what are linked genes and types of linked genes
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A pair or set of genes that are located on the same chromosome and usually inherited together are known as linked genes.
Types of linkage are:
Completely linked genes- When the genes are tightly linked on a chromosome and show very low percentage of recombination. In such condition, only parental or non crossing over type of gametes are formed. These genes are said to be completely linked and the linkage is known as complete linkage.Incompletely linked genes-When the genes were loosely linked on a chromosome and show higher percentage of recombination. In such condition, non parental type of gametes are formed. These genes are said to be incompletely linked and the linkage is known as incomplete linkage.
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When two genes are close together on the same chromosome, they do not assort independently and are said to be linked. Whereas genes located on different chromosomes assort independently and have a recombination frequency of 50%, linked genes have a recombination frequency that is less than 50%.
Types of Linkage:
Linkage is of two types, complete and incomplete.1. Complete Linkage (Morgan, 1919):
The genes located on the same chromosome do not separate and are inherited together over the generations due to the absence of crossing over. Complete linkage allows the combination of parental traits to be inherited as such. It is rare but has been reported in male Drosophila and some other heterogametic organisms.
A red eyed normal winged (wild type) pure breeding female Drosophila is crossed to homozygous recessive purple eyed and vestigial winged male. The progeny or F1 generation individuals are heterozygous red eyed and normal winged. When F1 males are test crossed to homozygous recessive female (purple eyed and vestigial winged), only two types of individuals are produced— red eyed normal winged and purple eye vestigial winged in the ratio of 1 : 1 (parental phenotypes only). Similarly during inbreeding of F1individuals, recombinant types are absent. In practice, this 1: 1 test ratio is never achieved because total linkage is rare.
In Drosophila, genes of grey body and long wings are dominant over black body and vestigial (short) wings. If pure breeding grey bodied long winged Drosophila (GL/ GL) flies are crossed with black bodied vestigial winged flies (gl/gl), the F2 shows a 3 : 1 ratio of parental phenotypes (3 grey body long winged and one black body vestigial winged).
This is explained by assuming that genes of body colour and wing length are found on the same chromosome and are completely linked.2. Incomplete Linkage:
Genes present in the same chromosome have a tendency to separate due to crossing over and hence produce recombinant progeny besides the parental type. The number of recombinant individuals is usually less than the number expected in independent assortment. In independent assortment all the four types (two parental types and two recombinant types) are each 25%. In case of linkage, each of the two parental types is more than 25% while each of the recombinant types is less than 25%.
A red eyed normal winged or wild type dominant homozygous female Drosophila is crossed to homozygous recessive purple eyed and vestigial winged male. The progeny or F1 individuals are heterozygous red eyed and normal winged. F1 female flies are test crossed with homozygous recessive males. It does not yield the ratio of 1: 1: 1: 1. Instead the ratio comes out to be 9: 1: 1: 8. This shows that the two genes did not segregate independently of each other. The data obtained by Bridges (1916) is as follows:
Only 9.3% recombinant types were observed which is quite different from 50% recombinants in case of independent assortment. This shows that in the oocytes of the F1, generation only some of the chromatids undergo cross-over while the majority is preserved intact. This produces 90.7% parental types in the progeny.
In Sweet Pea (Lathyrus odoratus) blue flower colour (B) is dominant over red flower colour (b) while the trait of long pollen (L) is dominant over round pollen (1). A Sweet Pea plant heterozygous for both blue flower colour and long pollen (BbLl) was crossed with double recessive red flowered plant with round pollen (bbll). It is similar to test cross. In case the genes of the two traits are unlinked, the progeny should have four phenotypes (Blue Long, Blue Round, Red Long, and Red Round) in the ratio of 1: 1: 1: 1 (25% each). In case the two genes are completely linked the progeny should have both the parental types (Blue Long and Red Round) in the ratio of 1: 1(50% each).
Recombinants should not appear. However, in the above cross Bateson and Punnett (1906) found both parental and recombinant types but with different frequencies in the ratio of 7: 1: 1: 7. (7 + 7 Parental and 1 + 1 recombinant types).Phenotype Progeny Observed
frequencyExpected frequency if complete linkage Expected frequency if Independent assortment Parental Types (i) Blue Long 43.7% 50% 25% Recombinant Types (ii) Red Round 43.7% 50% 25% (a) Blue Round 6.3% 0% 25% (b) Red Long 6.3% 0% 25%
Only 12.6% recombinant types were observed against the expected percentage of 50% in case of independent assortment. Therefore, the genes are linked but undergo recombination due to crossing over in some of the cases.
Morgan and his students have found that linked genes show varied recombinations, some being more tightly linked than others, (i) In Drosophila, crossing of yellow bodied (Y) and white eyed (W) female with brown bodied (Y+) red eyed (W+) male produced F1 to be brown bodied red eyed. On intercrossing of F1 progeny, Morgan observed that the two genes did not segregate independently of each other and, therefore, the F2 ratio deviated significantly from expected 9: 3: 3: 1 ratio. He found 98.7% to be parental and only 1.3% recombinants (Fig. 5.18). (ii) In a second cross in Drosophila between white eyed and miniature winged (wwmm) female with wild type or red eyed normal winged (w+w+m+m+) males, all the F1 were found to be of wild type, i.e., red eyed and normal winged. An F1female fly was then test crossed with white eyed and miniature winged male. 62.8% of the progeny was of parental types while 37.2% were recombinants (Fig. 5.19).Linkage Groups:
A linkage group is a linearly arranged group of linked genes which are normally inherited together except for crossing over.
It corresponds to a chromosome which bears a linear sequence of genes linked and inherited together. 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.
Fruit-fly Drosophila melanogaster has four linkage groups (4 pairs of chromosomes), human beings 23 linkage groups (23 pairs of chromosomes), Pea seven linkage groups (7 pairs of chromosomes), Neurospora 7 linkage groups (7 chromosomes), Mucor 2 linkage groups (2 chromosomes), Escherichia coli one linkage group (one pro-chromosome or nucleoid) while Maize has 10 linkage groups (10 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 longer linkage group. This is subject to the amount of heterochromatin present in the chromosome. Thus Y-chromosome of man possesses 231 genes while human chromosome 1 has 2969 genes.
because it does not allow them to freely bring all the desirable traits in one variety, (v) It dilutes the use of desirable traits if undesirable ones are also present on the same linkage group, e.g., low ginning and naked seeds or fuzzy seeds and high ginning in American Cotton, (vi) Marker genes or genes which express their effect in early growth can indicate the effect of a linked gene which is to express late, e.g., wavy lamina and larger panicle in MilletSex Linkage or Sex-Linked Inheritance:
Sex linkage or sex-linked inheritance is the transmission of characters and their determining genes alongwith sex determining genes which are borne on the sex chromosomes and, therefore, are inherited together from one generation to the next. Y-chromosome of the male carries fewer genes alongwith TDF. The X- chromosome which is common to male and female carries a number of genes.
All sex linked characters show criss-cross inheritance. It was discovered by Morgan (1910) when he studied the inheritance of red-white eye colour trait (locating genes on chromosomes). Two important sex-linked human diseases are haemophilia and colour blindness.
Sex Linked Traits:
They are those traits the determining genes of which are found on the sex chromosomes. All the sex-linked traits present on a sex chromosome are inherited together.
Sex Limited Traits:
They are autosomal traits which are expressed in a particular sex in response to sex hormones though their genes also occur in the other sex, e.g., milk secretion in mammalian females, pattern baldness in males. The gene for baldness behaves as an autosomal dominant in males and autosomal recessive in females.
Sex Influenced Traits:
The traits are not due to particular genes but are by-products of sex hormones, e.g., low pitched voice, beard, moustaches. In males, pattern baldness is related to both autosomal genes as well as excessive secretion of testosterone.Characteristics of Sex Linked Inheritance:
1. It is criss-cross 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. It is because the males have only one X-chromosome which is transferred to the female offspring. Only Y-chromosome of the father is transferred to the male offspring but this sex chromosome does not carry many alleles.
2. Mother passes the alleles of sex-linked traits to both sons and daughters.
3. Majority of the sex linked traits are recessive.
4. Sex linked traits are more apparent in males than in females.
5. As many sex-linked traits are harmful, males suffer more from sex-linked disorders.
6. Females generally function as carriers of sex-linked disorders because recessive genes can express themselves in females only in the homozygous state.
7. Traits governed by sex-linked recessive genes (a) Produce disorders in males more often than in females, (b) Express themselves in males even when represented by a single allele because Y-chromosome does not carry any corresponding alleles, (c) Seldom appear in both father and son. (d) Fail to appear in females unless their father also possesses the same and the mother is a carrier, (e) Females heterozygous for the trait function as carrier, (f) Female homozygous for the recessive trait, transfers the trait to all the sons.
8. Traits governed by sex-linked dominant genes (a) Produce disorders in females more often than in males, (b) All the female offspring will exhibit them if father possesses the same, (c) Do not get transmitted to son if mother does not exhibit them.