3. Inheritance and Variation

-CREATED BY-

PROF. DESHMUKH A. B.

ASST. TEACHER

AGASTI ARTS, COMMERCE AND DADASAHEB RUPWATE SCIENCE JUNIOR COLLEGE, AKOLE, TAL. – AKOLE, DIST. - AHMEDNAGAR

3.12 Genetic Disorders

A B’s. Biology

3.12 Genetic Disorders :

• Genetic Disorders are broadly grouped into two categories as, Mendelian disorders and chromosomal disorders.
• Mendelian disorders are mainly caused due to alteration or mutation in the gene.
• e.g.,
• Thalassemia,
• Sickle cell anaemia,
• Colour blindness,
• Haemophilia,
• Phenylketonuria etc.
• On the other hand, chromosomal disorders are caused due to absence or excess of one or more chromosomes or their abnormal arrangement.
• e.g.,

Down’s syndrome,

Turner’s syndrome,

Klinefelter’s syndrome etc.

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Thalassemia :

• Thalassemia is an autosomal, inherited recessive disease.
• Haemoglobin molecule is made of four polypeptide chains-
• 2 alpha (a) and
• 2 beta (b) chains.
• The synthesis of alpha chains are controlled by two closely linked genes (HBA1 and HBA2) on chromosome 16.
• The synthesis of beta chain is controlled by a single gene (HBB) on chromosome 11.

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• Depending upon which chain of haemoglobin is affected, thalassemia is classified as alpha-thalassemia and beta thalassemia.
• It is caused due to deletion or mutation of gene which codes for alpha (a) and beta (b) globin chains that result in abnormal synthesis of haemoglobin.
• In Thalassemia, person shows symptoms like anaemia, pale yellow skin, change in size and shape of RBCs, slow growth and development, dark urine, etc.
• Massive blood transfusion is needed to these patients.
• Thalassemia differs from sickle-cell anaemia.
• The former is a quantitative problem of synthesizing few globin molecule, while the latter is a qualitative problem of synthesizing an incorrectly functional globin.

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Down’s Syndrome (21st trisomy) :

• Down’s syndrome is named after the physician John Langdon Down who first described this autosomal chromosomal disorder in 1866.
• This Syndrome is caused due to an extra copy of chromosome number 21st.
• It shows presence of three copies of 21st chromosome instead of homologous pair.
• These individuals will have 47 chromosomes instead of the normal number 46.
• 21st Trisomy occurs due to non- disjunction or failure of separation of chromosomes (autosomes) during gamete formation.
• The incidence of non-disjunction is distinctly higher in mothers who are over 45 years old.

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Female Gamete

Male Gamete

= 47

= 47

Down’s Syndrome

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Symptoms:

• These patients have mild or moderate mental retardation and skeletal development is poor.
• Distinct facial features like
• small head, ears and mouth,
• face is typically flat and rounded with flat nose,
• open mouth and protruding tongue,
• eyes slant up and out with internal epicanthal folds,
• flat hands and stubby fingers and palm is broad with single palmer crease.

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Turner’s Syndrome (X monosomy / XO females):

• It is sex chromosomal disorder caused due to non-disjunction of chromosome during gamete formation.
• Individual born with Turner’s syndrome has 44 autosomes with XO.
• They are phenotypically female.
• They have a short stature (height) and webbed neck, lower posterior hair line, broad shield-shaped chest, poorly developed ovaries and breast, and low intelligence.

Female Gamete

Male Gamete

https://www.onlinesciencestudy.com

Klinefelter’s syndrome (XXY males) :

• It is chromosomal disorder caused due to extra X chromosome in males.
• Thus, genotype of individuals is 44 + XXY.
• They are described as feminized males.
• Extra chromosome is a result of non-disjunction of X-chromosome during meiosis.
• Individual is male and has over all masculine development.
• Voice pitch is harsh and have underdeveloped testis.
• They are tall with long arms, feminine development (development of breast i.e. Gynaecomastia) and no spermatogenesis, therefore, individuals are sterile.

Female Gamete

Male Gamete

https://www.onlinesciencestudy.com