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3. Inheritance and Variation

-CREATED BY-

PROF. DESHMUKH A. B.

ASST. TEACHER

SEC. & HIGHER SEC. SCHOOL, SHENDI (BHANDARDARA DAM), TAL. – AKOLE, DIST. - AHMEDNAGAR

3.9 Autosomal Inheritance

A. B’s. Biology

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3.9 Autosomal Inheritance :

  • Human somatic (2n) cell contains 23 pairs of chromosomes.

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  • They can be divided functionally as autosomes and sex chromosomes.
  • A single pair of chromosomes is involved in sex determination and remaining 22 pairs are called autosomes.
  • Autosomes control a variety of traits other than sex.
    • These traits are called autosome linked traits.
  • Transmission of body characters other than the sex linked traits from parents to their offsprings through autosomes, is called autosomal inheritance.

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  • Some characters are influenced by dominant genes while some other are by recessive genes, present on autosomes.
  • For example,

• Autosomal dominant traits like

Widow’s peak -

“V” shape hair line on forehead.

Huntington’s disease -

An inherited condition in which nerve cells in the brain break down over time.

Typically starts in a person’s 30s or 40s. Rare.

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• Autosomal recessive traits like

Phenylketonuria (PKU),

- A birth defect that causes an amino acid called phenylalanine to build up in the body & is excreted in urine.

Cystic fibrosis –

- An inherited life- threatening disorder that

damages the lungs & digestive system.

Sickle cell anaemia –

- A group of disorders that causes RBCs to become misshapen & break down.

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a. Widow’s peak :

  • A prominent “V” shaped hairline on forehead is described as widow’s peak.
  • It is determined by autosomal dominant gene.
  • Widow’s peak occurs in homozygous dominant (WW) and also heterozygous (Ww) individuals.
  • Individuals with homozygous recessive (ww) genotype have a straight hair line (no widows peak).
  • Both males and females have equal chance of inheritance.

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b. Phenylketonuria (PKU):-

  • It is an inborn metabolic disorder caused due to recessive autosomal genes.
  • When recessive genes are present in homozygous condition, phenylalanine hydroxylase enzyme is not produced.
  • This enzyme is essential for conversion of amino acid phenylalanine into tyrosine.
  • Due to absence of this enzyme, phenylalanine is not converted into tyrosine.
  • Hence, phenylalanine and its derivatives are accumulated in blood and cerebrospinal fluid (CSF).

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  • It affects development of brain and causes mental retardation.
  • Excess phenylalanine is excreted in urine, hence this disease is called phenylketonuria.
  • Autosomal recessive traits appear in both sexes with equal frequency.
  • These traits tend to skip generations.

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