Genetic Linkage

Gene linkage…1 This is the tendency of certain genes to be inherited together Caused by the occurrence of different genes on the same chromosome The genes are in close proximity to each other Genes/alleles that are physically close to one another on the same chromosome tend to stay together during meiosis Is the result of non-random assortment during meiosis Genes co-exist or occupy the same locus or position on the chromosome Two genes are said to be linked if they are located on the same chromosome. We assume that different chromosomes segregate independently during meiosis.

Gene linkage…2 Genes inherited as a single unit are said to be linked Form linkage groups Humans have 23 pairs of chromosomes and thousands of genes Each chromosome contains large numbers of genes Chromosomes are inherited as units, i.e. they pair and segregate during meiosis as gametes Genes specifying particular traits occupy distinct positions and tend to be inherited as units For example, in Drosophila, the genes affecting eye colour and wing length are inherited together because they appear on the same chromosome Linkage can be complete (tight) or incomplete (loose)

Complete linkage Two loci which are so close together that alleles occupying the loci don’t separate during crossing over During reproduction, chromosome pairs exchange sections of DNA Genes that were originally on the same chromosome may end up on different chromosomes - genetic recombination The closer the location of two genes on the DNA, the less likely they are to be separated Linked genes are closely associated and tend to transmit together Complete linkage has been observed in male Drosophila

Incomplete linkage Some linked genes may not transmit together Chromosomes may exchange segments of DNA during meiotic prophase crossing over Linked genes widely located on chromosomes have a higher probability of being separated by crossing over Has been observed in Zea mays, tomato, female Drosophila, mice, poultry and man Linkage is significant because it reduces the possibility of variability in gametes Unless crossing over takes place

Linkage: Complete - Incomplete

Gene linkage…3 Example 1 Model answer In Zea mays, long leaves (S) and green veins (Y) are dominant over short leaves (s) and yellow veins (y). The cross SSYY x ssyy resulted in an F1 SsYy. When F1 plants were selfed, the F2 consisted of 570 long-leafed, green-veined individuals and 190 short-leafed, yellow-veined ones. Are the S and Y genes linked? Model answer You first have to calculate the ratio SSYY : ssyy If it gives a 3:1 ratio, then there is complete linkage and no crossing over occurs If however you get the 9:3:3:1 ratio, then the genes are not linked Alternatively, you could conduct a test cross If you get a 1:1:1:1 ratio, then there is no linkage

Linkage Group Linked genes of a chromosome form a linkage group All genes of a chromosome have identical allelomorphs (allele) on the homologous chromosome Linkage groups of a homologous pair of chromosome is considered as one The number of linkage groups in an organism of a particular species corresponds to the haploid number of that species Example: Humans have 23 pairs of chromosomes and 23 linkage groups Zea mays has10 chromosome pairs and 10 linkage groups Drosophila has 4 pairs of chromosomes and 4 linkage groups Linkage is an exception to Mendel’s second law!!

Comparison: Linkage and Independent Assortment Genes are located on homologous chromosomes Dihybrid test cross ratio is 1:1 P1: AB / AB x ab / ab P1 gametes: (AB) (ab) F1: AB / ab Test cross: AB /ab x ab / ab Gametes: (AB) (ab) (ab) F2: ½ AB/ab ½ ab/ab Ratio 1:1 Genes are located on different chromosomes Dihybrid test cross ratio is 1:1:1:1 P1: AA BB x aa bb P1 gametes: (AB) (ab) F1: Aa Bb Test cross: Aa Bb x aa bb Gametes: (AB) (Ab) (aB) (ab) (ab) F2: ¼ AaBb:1/4 Aa bb;1/4 aa Bb:1/4 aa bb Ratio 1:1:1:1

Comparison: Linkage and Independent Assortment...2 For two genes located at different chromosomes, we may assume that their alleles also segregate independently. The chance that an allele at one locus co-inherits with an allele at another locus of the same parental origin is then 0.5 and such genes are unlinked.

Complete Linkage in male Drosophila, Morgan (1919)

Incomplete Linkage in female Drosophila, Morgan (1919)

Crossing over and linked genes

Chromosomal Theory of Inheritance...1 Proposed by Walter Sutton and others including Simon Boveri (1902) States that: chromosomes are the basis for inheritance and are passed on from one generation to the next Explains the mechanism underlying Mendelian genetics Identifies chromosomes as the unit of inheritance, which occur in pairs Hypothesizes that chromosomes are linear structures carrying genes, each occupying specific positions along their length

Chromosomal Theory of Inheritance...2 Chromosome Theory of Linkage Proposed by Thomas Hunt Morgan and his students (1911) Developed from Mendelian genetics and Boveri/Sutton’s theory States that: Genes which show linkage are situated on the same pair of chromosomes Linked genes are arranged linearly on chromosomes Each gene has its own arrangement which is definite and constant

Chromosomal Theory of Inheritance...3 The distance between linked genes determines the degree of linkage Genes which are close to each other show tight linkage Those further apart show loose linkage Linked genes remain in their original combinations during inheritance Low frequency of crossing over with tight linkage