Transcript Mendelian Genetics

Mendelian Genetics
Dihybrid Crosses
I.
Law of Independent Assortment
• A. Dihybrid Crosses
•
1. move from monohybrid to dihybrid cross
•
2. dihybrid-investigating the inheritance of
two traits at one time
•
3. question to be asked is
•
a. Are two genes inherited as a
package?
•
b. Are genes inherited independently
passing on to the next generation without being
affected by the first gene?
B.
Traits involved
• 1. Seed shape R (round) vs. r (wrinkled)
• 2. Seed color
Y (yellow) vs. y (green)
C. Parental generation
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1. true breeding round yellow X wrinkled green
2. list parental genotypes
3. list alleles carried in gametes of parental
4. list genotype of F1
5. allow F1 to self-pollinate
6. if traits are linked together would expect 3:1
ratio of round yellow : wrinkled green
• 7. if traits inherited differently would expect
something else
D.
•
Actual results
Round
yellow
315
parental
Wrinkled
yellow
101
recombinant
Round
green
108
recombinant
Wrinkled
green
32
parental
E.
Explanation
• 1. Analyze trait as two monohybrid crosses
• 2. Total number of round : wrinkled
• 3. Total number of yellow : green
• 4. both come out to a 3:1 ratio
• 5. evidence that the two genes are inherited
independently from one another
• 6. the two traits travel from one generation to the next
without influencing eachother
• 7. they are not tied together
• 8. What does this mean at the chromosomal level?
9.
Two chromosomes are traveling on different
chromosomes
• a. nucleus of F1
portrayed at right
• b. Two possible
alignments for
chromosomal pairs at
metaphase I
• c. these alignments
result in the production of
four different gametes
produced in equal
numbers
10.
Produce Punnett square for cross
Male/fe
male
RY
Ry
rY
ry
RY
RRYY
Ry
rY
ry
11. Complete square to determine phenotypic
ratio
offspring
Round
yellow
Round
green
Wrinkled
yellow
Wrinkled
green
Expected Actual
mendelian numbers
ratio
Actual
ratio
F.
Determining gametes-FOIL method
• 1. meaning of foil
• 2. examples of double hybrid
• 3. examples using other combinations
Practice problem
• The ability to roll one’s tongue is dominant over nonrolling. The ability to taste certain substances is also
genetically controlled. For example, there is a substance
called phenylthiocarbamate (PTC for short), which some
people can taste (the dominant trait), while others cannot
(the recessive trait). Determine the alleles to be used in
this problem.
• Suppose a woman who is both a
homozygous tongue-roller and a non-PTCtaster marries a man who is a
heterozygous tongue-roller and is a
heterozygous PTC taster. They have a
child. What are the chances that their first
child would be a non-taster who is
homozygous for tongue-rolling?
II.
Fate of Mendel
• A. discouraging
• B. became head of
monastery-very busy and
little time for experiments
• C. tried to replicate
results using native
hawkweed
– 1. very different results
– 2. requires pollen to begin
development
– 3. male and female
nucleus do not fuse
– 4. zygote is a haploid
being
Mendel and the boys
Biology humor
Old and discouraged Mendel
III.
Trihybrid crosses