Sex Linked Inheritance

• 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 chromosomes

• the female, XX. the male, XY

X-linked diseases

• X-linked diseases are those for which the gene is present on the X chromosome.

• 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.

X-linked dominant • Hereditary 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 abnormalities

– Alport syndrome, which involves progressive hearing loss and progressive kidney problems.

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 daughters

• Affected females produce 50% normal and 50% affected offspring.. >>>> heterozygous

• Males 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.

The pattern for the pedigree of X-linked dominant inheritance

Pattern for inheritance• Mating A Mating B

Pattern for inheritance• Mating A Mating B

X-linked recessive

• hereditary 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 offspring

• X-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.

The pattern for the pedigree of X-linked recessive inheritance

•  

• www.tylermedicalclinic.com/ pgd-presentation18...

Pattern of x linked recessive inheritance

staff.um.edu.mt/ acus1/02Monogenic.htm

Hemophilia

• The blood fails to clot normally• Lacking a blood clotting factor

VIII(antihemophilic globulin, AHG),IX• bleeding from even minor cuts• in 1,500 newborn males. Most (75%) have he

mophilia 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 f

actor concentrates control bleeding

Hemophilia A and B. Coagulation system

www.emedicine.com/.../ 14801480ped0962-01.jpg

Inheritance of hemophilia

www.hemophilia.ca/ en/images/figure5.gif

A Pedigree of Hemophilia in the Royal Families of Europe

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 the

ir daughters are obligate carriers.

• Sons of heterozygous females have a 50% chance of receiving the mutant allele.

• These disorders are typically passed from an affected grandfather to 50% of his grandsons.