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Chapter 15: The Chromosomal Basis of Inheritance
I. Relating Mendalism to Chromosomes
A. Mendelian inheritance has its physical basis in the behavior of chromosomes during sexual life
cycles: Mendelian genes have specific loci on the chromosomes
B. Morgan traced a gene to a specific chromosome: Science as a process
1. Morgan’s Choice of Experimental Organism: Morgan chose a species of fruit fly known as
Drosophila melanogaster
i. Advantages of the fly
a. The flies have a very fast life cycle, so new generations can be breeding a matter of
weeks
b. The flies only have four pairs of chromosomes
ii. After a relatively short time, Morgan was able to breed a single male fly with white eyes
instead of the normal red.
a. The normal red-eye was called the wild type
b. the mutant white eye is called the mutant phenotype.
2. Discovery of Sex Linkage: The wild, red color eye exhibited dominance over the mutant
white eye type, but with a catch.
i. The mutant was mated with a wild type, producing a completely red eyed F 1 generation.
ii. The F2 generation exhibited the classic 3:1 wild to white ratio, but only male flies displayed
the trait. From this observation, Morgan concluded that the white eye allele was located
exclusively on the X chromosome.
a. For a male to have white eyes, it needed but one recessive gene, because there was
no eye color locus on the Y chromosome
b. For a female to have white eyes, it needed two recessive genes, which was
impossible for Morgan’s F2 generation.
iii. Genes located exclusively on one sex chromosome are called sex-linked genes
C. Linked genes tend to be inherited together because they are located on the same chromosome
D. Independent assortment of chromosomes and crossing over produce genetic recombinants
1. The Recombination of Unlinked Genes: Independent Assortment of Chromosomes
i. Offspring inherit one of each chromosome from each parent.
ii. If two genes are located on different chromosomes, they will be inherited completely
independently of one another.
2. The Recombination of Linked Genes: Crossing Over: Genes on the same chromosome are not
necessarily inherited dependently.
i. During meiosis I, the tetrad formations allow nonsister chromatids to exchange segments in
a process called crossing over.
ii. Crossing over produces allelic combinations that may not be present in either parent.
E. Geneticists can use recombination data to map a chromosome’s genetic loci: The rough order of
the loci
1. As the distance increases between to loci, the chances that a crossing over will occur between
them increases.
2. By calculating the frequency of recombination between two loci, it is possible to determine
the relative distance between them.
3. The unit used in genetic maps is known as the centimorgan. Each centimorgan is equal to one
percent recombination frequency.
4. The maximum recombination frequency difference between any two loci is 50%, which
equates to independent assortment.
5. Because the frequency of crossing over varies from section to section, the linkage map is not a
picture of the chromosome; the frequencies of recombination do not linearly map to the
physical chromosome.
6. Cytological maps are based on the locations of loci relative to chromosomal features, and
provide a better picture of the physical positions.
II. Sex Chromosomes
A. The chromosomal basis of sex varies with the organism
1.
In humans, sex is determined by the inheritance of sex chromosomes; the mother contributes
an X chromosome, and the father contributes a Y or X.
i. if the ovum is fertilized with an X carrying sperm, the resulting zygote will be XX, and
female.
ii. If the ovum is fertilized with a Y carrying sperm, the resulting zygote will be XY, and male.
iii. Individual regions of the sex chromosomes have been linked with the appearance of sex
characteristics, such as the SYR location and the generation of testes.
2. In grasshoppers, roaches, cricks ant some other insects, the X-0 system is utilized. Males carry
no special chromosome, and sex is determined by the number of X chromosomes.
i. If the sperm carries an X chromosomes, the resulting offspring will be XX, and female.
ii. If the sperm carries no sex chromosomes, the resulting offspring will be X, and male.
3. In birds, some fishes, and some insects the sex-determinant chromosome is contributed by the
ova, a system called Z-W
i. If the ovum carries a Z chromosome, the resulting organism will be ZZ, and male.
ii. If the ovum carries a W chromosome, the resulting organism will be ZW, and female.
4. Most species of ants and bees carry no sex chromosomes because they use the haplo-diploid
system.
i. Females develop from fertilized ova, and are therefore diploid.
ii. Males develop from unfertilized ova, and are therefore haploid.
B. Sex-linked genes have unique patterns of inheritance
1. Rules
i. Sex based inheritance
a. Fathers can pass X linked traits to their daughters, but not to their sons
b. Mothers can pass X linked traits to all their offspring
ii. Expression
a. Recessive Sex linked traits are displayed more frequently in the sex with only one
locus for the gene, because they require only one recessive allele to be expressed.
b. Dominant sex linked traits are displayed less frequently in the sex with only one
locus for the gene, as there is no chance for the organism to inherit a dominant and
recessive allele.
2. Sex Linked Disorders in Humans
i. Red green colorblindness is an X linked disorder.
ii. Duchenne muscular dystrophy affects approximately one in 3500 males born in the United
States, and causes a progressive weakening of muscles and the loss of coordination. The
disorder’s symptoms stem from the lack of a key muscle protein known as dystrophin.
iii. Hemophilia is a sex linked recessive trait that causes the absence key blood-clotting
protein. Hemophiliacs bleed excessively due to their bloods inability to clot.
3. X-Inactivation in Female Mammals
i. The second X chromosome of female mammals inactivates shortly after fertilization (but
after the first divisions). The inactivation of the X chromosome involves the attachment of
methyl groups (--CH3) to the cytosine base.
ii. The randomly selected X chromosome condenses to a small point called the Barr body,
which lies along the edge of the nuclear envelope.
iii. Most of the genes contained in the Barr body are not expressed, but some do remain
active.
a. Female mammals are a mosaic of two types of cells, one representing each X
chromosome.
1. The calico patches of cats are caused by this mosaic effect.
2. A recessive X linked trait that causes the absence of sweat glands in the skin will
produce patches of normal skin, and skin lacking sweat glands.
b. Researchers have discovered a gene that is active only in the Barr body, called XIST,
or X-inactive specific transcript.
1. XIST produces an RNA molecule (oddly called “specific transcript”) which
attaches to the X chromosome.
2. Specific transcript may interact with the chromosome, and may initiate Xinactivation.
c. In some cells which give rise to the ovaries, the Barr body is reactivated.
III. Errors and Exceptions in Chromosomal Inheritance
A. Alterations of chromosome number or structure cause some genetic disorders
1. Alterations of Chromosome Number: Aneuploidy and Polyploidy: Occasionally, a tetrad or
chromosome will fail to separate, a meiotic error called nondisjunction.
i. Aneuploidy
a. If a nondisjunction occurs, one or more gametes receive a duplicate chromosome,
while another receives no copy.
1. If an abnormal gamete successfully fertilizes its opposite, the offspring will have
an abnormal chromosome number, a condition called aneuploidy.
i. If the aneuplodic cell has a chromosome in triplicate, it is said to be trisomic
for that chromosome.
ii. if the aneuplodic cell has only one copy of a chromosome, it is said to be
monosomic for that chromosome.
b. Nondisjunction can also occur during mitosis. If nondisjunction occurs early in the
embryonic period, a large number of body cells will be aneuplodic, and this may
have a substantial effect on the organism.
ii. Polyploidy:
a. Some individuals may have more than two complete sets of chromosomes, a
condition called polyploidy. The specific terms triploidic and tetraploidic, refer to the
3n and 4n individuals respectively.
b. Polyploidy is common in the plant kingdom
2. Alterations of Chromosome Structure
i. Types of alterations
a. Deletions occur when a chromosomal fragment lacking a centromere is lost during
division. The chromosome from which the fragment is lost will have an abnormal
number and arrangement of genes.
b. Duplications occur when a corresponding portion of a chromosome is lost, and then
joins to a homologous chromosome.
c. Inversions occur when the fragment reattaches to the original chromosome, but only
after flipping around.
d. Reciprocal translations occur when two portions of non homologous chromosomes
trade places.
ii. Organisms that have large deletion from all inherited copies of their sex chromosomes
usually lack a large number of essential genes, and usually die quickly.
iii. Inversions and reciprocal translations may alter phenotypes because a gene’s expression is
based on its position in relation to the genes around it.
3. Human Disorders Due to Chromosomal Alterations
i. Down syndrome is caused by trisomy 21, and affects one out of 700 children born in the
United States.
a. Down syndrome generates a wide variety of effects, including distinctive facial
characteristics, short stature, and a tendency toward heart defects, mental retardation,
Alzheimer’s disease, and sterility.
b. The chances of producing a Down syndrome child increase with mother’s age. For
this reason, mothers over the age of 35 take tests for trisomy 21.
ii. Aneuploidy of the sex chromosomes is more common than autosomal errors, as extra X
chromosomes inactivate as Barr bodies, and the Y chromosome carries few genes.
a. Extra Y chromosomes in males occur approximately once in every 2000 births, and
result in a man who has abnormally small, sterile testes, as well as the accentuation
of feminine body characteristics.
b. Trisomy X occurs once in every 1000 births, and has no apparent effects.
c. Monosomy X results in a phenotypically female offspring, but the sex organs do not
mature at adolescence, and secondary sex characteristics fail to develop. The
individuals are usually of short stature, and normal intelligence.
iii. Deletions in chromosomes can cause sever physical and mental abnormalities. One
example of a disorder resulting from deletions is cri du chat (cry of the cat) syndrome, which
is caused by a specific deletion in chromosome 5.
a. The child is mentally retarded, has a small head and unusual facial features.
b. The child’s cry sounds like a cats mew, and gives the disorder its name.
c. Individuals afflicted with cri du chat syndrome usually die in their infancy.
iv. Translocations also cause disorders, like chronic myelogenous leukemia (CML). CML
cases result from a translocation on chromosomes 22 and 9.
B. The phenotypic effects of some genes depend on whether they were inherited from the mother or
father (imprinting)
1. Genomic imprinting.
i. In mammals, certain genes are imprinted with the sex of the individual they are inherited
from.
a. In gamete producing cells, the maternal and paternal imprints are erased, and
replaced with those that correspond to the sex of the individual they currently reside
in.
b. Imprints are believed to consist of methyl groups added to certain loci on the
chromosomes. The genes with many methyl groups are inactivated, so the animal
probably uses the genes that are not imprinted.
ii. Researchers have discovered approximately 20 mammalian genes that are subject to
imprinting, most of which are critical to embryonic development.
2. One example of imprinting is Prader-Willi and Angelman syndrome, which both seem to be
caused by an idendical deletion on chromosome 15.
i. Prader-Willi syndrome is characterized by mental retardation, obesity, short stature, and
unusually small hands and feet. Prader-Willi syndrome results from the inheritance of a
defective chromosome 15 from the father
ii. Angelman syndrome sufferers exhibit uncontrollable laughter, jerky movements, and other
mental symptoms. Angelman syndrome results from the inheritance of a defective
chromosome 15 from the mother.
3. Fragile X syndrome is another disorder which results from imprinting.
i. Fragile X is named for the abnormal appearance of the X chromosome: The tip of the
chromosome is hanging by thin thread of DNA.
ii. Fragile X occurs once in 1500 males, and once in 2500 females, and the disorder is the
most common genetic cause of mental retardation.
iii. Fragile X results from the methylation of an allele in the mother, which, instead of
inactivating the gene, causes the syndrome.
C. Extranuclear genes exhibit a non-Mendelian pattern of inheritance: Extracellular genes, small
circles of DNA found in plastids and mitochondria, are not passed on following the same rules as
nuclear genes. Mitochondria and most plastids are passed from the mother, so the father’s
organelle makeup has no effect on the offspring.
1. Mitochondrial disorders can result in a lower than normal ATP production rate, and cause
energy deprivation and therefore weakness and muscle deterioration.
2. Other mitochondrial mutations may result in an increased risk of heart disease, diabetes, and
the debilitation of the geriatric.