How is sex determined in organisms?

Different species use very different strategies for determination of sex of new-born individual. In some animals the temperature at which the fertilized eggs are kept determines the sex of the newborn (e.g., crocodiles). In other animals, such as snails, individuals can change sex, indicating that sex is not genetically determined. However, in human beings, the sex of the individual is largely genetically determined.
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Do all variations survive?

No, variations do not survive. Variations that are not suitable or advantageous to the organism in the prevailing environmental conditions are not maintained. Environmental factors act on these variants and the variant most suited to the existing environment would survive.
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What are Mendel’s laws of inheritance?


Mendel's Laws of inheritance are-:
i. Law of dominance: When two homozygous individuals with one or more sets of contrasting characters are crossed the characters that appear in the F1 hybrids are dominant characters.

ii. Law of segregation: Contrasting characters brought together in a hybrid remain together without being contaminated and when gametes are formed from the hybrid the two separate  from each other and only one enters each gamete.

iii. Law of independent assortment: In inheritance of more than one pair of contrasting characters simultaneously, the factors for each pair of characters assort independently of other pairs.

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How does an asexually reproducing organisms produce variations among their progeny? Explain giving example.

In asexually reproducing organisms variation occurs due to inaccuracies in DNA copying at the time of nuclear division.
For example, one bacterium divides, it will give rise to two bacteria. These daughter bacteria would be similar in body design, but with subtle differences.[See Fig.]

In asexually reproducing organisms variation occurs due to inaccuraci
Fig. Appearance of variations over succeeding generations
The resultant bacteria divide again, and each bacterium will give rise to two bacteria in the next generation i.e., there would be four individuals as shown in Fig. (iii). In the bottom row i.e., Fig.(iii) the four individuals will be different from each other.
They may have differences of two types (i) some of the unique type of differences and (ii) differences inherited from, their respective parents, who were different from each other.





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How do traits get expressed?

A trait in an organism results from the action of proteins (e.g., growth hormone in plants is a protein which is manufactured from a gene). Each protein is made by information from the genes for that protein. In the example of pea plant, there are two genes for plant height–‘T’ and ‘t’ ‘T’ is dominant over ‘t’. In a heterozygous plant (Tt), only the gene ‘T’ will be able to make proteins as it is dominant over the ‘t’ gene. This protein would be the growth hormone which will result in the plant being tall.
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