Genes and Inheritance

Genes and Inheritance


  • Genes, made up of segments of DNA, control the characteristics of organisms.
  • Inheritance is the process where genetic information is passed from parents to their offspring.

Gene Structure

  • Each gene has a unique position on a specific chromosome known as a locus.
  • The variant forms of a gene known as alleles may result in different phenotypic outcomes.
  • Dominant alleles express themselves even in a heterozygous state while recessive alleles only express in a homozygous state.

Mendelian Inheritance

  • Proposed by Gregor Mendel, Mendelian inheritance includes key principles such as the law of segregation and the law of independent assortment.
  • The Law of Segregation states that during gamete formation, the two alleles for each gene separate, so each gamete carries only one allele.
  • The Law of Independent Assortment states that genes for different traits assort independently of one another in the formation of gametes.

Genetic Crosses

  • Monohybrid crosses involve one pair of contrasting traits, while dihybrid crosses involve two.
  • The Punnett square is a diagram used to predict the outcome of a particular cross or breeding experiment.
  • Test crosses are used to determine the genotype of an individual with a dominant phenotype.

Non-Mendelian Inheritance

  • This includes various forms of inheritance that don’t follow Mendelian principles such as co-dominance, incomplete dominance, and polygenic inheritance.
  • Co-dominance occurs when both alleles for a gene are expressed equally in the phenotype of the organism.
  • Incomplete dominance happens when the phenotype of the offspring is a blend of the parents’ traits.
  • In Polygenic inheritance, multiple genes determine the phenotype of a trait.

Genetic Disorders

  • Certain combinations of alleles can result in genetic disorders like cystic fibrosis or sickle cell anaemia.
  • Genetic conditions can be autosomal dominant, autosomal recessive, or X-linked.
  • Genetic screening can help identify carriers of genetic disorders for informed reproductive decisions.

Sex Determination

  • Humans have 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes, labeled as X and Y.
  • The Y chromosome carries the SRY gene which triggers male development.


  • Understanding genetics and inheritance has implications for medicine, agriculture, and understanding evolution. It can help in treating diseases, improving crop and livestock breeds, and deciphering the history and future of species.