If two genes are present on the same chromosome, they are known as linked genes. Genes whose loci are close to each other are less likely to be separated into different chromatids during chromosomal crossover process, of meiosis 1.
Mendel’s law of independent assortment is applicable to the genes that are situated on different chromosomes. When genes for different characters are located on the same chromosome, they are linked to one another and are said to be linked.
Human has more than 25000 genes and these genes are present on 23 pair of the chromosome and which means more than 1000 genes are located on a particular chromosome. These genes are known as linked genes.
10.1. Linkage group
The haploid number of chromosomes is called linkage groups. The number of linkage group is a sum total of the haploid number of chromosome and sex chromosome. The number of linkage group in the human female is 22+X. The number of linkage group in a male is 22+X +Y.
Mendel studied 7 characters and every pair of traits that he reported in his paper segregated independently. Mendel mentioned that he studied all the seven character in all possible ways and he mentioned that the dihybrid and trihybrid ratio for all the 7 characters in all possible ways remain the same. This is the main reason behind his principle of independent assortment.
However, Peas have n = 4 chromosomes. This makes it impossible that all the genes of 7 character are present on a different chromosome. The modern scientist identified the locus of all 7 genes studied by Mendel. The pod shape gene R (round shape vs. wrinkled shape) and pod colour gene (green pod vs. yellow pod) are both on chromosome V (= syntenic) but still segregate independently. This is because they are so far apart (ca. 50 cM) that there is an average one crossover between them in every meiosis. The dihybrid cross ratio in the above example is 9:3:3:1. This indicates that if genes are linked and if the genes are 50 cM apart they work as a normal independent. This makes them behave as if they are independent or unlinked. The plant height gene Le (tall, long internode vs. short internode, dwarf plant) and pod shape gene (inflated pod vs. constricted pod) are both on chromosome III. The dihybrid ratio between them has deviated from typical dihybrid ratio of 9:3:3:1.
10.2. The worlds first chromosomal map
T.H. Morgan was the first scientist who said linked genes are present on the same chromosome. i.e. we can map the genes on the chromosome. He demonstrated that the gene of the white eye in Drosophila is present on the X chromosome. Later other scientists showed many X-linked genes.
The linkage map is based upon the probability of crossing over between linked genes. Independent assortment occurs when chromosomes separate from each other independently during meiosis and therefore are inherited separately from each other. This is true if the genes for the observed phenotypes are found on different chromosomes or separated by large distances on the same chromosome so that recombination occurs greater than or equal to 50% of the time.
When genes occur on the same chromosome, they are usually inherited together or are “linked to each other.” The relative distance between two linked genes can be calculated by observing offspring. If two genes are far from each other their tendency to form a cross over between them is high. They behave as if they are present on a different chromosome. If two genes are linked to the chances of cross over between them is less. The cross over separate the linked gene. If the linkage is more the separation is less. That means chances of formation of recombinant gametes are less. Linkage keeps particular allelic combination together and crossing over mixes them up. The crossing over or recombination breaks up linked genes. This breaking is frequent if genes are apart. Crossing over results in recombination; it breaks up the associations of genes that are close together on the same chromosome. Linkage and crossing over can be seen as processes that have opposite effects:
- MENDEL'S LAW OF GENETICS
- REPRESENTATION OF MENDEL’S EXPERIMENTS
- FORKED-LINE METHOD
- TRIHYBRID CROSS
- EXTENSIONS AND MODIFICATIONS OF BASIC PRINCIPLES OF MENDEL LAW
- TEST CROSS AND THE BACKCROSS
- CHROMOSOMAL BASIS OF INHERITANCE
- EXTENSION OF MENDELIAN GENETICS
- LINKAGE MAPPING
- TETRAD ANALYSIS
- BACTERIAL GENETICS
- PEDIGREE ANALYSIS
- SEX INFLUENCE TRAIT
- SEX LIMITED TRAITS
- POLYGENIC INHERITANCE-MULTIPLE GENE INHERITANCE QUANTITATIVE INHERITANCE
- CHROMOSOMAL ABBERATIONS