Genetics and Inheritance

Mendelian genetics — key vocabulary

Gregor Mendel's experiments with pea plants established the basic rules of inheritance. Understanding these terms is essential:

• Gene — a section of DNA that codes for a particular characteristic.
• Allele — a different version of the same gene (e.g. an allele for tallness vs an allele for shortness).
• Genotype — the genetic makeup of an organism for a trait (e.g. Tt).
• Phenotype — the observable physical characteristic resulting from the genotype (e.g. tall).
• Homozygous — having two identical alleles for a gene (e.g. TT or tt).
• Heterozygous — having two different alleles for a gene (e.g. Tt).
• Dominant allele — masks the effect of a recessive allele when present (written as a capital letter, e.g. T); only one copy is needed to show the dominant phenotype.
• Recessive allele — only shows its effect in the phenotype when two copies are present (written as a lowercase letter, e.g. t).

Monohybrid crosses and the Punnett square

A monohybrid cross tracks the inheritance of a single gene. A Punnett square is a grid used to predict the possible genotypes (and their probabilities) of offspring from a cross between two parents.

Incomplete dominance and codominance

Not all alleles show simple dominant/recessive inheritance:

Sex-linked inheritance

Some genes are carried on the sex chromosomes (X and Y) rather than the other ("autosomal") chromosomes. Because the Y chromosome is much smaller and carries far fewer genes than the X chromosome, X-linked recessive conditions (such as red-green colour blindness and haemophilia) are much more common in males (XY) than females (XX) — a male only needs one copy of the recessive allele (on his single X chromosome) to show the condition, while a female needs two copies (one on each X chromosome) to be affected. A female with only one copy is a carrier — she does not show the condition but can pass the allele to her children.