Heredity And Evolution
Heredity is a process in which organisms acquire characteristics from their parents. These characteristics are called traits. Every individual is unique because they have a unique set of traits. The traits which are transmitted by the parent to its offspring during the process of fertilization are inherited traits. This inheritance is determined by certain rules of heredity. Inherited traits are coded in our DNA and hence can be passed on to the next generation. Example: eye colour, height, complexion, hair colour etc.
The variations that emerge as a result of reproduction may be inherited which causes an increase in the survival rate of entities. For the same trait, there are two copies of genes in individuals that reproduce sexually. In case the copies are not identical to each other, the characteristic or the trait, which is expressed phenotypically is referred to as the dominant trait while the other is referred to as the recessive trait. These variations observed in the species can either bestow survival benefits or offer to the genetic drift.
However, if there are alterations in the non-reproductive tissues that are due to environmental factors, these are not inherited. If the variation is linked with geographical isolation, then there are chances of speciation taking place. Evolutionary dynamics are evident in organisms when they are classified. Not only living species, but even the study of fossil fuels can help in the study of evolution. The evolution of complex organs can be attributed to the survival benefits that occur at the intermediate stages. During the event of evolution, features or organs can be modified and adapted to perform new functions such as the evolution of feathers from providing warmth to aiding birds in taking a flight. Hence, evolution cannot be merely termed as progress to higher forms or lower forms, instead, it appears to have provided with complicated body structures and designs simultaneously when simpler structures are still thriving.
Acquired traits, on the other hand, are characteristics that are acquired by an individual in his lifetime. It may be acquired due to his own activities or external influence. Unlike inherited traits, acquired traits cannot be genetically passed on to the next generation. Example, the ability to dance, cook well etc.
Inherited Traits Examples
Though human beings are unique, there are a set of common characteristics that we all share with the members of our family, with our peers etc. We possess a unique set of traits. While some traits are governed by genes which are inherited from parents to the offsprings, there are traits that are acquired through observing, learning, most of which are determined by a combination of environmental factors and genes. Listed below are a few examples:
- Tongue rolling
- Earlobe attachment
- Curly hair
- Hairline shape
- Green/Red Colourblindness
- Hand clasping
Rules For The Inheritance Of Traits – Mendel’s Contributions
Inheritance rules are based on the fact that both the father and the mother equally contribute genetic material to their child, which means every characteristic that is inherited is influenced by the paternal and maternal DNA. Hence, for each trait, there are two versions in each child, one from the father and the other from the mother. Gregor Mendel formulated rules for inheritance after thorough understanding and experimentation.
Mendel experimented using many physical characters of garden peas such as round/wrinkled seeds, white/violet flowers, tall/short plants etc. He took pea plants, with tall and short characteristics to produce progeny by crossing them to arrive at the results. Following are his observations:
- He noticed that there were no midway traits in F1 progeny, no plants with medium height
- All plants produced were tall
- Only one parental trait was observed and not a mixture of two.
- Checked if tall plants in F1 progeny and parent plant are same
- Tested this by getting both the parental plants and F1 tall plants to reproduce through self-pollination.
- Observed, F2 progeny(second-generation) obtained by the above cross, are all not tall, a quarter of them was short
- Deduced both tallness and shortness traits are inherited but only tallness was expressed
- Hence Mendel proposed that genes controlling traits are present in organisms who reproduce sexually.
- This reproduction depends on the parents, it can either be different or identical.
The table below will help you understand Mendel’s law:
T – Tall plant, t – short plant
TT X tt→ Tt (F1 Progeny)
According to this theory, both TT and Tt are tall plants, the only tt is a short plant. It helped him arrive at this conclusion:
- A gene containing a single ‘T’ is enough to make a plant tall
- While for a plant to be short, both it’s traits need to be ‘t’ i.e, ‘tt’
- Dominant traits are traits like ‘T’
- Recessive traits are traits like ‘t’
Mendel also went onto experiment with self and cross-pollination methods by taking tall plants with round seeds and short plants with wrinkled seeds, and all turned out to be tall and round seeded plants, thus stating that these were dominant traits. When F1 progeny was used to generating F2 progeny by self-pollination, he noticed that some F2 progeny are tall and round seeded plants while some are short and wrinkled seeded plants. It interestingly gave rise to new combinations as well such as tall wrinkled seeded plants and short round seeded plants, all these changes happen during zygote formation. He concluded that traits are independently inherited.
How Do Traits Get Expressed?
Information source for making proteins in a cell is Cellular DNA. A section of this DNA provides data for one protein and is called a gene for that protein. These genes control traits. As per the Mendelian law, both parents equally contribute genes and thus the child has a combination of a set of genes from both the parents. Gene set are present as separate independent pieces called chromosomes and not a single long thread of DNA. Thus, each cell will have two copies of the chromosome set, one from each parent. When germ cells combine, they will restore a normal number of chromosomes to ensure the stability of DNA in species.
Sexual reproduction involves the participation of both parents, unlike asexual reproduction. In a few organisms, sex is determined by various factors, physical too. For example, in reptiles, the sex of the newborn depends on the temperature at which eggs fertilize and are kept. Few organisms can change their sex, which means sex is not genetically determined. But in human beings, sex is determined by the genes.
Human beings have 23 pairs of chromosomes all of which are paired except the sex chromosomes which are odd and not perfect. Women have a perfect set of sex chromosomes i.e XX, while men have a normal-sized ‘X’ chromosome and a short-sized ‘Y’ chromosome forming XY chromosome. Hence, all children will inherit ‘X’ chromosome from their mother regardless of them being a girl or a boy. And from their father, they inherit either ‘X’ or ‘Y’. This clearly means the sex of a child is determined by what they inherit from the father.
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