2 Introduction A human female, has 23 pair of chromosomes A human male, has 22 similar pairs and one pair consisting of two chromosomes that are dissimilar in size and structure.The 23 rd pair in both the sexes is called sex chromosomesthe female, XX. the male, XY
3 X-linked diseasesX-linked diseases are those for which the gene is present on the X chromosome.X-linked diseases show inheritance patterns that differ from autosomal diseases.This occurs because males only have one copy of the X chromosome (plus their Y chromosome) and females have two X chromosomes.Because of this, males and females show different patterns of inheritance and severity of manifestation. While there are both dominant and recessive X-linked diseases, there are some characteristics that are common to X-linked disorders in general
4 X-linked genes are never passed from father to son. The Y chromosome is the only sex chromosome that passes from father to son.Males are never carriers – if they have a mutated gene on the X chromosome, it will be expressed.Males are termed hemizygous for genes on the X chromosome.
5 X-linked dominanthereditary pattern in which a dominant gene on the X chromosome causes a characteristic to be manifested in the offspring.X-linked dominant diseases are those that are expressed in females when only a single copy of the mutated gene is present.Very few X-linked dominant diseases have been identified (e.g. hypophosphatemic rickets, Alport syndrome, diabetes insipidus)hypophosphatemic rickets or vitamin D resistant rickets >>>low serum phosphorus, skeletal abnormalitiesAlport syndrome, which involves progressive hearing loss and progressive kidney problems.
6 Characteristics of X-linked dominant diseases include: Never passed from father to son.Affected males produce only affected females. An affected male only has one X chromosome to pass on to his daughtersAffected females produce 50% normal and 50% affected offspring.. >>>> heterozygousMales are usually more severely affected than females. Some X-linked dominant traits may even be lethal to males.Females are more likely to be affected. Since females have 2 X chromosomes, they have 2 “chances” to inherit the mutated allele.
9 Amelogenesis Imperfecta-1 : X-linked dominantGene name: Amelogenin, Chromosome location: Xp22.3-p22.1Altered Cellular function: Abnormal tooth enamelSymptoms: Very hard enamel, thin enamel, small teeth, and/or rough teethIncidence: Rare
10 X-linked recessivehereditary pattern in which a recessive gene on the X chromosome results in the manifestation of characteristics in male offspring and a carrier state in female offspringX-linked recessive diseases are those in which a female must have two copies of the mutant allele in order for the mutant phenotype to develop.Many X-linked recessive disorders are well-known, including color blindness, hemophilia, and Duchenne muscular dystrophy.
13 Hemophilia The blood fails to clot normally Lacking a blood clotting factor VIII(antihemophilic globulin, AHG),IXbleeding from even minor cutsin 1,500 newborn males. Most (75%) have hemophilia A, a lack of clotting factor VIII.Hemophilia B- "Christmas Disease" is a defect in clotting factor IX.Transfusions of fresh whole blood or plasma or factor concentrates control bleeding
15 Typical features of X-linked recessive inheritance Never passed from father to son.Males are much more likely to be affected because they only need one copy of the mutant allele to express the phenotype.Affected males get the disease from their mothers and all of their daughters are obligate carriers.Sons of heterozygous females have a 50% chance of receiving the mutant allele.
16 The Marker X syndromeFragile X syndrome (Marker X syndrome ) is a genetic condition involving changes in the long arm of the X chromosome. It is characterized by mental retardation.Alternative NamesMartin-Bell syndrome; Marker X syndrome
17 Causes, & Risk FactorsFragile X syndrome is the most common form of inherited mental retardation in males and a significant cause in females.The inheritance is different from common dominant or recessive inheritance patterns.A fragile area on the X chromosome tends to repeat bits of the genetic code.The more repeats, the more likely there is to be a problem.Boys and girls can both be affected, but because boys have only one X chromosome, a single fragile X is more likely to affect them more severely.
18 Fragile X Syndrome Symptoms & Signs Family history of fragile X syndrome, especially a male relativeMental retardationLarge testicles (macro-orchidism)Large sizeTendency to avoid eye contactHyperactive behaviorLarge forehead and/or ears with a prominent jaw
20 Sex-limited inheritance: A trait that appears in only one sex is called sex-limited.This is different from X-linked inheritance, which refers to traits carried on the X chromosome.Sex hormones and other physiologic differences between males and females may alter the expressivity of a gene.For example, premature baldness is an autosomal dominant trait, but presumably as a result of female sex hormones, the condition is rarely expressed in the female, and then usually only after menopause.Thus, sex-limited inheritance, perhaps more correctly called sex-influenced inheritance, is a special case of limited expressivity and penetrance
21 Sex limited traitssex linked traits are generally expressed much more often in males than in females.some traits which affect one sex more than another are not necessarily sex linked.Examples are cases of sex limited expression which might include genes affecting beard growth or breast size, and (in cattle), horn growth and milk yield.
22 Sex-influenced traits characteristic may appear in both sexes but expression of the phenotype differs.Example: Early balding (pattern baldness) in humans. Heterozygous men (b+/b) lose their hair; heteroyzgous women do not have significant hair loss.Homozygous men or women (b/b) become bald. The trait is therefore dominant in men, recessive in women. (We used b to designate the mutant baldness allele even though the allele is dominant in males.)