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CHAPTER-6: Linkage and Crossing Over

Prepared by: Dr. Md. Ariful Alam, Associate Professor

Linkage and Crossing over

Linkage

The numbers of genes of an organism exceed the number of its chromosomes. For instance, man has thousands of pairs of genes, but only 23 chromosome pairs. Hence each chromosome must contain a large number of genes. Further, because the chromosomes are inherited as units i.e., they pair and segregate in meiosis as units to gametes, so all of the genes which may specify various different phenotypic traits but are located in any given chromosomes, tend to be inherited together. All the genes on a chromosome are said to be linked to one another and belong to the same linkage group. The phenomenon of inheritance of linked genes in same linkage group is called linkage.

Difference in linkage and independent assortment:

s law of independent assortment is applied only to those genes which are located on separate chromosomes, because the linked genes of a linkage group inherit together. A dihybrid contains either linked genes or independently assorted genes, can be determined by test crossing it with a double recessive parent. The independently assorted genes give the test cross ratio of 1:1:1:1 and linked genes give the test cross ratio of 1:1 as have been illustrated by following examples: Example I. If genes occur on different chromosome they assort independently and give a test cross ratio of 1:1:1:1 as follows: Example II. The linked genes do not assort independently, but tend to stay together in the same combinations as they were in the parents. In the following figure, the genes on the left of the slash line (/) are on one chromosome and those on the right are on the homologous chromosome. The linked genes give the test cross ratio of 1:1 as follows:

Parents: AB/ab x ab/ab

Gametes: (AB) (ab) (ab) (ab)

F1: AB/ab : ab/ab or 1:1 (test cross ratio)

P1 AA BB X aa bb

P1 Gametes: AB ab

F1: AaBb

Test cross Aa Bb X aa bb

ab Gametes AB, Ab, aB, ab

F2 Aa Bb: Aa bb: aa Bb : aa bb

or 1:1:1:1 (Test cross ratio)

CHAPTER-6: Linkage and Crossing Over

Prepared by: Dr. Md. Ariful Alam, Associate Professor

Test Cross:

A test cross is the matting of an incompletely known genotype to a genotype which is homozygous recessive at all the loci under consideration. The phenotypes of the offspring produced by a test cross reveal the number of different gametes formed by the parental genotype under test. When all of the gametes of an individual are known, the genotype of that individual is also known. A monohybrid test cross gives a 1:1 phenotypic ratio indicating that one pair of factors is segregating. A dihybrid test cross gives a 1:1:1:1 ratio indicating that two pairs of factors are segregating and assorting independently. A trihybrid tests cross gives a 1:1:1:1:1:1:1:1 ratio, while three pairs of factors are segregating.

Arrangement of linked genes:

When two pairs of genes are linked, the linkage may be of either of two types in an individual heterozygous for both pairs (dihybrid):

1. The two dominant genes (eg., R and Ro of sweet pea) may be located on one

chromosome pair; with the two recessive gene (eg., r and ro) on the other or

2. The dominant allele of one pair and the recessive of other pair (R and ro) may be

located on one chromosome of the pair, with the recessive of the first gene pair and the dominant of the second gene (eg., r and Ro) on the other chromosome. The first arrangement with two dominants on same chromosome is called cis arrangement. This type of interrelationship of dominant and recessive linked genes (eg., RRo/rro) is called coupling phase. The second arrangement, one dominant and one recessive gene on the same chromosome is called trans arrangement. Such type of interrelationship of dominant and recessive genes (eg., Rro/rRo) is called repulsion phase.

CHAPTER-6: Linkage and Crossing Over

Prepared by: Dr. Md. Ariful Alam, Associate Professor Fig: Cis and Trans arrangement of two pairs of linked genes

Kinds of linkage:

The phenomenon of linkage is of following two kinds:

1. Complete linkage: When the linked genes are so closely located in chromosomes that

they inherit in same linkage groups for two or more generations in a continuous and regular fashion, then, they are called completely linked genes and the phenomenon of inheritance of completely linked genes is called complete linkage

2. Incomplete linkage: The linked genes do not always stay together because

homologous non-sister chromatids may exchange segments of varying length with one another during crossing over. The linked genes which are widely located in chromosomes and have chances of separation by crossing over are called incompletely linked genes and the phenomenon of their inheritance is called incomplete linkage.

Significance of linkage:

The phenomenon of linkage has one of the great significance for the living organisms that it reduces the possibility of variability in gametes unless crossing over occurs.

Crossing Over

The process of crossing over can be defined as a process which produces new combinations (recombinations) of genes by inter changing of corresponding segments between non-sister chromatids of homologous chromosomes. The chromatids in which crossing over has occurred have new combinations of genes and are called cross over.

Kinds of crossing over:

According to its occurrence in the germinal or somatic cells following two types of crossing over have been recognized:

Germinal or meiotic crossing over:

Commonly crossing over occurs in the germinal cells of reproductive organs during the process of gametogenesis which includes meiosis. This type of crossing over is called germinal or meiotic crossing over. It is universal in its occurrence and has great genetic significance.

Somatic or mitotic crossing over:

Sometimes crossing over may occur during mitosis or somatic cells. this type of crossing over in rare cases, has no genetic significance and is called somatic or mitotic crossing over. It has been observed in body cells of Drosophila.

CHAPTER-6: Linkage and Crossing Over

Prepared by: Dr. Md. Ariful Alam, Associate Professor According to the number of chiasma, crossing over may be of the following types:

1. Single cross over: When only one chiasma occur only at one point of the chromosome

pair. It produces two non-crossover chromatids and two crossover chromatids.

2. Double cross over: when crossing over occurs at two points between any two given

points in the same chromosome pair, it is called double crossing over. It produces FOUR crossovers. (a) Reciprocal chiasma- same two chromatids (b) Complementary chiasma- both chromatids take part (3) Multiple crossing over- Crossing overs occurs at three or more points

Significance of crossing over:

The crossing over provides origin of new characters due to the exchange of a segment from one chromosome to another and thus it is a source of genetic variation The crossing over provide the direct evidence that the genes are linearly arranged on the chromosome The frequency of crossing over is helpful in chromosomes mapping Crossing over gives an operational definition of a gene Crossing over has a great importance in the field of breeding to improve the varieties of plants and animals

Difference between crossing over and linkage

Crossing over Linkage

It leads to separation of linked genes It keeps the gene together

It involves non-sister chromatids of homologous

chromosome

It involves individual chromosome

Frequency of crossing over can never exceed

50%

Linkage group can never be more than

haploid chromosome number

It increase variability by forming new gene

combinations

It reduces variability

It provides equal frequency of parental and

recombinant types in test cross progeny

Provides higher frequency of parental

types than recombinant types in test cross progeny

CHAPTER-6: Linkage and Crossing Over

Prepared by: Dr. Md. Ariful Alam, Associate Professor

Genetic Recombination

Genetic Recombination

1. Due to genetic recombination, offspring have a different combination of genes than their

parents.

2. Without recombination, asexual organisms must rely on mutations to generate variation

among offspring; this is sufficient because they have great numbers of offspring.

Crossing-Over Introduces Variation

1. Crossing-over results in exchange of genetic material between non-sister chromatids.

2. At synapsis, homologous chromosomes are held in position by a nucleoprotein lattice

(the synaptonemal complex).

3. As the lattice of the synaptonemal complex breaks down at beginning of anaphase I,

homologues are temporarily held together by chiasmata, regions were the non-sister chromatids are attached due to crossing-over.

4. The homologues separate and are distributed to separate cells.

5. Due to crossing-over, daughter chromosomes derived from sister chromatids are no

longer identical.

Independent Assortment of Homologous Chromosomes

1. Independent assortment in a cell with only three pairs of chromosomes is eight possible

combinations.

2. In humans with 23 pairs of chromosomes, the combinations possible are 8,388,608

possible combinations.

Fertilization

1. Meiosis increases variation.

2. When gametes fuse at fertilization, chromosomes donated by parents combine.

3. Chromosomally different zygotes from same parents are 70,368,744,000,000

combinations possible without crossing over.

4. If crossing over occurs once, then 4,951,760,200,000,000,000,000,000,000

combinations of genetically different zygotes are possible for one couple.

Genetic recombination:

No two people look exactly alike as genes determine most of our physical characteristics, the exact combination of genes we inherit, and thus our physical traits, is in part due to a process our chromosomes undergo, known as genetic recombination.

CHAPTER-6: Linkage and Crossing Over

Prepared by: Dr. Md. Ariful Alam, Associate Professor Genetic recombination happens during meiosis, a special type of cell division that occurs during formation of sperm and egg cells and gives them the correct number of chromosomes. Since a sperm and egg unite during fertilization, each must have only half the number of chromosomes other body cells have. Otherwise, the fertilized cell would have too many. Inside the cells that produce sperm and eggs, chromosomes become paired. While they are pressed together, the chromosomes may break, and each may swap a portion of its genetic material for the matching portion from its mate. This form of recombination is called crossing-over. When the chromosomes glue themselves back together and separate, each has picked up new genetic material from the other. The constellation of physical characteristics it determines is now different than before crossing-over. Tracking the movement of genes during crossing-over helps geneticists determine roughly how far apart two genes are on a chromosome. Since there are more chances for a break to occur between two genes that lie far apart, it is more likely that one gene will stay on the original chromosome, while the other crosses over. So, genes that lie far apart are likely to end up on two different chromosomes. On the other hand, genes that lie very close together are less likely to be separated by a break and crossing-over.

CHAPTER-6: Linkage and Crossing Over

Prepared by: Dr. Md. Ariful Alam, Associate Professor Genes that tend to stay together during recombination are said to be linked. Sometimes, one gene in a linked pair serves as a "marker" that can be used by geneticists to infer the presence of the other (often, a disease-causing gene). there will be much more about this in the next section, on linkage. After the chromosomes separate, they are parceled out into individual sex cells. Each chromosome moves independently of all the others - a phenomenon called independent assortment. So, for example, the copy of chromosome 1 that an egg cell receives in no way influences which of the two possible copies of chromosome 5 it gets. Assortment takes place for each of the 23 pairs of human chromosomes. So, any single human egg receives one of two possible chromosomes 23 times, and the total number of different possible chromosome combinations is over 8 million (2 raised to the 23rd power). And that's just for the eggs. The same random assortment goes on as each sperm cell is made. Thus, when a sperm fertilizes an egg, the resulting zygote contains a combination of genes arranged in an order that has never occurred before and will never occur again.

CHAPTER-6: Linkage and Crossing Over

Prepared by: Dr. Md. Ariful Alam, Associate Professor

Difference between crossing over and linkage

Crossing over Linkage

It leads to separation of linked genes It keeps the gene together

It involves non-sister chromatids of

homologous chromosome

It involves individual chromosome

Frequency of crossing over can never

exceed 50%

Linkage group can never be more than

haploid chromosome number

It increase variability by forming new

gene combinations

It reduces variability

It provides equal frequency of parental

and recombinant types in test cross progeny

Provides higher frequency of parental

types than recombinant types in test cross progenyquotesdbs_dbs23.pdfusesText_29

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