Chapter 15 Inheritance | Short Notes

Chapter -15                                                                                                                  INHERITANCE

Short Notes

Genetics

Genetics is the branch of biology in which we study inheritance.

Inheritance

Inheritance means the transmission of characteristics from parents to offspring.

Trait

The characteristics which are controlled and transmitted to next generations through genes. For example: In man height, color of the eyes, intelligence etc. are all inheritable traits.

Gene

A gene is the basic physical and functional unit of heredity. Genes, which are made up of DNA, act as instructions to make molecules called proteins.

Homologous Chromosomes

A pair of chromosomes having same size and shape and carrying alleles for the same traits. In human body there are 23 pairs of homologous chromosomes.

Chromosome

Chromosome is made of chromatin material. Human have 46 chromosomes.

Chromatin

Chromatin is a complex material, made of DNA and proteins (mainly histone proteins).

Nucleosomes

DNA wraps around histone proteins and forms round structures, called nucleosomes. DNA is also present between nucleosomes. In this way, the nucleosomes and the DNA between them look like “beads on a string”. The fibers consisting of nucleosomes condense into compact forms and get the structure of chromosomes.

Watson-Crick Model of DNA

In 1953, James Watson and Francis Crick proposed the structure for DNA. According to the Watson - Crick Model, a DNA molecules consists of two polynucleotide strands. These strands are coiled around each other in the form of a double helix. There is a phosphate-sugar backbone on the outside of double helix, and the nitrogenous bases are on the inside. In double helix, the nitrogenous bases of opposite nucleotides form pairs through hydrogen bonds. This pairing is very specific. The nitrogenous base adenine of one nucleotide forms pair with the thymine of opposing nucleotide, while cytosine forms pair with guanine. There are two hydrogen bonds between adenine and thymine while there are three hydrogen bonds between cytosine and guanine.

DNA Replication

DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. During replication, the DNA double helix is unwound and the two strands are separated, much like the two sides of a zipper. Each strand acts as a template to produce another strand. Its N (nitrogenous) bases make pairs with the N-bases of new nucleotides. In this way, both template strands make new polynucleotide strands in front of them. Each template and its new strand together then form a new DNA double helix, identical to the original.

Central Dogma

The central dogma of molecular biology describes the two-step process, transcription and translation, by which the information in genes flows into proteins: DNA → RNA → protein.

Transcription

Transcription is the first step of gene expression, in which a particular segment of DNA is copied into RNA (messenger RNA - mRNA) by the enzyme RNA polymerase.

Translation

Translation is the cellular process in which proteins are produced by decoding, or translating, particular genetic information of the DNA using a messenger RNA (mRNA) intermediate as the template. The mRNA carries the sequence of its nucleotides to ribosome. The ribosome reads this sequence and joins specific amino acids, according to it, to form protein. This step is known as translation.

Loci

The locations or positions of genes on chromosomes are known as loci (Singular, locus).

Alleles

Individual carries at least one pair of gene for each trait. Both members of a gene pair can be same in some individuals and different in others. It means that a gene exists in more than one alternate forms. The alternate forms of a gene are called alleles.

Genotype

The specific combination of genes in an individual is known as genotype.

Homozygous Genotype

The genotype in which the gene pair contains two identical alleles (AA or aa), is called homozygous genotype.

Heterozygous Genotype

The genotype in which the gene pair contains two different alleles (Aa), is called heterozygous genotype.

Dominant Allele

When a gene pair is in a heterozygous condition one allele masks or prevents the expression of the other, it is called the dominant allele. A dominant allele only suppresses the expression of recessive allele. It does not affect its nature. Dominant alleles are represented by capital letters.

Recessive Allele

The allele which is not expressed is called recessive allele. These are represented by lower case letters.

Phenotype

The expression of the genotype in the form of trait.

Mendel’s organism selection for genetic experiments

He argued that an organism for genetic experiments should have the following features:

There should be a number of different traits that can be studied.

The organism should have contrasting traits e.g. for the trait of height there should be only two very different phenotypes i.e. tallness and shortness.

The organism (if it is a plant) should be self-fertilizing but cross fertilization should also possible.

The organism should have a short life but fast life cycle.

True Breeding

The individual with homozygous genotype.

Monohybrid Cross

A cross in which only one trait is studied at a time, is called as a monohybrid cross.

Dihybrid Cross

A cross in which two contrasting traits are studied at a time, is called as a dihybrid cross.

P1 Generation

The parental generation is denoted as P1 generation.

F1 Generation

The offspring of P1 generation are F1 generation (first filial).

F2 Generation

The cross in F1 generation produces F2 generation (second filial).

Mendel’s Law of Segregation

Mendel's first law (also called the law of segregation) states that during the formation of reproductive cells (gametes), pairs of hereditary factors (genes) for a specific trait separate so that offspring receive one factor from each parent.

Or in other words:

Mendel's law of segregation, states that allele pairs separate or segregate during gamete formation, and randomly unite at fertilization.

Mendel’s Law of Independent Assortment

It states that: “The alleles of a gene pair segregates (get separated and distributed to gametes) independently from the alleles of other gene pairs”.

Punnett Square

The Punnett square is a diagram that is used to predict an outcome of a particular cross or breeding experiment. It is named after R. C. Punnett (an English mathematician). The gametes of both parents having all possible genetic set-ups are determined. A checker board is used to cross all the possible gametes of one parent with all the gametes of other parent.

Co-dominance

A form of dominance in which the alleles of a gene pair in a heterozygote are completely expressed thereby resulting in offspring with a phenotype that is neither dominant nor recessive. As a result, the heterozygous organism shows a phenotype that is different from both homozygous parents. Example of co-dominance is the expression of human blood group AB.

In-complete Dominance

Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele, both the alleles express as a blend (mixture). This results in a third phenotype in which the expressed physical trait is a combination of the phenotypes of both alleles. Example of in-complete dominance is Four O Clock plant, in which a true breeding red flower plant (RR) and a true breeding white flower plant (rr) produces heterozygous plants of pink flowers (Rr).

Variation

Variation, in biology, any difference between cells, individual organisms, or groups of organisms of any species caused either by genetic differences (genotypic variation) or by the effect of environmental factors on the expression of the genetic potentials (phenotypic variation). Variation may be shown in physical appearance, metabolism, fertility, mode of reproduction, behavior, learning and mental ability, and other obvious or measurable characters.

Main sources of variations

Main sources of variations in sexually reproducing populations are described below:

•            The genetic recombination produced through crossing over results in gametes with variations.
•         Mutations (changes in DNA) are important source of variations. Mutation also happen during gametes formation through meiosis.
•      During fertilization, one of the millions of sperms combines with a single egg. The chance involved in this combination also act as the source of variations.
•      Gene flow i.e. movement of genes from one population to another is also an important source of variations.

Discontinuous Variations

Discontinuous variations show distinct phenotypes. The phenotypes of such variations cannot be measured. Blood groups are good example of such variations. In a human population, an individual has one of the four distinct phenotypes (blood groups) and cannot have in between. Discontinuous variations are controlled by the alleles of a single gene pair. The environment has little effect on this type of variations.

Continuous Variations

In continuous variations, the phenotypes show a complete range of measurements from one extreme to the other. Height, weight, feet size, intelligence etc. are example of continuous variations. Continuous variations are controlled by many genes and are often affected by environmental factors.

Organic Evolution / Biological Evolution

Organic evolution (biological evolution) is the change in the characteristics of a population or species of organisms over the course of generations. The evolutionary changes are always inheritable. The changes in an individual are not considered as evolution, because evolution refers to populations and not to individuals. Organic evolution included two major processes:

·         Alternation in genetic characteristics (traits) of a type of organism over time; and

·         Creation of new types of organisms from a single type.

Theory of Special Creation

The anti-evolution ideas support that all living things had been created in their current form only a few thousand years ago. It is known as the “theory of special creation”.

Natural Selection

Natural selection is the process by which better genetic variations become more common in successive generations of a population. The central concept of natural selection is the evolutionary fitness of an organism. Fitness means an organism’s ability to survive and reproduce. Favorable variations are selected for their transmission to next generations, while the unfavorable variations are selected against their transmission to next generations.

Artificial Selection

Artificial selection (or selective breeding) means intentional breeding between individuals for certain traits, or combination of traits. Selective breeding has revolutionized agricultural and livestock production throughout the world. In artificial selection. The bred animals are known as breeds, while bred plants are known as variations or cultivars.