Notes on the Principles of Inheritance and Variation for Class 12th?
Heredity- It can be defined as the process of transferring heritable traits from the parents to their offspring.
Inheritance- It is the basis of heredity through which traits are passed from the parents to the offspring. Continuity of the gene pool is controlled by the process of inheritance.
Genetics- It is that branch of biology which deals with the principles and mechanism of inheritance and variation.
Genes- They are the basic unit of Inheritance which are situated on chromosomes.
Variation- It occurs among individuals of one species. Variation happens due to crossing over, recombination, mutation and environmental effects on the genes located on chromosomes.
Mendel’s laws of inheritance
Gregor Johann Mendel is known as ‘’The father of genetics”.
Mendel performed many experiments on garden pea by taking 14 true-breeding plants of pea which have seven distinguishable characters having two opposite traits.
He termed genes as factors which are passed from parents to offspring.
Genes encoding a pair of opposite traits are called alleles.
He propagated three laws of inheritance based on his observation:
- Law of dominance– Here, one of the alleles is dominant and it expresses itself in the phenotype in the case of the heterozygote, e.g., when homozygous tall (TT) is crossed with dwarf (tt) plants, all tall plants having the genotype Tt is found. Therefore, tallness is a dominant trait over dwarfness.
- Law of segregation of genes– When the gametes are formed, each allele separates during meiosis. No blending occurs and characters are transferred to different gametes. Homozygotes make only one kind of gametes and heterozygotes make different kinds of gametes.
- Law of independent assortment– According to this law, alleles for different traits are inherited independently
Test cross– This is a method to take out the genotype of the plant showing dominant trait. The plant is crossed with the recessive homozygote. The two observations which were found out are:
- When the phenotype of offspring exhibits only the dominant trait, then the parent plant was homozygote to the dominant trait.
- When the offspring produced are both phenotypes, then the parent plant was heterozygote to the dominant trait.
Incomplete dominance
- When both the two alleles are dominant and the phenotype of the heterozygote does not have similar traits of any of the parents, the heterozygote exhibits intermediate or a mixture of two parents’ traits.
- Example; the flower color inheritance of snapdragon (dog flower). When true breeding red (RR) and white flower (rr) are crossed, then all pink color flowers in the F1 generation are derived, which on self-pollination will give red: pink: white flowers in the ratio 1:2:1 in the F2 generation.
Co-dominance
- When both the alleles show themselves together in an individual, they are known to be co-dominant.
- Example: the inheritance of the ABO blood group in humans is maintained by the gene I. the gene I contains three allelic forms, Ia, Ib and i.
Chromosomal theory of inheritance
- Sutton and Boveri agreed to Mendel’s observations and said that chromosomes are the carrier of genes.
- Chromosomes are a homologous pair and the two alleles of a gene are situated on the homologous pair of chromosomes as the same site.
- Homologous chromosomes get separated during meiosis during the process of gamete formation.
- Chromosomes separate and assemble independently.
- During the process of fertilization, gametes come together and make offspring with the dip loid no. of chromosomes, similar to the parent.
- Morgan studied fruit flies, Drosophila melanogaster and gave experimental evidence to support the chromosomal theory of inheritance.
Linkage and recombination
- When the genes located on a chromosome are associated physically, then it is known as linkage.
- In a dihybrid cross, when the two genes are compact or present on the same chromosome, the parental combination is more dominant than non-parental combinations or recombinants.
- The linkage and recombination depend on the distance between a pair of genes. When the distance is more, then there is a greater chance of recombination.
Multiple alleles– When a trait is ruled by more than two alleles, e.g., ABO blood group, it is known as multiple alleles.
Do you know the answer of the below question?
- An abnormal human baby with XXX sex chromosomes was born due to:
- A) formation of abnormal ova in the mother
- B) fusion of two ova and one sperm
- C) fusion of two sperms and one ovum
- D) formation of abnormal sperms in the father
Polygenic inheritance– When a trait is ruled by multiple independent genes, which have a similar or additive effect on the trait, it is called as polygenic inheritance, e.g., eye color, skin pigmentation, height, hair color, etc.
Pleiotropy- When a single gene governs many phenotype traits, it is known as a pleiotropic gene. The various phenotypic expressions are due to the result of the effect of a gene on metabolic pathways.
Sex determination
- There are several systems of sex determination which are present in different organisms.
- Henking observed X chromosome and named it X body.
- The chromosomes which are responsible for the development of sexual characters are called as sex chromosomes and the rest of the chromosomes are called as autosomes.
- Male heterogamety is when the male gives two different kinds of gamete, e.g humans, grasshoppers, etc.
- Female heterogamety is when the female gives two different kinds of gamete, e.g. birds.