If the number of dominant genes are more, then the degree of expression of the character will be more. This kind of inheritance is termed as Quatitative or Polygenic Inheritance.
Defined as two or more different pairs of genes which are non- allelic, having cumulative effect and are responsible for quatitative characters.
The effects of each contributing gene are cumulative or additive. Each contributing genes produces an equal effect. There is no dominance involved. The polygenes have pleiotropic effects, that is, one gene may modify or suppress more than one phenotypic trait. The environmental conditions have considerable effect on the phenotypic expression of polygenes.
The presence of melanin pigment in the skin of the man determines the skin color. The more pigment the darker is the skin color. The amount of the melanin developing individual is determined by two pair of genes. These genes are present at two different loci and each dominant gene is responsible for the synthesis of fixed amount of melanin.
In Man, tallness is recessive to shortness. Individual having a genotype of more dominant genes will have shortnesss as their phenotype. The heights of an adult generalyy range from 140 cm. to 203 cm.
Can be defined as a set of three, four or more alleles or allelomorphic genes, which have arisen as a result of mutation of a normal gene and which occupy the same locus in the homologous chromosomes.
Multiple alleles occupy the same locus within the homologous chromosomes. Since in a diploid cell only two chromosomes of each type are present, only two genes of the multiple series are found in a cell and also in an individual. Crossing over does not occur in multiple alleles. Multiple alleles control the same character. One is dominant and the rest are recessive or sometimes they are co-dominant.
Skin Color in Rabbit Eye Color in Drosophila ABO blood groups in Human
Why do blood from an individual cannot always be safely mixed with that of another? This is due to the fact that blood proteins of one individual differ from those of others.
The antigens or agglutinogens and antibodies or agglutinins. The Antigens A and B are located in the RBC and the Antibodies a and b in the Plasma. The antigens are found to be mucopolysaccharides. The clumping reaction occurs between antigen and antibodies and is also termed as Agglutination.
1. Chromosomal Sex Determining Systems2. Genic Balance System3. Hormonal System4. Environmental Sex Determination
It was given by E.B. Wilson and Steven (1902) The Chromosomal theory of inheritance states that genes are located on chromosomes, which serve as the vehicles of gene segregation in meiosis. In majority of animals a pair of sex chromosomes are found. These are represented by X and Y.
XX-XY Type or Lygaeus Type Female homogametic and Male heterogametic Female heterogametic and Male homogametic XX-XO Type or Protenor Type
By: Wilson and Stevens The cell of the male and the females have the same number of chromosomes but the females have two X chromosomes and the males have one X and the small Y chromosomes.
Female Homogametic Female Heterogametic and Male and Male Heterogametic Heterogametic Females are In fowls, other birds homogametic sex with and some fishes, some XX chromosomes. They amphibians, moths and produce eggs of same butterflies, the female type, carrying on ly X sex is heterogametic chromosomes. On the with X and Y other hand, males are chromosomes. XX-XY heterogametic with XY system, are labelled as chromosomes. Z and W.
In Squash Bug it was observed that males have 21 chromosomes, but females have 22 chromosomes. Thus all the eggs of the same type carrying X chromosomes while the Sperms of Two types; 50% with 11 chromosomes and 50% with 10 chromosomes. Fertilization of an egg by the sperm carrying 11chromosomes result in female, while fertilized egg by sperm with ten chromosomes produces male.
According to this theory, every individual whether male or female, possesses in its genotype genes for both male and female characteristics. Depending upon the number of genes of a particular sex the organism’s sex will be determined. If there is excess of female determining genes, female develops; if the male determining genes are in excess, a male is formed.
In higher vertebrates, the embryo develops some characters of the opposite sex together with the characters of its own sex chromosome. This is due to the hormones secreted by the reproductive organs of the animal. A large number of cases are known where sex is modified due to hormones secreted from sex organs. Some examples are: Sex reversal Freemartin
This case of sex reversal was explained by assuming that when the ovary is destriyed on removal, the ovarian hormones were stopped. In the place of ovary, the rudimentary gonad (this is a non- functional gonad in birds) started functioning as testis. The male hormones were produced and resulted in the appearance of the male secondary sexual characters and the formation of sperm.
Freemartin is the effect of the hormones of the male sex on the female. Studies show: F.R. Lilies found that in cattles, where twin calves of opposites sex (one male and the other female) are born, the male is normal but the female is sterile with many male characteristics. Such sterile females are known as Freemartin.
In number of organisms, sex is determined fully or partly by environmental factors. Environmental factors are also important in determining sex in many reptiles. Although most snakes and lizards have sex chromosomes, in many turtles, crocodiles and alligators, during embryonic development determines sex. Example: In turtles, warm temperature produces females during certain times of the year, whereas cool temperature produces males.
Prepared To: Miss Sheryllene J. Lat Bsed III-B Day