Transcript Ch 15: Chromosomal Basis of Inheritance

Chapter 15
Chromosomal Basis of Inheritance
Mendel & Chromosomes
• Mendel was ahead of his time. 19th C
cytology suggested a mechanism for his
earlier findings. What did they find?
 Chromosomes and genes are both
present in pairs in diploid cells.
 Homologous chromosomes separate
and alleles segregate during meiosis.
 Fertilization restores the paired
condition for both chromosomes and
genes.
Chromosome Theory of Inheritance
• Mendelian genes
have specific loci
on chromosomes
• Chromosomes are
what physically
undergo
segregation and
independent
assortment.
Morgan’s Fruit Flies
• Morgan first associated a specific gene
with a specific chromosome.
• Why fruit flies?
• Breed quickly (two week generations)
• 4 pairs of chromosomes (3 pair of
autosomes, 1 pair of sex chromosomes)
• Females = XX
• Males = XY
Morgan’s Fruit Flies
• Wild Type flies are the most common
natural phenotype. (Red Eyes)
• After a series of crosses, Morgan
produced mutants with white eyes.
• After a few generations, Morgan noted
that only males displayed the white eyes.
• He concluded that certain genes are
located on the sex chromosome and thus
linked to sex.
• Sex-linked genes (ie: hemophilia)
Sex-linked Traits
Sex-linked Traits
• Morgan concluded
the gene with the
white-eyed
mutation is on the
X chromosome.
Y chromosome =
no info
• Males (XY) only
need one copy of
recessive allele to
show trait.
Linked Genes
• All genes located on the same
chromosome tend to be inherited together.
• Chromosome passed on as a unit.
• Testcross results varied from those
predicted by the law of independent
assortment.
• This showed that certain genes will
assort together. (on same chromosome)
Linked Genes
Linked Genes
• Body color and wing shape are usually
inherited together (same chromosome)
Recombinants
•
Where did the other phenotypes come
from? (grey-vestigial and black normal)
• Genetic recombination= offspring with
new combinations of traits inherited from
two parents
• How??
• independent assortment of genes (nonhomologous)
• crossing over of genes (homologous)
Recombinants
Mendel’s dihybrid crosses produced
recombinant genotypes.
• 50% parental : 50% recombinant
genotypes typical for nonhomologues
• Metaphase I
• YR, Yr, yR, and yr
• Seed shape and color tetrads are
independent from one another
•
Recombinants
•
Linked genes tend to move together
during meiosis/fertilization
• If Independent assortment of genes
• Expect a 1:1:1:1 phenotype ratio
• If Complete linkage of genes
• 1:1:0:0 ratio (all parental)
• Observed 17% recombinant flies
• Suggested Incomplete linkage of genes
Crossing Over
 Prophase
I: homologous chromosomes
can “swap” alleles
 More variable gametes than simple
mendelian rules would predict
Therefore, Crossing Over Explains:
Linkage Maps
•
Ordered list of genetic loci along
chromosome
• Based on recombination frequencies
between two genes
• Higher % of recombination = farther apart
• More places in between genes for
crossing over to occur and separate the
genes
Linkage Maps
•The recombination frequency between
cn and b is 9%.
•The recombination frequency between
cn and vg is 9.5%.
•The recombination frequency between b
and vg is 17%.
Linkage Maps
•
Map units are the distances between
genes on a chromosome.
• 1 map unit = 1% recombination
• 50% recombination = so far apart that
crossing over is all but certain
• Remember, 50% recomb. = ind.
assortment (non-homologous)
• Linkage maps show relative
order/distance
• More recent studies show exact
distances and order
Sex Chromosomes
•
•
•
•
•
X-Y Sex Determination
X and Y behave as homologues
Each egg receives an X from XX mother
One sperm receives X and one Y
Results in 50/50 chance of male or female
SRY Gene
• Present (on Y) : gonads develop into
testes (male)
• Not present (no Y): gonads become
ovaries (female)
• SRY also regulates other genes
Sex-Linked Genes
•
Sex chromosomes also contain other
genes. (ie: drosophila eye color)
• Females must be homozygous recessive
to display trait (XX – second X can mask
recessive)
• Females can be carriers
• Males only need to inherit a single copy to
show trait
• Can a male be a carrier?
Sex-Linked Disorders
•
Duchenne Muscular Dystrophy
•
•
•
1/3500 males
Progressive muscular weakening
• Die by mid-20’s
Missing X-linked gene
• No production of dystrophin (muscle
protein)
Sex-Linked Disorders
•
Hemophilia
• Absence of one or more clotting
factors
• affected individuals cannot stop
bleeding normally
• treated with protein injections
Barr Bodies
•
Only one of the females X chromosomes
is active
• The other becomes a Barr body
• When assorted into an ovum, the Barr
body becomes activated again
• Which X becomes Barr body is random in
each cell
• Approx. 50% express each allele (if
hetero)
X-Inactivation in Females
Nondisjunction
•
•
Errors with meiotic spindle
• Meiosis I: Homologous tetrad doesn’t
separate OR
• Meiosis II: Sister chromatids don’t
separate
Some gametes receive two of the same
type of chromosome and another gamete
receives no copy
Aneuploidy
•
Results from fertilization involving
nondisjoined gamete(s)
•
Trisomy three copies of a particular
chromosome (2n + 1)
•
Monosomy only one copy of a particular
chromosome (2n – 1)
Down Syndrome
•
•
•
Three copies of
chromosome
21
1/700 children
born each year
Definite link
with maternal
age
Aneuploidy in Sex Chromosomes
•
XXY Male (Klinefelter’s Syndrome)
• Male sex organs, sterile w/
femininity
•
XYY Males
• Tend to be taller than normal
Aneuploidy in Sex Chromosomes
•
XXX Females
• Will develop as normal females
•
XO Females (monosomy – Turner
syndrome)
• Immature females
• 1/2500 live female births
Changes in Chromosomes