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Sex Linked Inheritance - types, X linked, Y linked, XY linked, Completely sex linked and Incompletely sex linked inheritance , cris-cross/ zig zag inheritance

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                                                                             Sex Linked Inheritance The transmission of body characters from parents to offspring along with a sex is called sex linked inheritance. It is also called sex linkage. The genes controlling body characters located on the sex chromosomes are called sex linked genes. The body characters (other than sex characters) controlled by genes located on the sex chromosomes are called sex-linked characters. Sex linked inheritance was discovered by T.H. Morgan in 1910. The following are the common examples for sex-linked inheritance: 1. Colour blindness 2. Haemophilia 3. Eye colour in Drosophila 4. Hypertrichosis (Hair in the ear pinna) 5. Ichthyosis hystrix. The sex linked genes are located on X chromosome or Y chromosome or both X and Y chromosome. The genes, controlling body characters, located on X chromosome are called X-linked genes. The inheritance of X-linked genes is called X-linked inheritance. The c
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Theories of Linkage:

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                                                                                  Theories of Linkage: VIEWS OF CLASSICAL GENETICISTS ON LINKAGE: Mendel could not notice the phenomenon of linkage because fortunately the seven pairs of factors or alleles studied by him in pea were located on seven different pairs of chromosomes. It was noticed and discovered by some other post-Mendelian geneticists who during their genetic investigations came across to linked genes. The evolution of linkage concept took place by the views of following classical geneticists: 1. SUTTON'S VIEWS ON LINKAGE: Sutton (1903) was the first classical geneticist who made certain predictions about the linkage merely by performing certain cytological investigations. He suggested that each chromosome must bear more than a single gene and that the genes represented by one chromosome must be inherited together. However, he could not prove his predictions by genetic experiments. 2. COUPLING AND REPULSION H
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Linkage - principles, types, arrangement of linked genes, significance, linkage group, factors affecting linkage

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                                                                                          Linkage Linkage is defined as the tendency of two or more genes to remain together in the original combination in the same chromo some during the process of inheritance for a number of generations. All the genes on a chromosome are said to be linked to one another. Linkage was discovered by T.H. Morgan. Linkage was found in Drosophila, sweet peas, maize, man, etc. In Drosophila, the genes of body colour and nature of wings are linked and located on the same chromosome. In sweet pea, the genes of flower colour and size of pollen grains are linked. In maize, the genes for colour and shape of seeds are linked. In man, linked. The genes for haemophilia and colour blindness are linked. Principles of linkage: T.H. Morgan (1911) proposed the theory of linkage based on his experiments in Drosophila. The theory of linkage explains the main principles of linkage. They are the following: 1. A chromos
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sex linked inheritance - Sex Limited Genes, Sex Influenced Genes, Sex Linkage, Holandric Genes, Criss-Cross Inheritance, Hemizygous, Sex Linked Lethal Genes

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 Sex Limited Genes: Sex limited genes express characters in only one sex. The sex limited genes may be located on any chromosome (sex linked genes are located only on the sex chromosome). Their expression in the vertebrates is governed by the sex-hormones. Sex limited genes are responsible for the secondary sexual characteristics s as well as primary sexual characters. Examples 1. In man, the beard is produced by sex limited genes. A woman, normally, does not have a beard, yet she surely carries all the genes necessary to produce a beard. But the expression of that particular e in ladies is prevented by the absence of a particular hormone. In rare cases, abnormalities in hormone secretion may occur in a woman gene which allow these genes to express themselves and the result is a bearded lady. 2. Breast development is normally limited to woman but hormone unbalance may cause breast development in a man. 3. The genes for the deep masculine voice and masculine musculature in man w
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Self-sterility in Nicotiana

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  Self-sterility in Nicotiana In plants, multiple alleles have been reported in association with self-sterility or self incompatibility.  Self-sterility means that the pollen from a plant is unable to germinate on its own stigma and will not be able to bring about fertilization in the ovules of the same plant.  East (1925) observed multiple alleles in Nicotiana which are responsible for self-incompatibility or self-sterility.  The gene for self-incompatibility can be designated as S, which has allelic series S ₁ , S ₂ , S ₃ , S ₄ and S ₅ .  The cross-fertilizing tobacco plants were not always homozygous as S ₁ S ₁ or S ₂ S ₂ , but all plants were heterozygous as S ₁ S ₂ , S ₃ S ₄ , S ₅, S ₆ .  When crosses were made between different S ₁ S ₂ plants, the pollen tube did not develop normally.  But effective pollen tube development was observed when crossing was made with other than S ₁ S ₂ for example S ₃ S ₄ .  When crosses were made between seed parents with S ₁ S ₂ and pollen par
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Linkage in Sweet Pea

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                                                                        Linkage in Sweet Pea Bateson and Punnet (1906) studied linkage in sweet pea ( Lathyrus odoratus ). In sweet pea, blue flower colour is dominant (BB) over red flower (bb). Long pollen grain is dominant (LL) over round pollen grain(ll). They crossed blue flowered, long pollen sweet pea with red flowered, round pollen plant. In these plants, the genes for flower colour and pollen shape are located in the same chromosome. That is, they are linked. The linked genotypes of the parents are written as follows: Blue flowered long pollen grained plant - BL/BL Red flowered round pollen grained plant-bl/bl The F1 plants were blue flowered with long pollen. The genotype is written as BL/bl. The F1 plant (BbLI) was test crossed with double recessive plant (bl/bl). In the F2 generation, the plants appeared in the ratio 7:1:1:7 instead of the Mendelian test cross ratio 1:1:1:1. Blue long-7 Blue round-1 Red long-
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Linkage in Maize

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                                                                            Linkage in Maize   Hutchison experimentally proved the existence of linkage in maize ( Zea mays ). In maize, coloured seed (CC) is dominant and colourless seed is recessive (cc). Full seed (endosperm) is dominant (FF) and shrunken seed is recessive (ff). A cross was made between coloured full seeded plant with colourless shrunken seeded plant. The genes for seed colour a shape are linked. i.e. C and F are linked; c and f are linked. T linked genotypes of the parents are written as follows. Colourless shrunken seeded plant cf/cf , Coloured full seeded plant CF/CF All the F1 plants are coloured and full and the genotype written as CF/cf A test cross is made by crossing the F1 plant (CF/cf) with double recessive plant (cf/cf). In the F2 generation, plants were obtained in the ratio of 48.2: 1.8: 1.8:48.2. Coloured full CF/cf - 48.2% Coloured shrunken Cf/cf - 1.8% Colourless full cF/cf - - 1.8% Col
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