Sex linked inheritance and hereditary defects

Contents: Introduction to sex linked inheritance. Chromosome carries which type of genes? Three types of traits Characteristics of sex linkage. Types of sex linkage. Sex linked disorders.

Sex linked inheritance : Discovered by Morgan in 1910 in case of eye -colour in Drosophila melanogester. Also known as sex linkage. It is the transmission of characters and their determining genes along with sex determining genes ; inherited together from one generation to another.

Y- Chromosome (male)- carries fewer genes along with TDF(testis determining factor). X-Chromosome – carries a number of genes.

TRAITS: SEX LINKED TRAITS- traits whose determining genes are found on sex chromosome. SEX LIMITED TRAITS- autosomal traits which are expressed in a particular sex. E.g: milk secretion in mammalian females , pattern baldness in males. SEX INFLUENCED TRAITS- not due to particular genes but are by by-products of sex hormone. E.g. low pitched voice , beard , moustaches.

CHARACTERISTICS OF SEX LINKAGE: It is criss cross inheritance. Mother passes alleles of sex-linked traits to both sons and daughters. Majority of sex linked traits are recessive. More apparent in males than females. Females generally function as carriers.

Criss cross inheritance holandric hologynic Sex linkage

Criss cross inheritance: Father passes the traits to grandson through his daughter (diagynic) while mother transfers traits to her grand daughter through her son (dia-andric). Holandric: traits transferred from father to son, e.g. hypertrichy, maleness. Hologynic: trais transferred from mother to daughter.

haemophilia Colour blindness DMD Night blindness Sex linked disorders haemophilia Colour blindness DMD Night blindness These sex linked disorders are caused by recessive gene located in X-chromosme and affect the males more than in females.

Haemophilia (John otto, 1803) Also known as bleeder’s disease as patient will continue to bleed even from a minor cut. Absence of antihaemophilic globulin or factor 8 (haemophilia-A) and plasma thromboplastin factor 9 (haemophilia-B). Royal disease-common in royal families of Europe. Its frequency is 1 in 7000 in human males and 1 in 10000000 in females.

Inheritance of haemophilia:

Colour blindness(horner,1876) Eye fails to distinguish red and green colour as shown in fig. It is more common in males(8%) as compared to female(0.4%) It shows criss-cross inheritance. Normal Vision is not affected.

Inheritance of colour blindness:

Night blindness: It is of two types: Acquired- due to Vitamin A deficiency. Congenital- due to visual purple deficiency that interferes with functions of retinal rods. It is inherited like red green colour blindness. Females become colour blind only if they receive two genes for defect.

Duchenne’s muscular dystrophy (DMD): Mutated gene is the largest gene with 2400kbp in man which is unable to produce a protein dystrophin. Due to its deficiency, calcium is not released from the muscle cell. As a result muscle contraction does not takes place. Deterioration of girdle muscles at an early age. Patient is unable to walk after the age of 12 followed by cardiomyopathy , mental impairment and death by age of 20 due to cardiac or respiratory failure.

Inheritance of DMD:

REFERENCES: TRUEMAN’S elementary biology Internet.org Wikipedia