Cyptoplasmic inheritence, Sex determination-XXX

Cytoplasmic Inheritance:-

> DNA is also found in chloroplasts and mitochondria in the cytoplasm. If a genetic trait is controlled by a gene present on this DNA, it is called cytoplasmic heredity.

> Both cytoplasm and nucleus are inherited from the mother while only the nucleus is inherited from the father.

>Thus cytoplasm passes to offspring only from mother. Hence it is also called maternal inheritance.

> If such a genetic trait is found in the father, then it is not transmitted to the next generation.

> If such a genetic trait is found in the mother, then it is 100% transmitted to the next generation.

Characteristics:-

i. Reciprocal Differences:- Cytoplasmic inheritance governs the characters with notable distinctions in reciprocal crosses in F₁.

ii. Maternal Effects:- Distinct maternal effects are observed due to more contribution of cytoplasm to the zygote by a female parent than a male parent.

iii. Mappability:- Chloroplast genes and mitochondrial genes have been mapped, which were very difficult to map. Chloroplast genes in Chlamydomonas and maize and mitochondrial genes in humans and yeast are some of the examples.

iv. Non-Mendelian Segregation:- A typical segregation pattern not followed in cytoplasmic inheritance exhibits in the Mendelian inheritance.

v. Somatic Segregation:- The characters, which are controlled by cytoplasmic genes, display segregation in somatic tissues. For example, segregation in somatic tissues like leaf variegation.

vi. Governed by plasma genes:- Chloroplast or mitochondria DNA governs or manages Cytoplasmic inheritance.

Significance:-

> Cytoplasmic inheritance helps determine the significance of cytoplasmic organelles in the passing on of characteristics in several organisms.

> Cytoplasmic inheritance helped map the chloroplast genome and mitochondrial genome in many species, such as humans, yeasts, maize, etc.

> Cytoplasmic male sterility has developed in crops like maize, cotton, pearl millet, etc. It has promoted the production of hybrid seeds.

> The role of mitochondria is gaining importance in heterosis, as heterosis results in vigorous growth of plants, which ultimately provides a higher yield of crops.

> The mutation leads to the generation of new variants of characteristics. In cytoplasmic inheritance, these mutations are observed in chloroplast and mitochondrial DNA.

Example:-

Plastid inheritance:- 

i. Mirabilis jalapa:- Leaf colour - green, white and variegated

ii. Iojap in Maize - Soap phenotype. Leaf colour - Green, white, striped

Mitochondrial inheritance:-

i. CMS (Cytoplasimc Male Sterility):- Maize, Cotton, Sorghum, Pearl millet

ii. Pokiness in Neurospora:- Two strains - wild (normal growth) and poky (slow growth)

iii. Petite in yeast:- Two strains - wild (normal size) and petite (very small size)

Sex Determination:- It is a biological system that determines the development of sexual characteristics in organisms. On the basis of these sexual characteristics, males and females can be differentiated in organisms.

1. Chromosomes:- There are 2 types of chromosomes found in organisms –

i. Somatic chromosomes or Autosomes:- These determine the somatic characteristics.

ii. Sex chromosomes or Allosomes:- These determine the sexual characteristics.

2. Environmental Sex Determination:-

Ø  Crepidula is a marine mollusk. When alone, it develops into a female. While with the female, it develops into a male.

Ø  Bonellia is a sea worm. When alone, it develops into a female of 3 cm long. While with the female, it develops into a 0.3 cm long parasitic male. In this, the male enters the female's body and lives there as a parasite.

Ø  In crocodiles and some lizards sex is determined by temperature:-

i. Males are developed at temperatures below 28 °C.

ii. Between 28 - 33 °C temperature, both males and females are developed in equal numbers.

iii. Females are developed at temperatures above 33°C.

3. Chromosomal Sex Determination:-

a. Heterogametic Male

b. Heterogametic Female

a. Heterogametic Male:-

i. XX – XY type:- In this, XY pattern is represented by the male and XX by the female.

Examples:- Humans, Drosophila, Melandrium , Coccinia, C. elegance

ii. XX – X0 type:- In this, the X0 pattern is represented by the male and the XX pattern by the female.

Examples:- Insects, Round worm, Dioscorea

b. Heterogametic Female:-

i. ZW – ZZ type:- In this, the ZZ pattern is represented by the male and ZW by the female.

Example:- Birds

ii. Z0 – ZZ type:- In this, the ZZ pattern is represented by the male and Z0 by the female.

Example:- Fishes

Note:- The Y-chromosome is smaller in size than the X-chromosome. TDF (Testis Determining Factor) is found on Y - chromosome. TDF is the smallest gene containing only 14 base pairs.

4. Sex Index:- The ratio of number of X-chromosome and sets of autosomes is called sex index. Its value determines the development of male or female. Its value of 0.5 determines the development of the male while its value of 1 determines the development of the female.

5. Gynandromorphs:- In Drosophila, sometimes half of the body is male and half is female. This state is called Gynandromorph.

6. Sex determination in Fragaria and Thalictrum:- In these plants, the XX pattern is represented by the male and the XY pattern by the female.

7. Sex determination in Maize:- In maize, 2 gene pairs are mainly related to sex determination –

bsbs = barren stalk [The cob is not formed so that the plants are male]

tsts = tassed seed [The cob is formed so that the plants are female]

8. Sex determination in Papaya:-

Ø  In papaya, spinach, grapes and satavar, sex is determined by 1 gene.

Ø  Sex is determined by a gene M in papaya. Which has 3 alleles –

i. M₁:- Dominant

ii. M₂:- Dominant

iii. m:- Recessive

Note:- If dominant alleles are present at both the loci, they are lethal to the plant.