Glossary: Principles of Inheritance & Variation | Biology for Grade 12 PDF Download

Mendel’s Laws of Inheritance

• Truebreeding line: A truebreeding line is one that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations. 

Inheritance of One Gene

• Filial Progeny: The offsprings produced as a result of a cross between two individuals is called the filial generation or progeny.
• Genes: Mendel proposed that something was being stably passed down, unchanged, from parent to offspring through the gametes, over successive generations. He called these things as ‘factors’. Now we call them as genes.
• Alleles: Genes which code for a pair of contrasting traits are known as alleles.
• Punnett Square: The production of gametes by the parents, the formation of the zygotes, the F1 and F2 plants can be understood from a diagram called Punnett Square.
• Test Cross: A test cross is a way to explore the genotpye of an organism. Early use of the test cross was an experimental mating test used to determine what alleles are present in the genotype.

Inheritance of Two Genes

• Mendel’s Law of Independent Assortment: The law states that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.
• Dihybrid Cross: A dihybrid cross is a breeding experiment between two organisms which are identical hybrids for two traits.
• Linkage: In linkage, two or more genes linked together are always inherited together in the same combination for more than two generations.
• Recombination: In recombination the genetic material is exchanged between different organisms which leads to the production of offsprings with the combination of traits.
• Pleiotropy
• Pleiotropic gene: When a single gene can exhibit multiple phenotypic expression. Such a gene is called a pleiotropic gene.

Sex Determination

• X body: ‘X body’ of Henking was a chromosome and that is why it was given the name X-chromosome.
• Autosomes: Due to the involvement of the X-chromosome in the determination of sex, it was designated to be the sex chromosome, and the rest of the chromosomes were named as autosomes.
• Male Heterogamety: When the male produces two types of gametes and have a role to play in sex determination, it is called male heterogamety, e.g. XY sex chromosome in male humans.
• Female Heterogamety: When the female determines the sex of the baby and produces two types of gametes having different sex chromosomes, it is called female heterogamety, e.g. ZW sex chromosome in female birds.

Mutation

• Mutation: Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism.

Genetic Disorders

• Pedigree Analysis: Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s), called aneuploidy.
• Polyploidy: Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organism and this phenomenon is known as polyploidy. This condition is often seen in plants.