Heredity and Evolution Revision Notes: Mendel's Contribution in Genetics

Heredity is the transmission of characteristics from one generation to the other. The process is done by genes, which define the characters in the organism. The study of heredity and variation is known as genetics.

Read Also: How do Organisms Reproduce?

Mendel and His Contribution in Genetics

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Gregor Johann Mendel is known as the ‘Father of Genetics’. He worked on Pisum sativum (garden pea plant) to understand his concept of heredity. His work laid the foundation of modern genetics.

He discovered the three basic laws of inheritance :

Note: Allele – each of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.

• Law of dominance – If two alleles at a locus differ, then one, the dominant allele, determines the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance.
• Law of Segregation – The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes.
• Law of Independent Assortment – Each pair of alleles segregates independently of other pairs of alleles during gamete formation.

Dominant traits: In Mendel’s experiment, we see that tall traits in pea plants tend to express more than the short trait. Therefore, the tall trait of the plant is said to be domshort plant over the short trait. The traits that express themselves in an organism in every possible combination and can be seen are called dominant traits.

Recessive traits: A trait that is not expressed in the presence of a dominant allele is known as recessive.

Monohybrid Cross: When one pair of contrasting characters was taken to cross two tea plants, it is known as a monohybrid cross. The ratio of characters, arising out of this cross, at F2 generation is called the monohybrid ratio, which is found to be 3:1. If a tall plant is crossed with a dwarf plant, we get 3 tall plants : 1 short plant at the end of F2 generation. The height of the plant is considered at a time.

Dihybrid Cross: When two pairs of contrasting characters are taken to cross two plants, it is known as a dihybrid cross. The ratio of characters, arising out of this cross, at F2 generation is called the dihybrid ratio, which is found to be 9:3:3:1. If a plant with round and green peas is crossed with a plant with wrinkled and yellow peas, the first generation plants would all have round and green peas.

Sex determination

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The process of determining the sex of an individual, based on the composition of the genetic material is called sex determination. In humans, sex determination happens on the basis of the presence or absence of the Y chromosome. An ovum (X chromosome) on fusion with Y chromosome containing sperm gives birth to a male child and upon fusion with X containing sperm, gives rise to a female child. In different animals, the sex of an embryo is determined by different factors. Different types of sex determination are XX-XY type (humans), XX-XO type( insects), ZW-ZZ type (chicken), and ZO-ZZ type (moths and butterflies).

Evolution:

Evolution is the gradual change in the heritable characteristics of a population over a million years to form a new species.

• J.B Lamarck was the first scientist who gave the theory on evolution. His theory is known as the “theory of inheritance of acquired characters”. The traits that are acquired by an Organism over the period of its lifetime are termed as acquired characters, Whereas the trades that are inherited from the parents are called inherited characters. The theory is based on the following postulates:

1. Living organisms tend to increase in size.
2. New needs lead to the formation of new organs.
3. Continued use of one organ makes it more developed and disuse of an organ leads to its degeneration.
4. New characters are acquired by individuals during their lifetime.

• Charles Darwin’s concept of evolution was based on the idea that new species were formed due to variations that occurred in the organisms Nature played an important role in selecting the organisms having suitable variations. The differences in the DNA sequences among every organism leading to the diverse gene pool are called genetic variations. Charles Darwin is also known as the “Father of Evolution”.

Postulates of Darwin Theory

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1. Speciation (formation of species) - Useful variations from generation to generation gives rise to the formation of new species. Speciation is the process of formation of a new species from existing ones due to several evolutionary forces like genetic drift, isolation of populations, natural selection, etc. Speciation leads to diversity in the ecosystem and diversity leads to evolution.
   1. Gene flow is the transfer of genes from one population to the next.
   2. Random fluctuations in gene variants are seen on many occasions. This phenomenon is known as genetic drift. Thus, genetic drift is a change in the frequency of an existing allele in a small population.
   3. Geographical barriers such as mountains, rivers can also lead to speciation. This is known as geographical isolation.

1. The struggle of existence - due to the multiplication of organisms and limited food and space, there exists competition among the organisms.
2. Survival of the fittest or Natural selection - Nature selects those characteristics or organisms that are useful and are best adapted to the prevailing conditions. The species/organisms which are better adapted to the changing conditions survive and reproduce i.e. selected by nature and species/organisms which cannot adapt perish i.e. rejected by nature. According to Darwin, natural selection also plays an important role in bringing about the evolution of new species of plants and animals. According to him, variations existed between the individuals of a population and some natural phenomena eliminated those individuals which were less adapted. The surviving population would pass the hereditary advantageous features to their offsprings. With time this process would give rise to organisms different from the original population and new species are formed.

Read More About Rules for the Inheritance of Traits - Mendel's Contributions

Evolution and Classification

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• Homologous organs: are the organs evolved from the same ancestors which have the same basic structural design and developmental origin but they have different functions. Example: forelimb of lizard, frog, bird, or human.
• Analogous organs: the organs from different ancestors which have a different basic design and developmental origin but have a similar appearance but have the same function. Example: wings of birds, insects, etc.
• Paleontological (fossil) evidence was given for evolution too. Fossils are dead remnants of the organisms. The study of fossils is important as the complete fossil record of animals like horses, camels, and man has helped us to study the stages of evolution.

Evolution by Stages :

Evolution is a continuous and gradual process, complicated organs did not evolve by a single DNA change but were formed by small changes over generations for example; complex organs like eyes were created by bit by bit changes. The structure of the eye in all organisms is different enough to have evolutionary origins. Some organs even developed for one particular function but later become useful for quite a different function, e.g the feathers developed to provide warmth to the animal but later helped in flight. Dinosaurs had feathers but couldn’t fly but birds made use of their feathers to fly.

Human evolution :

Human evolution has been studied with the help of excavation; time dating and fossil study. All human beings belong to single species i.e. Homo sapiens. Study of human evolution indicates that all humans belong to a single species that evolved in Africa and then spread across the world in places like West Asia, Central Asia, Eurasia, South Asia and East Asia, Indonesia, the Philippines, Australia, and America, forward and backward sometimes separating and sometimes coming back to mix with each other. They had come into being as an accident of evolution.

Stages of human development :