link of basic terminology(part1)
Chromosomal theory of Inheritance
According to this theory,inheritance of characters from one generation to another occurs due to chromosomes.
**Chromosomal theory and mendelian factors show parallelism with each other.
1) Both chromosome as well as mendelian
factors retain their number,structure and individuality during the process of transmission.
2)Both are neither get lost,nor mixed up.
3)Both occur in pair in diploid organisms and behave as a unit.
4)Both segregate from their respective paired unit at the time of gamete formation , hence gamete receive or contain only one allele af a gene pair and only one pair of homologous chromosome.
5)Paired condition of both chromosome as well as genes is restored after fertilisation.
6)Chromosome and mendelian factor show parrallism because of the fact that genes are present on the specific site of the chromosome.
6) The genes which are present at different loci on the same chromosome are called dependent genes /linked genes represent a linkage group.
6)There is a difference im the segregation of the chromosome and mendelian factors.
All the non- homologous chromosome segregate independently of each other but only independent genes(which are peesent on different chromosome)segregate independently .
T.H. Morgan
Father of experimentap genetics
***He provide experimental proof of chromosomal theory of inheritance.
**He experimented on Drosohila melanogoster(fruit fly)
**Fruit fly
2n= 8 chromosome
. =4 pairs
=3 pairs of autosomes +one pair is sex chromosome either XX or Xy
Sutiability of Drosophila as a experimentap tool
**Easy to culture and breed under lab condition on ripened fruits.
**Short life span of 2 weeks hence many generations can be studied within a single year.
** Single mating produce large number of offsprings.
**Many distinct external features.
*Sexual dimorphism i.e.Female is larger tha male and possess an ovipositor(egg laying structure).
**Smaller number of chromosomes;2n=4pair
**Heteromorphic sex chromosome in male is Xy are easy to understand transmission.
**Presence of polytene chromosome in the salivary gland cells of Cheronymous larva help in study of abnormal characters.
Linkage
Physical association of genes present on different loci of the same chromosome is called linkage.
**Exception of independent assortment.
**There are two types of arrangement of genes on chromosomes:
1)Cis arrangement: When dominant allele of different genes present together. or
when recessive alleles of different gene present together..Example: AABB, aabb
2)Trans arrangement: When dominant allele of one gene pair and recessive allele of another gene pair present together.Example: AAbb, aaBB
Bateson,punette 1st one who report linkage as coupling and repulsion..They experiment on Lathyrus odoratus .
Coupling: When dominant traits of different characters comes from the same parent i.e cis condition ; then they show a tendency to remain together called coupling.
Repulsion: When dominant traits og two different trait comes from the different parents i.e. trans condition; then they show tendency to segregate called repulsion.
Later,T.H. Morgan explain the coupling and repulsion as the two aspects of the smae phenomenon called linkage .
Linkage theory
All the genes preswnt on the same chromosome all together constitute a linkage group.
1) Number of linkage group = Haploid number of chromosomes.
2) Strength of linkage is inverselt proportional to the distancw between the genes.
3) Linked genes may segregate if crossing over takes place between them.Crossing over favours independent assortment but degenerate linkage .
4)Recombination frequency is directly proportional to distance between the genes and inversely proportional to the strength of linkage.
Alfred sturtvent
**Morgan student.
**prepared 1st chromosomal map for Drosophila by using recombination frequency.
** 1% recombinatiom frequency/cross over value(COV) = 1 unit distance between genes.
1 unit is map unit for prokaryotes.
Centimorgan (cm) map unit for eukatyotes.
**Alfred sturtvents were not accurate since recombination frequency is always less than 50%.
**This concept is applicable and valid to find out linear arrangement of genes in a chromosome.
Sex Determination in Organisms
It occurs at early stages of life through various adaptive strategies.
1)Fertility factor in Bacteria: on basis of F-plasmid contain F-factorm
If F-factor present -F+/donor/male
If F-factor absent - F-/recipient/female
2)Environmental mechanism: temperature dependent ,in reptiles/turtle .
In, Bonnelia virdis (sea worm)
***If larva develop into adult,being attached with the proboscis of male parent then individual is male.Since it receive Bonneline's hormone from male parenr.This hormone help in differentiation of male reproductive organ.
***If larva develop into adults freely on sea floor or mud then individual female.
3)Genic Balance Theory:Given by C.V. Bridges for Drosophila.
sex chromosome/autosome =X/A
This ratio determines the sex of an individual. Y has no role in sex determination but has a role im fertility.
If X/A =1 (normal female)
If X/A >1 (super female)
If X/A <1 to >0.5(intersex)
If X/A =0.5 (normal male)
If X/A <0.5 (super male)
3)Chromosomal theory of Sex inheritance:
***Henking discovered the X chromosome as 'X bodies'during spermatogenesis.In firefly,he observed that X bodies are transmitted to only 50%of the sperm.
***Later on Stevan discovered the Y chromosome .
wilson and Stevan proposed concept of sex chromosome.
two type of chromosomes:
(a) Autosomes/Homochromosomes:Each pair is identical in structure.No role in sex determination.
(b) Allosomes/Heterosomes/Sex chromosomes:Different in structure either in male or female.Role in sex determination.
Male (Xy): dissimilar ,heterogametic (two different type of gamete).
Female (XX):similar ,homogametic,only one type of gamete.
***Male heterogamety:
sex is determined by male,male produced 2 different types of gametes.
XX-XO TYPE:
Male nad female have unequal nylber of chromosomes.
female(2n) -AA+XX ,single type of gametes
male(2n-1) -AA+XO ,two type of gametes ;(A+X)n and (A+O)n-1
found in : most of the insects ,grasshopper,cockroach,bugs etc.,in roundworms and Dioscorea.
XX-Xy type:
***both male and female equal number of chromosomes.
male(2n) - AA+XX ,single type of gametes
female(2n) -AA+Xy ,2 types of gametes
found in Drosophila, mammals(human), in plants like Coccinia,Melandrium.
***Y chromosomes possess TDF(testes determining factor)which stimulates the differentiation of testes in young embryo.
***Female Heterogamety:
Female determines the sex of offsprings. Female have two different type of gametes.
ZZ- ZO type:
male and female have unequal number of chromosomes.
male(2n) - AA +ZZ ,single type of gametes
Female (2n-1) - AA+ZO, two type of gametes
found in:Pigeon,Duck ,Butterflies,Moth.
ZZ-ZW type:
both male and female possess equal number of chromosmes.
male(2n) - AA+ZZ ,single type of gametes
female(2n) - AA +ZW ,two type of gametes
found in : most of the birds,fishes,amphibians,some reptiles,in plants like Fragria clator (strawberry)
***Arrehenotoky:
Haploid-diploid mechanism
found in bees and flies
female(2n=32) -two types; Queen and workers;queen is fertile and workers is sterile.
male(n=16) - Drone
Sex linked and sex influence inheritance
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