births
  • testes are small; sterile
  • breast enlargement and other female body characteristics
  • subnormal intelligence
61

XYY

  • 1/1000 males
  • normal sexual development
  • no well defined syndrome
  • taller than average
62

XXX

  • trisomy X
  • 1/1000 female births
  • healthy
  • taller than average
  • at risk for learning disabilities
  • fertile
63

XO

  • monosomy X (Turners syndrome)
  • 1/2500 female births
  • only known viable monosomy in humans
  • sterile
  • estrogen replacement therapy will develop secondary sex characteristics
  • normal intelligence
64

disorders caused by structurally altered chromosomes

  • cri du chat ("cry of the cat")
  • CML
65

cri du chat

  • deletion in chromosome 5
  • severely intellectually disabled
  • small head with unusual facial features
  • cry that sounds like mewing of distressed cat "cry of the cat"
  • lifespan: infancy or early childhood
66

CML

  • chronic myelogenous leukemia
  • reciprocal translocation during mitosis of cells that will become white blood cells
  • exchange of a large portion of chromosome 22 with a small fragment of time of chromosome 9
  • produces a Philadelphia chromosome (shortened chromosome 22)
  • causes caner by activating a gene that leads to uncontrolled cell cycle progression
67

genomic imprinting

variation in phenotype depending on whether an allele is inherited from the male or female parent (most imprinted genes are on autosomes)(over 60 imprinted genes have been identified, with hundreds more suspected)

68

What exactly is a genomic imprint?

  • in many cases, it seems to consist of methyl (-CH3) groups that are added to cytosine nucleotides of one of the alleles (methylation)
  • methylation of certain cytosines on the paternal chromosome leads to expression of the paternal Igf2 allel, by an indirect mechanism involving chromatin condensation
69

not all of a eukaryotic cell's genes are located on nuclear chromosomes, or even in the nucleus; some genes are located in organelles in the cytoplasm

...

70

extracellular genes or cytoplasmic genes

genes outside the nucleus

71

organelle genes are not distributed to offspring according to the same rules that direct the distribution of nuclear chromosomes during meiosis, so they do not display mendelian inheritance (p. 309)

...

72

Carl Correns (year)

1909; Discovered that extranuclear genes exist; studied the inheritance of yellow or white patches on leaves of a green plant

73

parts of body most susceptible to energy deprivation

nervous system, muscles

74

mitochondrial diseases

  1. mitochondrial myopathy
  2. Leber's hereditary optic neuropathy
75

mitochondrial myopathy

  • mitochondrial disorder
  • causes weakness, intolerance of exercise, and muscle deterioration
76

Leber's hereditary optic neuropathy

  • mitochondrial disorder
  • can produce sudden blindness in people as young as their 20's or 30's
  • the four mutations found thus far to cause this disorder affect oxidative phosphorylation during cellular respiration
77

mitochondrial mutations inherited from a person's mother

  • some types of diabetes, heart disease, and Alzheimer's
  • believed to be a part of the normal aging process

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