Principles of Gene Segregation

Mendelian Genetics

By:Mustafa

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

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

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

Mendel’s Postulates

UNIT FACTORS IN PAIRS

Genetic characters (traits) are controlled by unit factors (alleles) that exist in pairs in individual organisms

Mendel’s 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.

Mendel’s Postulates

SEGREGATIONDuring 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)

What happens when genetic data no longer

coincides with Mendel’s ratios?

Mendel’s Postulates

QUESTION:

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

Mendel’s 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.

Mendel’s 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)

Mendel’s Postulates

QUESTION:

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

Mendel’s Postulates

ANSWER:

Selfing a plant that is heterozygous for two distinct traits yields the phenotypic ratio

9:3:3:1 where four different phenotypes

and nine different genotypes are

present in the offspring

Mendelian Genetics

TEST CROSS

• One-Factor Test Cross (monohybrid)

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

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

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

Independent Assortment

QUESTION

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

Independent Assortment

ANSWER

16 Different gamete combinations can be produced

Independent Assortment

QUESTION

?

Answer : 3

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?

Product Law (and rule)

ANSWER

p = 1/6 X 1/6 = 1/36

Product Law (and rule)

QUESTIONGenes 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?

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

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

Product Law (and rule)

QUESTIONGenes 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?

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

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.

Sum Law (either-or)

QUESTION

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

Sum Law (either-or)

ANSWER

p = 1/6 + 1/6 = 2/6