Transcript Chromosomes - Mrs. Dedam

Chromosomes
“Heritable factors” are on
chromosomes
• Scientists doubted Mendel’s theory without
physical evidence
• Behavior of chromosomes during meiosis
supports theory of inheritance.
Behavior of Chromosomes
•
•
•
•
Present in pairs
Separated during Meiosis I
Independent assortment at metaphase II
Mendel saw no evidence of crossing over
because the traits he looked at were on
different chromosomes
Thomas Hunt Morgan- The fly guy
• Fruitflies have only four pairs of
chromosomes
– 6 autosomes, 2 sex chromosome
• Used mutants to study heredity
• Observations didn’t always
match Mendel’s theory
– Chi-squared
The big one
• Crossed red eyed female with white eyed
male
• All offspring were red
• Is red dominant or recessive?
The second generation
• Crossed red male and red female
• What was the ratio of red: white offspring
???
The results
• All females were red, half males were
white, half were red
• Only males could have white eyes, white
eye factor had to be something that only
males had…y chromosome
• Genes are contained in chromosomes
Challenge
• Cross a red eyed, F1 hybrid female with a
white eyed male. What is the ratio of red:
white?
Linked Genes
• Linked genes tend to be inherited together
• Close together on the same chromosome
• Results deviate from Mendel’s ratios
The fly guy’s experiment
• Crossed double mutant (black body,
vestigial wings) with normal (gray body,
normal wings)
• Mutants are recessive to wild-type traits
• All offspring were gray with normal wings
• Crossed hybrids (b b+ vg vg+) from F1
with black bodied, vestigial winged (b b vg
vg)
– 965 Gray, normal (b+ b vg+ vg)
– 944 black, vestigial (b b vg vg)
– 206 gray, vestigial (b b+ vg vg)
– 185 black, normal (b b vg+ vg)
• More parental genotypes than
recombinants
Recombinants
• Crossing over separates linked genes, creates
recombinants
– In females most gamest have either (b+vg+) or (b vg)
– A few have either (b+ vg) or (b vg+)
• Female genotype always shows because male
contributes only recessive genes
Gene Maps
• % recombinant corresponds to distance
between genes on chromosomes
• The farther apart two genes are the
greater the possibility that they will be
separated by crossing over
• 1%=1 map unit
Constructing a gene map
– 965 Gray, normal (b+ b vg+ vg)
– 944 black, vestigial (b b vg vg)
– 206 gray, vestigial (b b+ vg vg)
– 185 black, normal (b b vg+ vg)
– 391 recombinant/ 2,300 total = 17%
• 50 % recombinant = different
chromosomes , assort independantly
Another cross
• Cross a hybrid normal winged, red eyed fly
with a vestigial winged, cinnebar eyed fly
– 457 normal, red
– 453 vestigial, cinnebar
– 50 normal, cinnebar
– 45 vestigial, red
– How far apart are the cinnebar and wing
genes?
Multiple gene linkages
• Cinnebar and wing =9.5 map units
• Body and wing = 17 map units
• Cinnebar and body =7.5 map units
b
cn
7.5
vg
9.5
Another mapping problem
• Genes A, B and C are located on the
same chromosome
• The results of the crosses are:
– Ab or aB  28 %
– Ac or aC 2%
– bC or Bc 30 %
– What order are these genes in?
Sex linked genes
• X chromosome contains many genes
– Blood clotting factors
– Male pattern baldness
– Color vision
• Y chromosomes contains one important
gene
– Testes determining factor (SRY)
X- linked traits
• Females can be homozygous or
heterozygous
• Males are hemizygous
• Any male that has one copy of recessive
gene will express it
• Females must have two copies of
recessive genes
• Males show x-linked phenotypes more
often
Inheritance of x-linked traits
• Females can be carriers of x-linked
diseases
• Men never pass x-linked traits to their
sons
Hemophilia in European royalty
X inactivation
• Only one X chromosome is necessary
– Males do fine with only one
• One X chromosome in each female cell
inactivates, forms a Barr Body
• Genes on Barr Body are inaccessible,
don’t show up
Females are Mosaic
• Inactivation occurs randomly at a certain
stage in embryonic development
• If a female is heterozygous for an x-linked
disease some of her
cells will express it and
others won’t
– Sweat glands