Chromosomal Inheritance

Basics of Chromosomal Inheritance

  • Genes are fundamental units of heredity, found on chromosomes located in the nucleus of cells.
  • Humans have 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes.
  • Each cell in the body, except sperm and egg cells, contains two copies of every chromosome, one inherited from each parent.

Mendelian Inheritance

  • The principles of inheritance were established by Gregor Mendel based on his experiments with pea plants.
  • Mendel identified the characters transmitted from parent to offspring as ‘factors’ (today known as genes).
  • According to the principle of segregation, each organism contains two factors (alleles) for each trait, and these segregate during the formation of gametes.
  • The principle of independent assortment states that alleles of different genes are inherited independently of each other.

Crosses and Genetic Diversity

  • Monohybrid crosses involve one pair of contrasting traits, whereas dihybrid crosses involve two.
  • Crossing over during meiosis produces varied combinations of genes on each chromosome, contributing to genetic diversity.
  • Random fertilization also adds genetic variability.

Sex-Linked Inheritance

  • Traits related to the sex chromosomes (X and Y) show sex-linked inheritance.
  • Male mammals are XY and females are XX, leading to different patterns of inheritance.
  • Males are more likely to express recessive traits linked to the X chromosome than females, as they have only one X chromosome.

Non-Mendelian Inheritance

  • Incomplete dominance occurs when the heterozygote has an intermediate phenotype between the two homozygotes.
  • Codominance happens when both alleles are fully expressed in the heterozygote.
  • The above inheritance patterns do not follow Mendel’s principles and show that inheritance is more complex.

Genetic Disorders

  • Certain genetic disorders are caused by the inheritance of specific alleles.
  • Cystic fibrosis and sickle cell anaemia are examples of autosomal recessive disorders.
  • Haemophilia and red-green colour blindness are examples of sex-linked disorders.

Pedigree Analysis

  • A pedigree chart traces the occurrence of a trait in a family.
  • It helps in predicting the inheritance of traits, identifying carriers of a disease, and eliminating any risk of genetic diseases while planning a family.

Genetic Testing and Personalised Medicine

  • Genetic testing can identify potential genetic disorders in embryos or adults.
  • This information can be used for personalised medicine – healthcare tailored to an individual’s genetic makeup.