Eyes and Brows

Topicwise Answers

Page No. 143 & 147

1. If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?

Ans: Trait B as it is present in more members of the population. It must have arisen earlier and has now spread to 60% of the population. whereas trait A is new and has spread to only 10% of the population.


2. How does the creation of variations in a species promote survival?

Ans: The variations ensure that the survival of the species as they allow the members of the population to adapt to the changing environment and conditions. The species are more suitable to the changing environment. Thus continuity of the species is maintained.

For example, bacteria variants which can withstand tolerate heat will survive better in a heatwave than variants which cannot heatwave. It proves that the creation of variations in a species promotes survival.


1. How do Mendel’s experiments show that traits may be dominant or recessive?

Ans: The factor which expresses itself over the other is called a dominant trait/factor.

Example: In the F2 generation of pea plants, the factor for tallness (T) is expressed in 75% individuals is called as dominant factor.


The factor which remains hidden or concealed in the generations is called recessive trait/factor.

Example: In the F2 generation, the factor for dwarfness (t) is expressed in 25% individuals which are remained hidden so it is called a recessive factor.

2. How do Mendel’s experiments show that traits are inherited independently?

Ans: According to the law of independent assortment “When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters”. Mendel took two pairs of alternative expressions of two traits of a pea plant and crossed them. The F1 progeny showed only the dominant two characteristics among each pair. The F2 progeny had phenotypes similar to parents but also new phenotypes that did not exist in the parents. This indicates that pairs of alternate characteristics behave independently of the other pair and are thus inherited independently of each other.

3. A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits – blood group A or O – is dominant? Why or why not?

Ans: The given information is not enough to tell us which of the traits - blood group A or O is dominant. In blood heredity, blood Type A is always dominant and blood Type O is always recessive. Here, the father’s Blood group can be AA (homozygous) or AO (heterozygous) genotypically, whereas that of the mother can be OA or OO. For daughter to be born with blood group O, she must receive O type gene one each from the father and mother. For this father must have a heterozygous AO blood group and the mother must have a homozygous blood group OO.


4. How is the sex of the child determined in human beings?


  • Humans show XY type of sex determination mechanism.

  • Out of 23 pairs of chromosomes, 22 are autosomes (same in both males and females).

  • Females have a pair of X chromosomes.

  • Males have an X and a Y chromosome.

  • During spermatogenesis Males produce two types of gametes with equal probability - sperm carrying either X or Y chromosome.

  • During oogenesis females produce only one type of gamete having X chromosome.

  • An ovum fertilized by the sperm carrying X-chromosome develops into a female (XX) and an ovum fertilized by the sperm carrying Y-chromosome develops into a male(XY).


Page No. 150

1. What are the different ways in which individuals with a particular trait may increase in a population?

Ans: Individuals with a particular trait may increase in population through; Natural selection - Variations brought during the course of time. Genetic drift - Accidental changes in the frequency of some genes in a population. Mutation - A sudden change in the genetic material of an organism in a population.

2. Why are traits acquired during the lifetime of an individual not inherited?

Ans: The traits acquired during the lifetime are changes in the non-reproductive cells of the organism and are not capable of being inherited onto the next generation. Changes that occur in the DNA of germ cells (sex cells) are only inherited.


3. Why are the small numbers of surviving tigers a cause of worry from the point of view of genetics?

Ans: The tigers have very less variation in genetic characteristics among each other and if the natural conditions would change drastically no tiger may survive. For example, if a deadly disease was to infect the tigers, all the tigers may die from it as there would be less variation in the genotype to survive the disease. A decrease in the population of tigers also indicates that tiger variants are not adapted to the existing environment and extinct soon.


Page No. 151

1. What factors could lead to the rise of a new species?

Ans: The main factors which lead to the rise of new species are natural selection, genetic drift, geographical isolation, and mutation. In speciation, a single evolutionary lineage gets split into two or more genetically independent lineages due to the actions of several environmental factors.


2. Will geographical isolation be a major factor in the speciation of a self-pollinating plant species? Why or why not?

Ans: In a self-pollinating plant species, geographical isolation cannot be a major factor in speciation because no new trait can become a part of the genetic makeup in a self-pollination plant species. However, there are some chances of some environmental changes which might lead to some variations.


3. Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually? Why or why not?

Ans: No, asexually reproducing organisms have very little variation over generations. Any change resulting from geographical isolation may not be passed on to to the next generation. As these variations in DNA are not enough to rise and new species.


Page No. 156

1. Give an example of characteristics being used to determine how close two species are in evolutionary terms.

Ans: Homologous organ and analogous organ are the characteristics that can be used to find how close two species are in evolutionary terms. For example, the structure of forelimb in a frog, lizard, bird and horse is similar with similar bones which are modified to suit the function they perform. from this, it can be safely assumed that they may have evolved from a common ancestor.


2. Can the wing of a butterfly and the wing of a bat be considered homologous organs? Why or why not?

Ans: The wing of a butterfly and the wing of a bat are used for the same function of flying. However, the wing of the butterfly is anatomically very different from wing of the bat which has bones and skin. Thus they are not homologous organs.


3. What are fossils? What do they tell us about the process of evolution?

Ans: Fossils are preserved traces of animals or plants of a past geologic age that has been embedded in Earth’s crust. Fossils tell about the organisms that lived long ago. They tell about the structure of such organisms and the time period during which they lived. The fossils found in the upper strata of rocks are considered to be newer than those found in the lower strata. It is also seen from the fossils that the organisms become more complex in the upper strata when compared to the lower strata which indicate towards the evolutionary process. Also, transitional fossils like Archaeopteryx are known which were intermediaries between reptiles and birds.


Page No. 158

1. Why are human beings who look so different from each other in terms of size, colour and looks said to belong to the same species?

Ans: Human beings who look so different from each other in terms of size, colour and looks are said to belong to the same species because:

  • They have similarities in the DNA sequences.

  • They have descended from the same ancestor.

  • They can reproduce among themselves.


2. In evolutionary terms, can we say which among bacteria, spiders, fish and chimpanzees have a ‘better’ body design? Why or why not?

Ans: No, we cannot say that there is a better body design as these organisms evolved according to their needs to survive in the environment. If a chimpanzee has strong limbs capable of multiple actions, the bacteria can survive in extreme conditions where other organisms cannot. Hence there is no better body design.