Core Concepts in Genetics

Mendelian Inheritance

The Double Helix

Origin of Mendelian Genetics

"In 1859 I obtained a very fertile descendant with large, tasty seeds from a first generation hybrid. Since in the following year, its progeny retained the desirable characteristics and were uniform, the variety was cultivated in our vegetable garden, and many plants were raised every year up to 1865." (Gregor Mendel to Carl Nägeli, April 1867, from Mendel [1950])

Knowledge at the time

Hybridization was a familiar term at the time in plant work. (Kölreuter, Gärtner, Herbert, Lecoq, Wichura)

However, amongst the numerous plant hybrid researchers there was no law or methodology that described the formation of hybrids.

Hence, Mendel sought to design detailed experiments to describe these phenomena – The Law of Combination.

Mendel’s Hypotheses

Mendel perceived his experiments as a means to explaining the evolution of organic forms of life.

Mendel conceptualized that “something” had to be transmitted across the generations of an organism that allowed observable characteristics to persist.

Mendel’s Design

Characteristics of Subjects

1. Possess constant differentiating characteristics (a.k.a True Breeding).

2. The hybrids of such plants must, during the flowering period, be protected from the influence of all foreign pollen, or be easily capable of such protection.

3. The hybrids and their offspring should suffer no marked disturbance in their fertility in the successive generations

Mendel’s Design

After numerous trials with several

members of the Leguminosae

family the genus Pisum was found

to possess all the afore mentioned

characteristics.

The Monohybrid Cross

By examining hybrids created by a pair of true breeding plants that differed on a single characteristic Mendel sought “to deduce the law according to which they appear in successive generations”.

P1(round seed) x P2 (wrinkled)

= F1 (all round seed)

Observations of the Monohybrid Cross (F1)

All the hybrid plants formed had the characteristics of a single parent.

Mendel concluded that the characteristics that persisted, unaltered after transmission were dominant, and those which became latent/obscured were recessive.

Reciprocal crosses indicated that dominant/recessive characters were not affected by the nature of the parent.

Observations of the Monohybrid Cross (F2)

Mendel then proceeded to self-pollinate all of the progeny of the F1 generation.

F1 (round) x F1 (round) = F2 ( ¾ round : ¼ wrinkled)

Progeny of the F2 generation always presented themselves with dominant and recessive characteristics.

The Monohybrid Cross

Observations of the Monohybrid Cross

Backcross Curious of the results Mendel

conducted test crosses to determine the properties of F2 progeny.

Backcross – F2 x P2 (recessive) Backcross results indicate that

of the ¾ round seeds 1/3 produce only round seeds and 2/3 produce round and wrinkled seeds.

The Test Cross

Conclusions of the Monohybrid Cross (P1-F2)

For every characteristic examined the ratio of the offspring of the F1 generation is fairly constant.

F1 hybrid is a heterozygote  P1 & P2 are homozygotes.

Conclusions of the Monohybrid Cross (P1-F2)

If A represents the dominant characteristic and a the recessive, then the expression: A + 2Aa + a, describes the ratio of the parental forms to the hybrid forms in the F2 generation.

Furthermore, Hybrids display a natural tendency to revert back to their parental forms.

However, do not entirely disappear.

The Dihybrid Cross

Mendel then proceeded to combine plants that varied by two characteristics.

His F1 hybrid results were similar to that of the monohybrid crosses.

His F2 results consistently occurred in a ratio of 9:3:3:1 which consisted of the two parental forms and two hybrid forms.

The Dihybrid Cross

Reproductive Cells of Hybrids

To complete his theory on the inheritance of characteristics Mendel’s last set of experiments demonstrated that the egg cells of the plants were the vector for the transmission of information across generations.

Mendel’s Law of Combination

The law of combination of different characters which governs the development of the hybrids finds therefore its foundation and explanation in the principle enunciated, that the hybrids produce egg cells and pollen cells which in equal numbers represent all constant forms which result from the combinations of the characters brought together in fertilization.

Culmination of Mendel’s Work

The Law of Segregation

There are two elements of heredity for each trait in each individual that segregate during reproduction. Offspring receive one of the two elements from each parent. Furthermore, one of these elements may dominate the other.

Culmination of Mendel’s Work

The Law of Independent Assortment

Based upon the constant results of dihybrid crosses the elements for one characteristic assort independent from the elements for another characteristic.

The Double HelixThe Complementary

Model

J.D. Watson

F.H. Crick

Evidence for Fibrous Nature of DNA: Physico-

chemical analysis

DNA is a long asymmetrical chain that consists of a 5-carbon sugar and phosphate backbone joined in a 3`-5` direction by phosphodisester bonds.

Each sugar has 1 of 4 bases attached to it Adenine, Guanine, Cytosine, and Thymine.

DNA Has Two Chemical Chains: X-ray

Crystallography Two alternating

forms of DNA exists. A crystalline form

that occurs at 75% humidity (top) with a 2.8 A reflexion about its meridian..

A para-crystalline form that occurs at higher humidity (bottom) with 3.4 A reflexion about its meridian.

Density data indicate possibly two distinct polynucleotide chains

Hypothesized Structure DNA is three

dimensional. DNA has two chains

that are coiled around a single axis and held together by hydrogen bonds.

Both chains follow right handed helices.

The phosphates and sugar groups are on the outside and the bases on the inside.

A repeat distance of 34A with a reflexion of spacing 3.4A.

Hypothesized Structure

The two chains are held together by hydrogen bonds between the bases.

The base pairing is specific.

Adenine-Thymine Guanine-Cytosine

Each chain complements the other because of the specific base pairing.

Evidence supporting the Complementary Model

X-ray pictures suggests that: DNA’s basic structure is

helical. High concentration of atoms on

the circumference of the helix. The polynucleotide chains are

not distinct from each other.

Evidence supporting the Complementary Model

Titration curves of DNA suggest: Hydrogen bond formation is

characteristic of DNA structure

Analytical data of the bases

suggests: The amount of A – T and C – G

is very close.

Characteristics of Genetic Material

1. Must be able to self-replicate.

2. Must exert a highly specific influence on the cell.

DNA Replication: The Complementary Model

Of the two characteristics Watson & Crick proposed only a practical mechanism for replication.

Complementary base pairing is the backbone of replication.

But how does the interior of the helix allow itself to be replicated?

DNA Replication: The Complementary Model

Proposed that DNA unwinds by breaking the hydrogen bonds between the strands.

The single strands would then serve as a template to which complementary free nucleotides would attach and form 2 helices where there was originally one.

They were unaware of the numerous protein machinery (helicase, ligase, polymerases, etc.) that facilitated this process.

DNA Replication Today

DNA replication