1. Define the following terms and utilize these terms in an appropriate
context:
allele
chromosome theory
dihybrid cross
dominant
F1 generation F2 generation
genotype
heterozygote
homozygote
independent assortment
monohybrid cross
multiple allele
P generation
phenotype
recessive
reciprocal cross
segregation
test cross
trihybrid cross
Punnett Square
2. State Mendel's four postulates of inheritance.
3. Correlate Mendel's four postulates with what is now known about homologous chromosomes, genes, alleles and the process of meiosis.
4. Distinguish between homozygosity and heterozygosity.
5. Demonstrate the ability to determine possible gametes to be
passed given a parents genotype in monohybrid and dihybrid circumstances.
6. Given the designations of + for wild type and bw for brown
eyes in Drosophila melanogaster, outline a monohybrid cross between normal
flies and brown eyed flies to produce the F1, and an F1 cross to produce
the F2. Give the possible genotypes and phenotypes in each generation,
and the expected ratios.
7. Explain what is done in a test cross, and how the possible results are interpreted.
8. Given select non-linked traits from the Drosophila genome, outline a dihybrid cross to produce an F1, and an F1 cross to produce the F2 generation. Include the expected genotypes, phenotypes, and ratios.
9. For any total of F2 individuals, calculate the number expected to be of each phenotype.
10. Determine the relationship between the number of gene pairs for which an individual is heterozygous and the number of possible gamete types which can be produced by that individual with respect to those genes.
11. Determine the relationship between the number of phenotypes which can be produced by a cross between individuals which are heterozygous for any number of genes.
12. Discuss the considerations of possibility, probability and reality when dealing with genetic crosses.
Resources: Text Chapter 3, Cartoon Guide pgs. 36-55
Mendelian Genetics/ Classical Transmission Genetics
Possibility vs. Probability vs. Reality
-Inherited characteristics
are the result of particulate factors called genes composed of DNA that
are transmitted from
generation
to generation on vehicles called chromosomes.
-1866 Gregor Mendel parented
formal discipline of genetics
22 different varieties of peas, objective, simple, mathematical
Mendels Postulates (Laws):
1. Genetic characters are controlled by unit
factors that exist in pairs in individual organisms.
2. when two unlike unit factors responsible
for a single character are present in a single individual,
one unit factor is
dominant
to the other, which is said to be recessive.
3. During the formation of gametes, the paired
unit factors separate or segregate randomly so that each gamete
receives
one or the other.
4. During gamete formation, segregating pairs of unit factors assort independently of each other.
**Important to consider that Mendel had no knowledge of chromosomes or meiosis
Modern modifications to Mendel:
Allele- alternate forms of a gene (a gene is a location
on a chromosome, an allele is the info that fits in that place,
like a
cassette tape)
-Dominant allele
-Recessive allele
Gene locus
Homozygous
Heterozygous
Genotype
Phenotype
Punnett Square
Monohybrid Inheritance
A. Crosses involving
only one trait
-if blending were true, offspring should all be intermediates of parents
B. P- parental generation; F1 first filial generation; F2 second filial generation
F = filial -pertaining to or approptiate to a son or daughter
Mendel- Experiment with Tall X Short produced all tall
then tall plants self pollinated and produced 3/4 tall, 1/4 short, 0 intermediate
Conclusions: 3:1 ratio
possible if:
1. F1 contained two seperate copies of each hereditary factor, one dom
one rec
2. Factors seperated when gametes were formed
3. random joining of gametes upon fertilization
Example
Normal vs. nerd (please correct me if I seem to be wandering from political
correctness!!)
Alleles = N - normal n - nerd
Genotypes vs. phenotypes
Cross a homozygous normal individual with a nerd
Monohybrid Reciprocal cross - reverse roles of male and female
Monohybrid
Test cross- used to identify pure lines of descent, identify
carriers of recessives
-an individual with a dominant phenotype is crossed with one having a
recessive phenotype. Allows one to
determine whether an individual is homozygous dominant or heterozygous
(determines genotype).
DIHYBRID INHERITANCE- cross of true breeding lines that differ in two traits
Assumptions:
1. if dominants segregated
together and recessives segregated together, we would only see two phenotypes
in F2
2. if dominants and recessives segregated independently then we would see 4 phenotypes in F2
Mendel's law of Independent
Assortment:
Members of one pair of factors segregate (assort) independently of members
of another pair of factors. Therefore,
all possible combinations of factors can occur in the gametes. (***we
now know these "factors" to be
chromosomes and genes, however, chromosomes are the predominant mode of
organization)
For each genotype, give all possible gametes:
Remember, mother nature says that sexually reproducing organisms must pass
on one set of information to the
offspring, and there are two contributors to the genetic component
of the offspring
Find all the ways to pass on one piece of each set of information:
A. ttGG B. TtGG C. TtGg D. TTGg
Dihybrid example:
T= tall t= dwarf R= rough
r= smooth
Mate a homozygous tall/rough with an individual that is dwarf/smooth
Find F1 and then mate two F1 to find the F2
Dihybrid
Testcross:
as in mono cross questionable dominant genotype with individual that is
homozygous recessive for both traits
From our example above, are our tall/rough individuals TTRR, TtRR, TTRr or TtRr?
Cross questionable with ttrr (can check for both traits or singles)
T_R_ X ttrr
Trihybrid inheritance and beyond
Probabilities for each seperately
AABBCC X aabbcc
ABC AbC ABc aBC Abc aBc abC abc
This is an 8 X 8 Punnett Square with 64 possible outcomes.
Multiple gene inheritance produces tremendous variability
with 2 or more genes calculations become complex
Simple rules for determining
gametes (and from there probability)
1. determine number of heterozygous gene pairs and let this = n
2. 2n is the number of different gametes that can fe formed by each parent
3. 3n is the number of different genotypes that result following fertilization.
4. 2n is the number of different phenotypes that are produced by these genotypes
#
of het gene pairs #of diff gametes
#of diff genotypes #of diff phenos
n
2n
3n
2n
1 2 3 2
2 4 9 4
3 8 27 8
4
16
81
16
Ex.
AaBbCc n= 3
AABbCc n= 2
AAbbCc n= 1
AABBCC n= 0