Transcript Chromosomal Basis of Inheritance

Chapter 15:
The Chromosomal
Basis of Inheritance
Timeline
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1866- Mendel's Paper
1875- Mitosis worked out
1890's- Meiosis worked out
1902- Sutton, Boveri et. al. connect
chromosomes to Meiosis.
Sutton
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Developed the “Chromosome Theory of
Inheritance”.
Mendelian factors or alleles are located on
chromosomes.
Chromosomes segregate and show independent
assortment.
Morgan
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Chose to use Drosophila melanogaster (fruit
flies) as a test organism in genetics. Why?
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Small
Cheap to house and feed
Short generation time
Many offspring
Few chromosomes
Allowed the first tracing of traits to specific
chromosomes.
Discovered sex linked traits
Life Cycle
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Egg
Larva
Pupa
Adult
Genetic Symbols
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Mendel - use of uppercase or lowercase letters.
T = tall
t = short
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Morgan: symbol from the mutant phenotype.
+ = wild phenotype
 Recessive mutation:
 w = white eyes
 w+ = red eyes
 Dominant Mutation
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Cy = Curly wings
Cy+ = Normal wings
Fruit Fly Chromosomes
Morgan Discovered
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There are many genes, but only a few
chromosomes.
Therefore, each chromosome must carry a number
of genes together as a “package”.
Linked Genes
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Traits that are located on the same chromosome.
Result:
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Failure of Mendel's Law of Independent Assortment.
Ratios mimic monohybrid crosses.
Body Color
and Wing type
Example
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:
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Independent Assortment of traits fails.
Linkage may be “strong” or “weak”.
Degree of strength related to how close the
traits are on the chromosome.
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Weak - farther apart
Strong - closer together
Crossing-Over
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Breaks up linkages and creates new ones.
Recombinant offspring formed that doesn't
match the parental types.
Genetic Maps
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Constructed from crossing-over
frequencies.
1 map unit = 1% recombination frequency.
Have been constructed for many traits in
fruit flies, humans and other organisms.
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Comment - only good for genes that are within 50
map units of each other. Why?
Sex Linkage in Biology
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Several systems are known:
Mammals – XX and XY
Diploid insects – X and XX
Birds – ZZ and ZW
Chromosomal Basis of Sex in Humans
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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.
SRY
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Sex-determining Region Y chromosome
gene.
If present - male
If absent - female
SRY codes for a cell surface receptor.
Sex Linkage
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Inheritance of traits on the sex chromosomes.
X- Linkage (common)
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Color blindness
Duchenne's Muscular Dystrophy
Hemophilia (types a and b)
Y- Linkage (rare)
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Hairy ear rims
Males
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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:
Traits
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Sex Limited Traits
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Traits that are only expressed in one sex.
Ex – prostate glands
Sex Influenced Traits
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Traits whose expression differs because of the hormones
of the sex.
Ex. – beards, mammary gland development, baldness
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.
Barr Body
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Inactive X chromosome observed in the
nucleus.
Way of determining genetic sex without doing
a karyotype.
Lyon Hypothesis
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Which X inactivated is random.
Inactivation happens early in embryo
development by adding CH3 groups to the
DNA.
Result - body cells are a mosaic of X
types.
Examples
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Calico Cats.
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XB = black fur
XO = orange fur
Calico is heterozygous, XB XO.
Why don’t you find many calico males?
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They must be XB XOY and are sterile.
Human examples are known such as a
sweat gland disorder.
Chromosomal Alterations
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Changes in number.
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Aneuploidy - too many or too few chromosomes,
but not a whole “set” change.
Polyploidy - changes in whole “sets” of
chromosomes.
Changes in structure.
Aneuploidy
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Caused by nondisjunction, the failure of a
pair of chromosomes to separate during
meiosis.
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Nondisjunction in Meiosis I produces 4 abnormal
gametes.
Nondisjunction in Meiosis II produces 2 normal
and 2 abnormal gametes.
Types of Aneuoploidy
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Monosomy: 2N – 1 (very rare)
Trisomy: 2N + 1 (more common)
Turner Syndrome
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2N - 1 or 45 chromosomes
Genotype: X_ or X0.
Phenotype: female, but very poor secondary sexual
development.
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Characteristics
 Short stature.
 Extra skin on neck.
 Broad chest.
 Usually sterile
 Normal mental development except for some spatial
problems.
Question
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Why are Turner Individuals usually sterile?
Odd chromosome number.
Two X chromosomes needed for ovary
development.
Other Sex Chromosome changes
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Kleinfelter Syndrome
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Meta female
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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
2N + 1 or 2N + 2
Genotype: XXX or XXXX
Phenotype: female, but sexual development poor. Mental
impairment common.
Supermale
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2N + 1 or 2N + 2
Genotype: XYY or XYYY
Phenotype: male, usually normal, fertile.
Trisomy events
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Trisomy 21: Downs Syndrome
Trisomy 13: Patau Syndrome
Both have various physical and mental
changes.
Question?
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Why is trisomy more common than
monosomy?
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Fetus can survive an extra copy of a
chromosome, but being hemizygous is usually
fatal.
Why is trisomy 21 more common in older
mothers?
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Maternal age increases risk of nondisjunction.
Polyploid
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Triploid= 3N
Tetraploid= 4N
Usually fatal in animals.
Question?
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In plants, even # polyploids are often
fertile, why odd # polyploids are sterile.
Why?
Odd number of chromosomes can’t be
split during meiosis to make spores.
Chromosome Structure Alterations
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Deletions
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Result
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Loss of genetic information.
Position effects: a gene's expression is influenced by
its location to other genes.
Duplications
Inversions
Translocations
Translocations
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Cri Du Chat Syndrome
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Part of p arm of #5 missing.
Good survival.
Severe mental retardation.
Small sized heads common.
Philadelphia Chromosome
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An abnormal chromosome produced by an
exchange of portions of chromosomes 9 and 22.
Causes chronic myeloid leukemia.
Parental Imprinting of Genes
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Gene expression and inheritance depends on which
parent passed on the gene.
Usually caused by different methylations of the
DNA.
Imprints are "erased" in gamete producing cells and
re-coded by the body according to its sex.
Example:
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Prader-Willi Syndrome and
Angelman Syndrome
Both lack a small gene region from chromosome 15.
 Male imprint: Prader-Willi
Female imprint: Angelman
Result
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Phenotypes don't follow Mendelian
Inheritance patterns because the sex of the
parent does matter.
Why have parental imprinting?
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Method to detect that TWO different sets of
chromosomes are in the zygote.
Summary
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Know about linkage and crossing-over.
Sex chromosomes and their pattern of
inheritance.
Variations of chromosomes and inheritance
patterns.
Be able to work genetics problems for this
chapter.