Principles of Gene Segregation

1
Principles of Gene Segregation

• Mendelian Genetics

2
Mendelian Genetics

• A. Johann Mendel (1822) 1843 Augustiniam
• Monastery of St. Thomas
• B. Educated in botany and physics at
• University of Vienna
• C. Taught physics and natural sciences for
• 16 years
• D. Used garden pea 1856 - 1868
• E. Died in 1884 of a kidney disorder

3
Mendelian Genetics

• 1. Height - Tall/Dwarf
• 2. Seed Shape - Round/Wrinkled
• 3. Seed Color - Yellow/Green
• 4. Pod Shape - Full/Constricted
• 5. Pod Color - Green/Yellow
• 6. Pod Arrangement - Axial/Terminal
• 7. Flower Color - Violet/White

4
Mendelian Genetics

• 1. Phenotype
• 2. Genotype
• 3. Dominant
• 4. Recessive
• 5. Alleles
• 6. Locus
• 7. Punnett Square
• 8. Homozygous
• 9. Heterozygous
• 10. Monohybrid
• 11. Dihybrid

• 12. Hemizygous
• 13. Selfing
• 14. Pure-Breeding
• 15. Principle of Segregation
• 16. Principle of Independent
• Assortment
• 17. Pedigree
• 18. Probability
• 19. Product Rule
• 20. Sum Rule
• 21. Chi-Square Analysis

5
Mendels Postulates
UNIT FACTORS IN PAIRS Genetic characters
(traits) are controlled by unit factors (alleles)
that exist in pairs in individual organisms
6
Mendels Postulates
DOMINANCE/RECESSIVENESS When two unlike 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.
7
Mendels Postulates
SEGREGATION During the formation of gametes, the
paired unit factors separate, or segregate,
randomly so that each gamete receives one or the
other with equal likelihood OR Two members of
a gene pair (alleles) segregate from each other
during the formation of gametes (Meiosis
II)
8
What happens when genetic data no longer
coincides with Mendels ratios?
9
Mendels Postulates

• QUESTION
• What experimental data led Gregor Mendel to the
  conclusion that unit factors (alleles) segregate
  independently?

10
Mendels Postulates

• ANSWER
• Recessive characters, which are masked in the F1
  from a cross between two true-breeding strains,
  reappear in a specific proportion in the F2.

11
Mendels Postulates
INDEPENDENT ASSORTMENT During gamete formation,
segregating pairs of unit factors (traits on
sister chromatids) assort independently of each
other OR The factors for different traits
assort independently of one another
(Meiosis I)
12
Mendels Postulates

• QUESTION
• What experimental data led Gregor Mendel to the
  conclusion that genes on different chromosomes
  behave independently in gamete production?

13
Mendels Postulates

• ANSWER
• Selfing a plant that is heterozygous for two
  distinct traits yields the phenotypic ratio
• 9331
• where four different phenotypes and
• nine different genotypes are present in the
  offspring

14
Mendelian Genetics

• TEST CROSS
• One-Factor Test Cross (monohybrid)
• Two-Factor Test Cross (dihybrid)
• Three-Factor Test cross (trihybrid)
• Forked-Line or Branch Methods

15
Independent Assortment

• General Rules
• 1. Number of possible gamete combination for
  specific traits of an organism used in a test
  cross (gamete combinations seen on the top of a
  punnett square)
• 2n where n equals the number of heterozygous
• gene pairs

16
Independent Assortment

• General Rules
• 2. Number of possible genotypic combination
  found in the offspring of a testcross
• 3n where n equals the number of heterozygous
  gene pairs

17
Independent Assortment

• QUESTION
• How many different types of gametes are produced
  by an individual of genotype AaBbCCddEeFFGg?
  Assume that all 7 genes assort independently.

18
Independent Assortment

• ANSWER
• 16 Different gamete combinations can be produced

19
Independent Assortment

• QUESTION
• ?
• Answer 3

20
Product Law (and rule)

• QUESTION
• If two dice are rolled at the same time, what is
  the probability of rolling a 3 and a 6?

21
Product Law (and rule)

• ANSWER
• p 1/6 X 1/6 1/36

22
Product Law (and rule)

• QUESTION
• Genes a, b, and c assort independently and are
  recessive to their respective alleles A, B, and
  C. Two triply heterozygous
• (Aa Bb Cc) individuals are crossed.
• a. What is the probability that a given
  offspring will be phenotypically A B C, that is,
  will exhibit all three dominant traits?

23
Product Law (and rule)

• ANSWER
• The probability of an offspring showing the A
  trait from Aa x Aa is 3/4 . The same probability
  exists for B and C. Therefore, the probability
  of a given progeny being phenotypically A B C is
• ¾ x ¾ x ¾ 27/64

24
Product Law (and rule)

• When two or more events occur independently, but
  at the same time, we can calculate the
  probability of possible outcomes when they occur
  together

25
Product Law (and rule)
QUESTION Genes a, b, and c assort independently
and are recessive to their respective alleles A,
B, and C. Two triply heterozygous (Aa Bb Cc)
individuals are crossed. b. What is the
probability that a given offspring will be
genotypically homozygous for all three dominant
alleles?
26
Product Law (and rule)
ANSWER The probability of an AA offspring from
Aa x Aa is ¼. The same probability is the
case for a BB offspring and for a CC offspring.
Therefore, the probability of an AA BB CC
offspring is ¼ x ¼ ¼ 1/64
27
Sum Law (either-or)

• This rule may be used when events are mutually
  exclusive. The probability that one of several
  mutually exclusive events will occur is the sum
  of the probabilities.

28
Sum Law (either-or)

• QUESTION
• What is the probability, when we throw a die, of
  it showing either a four or a six?

29
Sum Law (either-or)

• ANSWER
• p 1/6 1/6 2/6