First and Second Laws of Inheritance
* Gregor Johann Mendel carried out several hybridization experiments with true-breeding pea lines which had sharply contrasting characters.
* A monohybrid cross is the study of inheritance of one pair of contrasting characters.
* The Law of Dominance states that in a monohybrid, pure line cross between parents with contrasting traits, the dominant from appears in the F1 generation and both forms appear in a ratio of 3:1 in the F2 generation.
* The Law of Segregation states that alleles do not show any blending but segregate from each other during gamete formation into different gametes.
* Punnett Square is a graphical representation that helps calculate the probability of all possible genotypes of the progeny in a genetic cross.
* A test cross is a cross between individuals if the F1 generation and a homozygous recessive parent to determine the genotype of an individual.
Chromosomal Disorders
* Chromosomal disorders are caused due to changes in chromosomal number or chromosomal structure.
* Changes in chromosomal number occur due to non-disjunction of chromosomes or failure of chromatids to disjoin during cell-division.
* Aneuploidy is a condition where one or more chromosomes are either gained or lost.
* Euploidy is a ployploidic condition where more than two haploid sets of chromosomes are formed due to the failure of cytokinesis after the telophase stage.
* Some chromosomal disorders that occur due to changes in chromosomal number include Down’s Syndrome, Klinefelter’s Syndrome and Turner’s Syndrome.
* Some of the structural changes in chromosomes include deletion, duplication and translocation.
* Structural changes in a chromosome are inherited or occur accidentally when reproductive cells are being formed.
Incomplete Dominance
* Mendel’s laws were not universal in occurrence.
* The phenomenon where a dominant allele does not completely mask the phenotypic expression of the recessive allele in a heterozygote, resulting in a blending of both dominant and recessive traits in the F1 and F2 heterozygotes, is called incomplete dominance.
* In incomplete dominance, both the genotypic and phenotypic ratios are 1:2:1.
* Incomplete dominance causes a distortion of the normal phenotypic ratio.
Co- Dominance
* Co-dominance is a situation where a heterozygous organisms has a phenotype that demonstrates traits from both dominant as well as recessive genes.
* A co-dominant trait is not blended but is independently and equally expressed.
* According to the multiple alleles concept, more than two alleles govern the same character.
* Dominance is not an autonomous feature of a gene or gene product.
* Dominance, incomplete dominance or co-dominance depends on gene product and the production of a particular phenotype from this product.
* Dominance, incomplete dominance or co-dominance also depends on the phenotype that we wish to examine in case the same gene influences more than one phenotype.
Dihybrid Cross
* A cross between plants that differ in two traits or characters is known as a dihybrid cross.
* The Law of Independent Assortment states that when two pairs of traits or characters are united in a hybridization experiment, the segregation of one pair of characters is independent of the other pair of characters.
* In a dihybrid cross, the genes assort independently.
Chromosomal Theory of Inheritance
* Mendel’s work did not receive due recognition until the beginning of the 20th century.
* Theodore Boveri and Walter Sutton’s hypothesis came to be known as the BoveriSutton Chromosome Theory.
* Both Sutton and Boveri were of the opinion that pairing and separation of a pair of chromosomes would result in the segregation of a pair of genes or factors carried by them.
* Sutton combined the hypothesis of chromosomal segregation and Mendelian principles and called it the Chromosomal Theory of Inheritance.
Linkage, Crossing-Over and Recombination
* Thomas Hunt Morgan and his colleagues tried to verify the Chromosomal Theory of Inheritance through various experiments.
* For two genes lying on the same chromosome, the proportion of parental gene combinations is much higher than non-parental gene combinations or recombinant types.
* Genes that are located on the same chromosome and that are inherited together are known as linked genes.
* The process where chromatids of a homologous pair exchange segments between them is known as crossing-over.
* The likelihood of two genes separating during crossing-over is proportional to the distance between them.
* Morgan and his group explored the arrangement of genes on a chromosome and also discovered the exception to Mendel’s Law of Independent Assortment, when linked genes are considered.
Sex Determination
* Sex-determination chromosome are called sex chromosome while the rest of the chromosomes are called autosomes.
* Of the 23 pairs of chromosomes in human beings, just one pair is the sex chromosome while the remaining 22 pairs are autosomes.
* The very first indication that sex chromosomes were different from autosomes came from experiments conducted by German biologist Hermann Henking in 1891.
* The four important types of sex-determination systems are the XX-XY type, XX-XO type, ZW-ZZ type and the ZO-ZZ type.
* A system of sex determination where the genetic make-up of the sperm determines the sex of the offspring is known as male heterogamety.
* A system of sex determination where the genetic make-up of the ovum determines the sex of the offspring is known as female heterogamety.
Pedigree Analysis and Mutation
* A hereditary disorder is a condition that is genetically passed on to one’s offspring.
* Pedigree analysis plays a very important role in tracing the inheritance of abnormal traits.
* A pedigree analysis involves using a pedigree chart with standard symbols to represent the inheritance of a particular trait.
* Mutation in genetic material leads to changes in the genotype and phenotype of an organism.
* Physical and chemical factors responsible for changing the genetic structure or for inducing mutations are known as mutagens.
Mendelian Disorders
* Genetic disorders are of two types, Mendelian disorders and chromosomal disorders.
* Mendelian disorders include genetic disorders caused by alterations or mutations in a single gene.
* Chromosomal disorders include genetic disorders caused by an excess, absence or abnormal arrangement of chromosomes.
* Mendelian disorders are caused due to mutations in normal genes, where the genes follow the Law of Mendelian Inheritance.
* Cystic fibrosis, haemophilia, thalassaemia, sickle cell anaemia and phenylketonuria are some Mendelian disorders.
Introduction to Genetics
* Inheritance is the process of passing characters from parents to offspring.
* Variation is the degree by which an offspring differs from its parents.
* Advances in field of genetics were made by Gregor Johann Mendel in the mid-19th century.
* Gregor Mendel proposed several laws of inheritance by conducting extensive research and hybridisation experiments on garden peas for about seven years.
* Hybridisation is the process of crossing two individual differing in at least one character, resulting in a hybrid individual.