NOTES: Ch 15 - Chromosomes, Sex Determination

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NOTES Ch 15 - Chromosomes, Sex Determination
Sex Linkage
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Overview Locating Genes on Chromosomes

• ? A century ago the relationship between genes
  and chromosomes was not obvious
• ? 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

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The Chromosome Theory of Inheritance states that

• ? Mendelian genes have specific loci (positions)
  on chromosomes
• ? It is the chromosomes that undergo segregation
  and independent assortment!

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P Generation
Yellow-round seeds (YYRR)
Green-wrinkled seeds (yyrr)
Meiosis
Fertilization
Gametes
All F1 plants produce yellow-round seeds (YyRr)
F1 Generation
Meiosis
LAW OF INDEPENDENT ASSORTMENT
LAW OF SEGREGATION
Two equally probable arrangements of
chromosomes at metaphase I
Anaphase I
Metaphase II
Gametes
F2 Generation
Fertilization among the F1 plants
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Morgans Experimental Evidence Scientific Inquiry

• ? The first solid evidence associating a specific
  gene with a a specific chromosome came from
  Thomas Hunt Morgan, an embryologist

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Morgans Choice of Experimental Organism Fruit
Flies!

• ? Characteristics that 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

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• ? Morgan noted WILD TYPE, or normal, phenotypes
  that were common in the fly populations
• ? Traits alternative to the wild type are called
  mutant phenotypes

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Correlating Behavior of a Genes 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 31 redwhite eye
  ratio, but only males had white eyes
• ? Morgan determined that the white-eye mutant
  allele must be located on the X chromosome
• ? Morgans finding supported the chromosome
  theory of inheritance!

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P Generation
F1 Generation
F2 Generation
P Generation
Ova (eggs)
Sperm
F1 Generation
Ova (eggs)
Sperm
F2 Generation
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Linkage Gene Maps
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The Big Question

• ? It may be easy to see that genes located on
  DIFFERENT chromosomes assort independently but
  what about genes located on the SAME chromosome?

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Thomas Morgans Research

• ? Morgan identified more than 50 genes on
  Drosophilas 4 chromosomes.
• ? He discovered that many seemed to be linked
  together
• They are almost always inherited together only
  rarely become separated
• ? Grouped genes into 4 linkage groups

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Hmmm...
4 chromosomes
4 linkage groups
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Morgans Conclusion

• ? Each chromosome is actually a group of linked
  genes
• ? BUT Mendels principle of independent
  assortment still holds true
• ? It is the chromosomes that assort
  independently!!
• Mendel missed this because 6 of the 7 traits he
  studied were on different chromosomes.

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So

• ? If 2 genes are found on the same chromosome are
  they linked forever?
• NO!!
• ? CROSSING OVER during Meiosis can separate
  linked genes

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Testcross parents
Gray body, normal wings (F1 dihybrid)
Black body, vestigial wings (double mutant)
Replication of chromosomes
Replication of chromosomes
Meiosis I Crossing over between b and vg loci
produces new allele combinations.
Meiosis I and II No new allele combinations
are produced.
Meiosis II Separation of chromatids
produces recombinant gametes with the new
allele combinations.
Recombinant chromosomes
Sperm
Ova
Gametes
Ova
Testcross offspring
965 Wild type (gray-normal)
944 Black- vestigial
206 Gray- vestigial
185 Black- normal
Sperm
Recombination frequency
391 recombinants
? 100 17

2,300 total offspring
Parental-type offspring
Recombinant offspring
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Gene Maps

• ? Alfred Sturtevant was a graduate student
  working in Morgans lab part-time in 1911
• ? He hypothesized that the farther apart 2 genes
  are on a chromosome the more likely they are to
  be separated by crossing-over
• ? The rate of at which linked genes are separated
  can be used to produce a map of distances
  between genes

Alfred Sturtevant 1891-1970
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Gene Maps

• ? This map shows the relative locations of each
  known gene on a chromosome

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Linkage Maps

• ? 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

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Recombination frequencies
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9.5
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b
cn
vg
Chromosome
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I
IV
X
Y
II
III
Mutant phenotypes
Short aristae
Black body
Cinnabar eyes
Vestigial wings
Brown eyes
104.5
67.0
57.5
48.5
0
Red eyes
Normal wings
Red eyes
Long aristae (appendages on head)
Gray body
Wild-type phenotypes
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Sex-linked genes exhibit unique patterns of
inheritance

• ? In humans and other animals, there is a
  chromosomal basis of sex determination

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• ? Human somatic cells contain 23 pairs of
  chromosomes
• -22 pairs of autosomes (same in males females)
• -1 pair of sex chromosomes (XX or XY)
• -Females have 2 matching sex chromosomes XX
• -Males are XY

XX
XY
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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
• ? Sex-linked genes follow specific patterns of
  inheritance

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Sperm
Sperm
Sperm
Ova
Ova
Ova
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• ? Some disorders caused by recessive alleles on
  the X chromosome in humans
• -Color blindness
• -Duchenne muscular dystrophy
• -Hemophilia

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• ? When a gene is located on the X chromosome,
  females receive 2 copies of the gene, and males
  receive only 1 copy
• Example Color-blindness (c) is recessive to
  normal vision (C), and it is located on the X
  chromosome hemophilia

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EXAMPLE PROBLEM

• ? A female heterozygous for normal vision (we
  say she has normal vision,
• but is a carrier of the colorblindness allele)
  
• ? A male who is colorblind

XC Xc
Xc Y
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What is the probability that

• a) they will have a son who is colorblind?
•  
• b) they will have a daughter who is colorblind?
•  
• c) their first son will be colorblind?
•  
• d) their first daughter will be carrier?

XC Xc

1. 1/4 (25)
2. 1/4 (25)
3. 1/2 (50)
4. 1/2 (50)

Xc Y
Xc Xc
XC Xc
Xc Y
XC Y
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EXAMPLE PROBLEM

• ? Hemophilia is a hereditary disease in which the
  blood clotting process if defective. Classic
  hemophilia results from an abnormal or missing
  clotting factor VIII it is inherited as an
  X-linked recessive disorder (h).
• ? If a man without hemophilia and a woman who is
  a carrier of the hemophilia allele have children,
  what is the probability that

XH Xh
XH Y
x
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what is the probability that

• a) they will have a daughter with hemophilia?
•  
• b) they will have a son with hemophilia?
•  
• c) their first son will have hemophilia?
•  
• d) their first daughter will be a carrier?

XH Xh

1. 0/4 (0)
2. 1/4 (25)
3. 1/2 (50)
4. 1/2 (50)

XH Y
XH Xh
XH XH
Xh Y
XH Y
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Pedigree Charts
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Queen Victorias Legacy in Royal Families of
Europe
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X-inactivation in Female Mammals

• ? In mammalian females, one of the two X
  chromosomes in each cell is randomly inactivated
  during embryonic development
• ? If a female is heterozygous for a particular
  gene located on the X chromosome, she will be a
  mosaic for that character

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Two cell populations in adult cat
Active X
Early embryo
Orange fur
X chromosomes
Cell division and X chromosome inactivation
Inactive X
Inactive X
Black fur
Allele for orange fur
Active X
Allele for black fur
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Tortoise-shell cats! (a.k.a. Torties) XBXb
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So, what about the Y chromosome?
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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

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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

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Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n 1
n 1
n 1
n 1
n 1
n 1
n
n
Number of chromosomes
Nondisjunction of homologous chromosomes in
meiosis I
Nondisjunction of sister chromatids in meiosis I
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• ? Aneuploidy results from the fertilization of
  gametes in which nondisjunction occurred
• ? Offspring with this condition have an abnormal
  number of a particular chromosome

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• ? a TRISOMIC zygote has three copies of a
  particular chromosome
• ? a MONOSOMIC zygote has only one copy of a
  particular chromosome
• ? Polyploidy is a condition
• in which an organism has
• more than two complete
• sets of chromosomes

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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

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Deletion
A deletion removes a chromosomal segment.
Duplication
A duplication repeats a segment.
Inversion
An inversion reverses a segment within a
chromosome.
A translocation moves a segment from one
chromosome to another, nonhomologous one.
Reciprocal translocation
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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

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Down Syndrome

• ? 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

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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

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Disorders Caused by Structurally Altered
Chromosomes

• ? One 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

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Normal chromosome 9
Translocated chromosome 9
Reciprocal translocation
Philadelphia chromosome
Normal chromosome 22
Translocated chromosome 22