Types of inheritance
Three different types of traits can be observed in the phenotype. the qualitative, the quantitative and the threshold traits.A qualitative trait It is the type of inheritance in which a single dominant gene influences a complete trait. Presence of two such dominant genes does not alter the phenotype. A qualitative trait are the classical Mendelian traits of kinds such as form (e.g., round or wrinkle seeds of pea); structure (e.g., horned or hornless condition in cattles); pigments (e.g., black or white coat of guinea pigs); and antigens and antibodies (e.g., blood group types of man) and so on.
A quantitative trait shows continued variation. It is a type of inheritance controlled by one or more genes in which the dominant alleles have cumulative effect with each dominant allele expressing a part or unit of the trait, the full trait being shown only when all the dominant alleles are present. A quantitative trait are economically important measurable phenotypic traits of degree such as height, weight, skin pigmentation, susceptibility to pathological diseases or intelligence in man; amount of flowers, fruits, seeds, milk, meat or egg produced by plants or animals, etc. The quantitative traits are also called metric traits.
Definitions :
Allele: One of two or more forms a gene may take.Dominant: An allele whose expression overpowers the effect of a second form of the same gene.
Recessive: An allele whose effects are concealed in offspring by the dominant allele in the pair.
Gamete: A reproductive cell.
Heterozygous: A condition in which two alleles for a given gene are different from each other.
Homozygous: A condition in which two alleles for a given gene are the same.
Mendelian laws
Mendel's approach was to transfer pollen (which contains male sex cells) from the stamen (the male reproductive organ) of one pea plant to the pistil (female reproductive organ) of a second pea plant. As a simple example of this kind of experiment, suppose that one takes pollen from a pea plant with red flowers and uses it to fertilize a pea plant with white flowers. What Mendel wanted to know is what color the flowers would be in the offspring of these two plants. In a second series of experiments, Mendel studied the changes that occurred in the second generation. That is, suppose two offspring of the red/white mating ("cross") are themselves mated. What color will the flowers be in this second generation of plants? As a result of these experiments, Mendel was able to state three generalizations about the way characteristics are transmitted from one generation to the next in pea plants.The first step that takes place in reproduction is for the sex cells in plants to divide into two halves, called gametes. The next step is for the gametes from the male plant to combine with the gametes of the female plant to produce a fertilized egg. That fertilized egg is called a zygote. A zygote contains genetic information from both parents.For example, a zygote might contain one allele for white flowers and one allele for red flowers. The plant that develops from that zygote would said to be heterozygous for that trait since its gene for flower color has two different alleles. If the zygote contains a gene with two identical alleles, it is said to be homozygous.
Monohybrid ( single trait)
law of segregation.
the principle stating that during the production of gametes the two copies of eachhereditary factor segregate so that offspring acquire one factor from each parent.
law of dominance.
the principle stating that one factor in a pair of traits dominates the other ininheritance unless both factors in the pair are recessive.
Dihybrid ( two traits)
law of independent assortment.
the principle stating that the laws of chance govern which particularcharacteristics of the parental pairs will occur in each individual offspring.
Mendel's law of segregation describes what happens to the alleles that make up a gene during formation of gametes. For example, suppose that a pea plant contains a gene for flower color in which both alleles code for red. One way to represent that condition is to write RR, which indicates that both alleles (R and R) code for the color red. Another gene might have a different combination of alleles, as in Rr. In this case, the symbol R stands for red color and the r for "not red" or, in this case, white. Mendel's law of segregation says that the alleles that make up a gene separate from each other, or segregate, during the formation of gametes. That fact can be represented by simple equations, such as:
RR → R + R or Rr → R + r
Mendel's second law is called the law of independent assortment. That law refers to the fact that any plant contains many different kinds of genes. One gene determines flower color, a second gene determines length of stem, a third gene determines shape of pea pods, and so on. Mendel discovered that the way in which alleles from different genes separate and then recombine is unconnected to other genes. That is, suppose that a plant contains genes for color (RR) and for shape of pod (TT). Then Mendel's second law says that the two genes will segregate independently, as:
RR → R + R and TT → T + T
Mendel's third law deals with the matter of dominance. Suppose that a gene contains an allele for red color (R) and an allele for white color (r). What will be the color of the flowers produced on this plant? Mendel's answer was that in every pair of alleles, one is more likely to be expressed than the other. In other words, one allele is dominant and the other allele is recessive. In the example of an Rr gene, the flowers produced will be red because the allele R is dominant over the allele r.
Cytogenetics
Cytogenetics is a branch of genetics that is concerned with the study of the structure and function of the cell, especially the chromosomes.[1] It includes routine analysis of G-banded chromosomes. Giemsa banding is a technique used in cytogenetics to produce a visible karyotype by staining condensed chromosomes. It is useful for identifying genetic diseases through the photographic representation of the entire chromosome complement.
Banding type
Staining method
C-banding
constitutive heterochromatin
G-banding
Giemsa stain
Q-banding
quinacrine
R-banding
reverse Giemsa staining
T-banding
telomeric
Types of techniques
Karyotypes analysis.Florescent insitu hybridization.