Chromosomal Basis of Inheritance
Chromosomal Basis of Inheritance
Basis of Inheritance
1866- Mendel's Paper
1875- Mitosis worked out
1890's- Meiosis worked out
1902- Sutton, Boveri et. al. connect
chromosomes to Meiosis.
Developed the “Chromosome Theory of
Mendelian factors or alleles are located on
Chromosomes segregate and show independent
Chose to use fruit flies as a test organism in
Allowed the first tracing of traits to specific
Early test organism for genetic studies.
Cheap to house and feed
Short generation time
Fruit Fly Chromosomes
Mendel - use of uppercase or lowercase letters.
T = tall
t = short
Morgan: symbol from the mutant phenotype.
+ = wild phenotype
w = white eyes
w+ = red eyes
Cy = Curly wings
Cy+ = Normal wings
A male fly with a mutation for white eyes.
The white eye male with a normal red eye
The F1 offspring:
All had red eyes.
This suggests that white eyes is a genetic
F1 X F 1 = F2
Morgan expected the F2 to have a 3:1 ratio of
He got this ratio, however, all of the white eyed flies
Therefore, the eye color trait appeared to be linked
Sex linked traits.
Genetic traits whose expression are
dependent on the sex of the individual.
Fruit Fly Chromosomes
There are many genes, but only a few
Therefore, each chromosome must carry a
number of genes together as a “package”.
Traits that are located on the same chromosome.
Failure of Mendel's Law of Independent Assortment.
Ratios mimic monohybrid crosses.
and Wing type
b+b vg+vg X bb vgvg
(b+ linked to vg+)
(b linked to vg)
If unlinked: 1:1:1:1 ratio.
If linked: ratio will be altered.
Breaks up linkages and creates new ones.
Recombinant offspring formed that doesn't
match the parental types.
If Genes are Linked:
Independent Assortment of traits fails.
Linkage may be “strong” or “weak”.
Degree of strength related to how close the
traits are on the chromosome.
Weak - farther apart
Strong - closer together
Constructed from crossing-over
1 map unit = 1% recombination frequency.
Comment - only good for genes that are within 50
map units of each other. Why?
Have been constructed for many traits in fruit
flies, humans and other organisms.
Sex Linkage in Biology
Several systems are known:
Mammals – XX and XY
Diploid insects – X and XX
Birds – ZZ and ZW
Chromosomal Basis of Sex in
X chromosome - medium sized chromosome
with a large number of traits.
Y chromosome - much smaller chromosome
with only a few traits.
Human Chromosome Sex
Males - XY
Females - XX
Comment - The X and Y chromosomes are a
homologous pair, but only for a small region
at one tip.
Sex-determining Region Y chromosome
If present - male
If absent - female
SRY codes for a cell surface receptor.
Inheritance of traits on the sex chromosomes.
X- Linkage (common)
Y- Linkage (rare)
Hemizygous - 1 copy of X chromosome.
Show ALL X traits (dominant or
More likely to show X recessive gene
problems than females.
Duchenne's Muscular Dystrophy
Hemophilia (types a and b)
Samples of X-linked patterns:
Trait is usually passed from a carrier
mother to 1/2 of sons.
Affected father has no affected children,
but passes the trait on to all daughters
who will be carriers for the trait.
Watch how questions with sex linkage are
Chance of children?
Chance of males?
Can Females be color-blind?
Yes, if their mother was a carrier and their
father is affected.
Hairy ear pinnae.
Comment - new techniques have found a
number of Y-linked factors that can be
shown to run in the males of a family.
Ex: Jewish priests
Sex Limited Traits
Traits that are only expressed in one sex.
Ex – prostate glands
Sex Influenced Traits
Traits whose expression differs because of
the hormones of the sex.
Ex. – beards, mammary gland development,
Testosterone – makes the trait act as a dominant.
No testosterone – makes the trait act as a recessive.
Males – have gene = bald
Females – must be homozygous to have thin hair.
Inactive X chromosome observed in the
Way of determining genetic sex without doing
Which X inactivated is random.
Inactivation happens early in embryo
development by adding CH3 groups to the
Result - body cells are a mosaic of X
Human examples are known such as a
sweat gland disorder.
XB = black fur
XO = orange fur
Calico is heterozygous, XB XO.
Why don’t you find many calico males?
They must be XB XOY and are sterile.
Changes in number.
Changes in structure.
Aneuploidy - too many or too few
chromosomes, but not a whole “set” change.
Polyploidy - changes in whole “sets” of
Caused by nondisjunction, the failure of a
pair of chromosomes to separate during
Nondisjunction in Meiosis I produces 4
Nondisjunction in Meiosis II produces 2
normal and 2 abnormal gametes.
Types of Aneuoploidy
Monosomy: 2N – 1 (very rare)
Trisomy: 2N + 1 (more common)
2N - 1 or 45 chromosomes
Genotype: X_ or X0.
Phenotype: female, but very poor secondary
Extra skin on neck.
Normal mental development except for
some spatial problems.
Why are Turner Individuals usually sterile?
Odd chromosome number.
Two X chromosomes needed for ovary
Other Sex Chromosome
2N + 1 (2N + 2, 2N + 3)
Genotype: XXY (XXXY, XXXXY)
Phenotype: male, but sexual development may
be poor. Often taller than average, mental
development fine (XXY), usually sterile.
More X = more mental problems
May have been a Kleinfelter Syndrome
Much taller than average.
Produced no children.
2N + 1 or 2N + 2
Genotype: XXX or XXXX
Phenotype: female, but sexual development
poor. Mental impairment common.
2N + 1 or 2N + 2
Genotype: XYY or XYYY
Phenotype: male, usually normal, fertile.
Trisomy 21: Downs Syndrome
Trisomy 13: Patau Syndrome
Both have various physical and mental
Why is trisomy more common than
Fetus can survive an extra copy of a
chromosome, but being hemizygous is
Why is trisomy 21 more common in older
Maternal age increases risk of
Usually fatal in animals.
In plants, even # polyploids are often
fertile, why odd # polyploids are sterile.
Odd number of chromosomes can’t be
split during meiosis to make spores.
Loss of genetic information.
Position effects: a gene's expression is
influenced by its location to other genes.
Cri Du Chat Syndrome
Part of p arm of #5 missing.
Severe mental retardation.
Small sized heads common.
An abnormal chromosome produced by an
exchange of portions of chromosomes 9 and
Causes chronic myeloid leukemia.
Parental Imprinting of Genes
Gene expression and inheritance depends on
which parent passed on the gene.
Usually caused by different methylations of
Prader-Willi Syndrome and
Both lack a small gene region from
Male imprint: Prader-Willi
Female imprint: Angelman
Imprints are "erased" in gamete producing
cells and re-coded by the body according
to its sex.
Phenotypes don't follow Mendelian
Inheritance patterns because the sex of the
parent does matter.
Why have parental imprinting?
Method to detect that TWO different sets of
chromosomes are in the zygote.
Can fuse nuclei into mouse eggs.
If male/male = normal placenta, abnormal fetus
If female/female = abnormal placenta, normal fetus
If male/female = normal placenta and normal fetus
Inheritance of genes not located on the
DNA in organelles.
Mendelian inheritance patterns fail.
Maternal Inheritance of traits where the trait
is passed directly through the egg to the
Ragged Red-fiber Disease
Leber’s Optic Neuropathy
All are associated with ATP generation
problems and affect organs with high ATP
Gives non-green areas in leaves, called
Several different types known.
Very common in ornamental plants.
Cells can have a mixture of normal and
Result - degree of expression of the maternal
inherited trait can vary widely.
Know about linkage and crossing-over.
Sex chromosomes and their pattern of
Variations of chromosomes and inheritance
Be able to work genetics problems for this