TO BE WRITTEN IN THE NOTE BOOK HEREDITY AND EVOLUTION 1. How do Mendel’s experiments show that traits may be dominant or recessive? MONOHYBRID CROSS PARENTAL CROSS F1 CROSS F2 Generation Phenotype- 3 Tall : 1 dwarf Ratio of Monohybrid cross - 3:1 2. How do Mendels experiments show that traits are inherited independently? ✓ When Mendel first crossed pure bred pea plant having round green seed with pure bred pea plant having wrinkled yellow seed, he found that only round yellow seeds were produced in the first generation (F1). ✓ No wrinkled green seeds were obtained in the F1generation . ✓ From this it was concluded that round shape and yellow coloured seeds were dominant over the wrinkled shape and green coloured seeds. ✓ When the F1 generation pea plants having round yellow seeds were cross bred by self pollination , then 4 types of seeds having different combinations of shape and colour were obtained in the second generation (F2). ✓ These were round yellow , round green , wrinkled green and wrinkled yellow. ✓ Such a cross is known as dihybrid cross and two sets of contrasting characters were considered . ✓ 9→ Round, Yellow ✓ 3→Round green ✓ 3→ wrinkled Yellow ✓ 1→wrinkled green ✓ Thus , the ratio of each phenotype of the seeds in the F2 Generation is 9:3:3:1. ✓ Mendel observed that round yellow and wrinkled green are two new combinations of characteristics that appeared in the F2 generation. ✓ He concluded that though the two pairs of original characteristics combine in the F1 generation , they get separated and behave independently in the subsequent generations. DIHYBRID CROSS Parental Cross Parental phenotype F1 Generation phenotype→ Round Yellow x wrinkled green ALL ROUND AND YELLOW SEEDS F1 CROSS F1 Generation phenotype Round Yellow x F2 Generation ( self cross) Round Yellow PUNNETT SQUARE F2 Generation phenotype 9→Round Yellow 3→Round green 3→wrinkled Yellow 1→wrinkled green Ratio of dihybrid cross →9:3:3:1 3. 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? ✓ In asexual reproduction, the reproducing cells produce a copy of their DNA by mitotic division. ✓ However, this copying of DNA is not accurate and therefore, the newly formed DNA has some variations. ✓ It can be easily observed in the above figure that in asexual reproduction, very few variations are allowed. ✓ Therefore, if a trait is present in only 10% of the population, it is more likely that the trait has arisen recently. ✓ Hence, it can be concluded that trait B that exists in 60% of the same population has arisen earlier than trait A. 4. How does the creation of variations in a species promote survival? ✓ Sometimes for a species, the environmental conditions change so drastically that their survival becomes difficult. ✓ For example, if the temperature of water increases suddenly, most of the bacteria living in that water would die. ✓ Only few variants resistant to heat would be able to survive. ✓ If these variants were not there, then the entire species of bacteria would have been destroyed. ✓ Thus, these variants help in the survival of the species. ✓ However, not all variations are useful. Therefore, these are not necessarily beneficial for the individual organisms. 5. Explain how sexual reproduction gives rise to more viable variation than asexual reproduction. How does this affect the evolution of those organisms that reproduce asexually? ✓ In asexual reproduction the off springs are almost identical to their parents because they have the same genes as of their single parents. ✓ Thus much genetic variation is not possible in asexual reproduction. ✓ In sexual reproduction, the off springs although similar to their parents are not identical to them or to one another. ✓ This is because the off springs receive some genes from the mother and some from the father. ✓ Due to the mixing of genes of the mother and the father in different combinations, all the off springs will exhibit genetic variation. ✓ In this way sexual reproduction leads to a greater variation in the population. ✓ Thus genetic variations lead to the continuous evolution of various species to form better and still better organisms. 6. Give one example of each of the characters that are inherited and the ones that are acquired in humans. Mention the difference between the inherited and the acquired characters. ✓ Inherited Trait: Fused and Free ear lobes. ✓ Acquired Trait: Muscular body of a wrestler ✓ Difference: Acquired trait develops during the life time of an individual which affects somatic parts and dies with the death of the individual. Inherited trait is obtained from the parents, influences genes or germ cells and is passed on to the next generation. 7. The sex of a newborn child is a matter of chance and none of the parents may be considered responsible for it.” Justify this statement with the help of a flow chart showing the determination of sex of a newborn. ✓ Sex determination flow chart: ✓ A male has one X-chromosome and one Y-chromosome. Thus half the male gametes have X- chromosomes and the other half have Ychromosomes. ✓ A female has two X-chromosomes. Thus all female gametes have only X-chromosomes. ✓ If a sperm carrying Y-chromosome fertilises an ovum carrying Xchromosome, then the child born will be a boy. ✓ If a sperm carrying X-chromosome fertilises an ovum carrying Xchromosome, then the child born will be a girl. ✓ Therefore it is the sperm from the father which determines the sex of the child. ✓ Thus in human beings, the sex of the baby is determined by the type of 8. ✓ 9. ✓ ✓ ✓ ✓ ✓ sperm that fuses with ovum. ✓ As human male produces two types of sperms in equal proportion, there are 50% chances of a male baby and 50% chances of a female baby. ✓ Sex of the child cannot be changed afterwards by any method. ✓ It is determined at the time of conception. In the following Questions, an Assertion and Reason have been put forward. Read the statements carefully and choose the correct alternative from the following: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion. (b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion. (c) Assertion is true but the Reason is false. (d) The statement of the Assertion is false but the Reason is true. Assertion: When pure bred tall plants are crossed with pure bred short plants, all the plants in F1 progeny are tall. When the tall plants of F1 progeny are crossed, short plants re-appear in F2 progeny. Reason: Traits are independently inherited. (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion. Akshat and his wife have attached earlobe (recessive trait) and are professional dancers. They told their colleagues that their offspring would also have attached earlobe and will be a good dancer. Is their notion right? Support your answer with suitable reasons. Attached earlobe or free earlobe is an inherited trait. Also, both parents have attached earlobe which is a recessive trait, so the progeny produced will have attached earlobe. But, the ability to dance or being a good dancer is an acquired trait which an individual acquires during its lifetime. So, there is no certainty that the offspring produced will be a good dancer or not. Therefore, the notion they perceive is not right. ____________________________________________________________
