Genetics and Inheritance

Basic Principles of Genetics

Genetics is the study of how traits are passed from parents to offspring, a process known as inheritance.
Organisms have two copies of each gene - one from the mother and one from the father - and these copies might not be identical. These different forms of the same gene are called alleles.
Dominant alleles are those that are expressed even if only one copy is present (i.e., if the organism is heterozygous for that trait).
Recessive alleles are only expressed if two copies are present (i.e., the organism is homozygous for that trait).
An organism’s genetic makeup is known as its genotype, while the physical expression of those genes is its phenotype.

Scientists use a simple tool known as a Punnett Square to predict the probability of certain outcomes in genetic crosses.
In a Punnet square, the alleles that could be inherited from the mother are usually written along the top, while those that could be inherited from the father are written along the side.
The squares inside the Punnett square represent all possible combinations of alleles in the offspring.
A Punnett square is a simple way to calculate probabilities and is especially useful for looking at simple traits controlled by one gene, such as fur or hair colour in animals.

DNA (Deoxyribose Nucleic Acid) is a molecule that carries the instructions for building all of the proteins an organism needs. These instructions are stored in the form of a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T).
A gene is a section of DNA that carries the instructions for one specific protein. The location of a gene on a chromosome is its locus.
DNA is organised into structures called chromosomes. Each species has a specific number of chromosomes; for example, humans have 46, whereas dogs have 78.
Chromosomes come in pairs, with one member of each pair inherited from each parent. This means that for most genes, an individual will have two alleles.

Austrian monk and scientist Gregor Mendel conducted experiments on pea plants and formulated the basic laws of inheritance, known as Mendel’s Laws.
Law of Segregation states that the two alleles for each trait separate from each other during the formation of gametes, so each gamete carries only one allele for each gene.
Law of Independent Assortment suggests that different traits are inherited independently of each other, although we now know this only strictly applies to genes on different chromosomes or far apart on the same chromosome.

Genetic variation within a population increases the likelihood that at least some individuals will have traits that are suited to changing environments, enhancing survival and reproduction (fitness).
Mutations, which are changes to the genetic material, are one source of genetic variation. They can be beneficial, neutral, or harmful.
Mutations can occur spontaneously or in response to environmental factors such as chemicals or radiation. They may affect single genes or entire chromosomes.
Some mutations can cause genetic disorders in animals, such as cystic fibrosis in humans or hip dysplasia in dogs.