CLASS WORK-3

XD &XH                         

26-12-13

Reproductive processes form new individuals that are similar to the parents but have some differences. The similarities between parents and off springs are due to Heredity and the differences are due to variations.

Variations and their importance- Differences in the traits among individuals belonging to the same species are variations. They are important as-

1)       They enable the organism to adapt and survive   better in an environment.

2)       They get accumulated and lead to formation of new species.

3)       They from the basic cause of evolution.

Mechanism of Heredity:- Deoxyribonucleic acid (DNA) present in the cell is the hereditary material. It is the information source in the cell for making proteins. A part of DNA that provides information for one particular protein is called a gene for that protein. In fact, the genes determine everything about an organism, i.e. they control all the characteristics (traits) of the organism. A dominant factor of allele controls the synthesis of a fully functional structural protein or enzyme to produce its morphological or physiological effect. A recessive factor, on the other hand, is able to contribute partially and thus produces defective, incomplete and less efficient structural protein or enzyme it is, therefore, unable to express its effect in the presence of dominant factor. Effect of recessive factor only becomes apparent when it occurs in a pair.

e.g. height of a plant depends upon the growth hormone which is in turn controlled by the gene. If the gene is efficient and more growth hormone is secreted the plant will grow tall. If the gene for that particular protein gets altered and less of it is secreted then the plant will remain short.

Contribution of Genetic material by Both the Parents:- According to Mendel’s interpretations, both parents contributes equally to the DNA of the progeny during sexual reproduction. Also both parents help in determining the traits in the progeny. Genes occur in a number of chromosomes. The number of chromosomes in every organisms is fixed. These exist in pairs. Each cell has two copies of each chromosome (homologous chromosomes), one each from the male and female parents. When the germ cells are formed, every germ cell receives one chromosome from each pair of chromosomes and these may be of either maternal of paternal origin. When male and female germ cells combine during fertilization, normal number of chromosomes in the progeny is restored ensuring the stability of the DNA of the species.

The mechanism, by which the sex of an individual is determined as it begins life, is called sex determination.

Genetic or Chromosomal Determination of Sex

In diploid organisms having separate sexes, a specific pair of chromosomes in each diploid cell determines the sex of the individual. They are called sex chromosomes. All other chromosomes are termed autosomes as these have genes which control the somatic (body) characters. The two members of each pair of homologous autosomes are similar in size and shape, but this may not be true of sex chromosomes. For instance, in human beings, there are 46 chromosomes, Of these, 44 (22 pairs) are autosomes and 2 (one pair) are sex chromosomes, The sex chromosomes in human beings and also in fruitfly (Drosophila), are of two types – X chromosome and Y chromosome.

(i)    A male individual contains one X chromosome and one Y chromosome i.e. XY (heteromorphic sex chromosomes having different size and shape). Male produces two different kinds of gametes (sperms); half of the gametes having X chromosome and other half having Y chromosomes. Therefore, male is called heterogametic.

(ii)   A female individual contains two similar X chromosomes i.e. XX (homomorphic sex chromosomes of same size and shape). Female, therefore, produces same type of all gametes (ova or eggs). So female is called homogametic.

The sex of the child is determined at the time of fertilization when male and female gametes fuse to form zygote.

If a sperm (male gamete) carrying X chromosome fertilizes an egg or ovum (female gamete) carrying X chromosome, then the offspring will be a girl (female). This is because the offspring will have XX combination of sex chromosomes

If a sperm (male gamete) carrying Y chromosome fertilizes an egg or ovum (female gamete) which has X chromosome, then the offspring will be a boy (male). This is because the offspring will have XY combination of sex chromosomes

The sex of the child (offspring) is determined by the type of sperm that fuses with ovum at the time of fertilization. Therefore, there is 50% chance of a male child being born and a 50% chance of a female child being born.

This mechanism of sex determination in human beings and also in fruitfly (Drosophila) is called XX-XY mechanism.

Role of Environmental Factors in Sex Determination.

Sex determination is sometimes regulated by environmental factors also. In some reptiles, the temperature at which the fertilized egg is incubated before hatching, is important environmental factor for determining the sex of the offspring.

(i)    In turtle high incubation temperature above 33ºC results in development of female progeny while a temperature below 28ºC produces only males.

(ii)   In lizard high incubation temperature results in male progeny while a temperature below 28ºC produces only females.

(iii)  Some animals like snails can change sex indicating that sex is not genetically determined

CLASS WORK-3

Q.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. To explain the reason, is mentioned that there occurs no reshuffling of traits in asexually reproducing individuals of a population. Appearance of very few new traits in the population is due to small inaccuracies during DNA copying. These very few new traits will be in very small proportion than the traits already present. Thus, trait B which exists in 60% of population must have arisen earlier than trait A which occurs only in 10% of the population.

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

Ans. Variations in a species arise either due to errors in DNA copying or during sexual reproduction. Force of natural selection selects individuals with useful variations in the prevailing environment so as to ensure their survival. The individuals with useful variations then increase in numbers through differential reproduction in the population.

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

Ans.  No. The information provided is not enough to tell us whether the trait of blood group A or blood group O is dominant. Either can be possible because each individual carries two alleles. Recessive trait will appear only when the two alleles are similar. In this case, there are two possibilities.

Possibility I. Blood group A is dominant and blood group O is recessive. The blood group O in daughter can appear only when both the recessive alleles occur together in mother and father has one allele of A and other of O Blood group.

Possibility II. Blood group O is dominant and blood group A is recessive. In this possibility, father must carry both alleles of blood group A while the mother may be having either both alleles of O blood group or one of A blood group and other of O blood group.

Q.4 How is the sex of a new born determined in humans?

Ans. The sex of the individual is genetically determined that is genes inherited          from parents decide whether the new born will be a boy or a girl. A new born         who inherits an ‘X’ chromosomes from father will be a girl and one who inherits         a ‘Y’ chromosomes will be a boy.

Q.5 Do genetic combination of mothers play a significant role in determining the sex of a new?

Ans. No, because mothers have a pair of X chromosomes. All children will inherit         an ‘X’ chromosome from their mother regardless of whether they are boys or         girls.

Q.6 In human beings, the statistical probability of getting either a male or female Child is 50:50 Give a suitable explanation.

Ans. The sex of an infant is determined by the type of sex chromosome contributed        by the male gamete. Since the ratio of the male gametes containing   X chromosome and those containing Y chromosome is 50: 50, the  statistical probability of  male or a female infant is also 50:50.   

Q.7 A woman has only daughters. Analyse the situation genetically and provide a suitable explanation.

  Ans. a) The woman produces ova with ’X’ chromosome

           b) The man produces sperms with X and Y chromosomes which actually

               determines the sex of the baby.

         

                                                                                                            Seema Misra