Biology 112

1
Chromosomal Basis of Inheritance

• Biology 112

2
Locating Genes Along Chromosomes

• Mendels hereditary factors were genes, though
  this wasnt known during his time
• Today know 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

3
Mendelian inheritance has its physical basis in
the behavior of chromosomes

• The chromosome theory of inheritance states
• Mendelian genes have specific loci (positions) on
  chromosomes
• Chromosomes undergo segregation and independent
  assortment
• The behavior of chromosomes during meiosis can be
  said to account for Mendels laws of segregation
  and independent assortment

4
Fig. 15-2
P Generation
Yellow-round seeds (YYRR)
Green-wrinkled seeds ( yyrr)
y
Y
r
?
R
R
r
Y
y
Meiosis
Fertilization
r
y
R
Y
Gametes
All F1 plants produce yellow-round seeds (YyRr)
F1 Generation
R
R
y
y
r
r
Y
Y
LAW OF INDEPENDENT ASSORTMENT Alleles of genes on
nonhomologous chromosomes assort independently
during gamete formation.
LAW OF SEGREGATION The two alleles for each
gene separate during gamete formation.
Meiosis
R
r
r
R
Metaphase I
Y
y
y
Y
1
1
R
R
r
r
Anaphase I
Y
Y
y
y
Metaphase II
r
r
R
R
2
2
y
Y
y
Y
y
Y
Y
y
y
Y
y
Y
Gametes
r
R
r
r
r
R
R
R
1/4
1/4
1/4
1/4
yR
yr
Yr
YR
F2 Generation
An F1 ? F1 cross-fertilization
3
3
9
3
3
1
5
Thomas Hunt Morgan

• Drosophila melanogaster

6
Morgans Experimental Evidence

• The first solid evidence associating a specific
  gene with a specific chromosome came from Thomas
  Hunt Morgan
• Morgans experiments with fruit flies provided
  convincing evidence that chromosomes are the
  location of Mendels heritable factors
• Several characteristics 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

7

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

8
Sex-Linked Genes

• Sex-Linked genes Genes located on sex
  chromosomes.
• This term is commonly applied to genes on the X
  chromosome
• 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-eyed mutant
  allele must be located on the X chromosome

9
Fig. 15-4
EXPERIMENT
P
?
Generation
F1
All offspring had red eyes
Generation
RESULTS
F2
Generation
CONCLUSION

P
w
w
X
X
?
Generation
X
Y

w
w
Sperm
Eggs


F1
w
w

Generation
w
w

Sperm
w
Eggs


w
w

F2
w
Generation

w
w
w
w

w
10
Linked genes

• Each chromosome has hundreds or thousands of
  genes
• Genes located on the same chromosome that tend to
  be inherited together are called linked genes
• Linked genes tend to be inherited together
  because they are located near each other on the
  same chromosome

11
Linkage Affects Inheritance

• Morgan did some experiments with fruit flies to
  see how linkage affects inheritance of two
  characters
• Morgan crossed flies that differed in traits of
  body color and wing size

12

• Morgan found that body color and wing size are
  usually inherited together in specific
  combinations (parental phenotypes)
• He noted that these genes do not assort
  independently, and reasoned that they were on the
  same chromosome
• However, nonparental phenotypes were also
  produced
• Genetic recombinationoccured the production of
  offspring with combinations of traits differing
  from either parent

13
Fig. 15-UN1
b vg
b vg
?
Parents in testcross
b vg
b vg
b vg
b vg
Most offspring
or
b vg
b vg
14
Fig. 15-9-4
EXPERIMENT
P Generation (homozygous)
Wild type (gray body, normal wings)
Double mutant (black body, vestigial wings)
?
b b vg vg
b b vg vg
F1 dihybrid (wild type)
Double mutant
TESTCROSS
?
b b vg vg
b b vg vg
Testcross offspring
b vg
b vg
b vg
b vg
Eggs
Black- normal
Gray- vestigial
Black- vestigial
Wild type (gray-normal)
b vg
Sperm
b b vg vg
b b vg vg
b b vg vg
b b vg vg
PREDICTED RATIOS
1
If genes are located on different chromosomes
1
1
1



If genes are located on the same chromosome
and parental alleles are always inherited
together
1
1
0
0



965
185


944
206

RESULTS
15
Recombination of Unlinked Genes Independent
Assortment of Chromosomes

• Mendel observed that combinations of traits in
  some offspring differ from either parent
• 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

16
Recombination of Linked Genes Crossing Over

• 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

17
Fig. 15-10
Testcross parents
Gray body, normal wings (F1 dihybrid)
Black body, vestigial wings (double mutant)
b vg
b vg
b vg
b vg
Replication of chromo- somes
Replication of chromo- somes
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b vg
Meiosis I
b vg
Meiosis I and II
b vg
b vg
b vg
Meiosis II
Recombinant chromosomes
b vg
b vg
b vg
b vg
Eggs
Testcross offspring
965 Wild type (gray-normal)
944 Black- vestigial
206 Gray- vestigial
185 Black- normal
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b vg
Sperm
Parental-type offspring
Recombinant offspring
391 recombinants
Recombination frequency

? 100 17
2,300 total offspring
18
Fig. 15-10a
Testcross parents
Black body, vestigial wings (double mutant)
Gray body, normal wings (F1 dihybrid)
b vg
b vg
b vg
b vg
Replication of chromo- somes
Replication of chromo- somes
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b vg
Meiosis I
b vg
Meiosis I and II
b vg
b vg
b vg
Meiosis II
Recombinant chromosomes
b vg
b vg
b vg
b vg
b vg
Sperm
Eggs
19
Fig. 15-10b
Recombinant chromosomes
b vg
b vg
b vg
b vg
Eggs
965 Wild type (gray-normal)
944 Black- vestigial
185 Black- normal
206 Gray- vestigial
Testcross offspring
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b vg
Sperm
Recombinant offspring
Parental-type offspring
Recombination frequency
391 recombinants
? 100 17

2,300 total offspring
20
Mapping the Distance Between Genes Using
Recombination Data

• Genetic map, an ordered list of the genetic loci
  along a particular chromosome
• The farther apart two genes are, the higher the
  probability that a crossover will occur between
  them and therefore the higher the recombination
  frequency

21

• 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

22
Fig. 15-11
RESULTS
Recombination frequencies
9
9.5
Chromosome
17
b
cn
vg
23
Fig. 15-12
Mutant phenotypes
Short aristae
Cinnabar eyes
Vestigial wings
Brown eyes
Black body
0
48.5
57.5
67.0
104.5
Red eyes
Normal wings
Red eyes
Gray body
Long aristae (appendages on head)
Wild-type phenotypes
24

• 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

25

• Using methods like chromosomal banding,
  geneticists can develop cytogenetic maps of
  chromosomes
• Cytogenetic maps indicate the positions of genes
  with respect to chromosomal features

26
Sex systems in animals
27
Fig. 15-6
44 XX
44 XY
Parents
22 X
22 X
22 Y
or

Sperm
Egg
44 XX
44 XY
or
Zygotes (offspring)
(a) The X-Y system
22 XX
22 X
(b) The X-0 system
76 ZW
76 ZZ
(c) The Z-W system
32 (Diploid)
16 (Haploid)
(d) The haplo-diploid system
28
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
• In humans, sex-linked usually refers to a gene on
  the larger X chromosome

29

• Sex-linked genes follow specific patterns of
  inheritance
• For a recessive sex-linked trait to be expressed
• A female needs two copies of the allele
• A male needs only one copy of the allele
• Sex-linked recessive disorders are much more
  common in males than in females
• Some disorders caused by recessive alleles on the
  X chromosome in humans
• Color blindness
• Duchenne muscular dystrophy
• Hemophilia

30
Fig. 15-7
XnY
XnY
XNXN
XNXn
?
?
XNY
XNXn
?
Sperm
Sperm
Sperm
Y
Xn
XN
Y
Xn
Y
Eggs
Eggs
XNXN
Eggs
XNXn
XN
XNY
XN
XNY
XNXn
XNY
XN
XNXn
XNY
XnXN
XnY
XnXn
XnY
XN
Xn
Xn
(a)
(b)
(c)
31

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

32
X Inactivation in Female Mammals

• In mammalian females, one of the two X
  chromosomes in each cell is randomly inactivated
  during embryonic development
• The inactive X condenses into a Barr body
• If a female is heterozygous for a particular gene
  located on the X chromosome, she will be a mosaic
  for that character
• DNA Methylation is the mechanism for
  X-inactivation
• (methyl groups added to cytosine nucleotide in
  the DNA molecule)

33
Fig. 15-8
X chromosomes
Allele for orange fur
Early embryo
Allele for black fur
Cell division and X chromosome inactivation
Two cell populations in adult cat
Active X
Inactive X
Active X
Black fur
Orange fur
34
Chromosomal Alterations

• Large-scale chromosomal alterations often lead to
  spontaneous abortions (miscarriages) or cause a
  variety of developmental disordersbad news!

35
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
• Nondisjunction can also occur in mitosis

36
Fig. 15-13-3
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n 1
n 1
n 1
n
n
n 1
n 1
n 1
Number of chromosomes
(b) Nondisjunction of sister chromatids in
meiosis II
(a) Nondisjunction of homologous chromosomes
in meiosis I
Nondisjunction video
37
Aneuploidy

• Aneuploidy results from the fertilization of
  gametes in which nondisjunction occurred
• Offspring with this condition have an abnormal
  number of a particular chromosome
• A monosomic zygote has only one copy of a
  particular chromosome
• A trisomic zygote has three copies of a
  particular chromosome

38
Polyploidy

• Polyploidy is a condition in which an organism
  has more than two complete sets of chromosomes
• Triploidy (3n) is three sets of chromosomes
• Tetraploidy (4n) is four sets of chromosomes
• Polyploidy is common in plants, but not animals
• Polyploids are more normal in appearance than
  aneuploids

39
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

40
Fig. 15-15
A B C D E F G H
A B C E F G H
Deletion
(a)
A B C D E F G H
A B C B C D E F G H
Duplication
(b)
A B C D E F G H
A D C B E F G H
Inversion
(c)
A B C D E F G H
M N O C D E F G H
(d)
Reciprocal translocation
M N O P Q R
A B P Q R
41
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

42
Down Syndrome (Trisomy 21)

• 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.

43
Aneuploidy of Sex Chromosomes

• Nondisjunction of sex chromosomes produces a
  variety of aneuploid conditions
• Males
• Klinefelter syndrome is the result of an extra
  chromosome in a male, producing sterile XXY
  individuals (1 in 2000 births?)
• Extra Y (XYY)apparently normal males (1 in 1000
  births?)
• Females
• Monosomy X, called Turner syndrome, produces X0
  females, who are sterile it is the only known
  viable monosomy in humans (1 in 5000 births?)
• Triple X Syndrome (XXX)apparently normal females
  (1 in 1000 births?)

44
Genomic Imprinting

• For a few mammalian traits, the phenotype depends
  on which parent passed along the alleles for
  those traits
• Such variation in phenotype is called genomic
  imprinting
• Genomic imprinting involves the silencing of
  certain genes that are stamped with an imprint
  during gamete production
• Causes certain genes to be differently expressed
  in the offspring depending upon whether the
  alleles were inherited from the ovum or from the
  sperm cell.

45
Triplet Repeats

• Fragile-X syndrome (CGG)
• Huntingtons disease (CAG)

46
Extranuclear Genes

• Extranuclear genes (or cytoplasmic genes) are
  genes found in organelles in the cytoplasm
• Mitochondria, chloroplasts carry small circular
  DNA molecules
• Extranuclear genes are inherited maternally
  because the zygotes cytoplasm comes from the egg