Introduction to Genetics

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Ben Whitten

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What is genetics?

Genetics is described as a branch of biology concerned with the study of genes, genetic variation and heredity in organisms. The 'father of genetics' is Gregor Mendel, who first established the principles of hereditary inheritance in the 19th century. He crossed two pure breeding pea plants, including a tall one and a short one. The offspring mainly were tall with only some short, demonstrating both dominant and recessive inheritance.

Key terminology involved in genetics

There are some key terms which are necessary to be aware of when it comes to learning genetics. 

  • Pure breeding: Individuals bred among themselves, they always produce offspring like the parents; individuals are homozygous for this gene. 

  • Gene: A segment of DNA that transmits information from one generation to the next by coding for a protein. 

  • Allele: A different or unique variation of the same gene (at the same locus) which is determined by small differences in the DNA sequence of the gene. Pairs of alleles are found on homologous maternal and paternal chromosomes. 

  • Genotype: The specific combination of alleles belonging to an individual or cell. 

  • Phenotype: The actual form taken by a specific feature in a particular individual based on their genotype. It is the physical expression of a trait. 

  • Dominant: A phenotype that requires only one copy of its allele to be expressed, and masks any other allele. 

  • Recessive: A phenotype that requires two copies of its allele to be expressed as the presence of a dominant allele will mask the recessive. 

  • Heterozygous: When an organism contains two different alleles for a gene. 

  • Homozygous: When an organism contains identical alleles for a gene. 

  • Autosomal inheritance: Inheritance of non-sex chromosomes (autosomes) which affects both males and females. Humans have 22 pairs of autosomes. 

  • Sex-linked inheritance: inheritance of sex chromosomes (X chromosome only), which affects one sex more than the other. 

  • Punnett square: A table that displays all the possible offspring genotypes given the parental alleles which can be produced at fertilisation.

Mendelian theory: his pea plants

Image: Gregor Mendel - characteristics of pea plants - english image, Image by LadyofHats, Sourced Under a Creative Commons 4.0 License from Wiki Commons

In Mendel's theory, tall peas were represented by a capital letter T which represents the dominant alleles, and the short peas were shown as t which represents recessive alleles. The original pure breeding plants are called the parental generation (P), the first generation of offspring is the first filial generation (F1), and this continues onward. A cross between a purebred tall plant and a purebred short plant would produce a hybrid organism.

Introduction to Genetics
Monohybrid Cross
Test Cross
Multiple Alleles
Dihybrid Cross
Polygenic Inheritance
Dominance Patterns
Autosomal Inheritance
Sex-linked Inheritance
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