Class 10 · Chapter 8 · Biology
Heredity Cheat Sheet
Laws
Law of dominance
When two contrasting forms of a trait are present, only one of them — the dominant trait — appears in the F₁ generation; the other, the recessive trait, remains hidden.
In Mendel’s pea plants, tall (T) is dominant over short (t), so all F₁ plants are tall.
Law of segregation
Two copies of a factor (gene) controlling a trait are present in a sexually reproducing organism, and these two copies separate during gamete formation so that each gamete receives only one.
A Tt plant forms two kinds of gametes, T and t, in equal numbers.
Law of independent inheritance
Two different traits are inherited independently of each other — the inheritance of one trait does not affect the inheritance of another.
Mendel’s dihybrid cross gave a 9 : 3 : 3 : 1 ratio in the F₂ generation.
Definitions
- Heredity
- The passing of traits from parents to their offspring.
- Variation
- The differences in traits among individuals of the same species.
- Trait
- A particular form of a character; e.g. tall or short.
- Gene
- A unit of inheritance that controls a trait.
- Allele
- One of the two or more forms of a gene.
- Dominant trait
- A trait that is expressed even when only one copy of its gene is present.
- Recessive trait
- A trait that is expressed only when both copies of its gene are present.
- Genotype
- The genetic makeup of an organism, e.g. TT or Tt.
- Phenotype
- The visible characteristic of an organism, e.g. tall or short.
- Homozygous
- Having two identical copies of a gene, e.g. TT or tt.
- Heterozygous
- Having two different copies of a gene, e.g. Tt.
- P generation
- The parent generation used in a cross.
- F₁ generation
- The first generation of offspring obtained by crossing the parents.
- F₂ generation
- The generation obtained by self-pollinating the F₁ plants.
- Monohybrid cross
- A cross involving only one pair of contrasting traits.
- Dihybrid cross
- A cross involving two pairs of contrasting traits at the same time.
- Sex chromosome
- A chromosome that determines the sex of an individual; X and Y in human beings.
- Acquired trait
- A trait gained during an organism’s lifetime; it is not passed on to the next generation.
Key Points
- Reproduction produces variations because DNA copies are not always perfectly identical; variations are the basis of survival and evolution of a species.
- Mendel worked with garden peas using contrasting characters such as tall/short plants and round/wrinkled seeds.
- In a monohybrid cross the F₁ generation shows only the dominant trait and the F₂ generation shows a 3 : 1 ratio (TT : Tt : tt as 1 : 2 : 1).
- A dominant factor needs only a single copy to be expressed; a recessive factor needs both copies.
- Genotype is the gene combination (TT, Tt, tt) while phenotype is the visible appearance (tall or short).
- In a dihybrid cross the F₂ generation shows four combinations in the ratio 9 : 3 : 3 : 1.
- In human beings sex is determined by the sex chromosomes: XX in females and XY in males.
- A child’s sex is decided by the father, because the mother always gives an X chromosome while the father gives either X or Y.
- Acquired traits of the body are not inherited; only changes in the genes of germ cells are passed to the next generation.