Genetics is a very confusing BUT a very IMPORTANT part of the NEET syllabus. This article (and the next three) will clear your basics and help you understand (and retain) all the information about genetics that you need to know!
Genetics: The branch of biology that deals with inheritance of genes.
Inheritance: The process by which traits are passed from parent to progeny.
Traits: Characteristics of organism, either Dominant or Recessive.
eg: seed shape (round or wrinkled)
seed colour (yellow or green)
Dominant and Recessive: The character which appears in F1 generation is called dominant. The alternative character that fails to show itself in the presence of dominant gene called recessive.
Variation : This refers to differences among organisms of the same species due to the differences in the genes they inherit and the environment they survive in. There are two main types of variation ,namely, Environmental variation and Genetic variation.
About Gregor Johann Mendel: Father of Genetics.
Mendel was born on 22nd July, an Austrian monk. He concluded hybridisation experiments on fruits, trees, flowers, vegetables, and more on garden peas. He began his scientific investigation on inheritance in 1856 but his work was recognised only in 1900.
Mendel studied the genetics of pea plants, and he traced the inheritance of a variety of characteristics. To do so, he started by crossing pure-breeding parent plants with different forms of a characteristic, such as violet and white flowers.
The traits that Mendel studied are listed below:
1.Form of ripe seed (R) – Round or wrinkled
2.Color of seed albumen (Y) – yellow or green
3.Color of flower (P) – purple or white
4.Form of ripe pods (I) – inflated or constricted
5.Color of unripe pods (G) – green or yellow
6.Position of flowers (A) – axial or terminal
7.Length of stem (T) – tall or dwarf
In the parental, or P generation, Mendel crossed a pure-breeding violet-flowered plant to a pure-breeding white-flowered plant. When he gathered and planted the seeds produced in this cross, Mendel found that 100 percent of the plants in the next generation, or F1 generation, had violet flowers.
Results: The white flower trait had completely disappeared. He called the trait that was visible in the F1 generation (violet flowers) the dominant trait, and the trait that was hidden or lost (white flowers) the recessive trait.
Mendel then let the F1 plants self-fertilise. Among their offspring, called the F2 generation, he found a ratio of 3.15 violet flowers to one white flower, or approximately 3:1. For the other six characteristics that Mendel examined, both the F1 and the F2 generations behaved in the same way they did for flower colour. One of the two traits would disappear completely from the generation, only to reappear in the generation in a ratio of roughly 3:1
Why did Mendel choose the Garden Pea for his experiments ?
1.Its life cycle is comparatively short.
2.Plants are annual and easy to cultivate.
3.Peas have many distinct, well-defined and easily observable morphological characteristics (traits).
4.Flowers are bisexual and naturally self-fertilizing, but they can also be easily cross-fertilized.
5.The offspring of cross-fertilized plants are fertile.
Genes and alleles: Genes are the unit of inheritance. They contain the information required to express a trait. Alleles are contrasting traits in a gene. Eg: The gene that codes for eye colour is B, then the alleles may be Black (B) or Blue (b).
The set of alleles carried by an organism is known as its genotype.
Genotype determines phenotype, an organism’s observable features.
When an organism has two copies of the same allele (say, YY or yy), it is said to be homozygous for that gene.
If, instead, it has two different copies (like Yy), we can say it is heterozygous.
True breeding line: breeding line that having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations.
Polyhybrid Cross: The cross which involves more han two pairs of contrsting characters is called polyhybrid cross.
Reciprocal Cross: A pair of crosses between a male of one strain and a female of another, and vice versa is called reciprocal cross.
Mendel’s Laws of Inheritance:
Law I: Law of Dominance: It states that when two alleles of the same gene come together after fertilisation, only the droinant trait is expressed. In his garden pea experiment, Mendel had taken tallness of the plant as the dominant trait and dwarfness as the recessive trait.
Law II: Law of Segregation: Also called as law of purity of gametes, it states that during the production of gametes the two copies of each hereditary factor segregate so that offspring acquire one factor at random from each parent.
Confirmation of second law :
This technique is called a test cross. In a test cross, the organism with the dominant phenotype is crossed with an organism that is homozygous recessive (e.g., green-seeded):
Law III: Law of Independent Assortment:
Mendel’s law of independent assortment states that the alleles of two (or more) different genes get sorted into gametes independently of one another. In other words, the allele a gamete receives for one gene does not influence the allele received for another gene
Imagine that we cross two pure-breeding pea plants: one with yellow, round seeds (YYRR) and one with green, wrinkled seeds (yyrr). Because each parent is homozygous- the gametes made by the wrinkled, green plant all are ry, and the gametes made by the round, yellow plant are all RY.
That gives us F1 offspring that are all RrYy. There were four different categories of pea seeds: yellow and round, yellow and wrinkled, green and round, and green and wrinkled. These phenotypic categories appeared in a ratio of approximately 9:3:3:1.
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