7.3: Principles of Inheritance

The fundamental laws governing the transfer of traits from parent to offspring were discovered by Gregor Mendel, an Austrian monk who conducted groundbreaking experiments with pea plants in the mid-1800s. His insights into inheritance remain central to our understanding of animal breeding today.

Genes exist at specific locations called loci (singular: locus) on chromosomes. Each gene can occur in different forms called alleles. For example, a gene for coat color might have one allele for black and another for brown. When an animal inherits two identical alleles at a particular locus (such as AA or aa), it is homozygous at that locus. When it inherits two different alleles (such as Aa), it is heterozygous.

Chromosomes occur in pairs called homologous chromosomes—matching partners that are similar in size and shape and carry genes affecting the same traits. Normal body cells, or somatic cells, contain the full diploid number of chromosomes (two complete sets). In contrast, sex cells or gametes (sperm and eggs) contain only the haploid number—half the diploid complement—ensuring that when fertilization occurs, the offspring receives one complete set from each parent.

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Mendel discovered two fundamental principles that explain how genetic information passes from parents to offspring:

The Principle of Segregation states that when gametes form during meiosis, paired alleles separate so that each gamete receives only one allele (randomly selected) for each locus. This means that if a parent is heterozygous (Aa), approximately half of its gametes will carry the A allele and half will carry the a allele.

The Principle of Independent Assortment states that the segregation of alleles at one locus occurs independently of the segregation of alleles at other loci. In other words, inheriting a particular allele for one trait does not influence which allele is inherited for an unrelated trait, assuming the genes are on different chromosomes or far apart on the same chromosome.

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