• A linkage group is a linearly arranged group of linked genes which are normally inherited together except for crossing over during cell division. They act and move as a unit rather than independently.
• Because the two homologous chromosomes possess either similar or allelic genes on the same loci, they constitute the same linkage group. Therefore, the number of linkage groups present in an individual corresponds to number of chromosomes in its one genome (all the chromosomes if haploid or homologous pairs if diploid).
• It is known as principle of limitation of linkage groups.
• Fruitfly Drosophila melanogaster has four linkage groups (4 pairs of chromosomes), human beings have 23 linkage groups (23 pairs of chromosomes), Pea has seven linkage groups (7 pairs of chromosomes).
• The size of the linkage group depends upon the size of chromosome.The smaller chromosome will naturally have smaller linkage group while a longer one has larger linkage group.
• This is subject to the amount of hetero chromatin present in the chromosome. Thus Y-chromosome of man possesses 231 genes while human chromosome 1 has 2969 genes.
Strength of linkage
• The strength of linkage between any two linked genes depends upon the distance between them. It is calculated by the frequency of recombination (crossing over) between them.
• It means the strength of linkage is inversely proportional to the distance between the genes or the percentage of crossing over between them.
• There are many factors affecting strength of linkage besides distance including many physiological as well as environmental factors such as:
– Age of animal: With increasing age the chance of crossing over are reduced and, therefore, strength of linkage increases.
– Temperature : The increase in temperature increases the frequency of chiasmata formation and, therefore, it decreases the strength of linkage.
– X-rays : Exposure to X-rays reduces the strength of linkage.
• Sex linkage or sex-linked inheritance is the transmission : of characters and their determining genes along with sex determining genes which are borne on the sex chromosomes and, therefore, are inherited together from one generation to the next.
• The sex linkage was discovered by Morgan (1910) when he studied the inheritance of red-white eye colour trait (locating genes on chromosomes).
• Most sex-linked genes are located on the X-chromosome, forming X-linkage. A few genes occur on the Y-chromosome, forming Y-linkage. The Y-linked traits are transmitted only through the male, for example, gene for sex determination in humans.
• Some animals, however, may carry a few genes on the Y chromosome that produce visible effects on the phenotype of the organism. Such ‘Y-linked” or holandric genes would be transmitted directly from father to son and never appear in females, Y linkage is clearly very rare in higher animals, particularly mammals.
• X linkage, on the other hand, is very common in all mammals that have been studied the mammalian X chromosome contains a large number of genes with major effects on phenotype.
• Two important sex-linked human diseases are haemophilia and colour blindness.
• Sex limited traits: They are those specific phenotypic traits which are expressed in a particular sex. There are certain autosomal genes which are expressed in one sex, not in the other due to hormonal differences or anatomical dissimilarities between male and female sexes. Since the expression of such genes are limited to one sex, therefore they are known as sex-limited genes and the traits controlled by them are called sex-limited traits. Genes for secondary sexual characters have sex-limited inheritance, e.g., plumage pattern in birds.
• Sex-influenced inheritance: The expression of certain autosomal and sex-linked genes is dependent on the hormone constitution of the individual. When they come in heterozygous condition they may be expressed in one sex not in other, e.g., pattern baldness in human. The gene for baldness behaves as autosomal dominant in males and autosomal recessive in females.
Characteristics of sex-linked inheritance
– It shows criss-cross pattern of inheritance. Father does not pass the sex-linked allele of a trait to his son. The same is passed to the daughter, from where it reaches the grandson, i.e., diagynic inheritance.
– Mother passes the alleles of a sex-linked trait to both sons and daughters.
– Majority of the sex-linked traits are recessive.
– Sex-linked traits are more apparent in males than in females. As many sex-linked traits are harmful, males suffer more from sex-linked disorders.
– Females generally function as carriers of sex-linked disorders because recessive genes can express themselves in females only in the homozygous state.
• Traits governed by sex-linked recessive genes:
– Produce disorders in males more often than in females. They express themselves in males even when represented by a single allele because Y-chromosome does not carry any corresponding alleles.
– Seldom appear in both father and son.
– Fail to appear in females unless their father also possesses the same and the mother is a carrier.
– When present in heterozygous condition in a female, she functions as carrier.
– When present in homozygous condition in a female, she transfers the trait to all the sons.
• Traits governed by sex-linked dominant genes:
– Produce disorders in females more often than in males.
– Are present in all the female offsprings if father possesses the same.
– Do not get transmitted to son if mother does not exhibit them.
X-linkage in Drosophila
• In 1910, Morgan fortuitously discovered a single fruitfly with white eyes that did not result from any of his treatments. (Normal Drosophila have red eyes, not white eyes.) Morgan immediately crossed this white-eyed male Drosophila to its red-eyed sisters.
Interestingly, when Morgan later inbred the heterozygous F, red-eyed flies, the traits of the F2 progeny did not assort independently. Morgan expected a 1:1:1:1 ratio of red-eyed females, red-eyed males, white-eyed males, and white-eyed females. Instead, he observed the following phenotypes in his F2 generation:
– 2,459 red-eyed females
– 1,011 red-eyed males
– 782 white-eyed males
There were no white-eyed females, Morgan wondered whether this was because the trait was sex-limited and only expressed in male flies.
To test whether this was indeed the case, Morgan did a reciprocal cross between a red-eyed male and white eyed female, the F, offsprings instead of being all redeyed consisted of 50% red-eyed and 50% white-eyed and all the red eyed offsprings were females and all the male offsprings were white-eyed.
When these F, offsprings were interbred, their F2 offsprings consisted of following phenotypes:
– 129 red-eyed females
– 132 red-eyed males
– 88 white-eyed females
-86 white-eyed males Thus, the results of this cross did produce white eyed females, and the groups had approximately equal numbers.
Morgan therefore hypothesized that the eye-color trait was connected with the sex factor. This in turn led to the idea of genetic linkage, which means that when two genes are closely associated on the same chromosome, they do not assort independently.
X-linkage in humans
• In humans, many genes and the traits controlled by them are recognized as being linked to the X chromosome.
• These X-linked traits can be easily identified in pedigrees, characterized by a criss-cross pattern of inheritance.
• Many X-linked human genes have been identified, like the genes controlling two forms of hemophilia and two forms of muscular dystrophy are located on the X chromosome.
• Because of the way in which X-linked genes are transmitted, unusual circumstances may be associated with recessive X-linked disorders in comparison to recessive autosomal disorders.
• For example, if an X-linked disorder debilitates or is lethal to the affected individual prior to reproductive maturation, the disorder occurs exclusively in males.
• This is the case because the only sources of the lethal allele in the population are heterozygous females who are “carriers” and do not express the disorder. They pass the allele to one-half of their sons who develop the disorder because they are hemizygous who rarely, if ever, reproduce.
• 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.
• A linkage or gentic 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 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! H. Morgan who is considered to be the father of experimental genetics).