Heterozygous females also pass the allele to one-half of their daughters, who become carriers but do not develop the disorder. an
example of such an X-linked disorder is the Duchenne form of muscular dystrophy. The disease has an onset prior to age 6 and is
often lethal prior to age 20. It normally occurs only in males.
Linkage MaP
a linkage or genetic or chromosome map is a linear graphic representation of the sequence and relative distances of the various
genes present in a chromosome. It is constructed by making crosses and observing whether certain characteristics tend to be
inherited together.
The first chromosome maps were prepared by Sturtevant in 1911 for two chromosomes and in 1913 for all the four chromosomes
of Drosophila.
Construction of linkage map is based on the following facts:
(i) Genes present in a chromosome are arranged in a linear sequence.
(ii) The frequency of crossing over and hence recombination between two genes is directly proportional to the physical distance
between the two.
Map unit is a unit for measuring distance between genes (or other loci) on a chromosome according to the frequency of recombination
between them due to crossing over.
1% crossing over between two linked genes is known as 1 map unit or centiMorgan (cM). 100% crossing over is termed as
Morgan (M) and 10% crossing over as deciMorgan (dM; after T. H. Morgan who is considered to be the father of experimental
genetics).
quaLitative anD quantitative inheritance
- Qualitative inheritance (monogenic inheritance) : 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. - Quantitative inheritance (polygenic multi-factorial inheritance) : It is a type of inheritance controlled by one or
more genes in which the dominant alleles have a 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.
qualitative traits
a qualitative trait is expressed qualitatively, which means that the phenotype falls into different categories. These categories do not
necessarily have a certain order. The pattern of inheritance for a qualitative trait is typically monogenetic, which means that the
trait is only influenced by a single gene. Many such inherited traits exist, and they frequently have profound effects on plant
value and utilisation. examples are starchy versus sugary kernels (characteristic of field and sweet corn, respectively) and determinant
versus indeterminant habit of growth in green beans (determinant varieties are adapted to mechanical harvesting).
Such differences can be seen easily and evaluated quickly, and the expression of the traits remains the same regardless of the
environment in which the plant grows. Traits of this type are termed ‘highly heritable’.
The qualitative traits are the classical Mendelian traits of kinds such as form (e.g., round or wrinkled seeds of pea); structure (e.g.,
horned or hornless condition in cattle); pigments (e.g., black or white coat of guinea pigs); and antigens and antibodies (e.g., blood
group types of man) and so on.
The organisms possessing qualitative traits have distinct (separate) phenotypic classes and are said to exhibit discontinuous
variations.
quantitative traits
The quantitative traits, are economically important characters that exhibit continuous variations like height, weight, skin
pigmentation, susceptibility to pathological diseases or intelligence in man; amount of flowers, fruits, seeds, milk, meat or eggs
produced by plants or animals, etc. These traits can usually be quantified by measuring, weighing, counting etc. The quantitative traits
are also called metric traits. They do not show clear cut differences between individuals and form a spectrum of phenotypes
which blend imperceptively from one type to another to cause continuous variations.
In contrast to qualitative traits, the quantitative traits may be modified variously by the environmental conditions and are usually
governed by many factors or genes (perhaps 10 or 100 or more), each contributing such a small amount of phenotype that their
individual effects cannot be detected by Mendelian methods but by only statistical analysis.