How traits are inherited from parents to offspring
Gregor Mendel studied pea plants and found that traits are controlled by factors, now called genes, that come in pairs of alleles. In a monohybrid cross between tall (TT) and dwarf (tt) plants, the F₁ generation is all tall, and the F₂ generation shows a 3 : 1 phenotypic ratio and a 1 : 2 : 1 genotypic ratio. This gives the law of dominance and the law of segregation. A dihybrid cross gives a 9 : 3 : 3 : 1 ratio, explained by the law of independent assortment. A test cross with a recessive parent reveals an unknown genotype.
Not all inheritance follows Mendel’s simple rules. Incomplete dominance gives an intermediate phenotype, as in the pink snapdragon, and codominance shows both alleles, as in AB blood groups. The chromosomal theory of inheritance places genes on chromosomes, and linked genes are inherited together unless separated by crossing over. Sex is determined by chromosomes, XX in human females and XY in males. Pedigree analysis tracks traits in families, revealing disorders such as haemophilia and colour blindness (sex-linked), sickle-cell anaemia (autosomal recessive) and Down’s syndrome (trisomy 21).
Why it matters
Genetics explains why children resemble their parents, helps doctors predict and counsel families about inherited disorders, and lets breeders develop better crops and livestock. Blood group inheritance matters for transfusions. Genetic testing is increasingly used in medicine, and this topic leads into molecular biology and biotechnology.
Key terms
- Allele
- One of the alternative forms of a gene.
- Genotype
- The genetic makeup of an organism for a particular trait.
- Phenotype
- The observable characteristics of an organism.
- Sex-linked trait
- A trait controlled by a gene located on a sex chromosome.