2X Linked (XL) disorders are encoded on the X chromosome. Males inherit their X chromosome from their mother, and Y chromosome from their father. So, males don’t have the second X chromosome and they are hemizygote for the genes on X chromosome.
3There is a characteristic inheritance pattern of the phenotypes of the genes on X chromosome. Almost 500 genes are localized on X chromosome and 70% of these genes are associated with disease phenotypes.
4In XL inheritance, males can NEVER be carriers In XL inheritance, males can NEVER be carriers. XL disorder manifests in males who are hemizygotes (whatever the disorder is, homozygote or heterozygote).Males can not inherit their X chromosome from their father, so XL trait NEVER transmits from fathers to sons.Pedigree shows no male-to-male transmission.
6Both of the X chromosomes must inherit the disease if a female is affected. If only one X chromosome inherits, then this female is called as a carrier.Males can show the symptomes of the disease because of hemizygosity.
71- It is more common in males according to females. Basic Criteria1- It is more common in males according to females.2- All daughters of an affected male are obligate carriers and none of his sons inherit the conditions.3- 50% of sons from carrier females are affected.4- There can be carrier females for throughout generations.5- Affected males are relatives over females.6- Heterozygote females are usually not affected, but they can show the symptomes of the disorders in a varying degree.
8Sex-Linked Recessive Traits An affected son can have parents who have the normal phenotype. (XAY x XAXa)For a daughter to have the trait, her father must also have it. Her mother must have it or be a carrier (XaY, XaXa, XAXa)The trait often skips a generation from the grandfather to the grandson.If a woman has the trait (XaXa), all of her sons will be affected.Pedigrees show only female carriers but no male carriers.
13Hemophilia A Probable patient Maternal X x Carrier X XY Xx XX 1/4 PaternalProbable patientaffectedYxY1/2F1/2MnormalPatient
14It skips generationIt affects males-- Hemophilia (no factor VIII)
15If X linked disease is seen for the first time in a family, then there should be a new mutation, in other words mother is not carrier and the siblings of this child are not affected by this disease.
16If a female has a X linked recessive disease, then.. a)The karyotype can be 46,XY. But, external genitalia is female (testicular feminization).b)The karyotype can be 45,XO (Turner syndrome). There is only one X chromosome, so this female should be affected.c) There can be a translocation between the mutated X chromosome and autosomal chromosomes. In this situation, translocated X should be active, and the normal X should be inactive.d)Female should be homozygote for this trait ( mother is carrier and father is affected)e)Normal X should be inactive and mutated X is active in Lyonization.
17LYON HYPOTHESIS (X Inactivation) One of the X chromosomes in somatic cells of females is active.The second X chromosome is condensed and inactive. This inactive X chromosome is called as “Barr body”.X inactivation is the process by which dosage compensation of X-linked genes in females is achieved by the transcriptional silencing of one of the two X chromosomes during early development (from day 9 post-fertilization when the inner cell mass of the blastocyst contains 64 cells).
19Although there is only one copy or allele of genes on X chromosome in males and two copies in females, both of them are equal in terms of genes.
20The mechanism of X inactivation is DNA methylation.The genes on the inactive X chromosome are inactivated by the addition of methyl group to the cytosines of DNA methyltransferase enzyme.
21The goal of X-inactivation is dosage compensation. The early events in X-inactivation are under the control of the X chromosome inactivation center (Xic). The XIST (X-inactive specific transcript) gene in the Xic at Xq13.2 is the only gene transcribed exclusively from the inactive X chromosome and is known to play an essential role in the initiation of X-inactivation. XIST gene on the inactive X chromosome is activated by hypermethylation.
23Typical LyonisationThe inactive X chromosome can be either the one that comes from the mother or father. This is chosen randomly and called as “Typical Lyonisation”. But once an X chromosome is inactivated in a cell, then all of the inactivated X chromosomes in this cell line should be the same X chromosome. In other words, inactivation is determined randomly, but it is permanent (irreversible).
24Atypical LyonisationIf one of the X chromosomes has a structural abnormality, this X chromosome is inactivated primarily. In this situation, inactive X is not selected randomly.If there is a translocation between an autosomal chromosome and X chromosome, the X chromosome that doesn’t have translocation is inactivated.
25the expression variability in heterozygote females X inactivation has some important consequences in terms of clinics and genetics. These are:dosage compansationthe expression variability in heterozygote femalesmosaicism (Mosaicism is the presence of two or more genetically different cell lines in an individual, all derived from a single zygote)
26There are still some genes that are expressed in both of the X chromosomes and escaped from X inactivation.
271961 Mary Lyon a) One of the X chromosomes is inactive b) Randomly inactivationmaternal 50%paternal 50%c) Irreversible inactivation
281- It is more common in males according to females. 2- All daughters of an affected male are obligate carriers and non of his sons inherit the conditions.3- For a carrier female, 50% of her sons will be affected and 50% of her daughters will be carriers.4- The trait is inherited by carrier females. So, the persons who have the trait are the relatives of the female.5- Heterozygote females are not affected. The degree to which females express the disorder is rarely related to X-inactivation patterns.6- Pedigree shows an oblique pattern.7- There is no transmission from male to male.
29Examples of Sex-Linked Recessive Disorders Red/Green Colorblindness – Difficulty perceiving differences between colors (red or green, blue or yellow).Hemophilia – Absence of one or more proteins necessary for normal blood clotting.Cataracts – opacity in the lens that can lead to blindnessNight blindness – (Nyctalopia) rods do not work so that can not see in the darkGlaucoma – pressure in the eye that can lead to optic nerve damage and blindness
30Duchenne Muscular Dystrophy – progressive weakness and degeneration of skeletal muscles that control movement due to absence of dystrophin (protein that maintains muscle integrity). Mainly in boys, onset 3-5 yrs, by 12 years can’t walk, and later needs respirator.
32There are also a few genes that have a dominant character on X chromosome and these cause some symptomes in females. The disease can be seen in both males and females. But, the symptomes are more serious in males, sometimes it can be lethal. X linked dominant inherited diseases are very rare. The best known disease is vitamin D resistant rickets.
34All children of an affected female has a risk of 50%. Males and females are affected, females usually less severely affected than malesAll daughters of affected male are affected but no male to male transmission. If one of the daughters is not affected or one of the sons is affected , then this shoud be an autosomal inheritance type.
42Some characters can only be inherited by the genes (holandric genes) on Y chromosome. For example a disease called as ichthyosis hystrix was firstly seen in Lambert family in England in It was inherited to the other generations in that family, but there was only male to male transmission.
43Holandric genes are also responsible from adherent fingers or toes (syndactyly). 1st chromosome has the most genes (2968) and Y chromosome has the least (231).