Mapping Our Genes – Maharashtra Board Class 10 Solutions for Science and Technology (English Medium)
- In asexual mode of reproduction the offspring are with minor differences.
- Both the parents contribute equal amount of genetic material to the offspring.
- Dominant character masks the recessive characters.
- Selection by nature is not deliberate but natural.
- In sexual mode of reproduction, two parents are involved.
- Each parent has its own set of DNA copies that are passed on to the offspring through their nuclei at the time of fertilisation of gametes.
- This gives rise to various combinations of father’s genes and mother’s genes in the offspring. These differences in genes lead to variations in the offspring.
- At the time of gamete formation, the process of meiosis also results in variations.
- Hence, in sexual mode of reproduction greater diversities are generated.
- A phenotype is the appearance or any detectable characteristic feature of an individual. Example: Red and white flowers.
- A genotype refers to the genetic composition of an individual. Examples: RR, Rr, rr.
- Phenotypically, all individuals can look similar, but genotypically, some may be homozygous and some may be heterozygous.
- Example: In Mendel’s experiments on pea plant, the monohybrid phenotypic ratio in the F2 generation was 3:1, while the genotypic ratio was 1:2:1. This means phenotypically there were two types of F2 plants – red and white. However, genotypically there were three types of plants, homozygous dominant (RR), heterozygous (Rr) and homozygous recessive (rr).
- Therefore, phenotypic and genotypic ratios are different.
- In human beings, there are 22 pairs of autosomes and 1 pair of sex chromosomes.
- Human males produce two types of gametes, one with 22 autosomes and a single X chromosome and the other with 22 autosomes and a single Y chromosome.
- Human females produce the same type of gamete with 22 autosomes and a single X chromosome.
- The sex of the baby will depend on the kind of sperm which fertilises the egg.
- If X-bearing sperm fertilises the egg, a female child is conceived. If a Y-bearing sperm fertilises the egg, a male child is conceived.
- Therefore, in human beings, the gametes from the male parent decide the sex of the baby.
- Fossils are collected from different levels of depths while excavating the Earth.
- A systematic study of fossils revealed the order in which the animals appeared on the Earth.
- Deepest layers of the Earth will have the fossils of older animals, while the layers above them will have fossils of animals which are comparatively buried recently.
- Because the fossils of invertebrates were buried deep down as compared to the fossils of vertebrates, it suggests that invertebrates came into existence before vertebrates.
Tallness is a dominant character, while dwarfness is a recessive character. Similarly, red flower colour is a dominant trait, while white flower colour is a recessive trait.
Therefore, the gene combinations responsible for the characters detailed above are
Tallness = TT
Dwarfness = tt
Red flowers = RR
White flowers = rr
1. P generation = TTRR (Tall and red), ttrr (dwarf and white)
Gametes produced by parents = TR × tr
2. F1 generation = TtRr (Tall red)
In the F1 generation, all the plants will be with the genotype TtRr and their phenotype will be tall red.
3. F2 generation = Cross between TtRr × TtRr
In the F2 generation, the ratio obtained was 9:3:3:1 in the following proportion:
9 (Tall red) : 3 (Tall white) : 3 (Dwarf red) : 1 (Dwarf white)
In the F2 generation:
Phenotypic ratio: 9:3:3:1
Genotypic ratio: 1:1:2:2:4:2:2:1:1
Darwin’s theory of evolution
- Darwin’s theory of evolution is based on natural selection.
- It is also known as the theory of natural selection as during evolution only those characteristics are selected which contribute to the survival of a species.
- Darwin suggested that only the fittest organisms survive in the long run. Plants and animals which are not fit for the changing environment eventually die.
- Fit species reproduce in large numbers and pass on their genes to the next generation. In the struggle for existence, only those organisms survive which have characters useful for their survival. Others with unfavourable characters perish during the process of evolution.
- The process of natural selection takes place for several generations. As a result, the species which are better adapted for survival are selected. The individuals of these species may be different from the original ones. If this changed species shows successful adaptation and growth in the given environment, then it gets evolved. Thus, new species are developed.
- The process of natural selection is not deliberate but natural and gradual.
- Darwin’s theory, however, failed to explain how the useful characters appear only in some individuals of the next generation.
- Despite this drawback, Darwin’s theory is an universally accepted theory of evolution.