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 Drosophila melanogaster (fruit
flies) as a test organism in genetics. Why?
Cheap to house and feed
Short generation time
Allowed the first tracing of traits to specific
Discovered sex linked traits
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
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.
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
Breaks up linkages and creates new ones.
Recombinant offspring formed that doesn't
match the parental types.
Constructed from crossing-over
1 map unit = 1% recombination frequency.
Have been constructed for many traits in
fruit flies, humans and other organisms.
Comment - only good for genes that are within 50
map units of each other. Why?
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 Humans
X chromosome - medium sized chromosome
with a large number of traits.
Y chromosome - much smaller chromosome
with only a few traits.
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)
Duchenne's Muscular Dystrophy
Hemophilia (types a and b)
Y- Linkage (rare)
Hairy ear rims
Hemizygous - 1 copy of X chromosome.
Show ALL X traits (dominant or recessive).
More likely to show X recessive gene
problems than females.
Samples of X-linked patterns:
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, baldness
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
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.
Human examples are known such as a
sweat gland disorder.
Changes in number.
Aneuploidy - too many or too few chromosomes,
but not a whole “set” change.
Polyploidy - changes in whole “sets” of
Changes in structure.
Caused by nondisjunction, the failure of a
pair of chromosomes to separate during
Nondisjunction in Meiosis I produces 4 abnormal
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 sexual
Extra skin on neck.
Normal mental development except for some spatial
Why are Turner Individuals usually sterile?
Odd chromosome number.
Two X chromosomes needed for ovary
Other Sex Chromosome changes
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
More X = more mental problems
2N + 1 or 2N + 2
Genotype: XXX or XXXX
Phenotype: female, but sexual development poor. Mental
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 usually
Why is trisomy 21 more common in older
Maternal age increases risk of nondisjunction.
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.
Chromosome Structure Alterations
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 22.
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 the
Imprints are "erased" in gamete producing cells and
re-coded by the body according to its sex.
Prader-Willi Syndrome and
Both lack a small gene region from chromosome 15.
Male imprint: Prader-Willi
Female imprint: Angelman
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.
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