B Sc III Sex Determination Concept 

Development of sexual characteristics

A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Two sexes are present in most sexual organisms. Because of the sexual chromosome differences the sex determination.

SEX DETERMINATION AND SEX CHROMOSOMES

I. The Chromosome Theory of Inheritance and Sex Linkage

A. Sutton and Boveri’s chromosome theory of inheritance proposed in 1902—Genes are located on chromosomes
B. Just previous to this (end of 19th century) biologists had discovered that half of all sperm cells carry a structure called an X body.
C. In 1905 the X bodies were determined to be chromosomes—X chromosomes.
D. Then the Y chromosome was also discovered in 1905.
E. Together the X and Y chromosomes are known as the sex chromosomes.
F. All other chromosomes are called autosomes.

G. Systems of sex chromosomes

1) XX-XO system (S H 5)
(a) Female—XX
(b) Male—X
(c) Occurs in some insects like grasshoppers

2) XX-XY System (S H 5)
(a) Female—XX (homogametic sex)
(b) Male XY (heterogametic sex)
(c) Occurs in Drosophila, mammals and some plants

3) ZZ-ZW System(S H 5)
(a) Female—XY (heterogametic sex)
(b) Male—XX (homogametic sex)
(c) Occurs in birds, butterflies and some fishes

4) X-Y-XY System(S H 5)
(a) Occurs in organisms with alteration of generations (e.g., liverworts and vascular plants)
(b) Male gametophytes—Y
(c) Female gametophytes—X
(d) Sporophytes—XY

H. Morphology and pairing of X and Y





1) Each type of sex chromosome has two regions
(a) Pairing region
(i) During synapsis of meiotic prophase I, the pairing regions combine
(ii) Some genes occur in these pairing regions
(iii) These genes exhibit X-and-Y linkage

(b) Differential region
(i) Differential regions do not pair during synapsis
(ii) Differential region genes are either
1. X-linked
2. Y-linked

(iii) Any gene X or Y chromosome is said to be sex linked.

I. Sex determination in Drosophila

1) Sex is determined by the ratio of the number of X chromosomes to number of autosomal sets

2) Scheme
(a) X/A = 1.0—female
(b) X/A = 0.5—male
(c) X/A > 1.0—metafemale
(d) X/A < 0.5—metamale

J. Sex determination in humans

• The presence or absence of the Y chromosome determines sex
• XX determines the female and XY determines the male.

Source: http://english.eagetutor.com/home/sex-determination-system-sp-193278729

Sex determination in humans:

In human beings, sex is determined by genetic inheritance. Genes inherited from the parents determine whether an offspring will be a boy or a girl.

Genes for all the characters are linearly arranged on chromosomes. These include the genes for sexual characters.

Generally, characters related to the reproductive system are called sexual characters and those that are not are called vegetative characters. The chromosomes that carry genes for sexual characters are called sex chromosomes, while those that carry genes for the vegetative characters are called autonomies.

A sex chromosome that carries the genes for male characters is called Y chromosome and one which carries the genes for female characters is called X Chromosome.

We have a total of 46 chromosomes. Half of them come from the mother and the rest, from the father. Out of these 46 chromosomes, 44 are autonomies and 2 are sex chromosomes. The sex chromosomes are not always a perfect pair.

In females there are 44 autonomies and two X chromosomes, in males there are 44 autonomies, one X chromosome and one Y chromosome. So the chromosomes

In woman are 44 + XX, while the chromosomes in man are 44 + XY. Let us see the inheritance pattern of X and Y chromosomes.

During gamete formation, the normal diploid chromosome number is halved. This is called the haploid condition. All the eggs of a female have 22 + X chromosomes. A male produces two types of sperms—one type bears the 22 + X composition and the other, 22 + Y. Therefore, in every 100 sperms, 50 have Y chromosomes and 50 have X chromosomes.

Any one of the two types of sperms can fertilize the egg. If a Y-bearing sperm fertilizes the egg, the zygote has the 44 + XY composition, and the resulting embryo grows to be a boy. When an X-bearing sperm fertilizes the egg, the resulting zygote has the 44 + XX composition. This embryo develops into a girl. All the children inherit one X chromosome from the mother.

Therefore, sex is always determined by the other sex chromosome that they inherit from the father. One who inherits the X chromosome of the father is a girl, while one who inherits the Y chromosome of the father is a boy.

Role of environment in sex determination:

Environmental conditions such as temperature around the developing embryo may determine sex in some animals. Such conditions may override the genetic basis. Some animals such as snails can even change their sex, showing that their sex is not genetically determined.

Incubation of the eggs of the turtle Chrysema picta at a high temperature produces females. But the incubation of the eggs of the lizard Agama at a high temperature produces males.

http://www.biologydiscussion.com/essay/determination-of-sex-in-human-beings/1643

B Sc II

Development of Male gametophyte (Before pollination):

Microspore or pollen grain is the first cell of the gametophyte. The microspore germinates in situ i.e. while within the microsporangium. Each microspore divides asymmetrically into a 2-cells: a smaller prothallial cell and a larger antheridialcell. The prothallial cell does not divide further while the antheridial cell divides into a smaller generative cell near the prothallial cell and a larger tube cell. Finally pollination takes place at 3-celled stage (a prothallial cell, a generative cell and a tube nucleus) (Fig. 9.12).

Pollination:

In Cycas pollination is anemophilous (by wind). The 3-celled microspores liberate from mega-sporangia are blown away by wind. Finally microspores reach on ovules and get enlarged in the pollination drop (ooze) of micropyle. As the ooze dries up, the microspores are drawn into the pollen chamber.

(d) Development male Gametophyte (After polli­nation):

After a gap of about 4 months, post-pollination development of male gam­etophyte occurs. The exine ruptures and the intine grows out in form of apollen tube. The pollen tube acts as a haustorium, i.e. absorb food while penetrating through the nucellus and hang in the archegonial chamber. In the pollen tube, generative cell divides into a stalk cell and a body cell. Finally, the body cell divides into two male gametes or antherozoids. Thus, a fully developed male- gametophyte consists of a disorganized prothallial cell, stalk cell, tube nucleus and 2 male gametes (Fig 9.13 )

Each male gamete appears top-shaped with 5-6 spiral bands of cilia. The size of male gamete in Cycas varies from 180-210µm (largest, 400«m reported from Chigua, a cycad).

Source : http://www.biologydiscussion.com/life-cycle/life-cycle-of-cycas-vegetative-and-sexual-life-cycle/5766

Development of female gametophyte (Endosperm):

Inside the nucellus, one cell differentiated into megaspore mother cell. It undergoes reduction division (meiosis) to form a linear tetrad of four haploid megaspores. Usually, the upper 3 megaspores towards micropyle degenerate while the lower most functional megaspore (embryo sac cell) undergoes free nuclear division followed by wall formation to form a cellular female gametophyte or endosperm.

Hence, the formation of female gametophyte is monosporic, i.e develops from a single megaspore. During formation of endosperm nucellus is utilized. It should be noted that in gymnosperms the endosperm develop before fertilization and is haploid (n) while in angiosperms it is triploid (3n) and formed after fertilization (Fig. 9.10).

At the micropylar end of female gametophyte 2-8 archegonia develop. All the necks of archegonia open into an archegonial chamber formed by a depression in female gametophyte (Fig. 9.11). Each archegonium develops from single superficial cell called archegonial initial.

It gets enlarged and divides transversally into outer primary neck cell and inner central cell. The primary neck cell divides anticlinally to form two neck cells. The inner central cell enlarges and its nucleus divides into venter canal nucleus and egg nucleus. Soon the venter canal nucleus disorganizes. Thus, a mature archegonium has two neck cells and an egg. Neck canal cells are not formed. The egg cell in Cycas is largest in the plant kingdom (Fig. 9.11).

Source : http://www.biologydiscussion.com/life-cycle/life-cycle-of-cycas-vegetative-and-sexual-life-cycle/5766

B Sc III Students - Comb shapes in fowl

B Sc II students 
Cycas Female reproductive structure

B Sc II students 
These are some of the images from various sources of internet. Folllow male reproductive structures in Cycas carefully

B Sc II students:

Read this article to learn about the Cycas: Occurrence, Morphological Features and Economic Importance !

Division : Cycadophyta

Order : Cycadales

Genus : Cycas

Class : Cycadopsida

Family : Cycadaceae

Occurrence:

Sixteen species of Cycas are found in the tropical and subtropical parts of the earth. The name Cycas has been derived from a Greek word Kykas = Cocopalm. It is an evergreen plant. In India Cycas is represented by six species— C. revoluta, C. pectinata, C. siamensis, C. beddomei, C. rumphiand C. circinalis.

Cycas revoluta is the most commonly cultivated species of Indian gardens. It also occurs in China, Africa, Nepal, Sri Lanka and Japan. Its natural habitat is in open, sunny, well-drained situations. It is a palm-like tree with size varies from 1-2 meters. As it yields ‘sago’, from the mucilage in its main trunk. It is popularly called ‘sago palm’. Leaflets are 6-8 mm broad with revolute margins.

Morphology:

Cycas sporophyte is an evergreen palm-like tree that attains a height of 0.4 to 2.0 mt. Sporophyte is dioecious i.e. male and female plants are separate. Both the plant body is differentiated into roots, stem and leaves (Fig. 9.2).

(i) Roots:

Roots are of two types, i.e., normal tap root and coralloid root. Tap root is positively geotropism, non-green, without root hair. It helps in anchorage and absorbs water and minerals. Coralloid roots are lateral branches of tap root. It grows first horizontally (diageotropic) then, repeatedly divide dichotomously, come out of the soil surface (apogeotropic) in the form of coral like mass.

These are devoid of root hair and root cap. Its anatomy is similar to normal tap root, but its cortex is differentiated into three zones i.e., Upper cortex, Middle cortex and Lower cortex Middle cortex carries BGA (Anabena cycadcearum, Nostoc) symbiotically. Coralloid root helps in nitrogen fixation as well as respiration (due to lenticels).

(ii) Stem:

The stem is erect, columnar, woody and un-branched. It is covered with alternate whorls of leaf bases of foliage leaves and scale leaves. The stem apex bears a crown of leaves and sex organs at maturity.

(iii) Leaves:

Leaves are dimorphic i.e. 2 types, assimilatory or foliage leaves and scaly leaves or cataphylls. Both the leaf types are born at the stem apex in alternate spirals. Foliage leaves are large (1 -3 m), green, pinnately compound. Petiole has 2 rows of spines. Rachis bears 80-100 pairs of pinnae or leaflets.

Leaflets are sessile, elongated, tough, and lanceolate in shape with a spiny apex, revolute or smooth margins. Young leaves have circinate vernation like those of ferns and always covered with brown hairs called ramenta (Fig. 9.3). Scale leaves are small, dry, brown, triangular leaves covered with ramenta. They are non- photosynthetic and protect the stem apex (Fig. 9.3).

Economic Importance of Cycas:

1. Cycas plants are highly valued for their ornamental looks, and hence grown in homes and garden as an ornamental plant.

2. A starch called ‘sago’ is obtained from the stem pith of Cycas revoluta. That is why this species of Cycas is also known as “Sago Palm”. Sago starch is used in the preparation of “Sabbodana”.

3. In Sri Lanka, the starch obtained from the seeds is used in cakes.

4. Soft young leaves of Cycas are used as vegetable also in some parts of the world.

5. Seed of some species of Cycas are used as fodder for animals.

6. Boiled seeds of Cycas rumphi are eaten by inhabitants of Andaman.

7. Its leaves are used for making mats, hats, L rooms and baskets.

8. Extract of young Cycas leaves is used in the treatment of many skin diseases, blood vomiting and stomach disorders.

9. The decoction of seeds is used as purgative.

10. Tincture prepared from its seeds is used in some areas in headache, nausea, bad throat etc.

11. The terminal buds of Cycas circinalis are used to treat ulcerated wounds and swollen glands.

Source: http://www.biologydiscussion.com/articles/cycas-occurrence-morphological-features-and-economic-importance/5720

B Sc II students: additional reading material follow this link for gymnosperms 

http://davesgarden.com/guides/articles/view/3608#b