WHY MENDEL? WHY PEAS?

genetics: the branch of biology that studies the transmission of hereditary information from parents to offspring

WHY MENDEL? WHY PEAS?

Mendel was a monk who was educated--he knew MATH! He also had a lot of free time.
As for the peas? Well, they were:
* easy to grow
* easy to pollinate (either self or cross)
* easy to study their TRAITS (characteristics) such as: height, seed pod shape, seed color, seed pod color, seed texture,flower position, and seed shape

Mendel kept very accurate records for hundreds of individual plants. He used statistical analysis (MATH) to study the traits of different generations. Through his findings, he proposed that the traits were passed on by some kind of hereditary factors (now know to us as DNA).

Here's basically what he did:

One trait he studied was plant height. Mendel pollinated all tall plants for many generations to get a pure population of tall plants. He also pollinated all short plants to get a pure population of small plants. See picture below...

X means to cross
(as in cross-pollinate)

tall plants X tall plants as well as short plants X short plants

He then cross-pollinated a member of the tall pure population with a member of the short pure population...

P1 (pure parent generation)

P1 TALL PLANT X P1 SHORT PLANT

ALL TALL PLANTS

F1 (first filial [family] generation)

Where did the short trait go? Mendel then crossed two tall members of the F1 generation.

F1 TALL PLANT X F1 TALL PLANT
THIS IS CALLED A MONOHYBRID CROSS (hybrids known for one trait)

F2 (second filial generation)

For every 3 TALL PLANTS there was 1 SHORT PLANT

So, the short trait did not disappear. It was present in the F1 generation, but IT WAS NOT EXPRESSED (this means it did not show up!) Somehow, it was expressed (visible) in the F2 generation.

I. Mendel's Principal of Dominance

* because the tall trait showed up more than short, Mendel call this trait DOMINANT

* the short trait, because it seemed 'weaker' than the tall trait, was called RECESSIVE

After this discovery...well, um...NOBODY CARED! Later on, when scientists studies meiosis in Drosophila (genus name for fruit flies), they linked together Mendel's factors with the chromosomes in gametes.

* Mendel's factors are now called GENES

* genes are segments of chromosomes (DNA) that code for a characteristic

* these characteristics can be:

a) physical traits (can be seen like eye color, hair color, height, etc.)

b) chemicals produced in the body (like for hormones or enzymes)

genes are lined up on chromosomes in a certain order, like beads on a string

* homologous (similar) chromosomes have the same order of genes...HOWEVER..

* these chromosomes might have the same forms of a gene

ex. trait=eye color can be blue, black, brown, hazel, green, etc.

* these different forms of a gene are called alleles

* in simple patterns of inheritance, there are 2 different forms of a gene (alleles), where one is dominant and one is recessive

* DOMINANT IS SHOWN BY CAPITAL LETTERS

* recessive is shown by lowercase letters

ex. in Mendel's peas T = tall t = short

* diploid (having 2 sets of chromosomes) organisms have 2 copies of genes, one on each chromosome they have

* the 2 copies can be the same--HOMOZYGOUS, like TT (homozygous dominant) or tt (homozygous recessive)

*the 2 copies can be different--HETEROZYGOUS Tt (heterozygous)

Notice that what the organism looks like and the kind of genes it has are two separate ways to classify an organism.

phenotype: what the appearance of an organism is (how it looks) ph = physical

genotype: what the genetic makeup of an organism is (what genes it has) gen = genes

genotype can be:
homozygous dominant----------> TT
heterozygous (a hybrid--a mix)---> Tt
homozygous recessive----------> tt

recessive phenotypes can only be expressed (show up) when there are 2 copies of a recessive gene present at the same time...if a dominant gene was there, then that would be expressed, hiding the recessive gene!

About characteristic

This diagram will explain to you about which one is the dominant and recessive.


In Gregor Mendel and his pea plant the P1 stands for parent generation, F1 stands for first filial (daughter) generation, and F2 stands for second filial (daughter) generation. So, for the Parent Cross, or the very first cross-breed Mendel did, which was between tall and short plants, the offspring were all Tt, each having a dominant allele and a recessive. Each F1 plant produces two types of gametes those with allele for tallness and shortness. So basically F1 inhearited P1's tallness, which is dominant, so all F1 plants are tall.

Konsep Pewarisan Berdasarkan Eksperimen Mendel

Konsep Pewarisan Berdasarkan Eksperimen Mendel

Semua organisma boleh menurunkan ciri-cirinya kepada zuriat masing-masing.

Pemindahan ciri-ciri induk kepada anaknya dari satu generasi ke satu generasi yang lain dikenali sebagai pewarisan (inheritance).

Kajian tentang pewarisan secara saintifik dipanggil sebagai genetik (genetic).

Sesuatu ciri (characteristics) adalah sifat tersendiri bagi sesuatu organisma. Ciri ini boleh diwarisi dari satu generasi ke satu generasi berikutnya. Contohnya, ketinggian dan warna kulit.

Sesuatu trait pula adalah varian bagi ciri tertentu.

Setiap ciri yang diwarisi terdiri daripada trait tertentu. Misalnya, ketinggian adalah suatu ciri manakala tinggi atau rendah adalah traitnya.

Jadual di bawah menunjukkan ciri dan trait pokok kacang pea yang dikaji olehGregor Mendel.


Ciri dan trait pokok kacang pea.

Beliau adalah ahli sains yang pertama menyarankan teori bahawa ciri yang terdapat pada induk (parent) diturunkan kepada zuriat melalui gamet jantan dan gamet betina (male and female gametes).

Ini menunjukkan bahawa nukleus (nucleus) gamet jantan dan gamet betina membawa maklumat genetik pewarisan ciri tersebut.

Faktor pewarisan (hereditary factor) yang menentukan sesuatu ciri (characteristics) itu dipanggil gen (gene).





Contoh di atas adalah apa yang dinamakan sebagai Punnet Square ini adalah untuk membantu pelajar yang keliru untuk membahagikan anak yang akan di hasilkan. Pada bahagian atas (RR) adalah dari tumbuhan A manakala (WW) dari tumbuhan B. Kemudian pecahkan kepada bentuk genotype atau gamete. Lepas itu buat cross polination walllaaa... terhasil la tumbuhan generasi pertama atau first filial generation (F1).

Primary Growth in Plant

This is the part which i already said in the class about 3 part of growth cell division, cell elongation and cell differentiation. Above is for shoots while below is for the roots.

Fruits

This is what is going to happen when it become a fruits.

Development of Pollen and Embryo sac

Flower

The flower is concerned with the function of sexual reproduction in angiosperms.

Development of Male Gametophyte or Pollen

Stamen forms the male reproductive unit and is made up of anther and the filament. The anther is generally bi lobed with chamber containing several pollen grains.

Pollen grains are produced in the anther. A very young anther comprises a mass of undifferentiated thin - walled cells bounded by an epidermis. The anther becomes four lobed as the anther grows, and the lobes are joined by a sterile tissue called connective. Each lobe contains an elongated chamber which is termed as microsporangium or pollen sac.

(A) T.S. of young anther showing four microsporangia and a vascular stand; 

(B) Detailed structure of one microsporangium; 

(C) T.S. of mature anther

A cross section of a young anther reveals that the microsporangium is surrounded on the outside by a single layered epidermis. Beneath the epidermis is a layer of cells called the endothecium. The sporangial wall encloses a mass of cells characterised by their large size abundant cytoplasm and prominent nuclei. These cells are called sporogenous cells. These sporogenous cells undergo a few mitotic divisions to increase their number and form the microspore mother cells or microsporocytes. The microsporocytes are diploid and usually closely packed. Each diploid microsporocyte undergoes meiosis, giving rise to a group of four haploid cells, known as microspores or pollen grains. Such a group of 4 spores is known as a tetrad.

(A-C), Pollen grains or microspores.

(A-B), Entire pollen grains; 

C, Pollen grain in section 

At maturity, the partition wall between two adjacent microsporangia breaks down and the cavities of the two microsporangia merge into one pollen sac. When the pollen grains are fully formed, the ripe anther splits to liberate the pollen.

Structure of Pollen Grains

Each pollen grain is a haploid, unicellular mass of protoplast with a single nucleus. It is surrounded by a thick wall differentiated into two layers; the outer thick exine and the inner thin intine. The pollen grains reveal a wide range of microsculpturing of the exine under a scanning microscope. The exine is made up of a complex substance called sporopollenin. The pollen grains are the best preserved structures because the sporopollenin is one of the most resistant biological materials known. The size, form and ornamentation of the exine is quite characteristic of the plant and is of great taxonomic value.

The branch of science that deals with the study of the characteristics of the pollen grains is called palynology.

At one or more places, the exine is very thin or absent. These spots are called germ pores and are the points from which the pollen tubes emerge during pollen germination.

Development of Female Gametophyte

Pistil is the female reproductive unit, and is made up of a basal swollen part called the ovary, a stalk called the style and a terminal receptive disc called the stigma. The ovule begins its development as a minute protuberance on the placenta and forms a thick mass of cells termed as the nucellus. As the growth and development of the ovule proceeds, the nucellus is elevated on a short stalk like structure called the funiculus. The point at which the body of the ovule is attached to the funiculus is called as hilum. As the ovule develops further, one or two protective layers called the integuments grow from the base of the nucellus known as chalaza and surround the nucellus except for a narrow opening, the micropyle. This opening serves as a passage for the entry of a pollen tube into the ovule.

Development of embryo sac and female gamete (in an anatropous ovule)

A hypodermal cell of the nucellus enlarges and becomes differentiated into a megaspore mother cell or megasporocyte. This diploid megaspore mother cell increases in size and undergoes meiosis to form a linear tetrad of 4 haploid megaspores, 3 of which degenerate and the 4th becomes the functional megaspore.

Female Gametophyte

The nucleus of the megaspore undergoes three successive mitotic divisions forming eight nuclei. The megaspore enlarges into an oval shaped structure called the embryo sac. The eight nuclei of the embryo sac arrange themselves in 3 groups.

(A I), (A - D), stages in the development of an ovule and functional megaspore;

E I, stages in the development of an embryo sac from the functional megaspore

Three nuclei are towards the micropylar end, and form the egg apparatus with a central egg cell surrounded by 2 synergids. Three nuclei are at the chalazal end and form the antipodals. One nucleus from each pole moves to the central position of the embryo sac and are called as polar nuclei. These nuclei may fuse together and form a diploid secondary nucleus. A fully developed embryo sac with the nucellus, integuments and funiculus, together forms the structure called the mature ovule.

Longitudinal Section of a Mature Ovule (anatropous type)

This is the picture where the fertilization takes place.

Both of this picture show what is the meaning or what you have to know in double fertilization.

Flowering Vascular Plants that Produce Seeds: Angiosperms

I. Background

A. Flowers

1. Flowers are the organ of sexual reproduction

            a. Seeds within a fruit

            b. Require double fertilization

2. Angiosperms have vessel elements in their xylem and sieve-tube members in their phloem

B. Classes of flowering plants

1. Monocots

            a. Typically herbaceous

            b. Examples: Lilies, grasses, corn, palms, and orchids

            c. Parallel leaf venation

            d. Floral parts in 3s or multiples of 3

            e. Fibrous root system

            f. Scattered vascular bundles

            g. One cotyledon in the seed

            h. Persistent endosperm in the mature seed

2. Dicots

            a. May be herbaceous or woody

            b. Examples: Roses, oak trees, potatoes, and daisies

            c. Netted leaf venation

            d. Floral parts in multiples of 4 or 5

            e. Taproot system

            f. Vascular bundles arranged in a ring

            g. Two cotyledons in the seed

            h. Endosperm absent in the mature seed

           

II. Morphology of Flowering Plants

1. Flowers consist of sepals, petals, stamens, and carpels arranged in whorls around a stalk (peduncle)

            a. Sepals and petals are sterile modified leaves

            b. Stamens and carpels are fertile modified leaves

            c. A flower with all four parts is complete

            d. A flower missing any of the four parts is incomplete

            e. A flower with both stamens and carpels is perfect

            f. A flower having stamens or carpels (but not both) is imperfect

2. Sepals

            a. Sepals function in protecting the other flower parts while it is a bud

            b. The sepals are collectively referred to as the calyx

3. Petals

            a. The petals are typically brightly colored to attract animal pollinators

            b. The petals are collectively referred to as the corolla

4. Stamen

            a. The stamens each have a filament (stalk) and an anther

            b. In the anther microspores develop into pollen grains

                        i. Each pollen grain has two sperm        

5. Carpel

            a. Female reproductive organs located in the center of the flower

            b. Produce ovules which ultimately may develop into seeds

            c. The female portion of the flower is sometimes referred to as a pistil

                        i. The pistil may consist of a single carpel (simple pistil) or several fused carpels (compound pistil)

            d. Each pistil has a stigma at the distal end

                        i. Pollen grain lands

            e. Each pistil has a style (neck) and an ovary

                        i. Contains the ovule

            f. Each ovule contains a female gametophyte that produces one egg and two polar nuclei

            g. After fertilization the ovule develops into a seed

III. Life Cycle of Flowering Plants

A. Double fertilization

1. Flowering plants have a dominant sporophyte generation

            a. Gametophyte is reduced to just a few cells

2. Ovules contain a megasporocyte

            a. Megasporocyte undergoes meiosis to produce 4 haploid megaspores

3. Only one megaspore develops

            a. Megaspore divides mitotically

            b. Develops into the female gametophyte (embryo sac)

4. The female gametophyte consists of only 8 cells

            a. Three of these cells participate in fertilization

                        i. One egg

                        ii. Two haploid polar bodies

5. The anther contains microsporocytes

            a. Microsporocytes undergo meiosis to produce 4 haploid microspores

6. Each of the microspores develops into the male gametophyte

            a. The male gametophyte is the pollen grain

7. Pollination involves transfer of the pollen grain to the style of the flower

            a. Pollen grain lands on a style of the same species

            b. Pollen grain grows a pollen tube down through the stigma to the ovaries

8. Both sperm enter the embryo sac

9. Fertilization involves both sperm

            a. Referred to as double fertilization

                        i. One sperm fertilizes the egg resulting in a diploid zygote

                        ii. The other sperm fuses with the two haploid polar nuclei forming a triploid cell

10. The triploid cell develops into endosperm

            a. Endosperm is a nutritive material for the embryo

            b. Endosperm is persistent in monocot seeds

            c. Endosperm is absorbed into the cotyledons in dicot seeds

IV. Development of Seeds and Fruit

A. Seeds and fruits develop after fertilization

1. Fruits protect the seeds from desiccation

2. Fruits may aid in dispersal of seeds by wind, water, or animals