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Chapter 15
The Chromosomal Basis of
Inheritance
Rob Swatski
Associate Professor of Biology
HACC – York Campus
1
Overview: Genes and Chromosomes
We know Mendel’s “hereditary factors” are genes
We know genes are located on chromosomes in specific regions (loci)
- Fluorescent “tagging”
2
Mitosis & meiosis were first described in the late 1800’s
The chromosome theory of inheritance:
- Mendelian genes have specific loci on chromosomes
- Chromosomes undergo segregation & independent assortment
Chromosome behavior during meiosis accounts for Mendel’s laws
3
P Generation
Gametes
Meiosis
Fertilization
Yellow-round seeds (YYRR)
Green-wrinkled seeds ( yyrr)
All F1 plants produce yellow-round seeds (YyRr)
y
y
y
r
r
r
Y
Y
Y R
R R
4
0.5 mm
Meiosis
Metaphase I
Anaphase I
Metaphase II
Gametes
LAW OF SEGREGATION The two alleles for each gene separate during gamete formation.
LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation.
1 4 yr 1 4 Yr 1 4 YR
3 3
F1 Generation
1 4 yR
R
R
R
R
R R
R
R R R R
R
Y
Y
Y Y
Y
Y Y
Y
Y Y
Y Y
y r r
r r
r r
r r
r r r r
y
y
y
y
y
y y
y y y y
All F1 plants produce yellow-round seeds (YyRr)
1
2 2
1
5
F2 Generation
3 Fertilization recombines the R and r alleles at random.
Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation.
An F1 F1 cross-fertilization
9 : 3 : 3 : 1
LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT
3
6
Morgan’s Experimental Evidence
• The 1st solid evidence associating a specific gene with a specific chromosome came from the embryologist, Thomas Hunt Morgan
• Morgan’s experiments with fruit flies provided convincing evidence that Mendel’s heritable factors are located on chromosomes
7
Why Fruit Flies?
- Fast breeding rate & lots of offspring
- New generation produced every 2 weeks
- The have only 4 pairs of chromosomes
8
Wild type (normal) phenotypes were common in the fly populations
Traits alternative to the wild type are called mutant phenotypes
wild type mutant
9
Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair
In one mating experiment, Morgan crossed:
- male white eyes (mutant) with female 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
- The white-eyed mutant allele must be on the X chromosome
Morgan’s finding supported the chromosome theory of inheritance
10
F2 Generation
P Generation
Eggs
Eggs
Sperm
Sperm
X w
CONCLUSION
X X Y
w
w w w
w
w w w
w
w
w
w
w
w w w
F1 Generation
12
The Chromosomal Basis of Sex
In humans & other mammals, there are 2 varieties of sex chromosomes:
- larger X chromosome
- smaller Y chromosome
Only the ends of Y chromosomes are homologous with corresponding regions of the X chromosomes
The SRY gene on the Y chromosome codes for the development of testes
X
Y
13
(a) The X-Y system: females are XX and males are XY
44 + XY
44 + XX
Parents
44 + XY
44 + XX
22 + X
22 + X
22 + Y or
or
Sperm Egg
+
Zygotes (offspring)
14
(b) The X-0 system (insects)
22 + XX
22 + X
(c) The Z-W system (birds, fishes, insects) - Sex chromosomes are present in the egg (not sperm)
76 + ZW
76 + ZZ
X0
15
Inheritance of Sex-Linked Genes
Sex-linked gene: a gene that is located on either sex chromosome
- in humans, this usually refers to x-linked genes, which are genes located on the X chromosome
(there are few Y-linked genes)
- X chromosomes have genes for many characters unrelated to sex
17
X-linked genes follow specific patterns of inheritance
For a recessive sex-linked trait to be expressed:
- A female needs 2 copies of the allele (XnXn) = homozygous
- A male needs only 1 copy of the allele (XnY) = hemizygous
Sex-linked recessive disorders are much more common
in males than in females – why? 18
(a) (b) (c)
XNXN XnY XNXn XNY XNXn XnY
Y Xn Sperm Y XN Sperm Y Xn Sperm
XNXn Eggs XN
XN XNXn
XNY
XNY
Eggs XN
Xn
XNXN
XnXN
XNY
XnY
Eggs XN
Xn
XNXn
XnXn
XNY
XnY
Transmission of Sex-Linked Recessive Traits
19
Sex-linked recessive disorders in humans include:
- Color blindness
- Duchenne muscular dystrophy
- Hemophilia
20
X Inactivation in Female Mammals
In female mammals, 1 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
21
X chromosomes
Early embryo:
Allele for orange fur
Allele for black fur
Cell division &
X chromosome inactivation
2 cell populations in adult cat:
Active X
Active X Inactive X
Black fur Orange fur
22
Each chromosome has 100’s or 1000’s of genes
(except for the Y chromosome)
Linked genes: located near each other on the same chromosome
- Linked genes are usually inherited together
23
How Linkage Affects Inheritance
Morgan experimented with fruit flies to see how linkage affects the inheritance of 2 characters
He crossed flies that differed in:
- body color & wing size
24
P Generation (homozygous)
Wild type (gray body, normal wings)
F1 dihybrid (wild type)
Testcross offspring
TESTCROSS
b b vg vg
b b vg vg
b b vg vg
b b vg vg
Double mutant (black body, vestigial wings)
Double mutant
Eggs
Sperm
EXPERIMENT
RESULTS
PREDICTED RATIOS
Wild type (gray-normal)
Black- vestigial
Gray- vestigial
Black- normal
b vg b vg b vg b vg
b b vg vg b b vg vg b b vg vg b b vg vg
965 944 206 185
1
1
1
1
1
0
1
0
If genes are located on different chromosomes:
If genes are located on the same chromosome and parental alleles are always inherited together:
:
:
:
:
:
:
:
:
:
b vg
25
Conclusion:
body color & wing size are usually inherited together in specific combinations
(parental phenotypes)
These genes do not assort independently & Morgan reasoned that they were on the same chromosome
26
Most offspring
F1 dihybrid female & homozygous recessive male in testcross
or
b+ vg+
b vg
b+ vg+
b vg
b vg
b vg
b vg
b vg
Results: a much higher % of parental phenotypes than would be expected by independent assortment 27
However, non-parental phenotypes were also produced
Understanding this result involves exploring genetic recombination
- the production of offspring with combinations of traits different from either parent
Both findings of Mendel & Morgan relate to the chromosomal basis of recombination
28
Recombination of Unlinked Genes: Independent Assortment of Chromosomes
Mendel observed that combinations of traits in some offspring differ from either parent
- Parental types: offspring with a phenotype matching 1 of the parental phenotypes
- Recombinant types (recombinants): offspring with non-parental phenotypes (new combinations of traits)
A 50% frequency of recombination is observed for any 2 genes on different chromosomes
29
YyRr
Gametes from green- wrinkled homozygous recessive parent ( yyrr)
Gametes from yellow-round heterozygous parent (YyRr)
Parental- type
offspring
Recombinant offspring
yr
yyrr Yyrr yyRr
YR yr Yr yR
30
Recombination of Linked Genes: Crossing Over
Morgan discovered that genes can be linked, but the linkage was incomplete (as shown by the appearance
of recombinant phenotypes)
Morgan proposed that some process must sometimes break the physical connection between genes on the
same chromosome
= the crossing over of homologous chromosomes
31
Testcross parents
Replication of chromo- somes
Gray body, normal wings (F1 dihybrid)
Black body, vestigial wings (double mutant)
Replication of chromo- somes
b+ vg+
b+ vg+
b+ vg+
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b+ vg+
b+ vg
b vg+
b vg
Recombinant chromosomes
Meiosis I and II
Meiosis I
Meiosis II
Eggs Sperm
b+ vg+ b vg b+ vg b vg b vg+
Crossing-over
32
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+
Sperm b vg
Parental-type offspring Recombinant offspring
b vg
b+ vg b vg+
b vg+
Recombination frequency =
391 recombinants
2,300 total offspring 100 = 17%
Eggs
Recombinant chromosomes
33
Recombinant chromosomes bring alleles together in new combinations in gametes
Random fertilization increases even further the number of variant combinations that can be
produced
This abundance of genetic variation is the raw material upon which natural selection works
New Combinations of Alleles: Variation for Normal Selection
34
Genetic Maps
Alfred Sturtevant, one of Morgan’s students, constructed a genetic map: an ordered list of loci
along a particular chromosome
Sturtevant predicted that:
the farther apart 2 genes are, the higher the probability that a crossover will occur between them & therefore the higher the recombination frequency
35
Linkage Maps
Genetic map of a chromosome based on recombination frequencies
Distances between genes expressed as map units:
1 map unit (centimorgan)
= 1% recombination frequency
Map units indicate relative distance & order, not precise locations of genes
36
Genes that are far apart on the same chromosome can have a recombination frequency near 50%
Such genes are physically linked, but genetically unlinked, & behave as if found on different
chromosomes
38
Sturtevant used recombination frequencies to make linkage maps of fruit fly genes
Using methods like chromosomal banding, geneticists can develop cytogenetic maps of chromosomes
- Indicate positions of genes with respect to chromosomal features
39
Mutant phenotypes
Short aristae
Black body
Cinnabar eyes
Vestigial wings
Brown eyes
Red eyes
Normal wings
Red eyes
Gray body
Long aristae (appendages on head)
Wild-type phenotypes
0 48.5 57.5 67.0 104.5
A partial linkage map of a Drosophila chromosome 40
Large-scale chromosomal alterations often lead to spontaneous abortions (miscarriages) or cause a
variety of developmental disorders
Plants tolerate such genetic changes better than animals do
Alteration of Chromosome Number or Structure Cause Some Genetic
Disorders
41
Abnormal Chromosome Number
Nondisjunction: pairs of homologous chromosomes do not separate normally during meiosis
- As a result, one gamete receives 2 of the same type of chromosome & another gamete receives no copy
42
Meiosis I
Nondisjunction
(a) Nondisjunction of homologous chromosomes in meiosis I
(b) Nondisjunction of sister chromatids in meiosis II
Meiosis II
Nondisjunction
Gametes
Number of chromosomes
n + 1 n + 1 n + 1 n – 1 n – 1 n – 1 n n
43
Aneuploidy: results from the fertilization of gametes in which nondisjunction occurred
- offspring have an abnormal number of a particular chromosome
Monosomic zygote: has only 1 copy of a particular chromosome
Trisomic zygote: has 3 copies of a particular chromosome
44
Polyploidy: organism has more than 2 complete sets of chromosomes
- Common in plants, but not animals
- Polyploids are more normal in appearance than aneuploids
- Triploidy (3n): 3 sets of chromosomes
- Tetraploidy (4n): 4 sets of chromosomes
45
Alterations of Chromosome Structure
Breakage of a chromosome can lead to 4 types of changes in chromosome structure:
- Deletion: removes a chromosomal segment
- Duplication: repeats a segment
- Inversion: reverses the orientation of a segment within a chromosome
- Translocation: moves a segment from 1 chromosome to another
46
(a) Deletion
(b) Duplication
(c) Inversion
(d) Translocation
A deletion removes a chromosomal segment.
A duplication repeats a segment.
An inversion reverses a segment within a chromosome.
A translocation moves a segment from one chromosome to a nonhomologous chromosome.
A B C D E F G H
A B C E F G H
A B C D E F G H
A B C D E F G H B C
A B C D E F G H
A D C B E F G H
A B C D E F G H M N O P Q R
G M N O C H F E D A B P Q R
47
Human Disorders Due to Chromosomal Alterations
Some types of aneuploidy have higher survival rates, resulting in individuals surviving to birth & beyond
These survivors have a set of symptoms (syndrome) characteristic of the type of aneuploidy
48
Down Syndrome (Trisomy 21)
Aneuploidy due to 3 copies of chromosome 21
- affects 1 out of every 700 US children
- frequency increases with mother’s age…
49
Aneuploidy of Sex Chromosomes
Due to nondisjunction of sex chromosomes
Klinefelter syndrome: results from an extra X chromosome in a male, producing XXY individuals
Monosomy X (Turner syndrome): produces sterile X0 females
- the only known viable monosomy in humans
51
Disorders Caused by Structurally Altered Chromosomes
Cri du chat syndrome: due to a specific deletion in chromosome 5
- mental retardation, catlike cry
- individuals usually die in infancy or early childhood
54
Disorders Caused by Structurally Altered Chromosomes
Certain cancers are caused by translocations of chromosomes
- Chronic myelogenous leukemia (CML)
55
Normal chromosome 9
Normal chromosome 22
Reciprocal translocation
Translocated chromosome 9
Translocated chromosome 22 (Philadelphia chromosome)
56
There are 2 normal exceptions to Mendelian genetics
1. One involves genes located inside the nucleus (genomic imprinting)
2. The other involves genes located outside the nucleus (extranuclear genes)
In both cases, the sex of the parent contributing an allele is a factor in the pattern of inheritance
57
Genomic Imprinting
For a few mammalian developmental traits, phenotype depends on which parent passed along
those alleles
- this phenotype variation is called genomic imprinting
- involves the silencing of certain genes that are “stamped” with an imprint during gamete
production
- due to the methylation (addition of –CH3) of cysteine nucleotides of DNA
58
Normal Igf2 allele is expressed
Paternal chromosome
Maternal chromosome
(a) Homozygote
Wild-type mouse (normal size)
Normal Igf2 allele is not expressed
Genomic imprinting of the mouse Igf2 gene 59
Mutant Igf2 allele inherited from mother
Mutant Igf2 allele inherited from father
Normal size mouse (wild type)
Dwarf mouse (mutant)
Normal Igf2 allele is expressed
Mutant Igf2 allele is expressed
Mutant Igf2 allele is not expressed
Normal Igf2 allele is not expressed
(b) Heterozygotes 60
Inheritance of Organelle Genes
Extranuclear (cytoplasmic) genes: found in organelles in the cytoplasm
- mitochondria, chloroplasts, & other plant plastids carry small circular DNA molecules
- extranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the egg
- the first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches
on leaves of an otherwise green plant
61
Some defects in mitochondrial genes prevent normal ATP
synthesis
- result in diseases that affect the muscular &
nervous systems
- mitochondrial myopathy
63
64
Credits
by Rob Swatski, 2013
http://robswatskibiology.wetpaint.com
Visit my website for more Biology study resources!
http://www.flickr.com/photos/rswatski
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