Definition
Inheritance is the transmission of characteristics from parents to their children.
Children inherit characteristics from both parents, but they do not look completely identical to either parent. Characteristics such as pupil colour, hair type, and blood group are passed down through genes located on homologous chromosomes.
Characteristics and Traits
A characteristic is a tangible property that is inherited and determined by genes. A trait is a variant of a specific characteristic that differentiates individuals.
Genes and Alleles
Genes are the basic units of inheritance found on chromosomes that control specific characteristics. An allele is a pair of genes located at the same locus or position on homologous chromosomes that control one trait of an organism. Alleles exist in dominant or recessive forms. For every trait, an individual inherits two copies of genes: one from the mother and one from the father.
Dominant and Recessive Alleles
A dominant allele expresses its characteristic and covers the effect of the recessive allele. It is represented by a capital letter, for example \(T\) for tall or \(B\) for the ability to roll the tongue.
A recessive allele has its characteristic covered in the presence of a dominant allele. It only shows its characteristic when the dominant allele is absent, meaning both alleles must be recessive, such as \(tt\) or \(bb\). It is represented by a small letter, for example \(t\) for dwarf or \(b\) for the inability to roll the tongue.
Examples of dominant and recessive traits in humans.
| Characteristic |
Dominant trait |
Recessive trait |
| Ability to roll tongue |
Ability to roll tongue |
Inability to roll tongue |
| Hair colour |
Black hair |
Blonde hair |
| Earlobes |
Free earlobes |
Attached earlobes |
Gregor Mendel and the Monohybrid Cross
Gregor Mendel (1822–1884), known as the "Father of Genetics", was the first to systematically discover the mechanism of inheritance using pea plants in monohybrid crosses.
A monohybrid cross involves the study of one characteristic at a time, for example plant height: tall versus dwarf.
Genotype and Phenotype
Genotype is the genetic information or gene combination in an organism, for example \(TT\), \(Tt\), or \(tt\). Phenotype is the physical characteristics displayed by an organism, for example tall or dwarf.
Mendel's Monohybrid Cross for Height
Parents: Pure-breeding tall (\(TT\)) \(\times\) Pure-breeding dwarf (\(tt\)).
Gametes: Meiosis separates alleles into gametes containing \(T\) and \(t\).
First Filial Generation (\(F_1\)): Genotypes are all \(Tt\) (heterozygous). Phenotypes are all tall pea plants.
Second Filial Generation (\(F_2\)): Crossing \(F_1 \times F_1\), meaning \(Tt \times Tt\), produces gametes \(T\) and \(t\) from each parent. Genotypes are \(TT\), \(Tt\), \(Tt\), and \(tt\), giving a ratio of 1 \(TT\) : 2 \(Tt\) : 1 \(tt\). Phenotypes are 3 tall : 1 dwarf, giving a phenotypic ratio of 3:1.
Human Example: Ability to Roll Tongue
A pure-breeding father able to roll his tongue (\(BB\)) \(\times\) a mother unable to roll her tongue (\(bb\)) produces all \(F_1\) offspring with genotype \(Bb\) and the phenotype of being able to roll their tongue.
Sex Determination in Humans
Humans have a total of 46 chromosomes, arranged as 23 pairs: 44 autosomes plus 2 sex chromosomes. Females have \(44 + XX\), while males have \(44 + XY\).
During gamete formation by meiosis, a sperm has half the parent chromosome number, 23 chromosomes: either \((22 + X)\) or \((22 + Y)\). An ovum also has 23 chromosomes and is always \((22 + X)\).
The gender of the offspring depends on the type of sperm fertilising the ovum. If a \((22 + X)\) sperm fertilises a \((22 + X)\) ovum, the child is female (\(44 + XX\)). If a \((22 + Y)\) sperm fertilises a \((22 + X)\) ovum, the child is male (\(44 + XY\)). Each pregnancy therefore has an equal chance of producing a male or female child: 50% female : 50% male, a 1:1 ratio.
