08. Heredity

Variations. : The differences between the characters among the individuals of the same species  and also by their offsprings are called variations.

Variations aries usally during the process Reproduction.  Asexual reproduction create few amount of Variations but sexual reproduction create greater amount of Variations 

Genetics : The study of heredity and variations is known as a Genetics 

Heredity – Transmission of various characters in living organisms from one generation to another generation is called heredity or inheritence.
Inherited traits : Those characteristics which are transmitted from parents to the offspring genetically are called hereditary traits. Examples Eye color, Free and attached earlobes.

Why mendel choose pea plant

Pea plant  have short life span  So, several generations can be studied in a short period

Pea plant Requires less area to grow

Pea plant have 7 pairs of contrasting characteristics.

Pea plants  have bisexual flowers which are mainly self-pollinating. It can be cross-pollinated easily

Seven pairs of contrasting characteristics of pea plants
S.N.  characteristics     Dominant trait   Recessive trait
1.     Plant height           Tall                     Dwarf
2.     Position of flowers: Axial                  Terminal.
3.     Pod shape:             Constricted         Inflated.
4.     Pod color:              Green                 Yellow.
5.     Flower color:          Purple                White.
6.     seeds shape:          Round                Wrinkled.
7.     Seed color:             Green                 Yellow.

Some important genetic terms

Gene : A section of DNA that contains information for protein synthesis is called the gene. Gene is the genetic unit of organisms. It is functional unit of heredity

Allele : Two or more  alternative forms of gene is call allele . For example, the gene controlling the height of a plant has two alleles T and t.

Dominant allele : The allele which can expresses itself even in the presence of recessive allele is called dominant allele

Recessive allele :  The allele which can expresses itself only in the absence of  dominant allele.

Homozygous : An organism is homozygous for a trait, when it has two copies of same allele. Genotype for a homozygous possesses either dominant or recessive, but never both the alleles. For example, RR or rr. It always produces only one kind of gametes.

Heterozygous : An organism is heterozygous for a trait when it has two different alleles. Genotype for heterozygous possesses both dominant and recessive alleles.  It produces two different kinds of gametes.

Phenotype : The external appearance of an individual for any trait is called phenotype.

Genotype : The genetic composition of an individal for a given trait is called genotype.

Mono Hybrid Cross : The hybridization in which the inheritance of one pair of contrasting characters is studied is called monohybrid cross.

When hybridization is done between homozygous tall plant “TT” and homozygous short plant “tt”, then in the F₁ generation all the plants are obtained tall.It means in the F₁ generation out of the two traits, only the trait of length is visible. So that the trait of tallness is the traits are called dominant trait whereas the trait of shortness is called recessive trait.
The Phenotypic ratio in F₁ – All tall
The Genotypic ratio in F₁ – All Tt

When hybridization is done between two heterozygous tall plants “Tt” which are obtained in the F₁ generation, then in the F₂ generation 1/4th plants are obtained short and 3⁄4 plants are obtained tall.

The Phenotypic ratio in F₂ – 3: 1 (3 tall : 1 short)
The Genotypic ratio in F₂ – 1: 2: 1 (TT : Tt : tt)
Dihybrid hybridization : The hybridization in which inheritance of two pairs of contrasting characters is studied is called dihybrid hybridization.

when a pea plant with round and green seeds is crossed with a pea plant with wrinkled and yellow seeds, then in the F₁ generation all the plants are obtained with round and green seeds. But when hybridization is done between plants of F₁ generation, some plants of F₂ generation exhibit new combinations.
Some plants are found with wrinkled and green seeds and some plants are found with round and yallow seeds. Therefore, the round/wrinkled trait and the green/yellow trait are independently inherited
The Phenotypic ratio in F₁ – All plants are with round and green seeds.
The Genotypic ratio in F₁ – All RrYy
The Phenotypic ratio in F₂ – 9:3:3:1
The Genotypic ratio in F₂ – 1:2:2:4:1:2:1:2:1

Rules for the Inheritance of Traits – Mendel’s Contributions
Gregor Johann Mendel, known as the ‘Father of Genetics’ who conducted experiments on pea plants to understand the concept of heredity.
The rules of inheritance of traits in humans are based on the fact that both mother and father contribute equal amounts of genetic material to the offspring.
Mendel studied many contrasting traits of the pea plant and made three basic laws of inheritance :
The Law of Dominance
The Law of Segregation
The Law of Independent Assortment.
The Law of Dominance : This law is based on monohybrid hybridization. According to this law, when hybridization is done between a pair of homozygous plants having contrasting traits, then the trait which is expressed in the F₁ generation is called dominant trait and the trait which is not expressed in the F₁ generation is called recessive trait.
2. Mendel's law of segregation
The law of segregation is also called the law of purity of gametes. This rule is based on monohybrid Cross. According to this law, the two allele of a gene separate/ segregate from each other during formation of gametes in heterozygous plants obtained in the F₁ generation, and only one allele enters in a gamete. Hence each gamete is “pure” and always contain only one allele of a gene.

3. The Law of Independent Assortment.
This law is based on dihybrid hybridization. According to this law , when hybridization is done between two or more contrasting traits plants having, then the inheritance of one trait has no effect on the other trait, That is, the alleles of each trait are not only separate, but the alleles of different traits behave independently towards each other.

Sex determination
In some animals like reptiles, sex determination depends on the incubation temperature of the fertilized egg (gamete)
In other animals, such as snails, individuals can change sex, indicating that sex is not genetically determined.
Sex determination in humans is based on genetics. In humans, sex is determined by chromosomes. In humans, there are 46 (23 pairs) chromosomes in the nucleus of cells, out of which 44 (22 pairs) chromosomes are called autosomes and 2 (one pair) chromosomes are called sex chromosomes. Both chromosomes of each pair of Autosomes chromosomes are identical, but the sex chromosomes are XX in females and XY in males. These sex chromosomes are responsible for sex determination in humans. At the time of gamete formation, two types of gametes (sperm) are formed in the male having 22 +X and 22+Y. But in females, only one type of gametes (eggs) with 22+X chromosomes are formed.
At the time of fertilization, if the egg meets the male sperm having 22+Y chromosome. So the child born from this will be a son and if at the time of fertilization the egg meets the sperm having 22+X chromosome of the male then the child born from this will be a daughter. Therefore, the probability of a human being having a son or daughter is 50% - 50%

1. Exchange of genetic material takes place in 
   [A] vegetative reproduction 
   [B] asexual reproduction 
   [C] sexual reproduction 
   [D] budding                 [C]
2. Sugarcane field has little variation because of
   [A] sexual reproduction  
   [B] asexual reproduction 
   [C] Both [A] and [B]  
   [D] None of these [B]
3. When two parents are crossed, the offsprings are referred to 
   [A] F1  generation   
   [B] F2 generation 
   [C] Either F1  or F2    
   [D] None of these    [A]
4.  A cross between two individuals result are in ratio of 9:3:3:1. The cross is 
   [A] Monohybrid cross   
   [B] Dihybrid cross 
   [C] Cross-pollination   
   [D] Self pollination  [B] 
5. A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. In the progeny, all bore violet flowers, but almost half of them were short. This suggests that the genetic makeup of tall plant can be depicted as 

   [A] TTWW 
   [B] TTww 
   [C] TtWW 
   [D] TtWw  [C]
6. Two pea plants one with round green seeds (RRyy) and another with wrinkled yellow (rrYY) seeds produce F1  progeny that have round, yellow (RrYy) seeds. When F1  plants are selfed, the F2 progeny will have new combination of characters. Choose the new combination from the following. 

   (i) Round, yellow   (ii) Round, green (iii) Wrinkled, yellow   (iv) Wrinkled, green 

   [A] (i) and (ii) 

   [B] (i) and (iv) 

   [C] (ii) and (iii)  

   [D] (i) and (iii)   [B] 
7. Select the statements that describe characteristics of genes 
   (i) genes are specific sequence of bases in a DNA molecule 
   (ii) a gene does not code for proteins 
   (iii) in individuals of a given species, a specific gene is located on a particular chromosome 
   (iv) each chromosome has only one gene 
   [A] (i) and (ii) 
   [B] (i) and (iii) 
   [C] (i) and (iv) 
   [D] (ii) and (iv)         [B]
8. The maleness of a child is determined by 
   [A] The X chromosome 
   [B] The Y chromosome   
   [C] The cytoplasm 
   [D] by chance.         [B]

1. Assertion: If one bacterium divides then two resultant bacteria divide again to generate four individual bacteria which will be highly identical.  Reason: It is due to asexual reproduction. 
   Both ‘A’ and ‘R’ are true and ‘R’ is correct explanation of the assertion.  
2. Assertion: Two pink coloured flowers on crossing resulted in 1 red, 2 pink and 1 white flower progeny. 
   Reason: It is due to double fertilisation. 
   ‘A’ is true but ‘R’ is false because it is due to self pollination.
3. Assertion:  The sex of a child in human beings will be determined by the type of chromosome he/she inherits from the father.  
   Reason: A child who inherits ‘X’ chromosome from his father would be a girl (XX), while a child who inherits a ‘Y’ chromosome from the father would be a boy (XY).
   Both ‘A’ and ‘R’ are true and ‘R’ is correct explanation of the assertion.
4. Assertion:  Dominant traits express itself with or without the presence of recessive trait.  
   Reason: Recessive trait can express itself only in absence of dominant trait.
   Both ‘A’ and ‘R’ are true but ‘R’ is not correct explanation of the assertion.  
5. Assertion:  A geneticist crossed a pea plant having violet flowers with a pea plant with white flowers, he got all violet flowers in first generation.  Reason: White colour gene is not passed on to next generation. 
   ‘A’ is true but ‘R’ is false 
6. Assertion [A]: Mendel choose a number of varieties of garden pea as plant material for his experiments.  
   
   Reason (R): Garden pea has well defined characters and is bisexual.  
   Both A and R are true and R is the correct explanation of A. 
7. Assertion [A]: In humans, males play an important role in determining the sex of the child.  
   Reason (R): Males have two X chromosomes.
   A is true but R is false. [D] A is false and R is true. 
8. Assertion: A zygote with two X chromosomes develops into a boy.  
   Reason: If the egg cell carrying an X chromosome fuses with the sperm carrying a Y chromosome, the resulting child would be a boy.  
   A is false and R is true.
9. Assertion [A]: Mendel proposed the law of inheritance of traits from the first generation to the next generation.  
   Reason (R): Mendel’s Law of segregation  is also known as “Law of purity of gametes. 
   Both A and R are true but R is NOT the correct explanation of A
10.  Assertion [A]: Mendel in his experiment selected only two characters of seed.  
    Reason (R): He studied single character at one time. 
    A is false and R is true. 

1. A normal baby girl receives her X chromosome from whom : mother, father, both mother and father or either from mother or father? 
   From both mother and father. 
2. How many pairs of sex chromosomes are there in a human zygote?
   one
3. Write the sex of the baby that inherits Y-chromosomes from the father. 
   Male. 
4. What is the phenotypic ratio of a dihybrid cross in F₂ generation? 
   9 : 3 : 3 : 1.
5. Give an example where sex determination is regulated by environmental factors. 
    In turtles, sex is determined by environmental factors (temperature). 
6. Write full form of DNA
   Deoxyribonucleic acid
7. Who is called father of genetics?
   Gregor Mendel. 
8. Name the term used for the traits that are exhibited externally.
   Phenotype. 
9. The mechanism by which variations are created and inherited". Give the respective scientific terms used for studying?
   Heredity
10. How are characters or traits controlled? 
    Characters or traits of an organism are controlled by the genes.
11. IF a round green seeded pea plant (RRYY) is crossed with wrinkled yellow seeded pea plant (rryy). which type seeds to be produced in F₁ generation will be?
    Round and Green seeds.
12. Name the plant on which Mendel performed his experiments. 
    Garden pea (Pisum sativum).
13. If YYRR is round yellow, what do the following represent? yyrr yyRR 
    yyrr - Wrinkled, green seeds 
    yyRR — Round, green seeds  
14. What is heredity? Who is its founder?
    Transmission of various traits in living organisms from one generation to another generation is called heredity. Gregor Mendel was the founded of heredity. 
15. No two individuals are absolutely alike in a population. Why? 
    All individuals have different kind of DNA. So all individuals are not alike to each other. 
16.  (i) What is the genetic constitution of human sperm? 
    22 + Y or 22 + X 
    (ii) Mention the chromosomes pair present in zygote determining the sex of a male child.
    (22 + Y + 22 + X) 
17. What is genotype? 
    The genetic composition of an individal for a given trait is called genotype.
18. Where are the genes located? What is the chemical nature of gene? 
    Genes are located on chromosomes in linear sequence and at fixed positions. Chemically genes are acidic in nature since they are nucleic acids which constitute DNA. 
19. How does the creation of variations in a species promote survival ? 
    Variations in a species help them to adapt to the changes in the environment  In this way, variation promotes survival of species.
20. A Mendelian experiment consisted of breeding pea plants bearing violet flowers with pea plants bearing white flowers. What will be the result in F₁ progeny? 
    All will bear violet flowers. 
21. Why is the progeny always tall when a tall pea plant is crossed with a short pea plant?
    The trait which represents the tallness in a pea plant is dominant over the another trait, shortness (dwarf). 
22. What is a gene?
    A section of DNA that contains information for  protein synthesis is called the gene. Gene is the genetic unit of organisms. It is a functional piece of DNA that is responsible for expression of a trait in the organisms.
23. What is F₂ generation?
    The generation produced by the offsprings of F₁ generation i.e., first generation as parent is called F₂ or second generation.
24.  What type of traits are phenotype? 
    The traits which are visible to us are called phenotype, e.g. tall or dwarf (short) are phenotype traits of the plants. 
25. What is variation? 
    The differences between the characters among the individuals of the same species  and also by their offsprings are called variations.
26. Define homozygous and heterozygous. 
    Homozygous : An organism is homozygous for a trait, when it has two copies of same allele.  For example, RR or rr. 
    Heterozygous : An organism is heterozygous for a trait when it has two different alleles.  For example, Rr , Tt 
27. “The chromosome number of the sexually producing parents and their offspring is the same”. Justify this statement. 
    In sexual reproduction, both the gametes (male and female) contain half the number of chromosomes (haploid or n) and by the fusion of these gametes,  the zygote have full set (diploid 2n) chromosomes.
28. In a pea plant, find the contrasting trait if: 
    (i) The position of flower is terminal. 
    Axial position of flower.
    (ii) The flower is white in colour. 
    Purple colour of flower
    (iii) Shape of pod is constricted. 
    Inflated shape.
29. Give the pair of contrasting traits of the following characters in the pea plant and mention which is dominant and which is recessive.
    (i) Yellow seed  (ii) Round seed
    Yellow – dominant,  Green – recessive
    Round – dominant, Wrinkled – recessive
30. Give differences between dominant and recessive traits, 
    Dominant trait: The trait which is able to express itself even in the presence of contrasting trait is called dominant trait.
    Recessive traits : The trait which can express itself only in absence of contrasting trait is called recessive trait.
31. (i) How many types of sex chromosomes are found in human beings? 
    Two types
    (ii) How many pairs of chromosomes are present in human beings? 
    23 pairs of chromosomes
    (iii) Out of these how many are sex chromosomes? 
    One pair,
32. (i) What are sex chromosomes? 
    Sex chromosomes are set of chromosomes present in human beings whichare responsible for sex determination in humans. 
    (ii) Which sex chromosomes are found in male and female human beings? 
    XX and XY sex chromosomes are found in male and female human beings
    (iii) State the chromosome responsible for the development of male child in human beings? 
    Y chromosome responsible for the development of male child in human beings
33. A violet pea plant [VV] was crossed with a white one[vv]. F₁ generation was allowed to self pollinate and F₂ generation was also obtained, Answer the following questions: 
    (i) What would be the phenotype of plants in F₁ generation? 
    All violet flowers
    (ii) What would be the percentage of plants with white flowers in F₂ generation?
    25% 
    (iii) What would be the ratio of vv: Vv in F₂ generation. 
     vv: Vv = 1 : 2.
34. A pea plant with blue colour flower denoted by BB is cross-breed with a pea plant with white flower denoted by ww. 
    (i) What is the expected colour of the flowers in their F₁ progeny? 
    F1 generation — Blue
    (ii) What will be the percentage of plants bearing white flower in F₂  generation, when the flowers of F₁ plants were selfed? 
    25% 
    (iii) State the expected ratio of the genotypes BB and Bw in the F₂ progeny. 
    BB : Bw = 1 : 2.
35. A cross was carried out between a pure bred tall pea plant and a pure bred dwarf pea plant and F₁ progeny was obtained. Later, the F₁ progeny was selfed to obtain F₂ progeny. Answer the following questions: 
    (i) What is the phenotype of the F1 progeny and why? 
    Tall, because genes responsible for tallness are dominant over dwarf trait. 
    (ii) Give the phenotypic ratio of the F₂ progeny. 
    3: 1
    (iii) Why is the F₂ progeny different from the F₁ progeny?” 
     Because in F₂ generation, recessive genes got expressed in homozygous condition.
36. An angiosperm plant having red coloured flowers when crossed with the other having the same colour produced 40 progenies, out of which 30 plants were with red coloured flowers, 10 plants were with white coloured flowers. Find out: 
    (i) What is the possible genotype of parent plants?
    Rr and Rr. 
    (ii) Which trait is dominant and recessive? 
    Red colour of flowers is the dominant trait while white colour is the recessive trait.
    (iii) What is this cross called as and what is its phenotypic ratio? 
    Monohybrid cross, phenotypic ratio is 3 : 1. 
37. (i) “Chromosomes are heredity carriers.” Why do we say so?
    Chromosomes are made-up of DNA. Genes are located on the chromosomes. It is the DNA copy which transfers from both the parents to their offspring. Therefore, chromosomes made-up of DNA and containing genes are said to be the heredity carriers
    (ii) Which vital function is not controlled by autosomes?. 
    Sex of the child is not controlled by autosomes. 
38. (i) On what rules inheritance is based? 
    The rules of inheritance is based on the fact that the traits in the progeny are carried out by DNA’s of both the parents (mother and father). 
    These rules are known s ‘Mendel’s Laws of Inheritance’. 
    The rules are: (i) Law of dominance, (ii) Law of segregation, and (iii) Law of independent assortment.
    (ii) Is each trait influenced by both paternal and maternal DNA? 
    Yes, it is true that each trait is influenced by both paternal and maternal DNA
39. A tall pea plant was crossed with a dwarf one. F₁ generation was allowed to self pollinate and F₂ generation was also obtained. Answer the following questions:

    (i) What would be the phenotype of plants in F₁ generation? 

    All tall

    (ii) What would be the phenotypic ratio in F₂ generation?

    3:1

    (iii) Give reason for your observation in F₁ generation.

    Reason: The tall trait of pea plant is dominant trait over short trait, a recessive trait. 
40. (i) What are monohybrid and dihybrid cross? 

    a. A monohybrid cross is the cross between two homozygous parents, which differ in only one contrasting trait

    A dihybrid cross is a cross between two heterozygous parents which differ in two contrasting traits. 

    (ii) How Mendel proved that tallness is the dominant trait and dwarfness is recessive in a pea plant?. Explain with the help of a monohybrid cross.

    Mendel took pea plants with different characteristics - a tall plant and a short plant, produced progeny from them and calculated the percentage of tall or short progeny. It was found that all plants were tall. This proves that tallness is the dominant trait while dwarfness is the recessive trait.
41. Explain the manner in which sex is determined in human beings? 

    or Explain how equal genetic contribution of male and female parents is ensured in progeny? 

    Sex determination in humans is based on genetics. In humans, sex is determined by sex chromosomes. In humans, there are 46 (23 pairs) chromosomes in the nucleus of cells, out of which 44 (22 pairs) chromosomes are called autosomes and 2 (one pair) chromosomes are called sex chromosomes. Both chromosomes of each pair of Autosomes chromosomes are identical, but the sex chromosomes are XX in females and XY in males. These sex chromosomes are responsible for sex determination in humans. At the time of gamete formation, two types of gametes (sperm) are formed in the male having 22 +X and 22+Y.  But in females, only one type of gametes (eggs) with 22+X chromosomes are formed.

    At the time of fertilization, if the egg meets the male sperm having 22+Y chromosome. So the child born from this will be a son and if at the time of fertilization the egg meets the sperm having 22+X chromosome of the male then the child born from this will be a daughter. Therefore, the probability of a human being having a son or daughter is 50% - 50%.
42. Woman are often blamed for bearing daughters. As a student with knowledge in science how will you explain it to your fellow students that the sex of the child is not determined by mother’s genetic contribution? 

    Sex of the child is not determined by mother’s genetic contribution. 

    In humans, sex is determined by sex chromosomes.

    The sex is determined by a pair of sex-chromosomes in humans . In female, this pair consists of two similar chromosomes denoted as XX. Hence, females produce only one type of gametes each having an ‘X’ chromosome. Whereas male has two different types of sex chromosomes  X and Y. Hence two types of gametes are formed in the male having 22 +X and 22+Y.  If the egg meets the male sperm having 22+Y chromosome then male child is born and If the egg meets the male sperm having 22+X chromosome then female child is born.Therefore, the father’s genes is responsible for the determination of the sex of the child.
43. The chromosomal number of the sexually producing parents and their offspring is the same. Justify this statement. 

    Offspring always takes 23 chromosomes from father and 23 chromosomes from mother and in total has 46 chromosomes. It means that total number of chromosomes remain constant. 
44. Sex determination in man depends upon 23rd pair of chromosomes, called the sex  chromosomes.  If it were a homologous pair-XX, it would be a female. If it were a heterologous pair – XY, it would be a male. Based on this statement, answer these questions:

    (i) How many types of eggs/ova and how many type  of sperms female and male human  being will produce respectively ?

    Male produces two genetic types of sperms, half with X and other half with Y chromosome. The female produces only one genetic type of ova, all carrying X chromosome. 

    (ii) Which parent’s contribution of sex chromosomes determines sex of the child?  

    Sex of children will be determined by what they inherit from their father, and not from their mother
45. How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism? 

    OR ‘‘It is possible that a trait is inherited but may not be expressed.’’ Give a suitable example to justify this statement.   

    Some traits that are inherited anot express themselves. Such hidden traits are known as recessive traits. Mendel explained this phenomenon with the help of monohybrid cross. In a monohybrid cross performed by Mendel, a tall plant was crossed with a dwarf plant which produced all tall plants in F1 progeny.  However, when these F1  tall plants were crossed with each other, ‘dwarf’ trait, which was not observed in the Fl generation, reappeared in the F2 progeny. 

    it is concluded that dwarfness is a trait of parent pea plant that was not lost. It was suppressed in the F1  generation by the tallness trait and reappeared in the F2 generation. So, we can say that a trait that is inherited may not be always expressed in an organism.