WBBSE CLASS 10 LIFE SCIENCE CHAPTER 3 NOTE
Madhyamik Life Science Chapter 3
HEREDITY AND COMMON GENETIC DISEASES
Madhyamik Life Science Heredity and Common Genetic Diseases |
Heredity and common genetic diseases (How are the characteristics of an organism transmitted from one generation to another ?)
Heredity and Variation :
The resemblance of offspring to their parents is termed heredity. Family members share many similarities in appearance, such as height, eye colour, hair colour. There are differences in the manner how characteristics are inherited to offsprings. Offsprings do not look exactly like their parents. So, the offspring resemble their parents in major respect but also differ from the parents in many minor respects. An Austrian monk, Gregor Johan Mendel performed a series of simple experiments and discovered how heredity worked.
Definition: Heredity is the transmission of characteristics or traits through genes from one generation to another by reproduction.
The term 'genetics' was coined by Bateson (1905). It is derived from the Greek word 'gen', which means "to grow into", whereas the term 'gene' was first proposed by Johannsen (1909).
Definition: The science of Heredity and the principles governing the inheritance of characters from parent to progeny is called Genetics.
Mutation - Sometimes there may be some change in chromosome structure or chromosome number or alteration in the composition of the DNA of a gene.
Definition: This change in the genetic constitution of an organism is called Mutation. The mutation is the cause of variation among organisms.
Variation - The permanent change in chromosome or DNA may result in a difference in
expression of characters called a variation.
Genotypic variation may result in phenotypic variation.
So, the mutation is the cause of variation whereas variation is the effect of the mutation.
Examples of variation in man:
1. Free and attached ear lobe -
2.Roller and normal tongue- PICTURE
Key terms associated with heredity :
(i) Characteristics:
Definition-All special features of an organism that are transmitted through genes from one generation to the next are called characteristics. e.g. Eye colour, the height of a person, gender (sex), blood groups, skin colour, etc.
(ii) Traits:
Definition-The phenotypic expression of a particular characteristic is called trait e.g. Eye colour is the characteristic whereas a trait would be blue eyes or brown eyes etc.
(iii) Allele or Allelomorph:
Definition-The contrasting pair of genes located in the same locus of the homologous chromosome and control the same characteristic are called allele or allelomorph. e.g. In a hybrid tall plant, T (Tall plant) and t (dwarf plant) genes are alleles. Similarly, the gene for brown eye colour (B) and blue eye colour (b) are also alleles.
(iv) Locus:
Definition-The point or place on a chromosome where a particular gene is present is known as locus of that gene. e.g. a chromosome may contain many loci linearly present on a chromosome. Each gene is present at a definite locus on the chromosome.
(v) Unit of inheritance (Factor/Gene):
Definition-Gene is a part of DNA, located on a particular point (locus) of a chromosome, functions as a hereditary unit (responsible for the inheritance of characteristics from parents to offsprings), also controls biological functions. G. J. Mendel, the father of genetics, called these genes 'factors' that control all the characteristics of an organism. e.g. Gene for tallness and gene for dwarfness in pea plant; the gene for black hair colour and white hair colour in guineapig, etc.
(vi) Monohybrid cross:
Definition- The genetic experiment of cross-breeding where only one pair of contrasting character (i.e. two alternate traits of a particular character) is considered is known as a monohybrid cross. e.g. In Mendel's experiment, the cross between pure tall and pure dwarf pea plant; in guineapig, the cross between pure black hair and pure white hair etc.
(vii) Dihybrid cross:
Definition- The genetic experiment of cross-breeding where two pairs of contrasting characters (i.e. two alternate forms of two different characters) are considered is known as dihybrid cross. e.g. In Mendel's dihybrid cross with pea plant, he considered shape of seed (Round or Wrinkled) as well as the colour of seed (Yellow or Green); in Guineapig, hair colour (Black or White) as well as the texture of hair (Rough and Smooth).
(viii) Homozygous organisms:
Definition-When the pair of genes present in the corresponding locus of the homologous chromosome is exactly the same, the organism is called a homozygous organism. e.g. homozygous tall pea plant (TT), homozygous dwarf pea plant (tt) etc.
(ix) Heterozygous organism:
Definition-When the pair of genes (alleles) present in the corresponding locus of the homologous chromosome is of contrasting type, the organism is called a heterozygous organism e.g. heterozygous tall pea plant (Tt); heterozygous black guineapig (Bb) etc.
(x) Hybridization:
Definition-It is the process of interbreeding between two genetically different (divergent) individuals. They may be of the same species (intraspecific hybridization) or different species (interspecific hybridization). e.g. In Mendel's monohybrid cross, there was cross-breeding or cross-pollination between a pure tall plant and a pure dwarf plant.
(xi) pure and hybrid:
Definition-Homozygous organisms are known as pure organisms and
(xii) Parental generation:
Definition- The first set of parents used in the cross of genetic experiments called parental generation
.
(xiii) Filial generation (F1, F2):
Definition- In a genetic experiment, all the offspring produced from the parental generation (P) are together known as a filial generation.
(xiv) Dominant characteristics:
Definition-Out of two contrasting alleles in a hybrid organism, one is expressed and other is suppressed-the expressed phenotype is known as dominant characteristic, the expressed gene is called dominant gene and the phenomenon is known as dominance. e.g. In Mendel's experiment, gene (factor) for tallness of plant (T) is dominant over the gene for dwarfness of the plant (t); in Guineapig, black hair colour (B) is dominant over white hair colour (b).
(xv) Recessive characteristics:
Definition-In a hybrid organism, out of two contrasting alleles, one is expressed and other is suppressed-the suppressed phenotype is known as recessive characteristic, the suppressed gene is called a recessive gene and the phenomenon is known as recessiveness. e.g. In Mendel's monohybrid cross, gene (factor) for dwarfness (t) is recessive to a gene for tallness (T); in Guineapig, white hair colour (b) is recessive to black hair colour (B).
(xvi) Phenotype and Genotype:
Definition-The externally visible (or measurable) characteristics of an organism are called the phenotype. e.g. Tall plant, dwarf plant, red flower, white flower, black eye, blue eye etc.
The definition-The genetic constitution of an organism is called the genotype. It is represented
by some symbols to explain the composition of genes of a particular character. e.g. gene for
tallness is represented by (T), that of dwarfness by (t); the genotype of a pure tall plant is
TT, that of the hybrid tall plant is (Tt) etc.
In Mendel's monohybrid cross, phenotype and genotype can be explained as follows:
Table
Differences between Homozygous (pure) and Heterozygous (hybrid):
Differences between Dominant and Recessive characters:
Differences between Phenotype and Genotype:
Mendel's work on Pea plant:
Self and Cross-pollination with pea plants: Mendel selected Garden pea (edible pea) plant (Pisum sativum; 2n=14) for his experiments. Mendel performed his experiment at a time when there was no concept of the chromosome, DNA, gene, meiosis and so on.
For Self pollination, Mendel covered the bisexual flowers of the pea plant by paper bags so that no foreign pollen grain could contaminate the flowers during the experiment.
For Cross-pollination, Mendel followed three steps:
（1) Emasculation-
From the selected bisexual flowers, the stamens were cut off by a Scissor as if the masculine part of the flower was removed.
（2） Dusting-
With the help of a brush, the pollen grains of one flower was transferred to the stigma of another flower and vice versa.
(3) Bagging -
After cross-pollination, all the flowers were covered with paper bags to avoid any contamination of undesired pollen grains.
Finally, after fertilization, the ovary of the flower was modified into fruit (pea pod) and ovules were modified into seeds. When the seeds were matured, they were collected and put into the soil, then the seeds germinated into new saplings (F1 generation).
Reasons behind Mendel's success :
Mendel performed his experiments when biologists were unaware of the chromosome, DNA, genes, etc, and also the process of cell division (mitosis or meiosis). Even the process of union between gametes during sexual reproduction was not very clear. Simply on the basis of his breeding (hybridization) experiments, Mendel came to several conclusions (known as Mendel's law) which constituted the foundation of the modern science of genetics.
The reasons behind Mendel's success can be explained as follows:
Mendel repeated his experiments many times to accumulate a good deal of data for statistical analysis.
He analysed the data by applying the principles of Mathematics and Statistics and then confirmed his conclusions or laws.
Mendel conducted his experiment with extreme care and considered one or two pairs of contrasting character for monohybrid and dihybrid cross respectively.
He considered only distinctive contrasting pairs of characters for his hybridization experiment. So, there was no confusion on the results e.g. Tall plant was 6'-7' whereas the dwarf plant is 1'-11/2’.Hence there is no possibility of any overlapping result of the tall and dwarf plant.
By repeated inbreeding (self-pollination) over the generations, Mendel selected only pure breeding varieties of pea plants for his experiment.
Mendel took enough care to avoid contamination of foreign pollen grain by covering the flowers with paper bags.
Mendel's Law :
From monohybrid cross, Mendel formulated his first law known as Law of Segregation.
1st Law of Mendel: Law of Segregation-Factors for contrasting character in a
zygote do not blend or contaminate with each other but segregate and pass into different
gametes and offsprings randomly.
From dihybrid cross, Mendel formulated his second law known as Law of Independent
Assortment.
2nd Law of Mendel: Law of Independent Assortment- Two or more contrasting
pairs of factors (characters) are assorted independently and may recombine randomly in all
possible combinations governed by chance alone.
Deviation of Mendel's Laws of Heredity:
In Mendel's monohybrid cross, the tall (T) trait was completely dominant over the dwarf (t) trait in
the pea plant. That's why, in F1 hybrid tall plants (Tt), 'T' was fully expressed, and 't' was totally
suppressed. This is known as complete dominance.
After Mendel, a long time passed away. Many scientists made their research work with
various plants and animals. A lot of new genetic phenomenon has been discovered. One such
interesting discovery is incomplete dominance.
Definition: In a heterozygous organism, out of contrasting dominant and recessive allele,
if the dominant gene cannot express its dominant phenotype completely but an intermediate
or mixed phenotype between dominant and recessive gene is expressed, the phenomenon
is known as incomplete dominance or partial dominance or semi dominance.
Sex determination in man :
Definition: The mechanism by which the male and female individuals of a species are differentiated is known as sex determination. In man, the mechanism of sex determination is known as XX-XY mechanism.
（i) In humans (Homo sapiens), the chromosome number is 2n = 46. There are 22 pairs autosome
and 1 pair sex chromosome.
（ii) Sex chromosomes determine sex. In the human female, the sex chromosomes (allosome) are homologous (XX) but in the human male, the sex chromosomes are nonhomologous (XY).
（iii) Female (44A + XX) is homogametic and produces only one type of ovum by meiosis (during oogenesis) (22A+x). ('A' stands for autosome and ‘X' for allosome).
（iv) Male (44A+XY) is heterogametic and produces two different types of sperm by meiosis (during spermatogenesis)-(22A+X) and (22A +Y). ('A' for autosome and X, Y designate allosome). X containing sperm is known as gynosperm and Y containing sperm is known as androsperm.
(v) During fertilization between sperm and ovum, any type of sperm may fertilize the ovum. Whether gynosperm (22A + X) or androsperm (22A + Y) would fertilize the ovum-is purely a matter of chance. Nothing can be predicted earlier.
（vi) The principle of sex determination in man is that presence of Y in the zygote indicates maleness whereas the absence of Y in zygote indicates femaleness.
（vii) If gynosperm (22A + X) fertilizes the ovum (22A + X), the zygote would be (44A +XX) which gives birth to a female baby. On the contrary, if androsperm (22A + Y) fertilizes the ovum (22A + X), the zygote would be (44A + XY) which gives birth to a male baby.
（viii) So, whether a male baby (son) or a female baby (daughter) will be born (Sex determination of the baby), depends completely on father and never on the mother (in no way).
For reference only:
The Y-chromosome in human male contains TDF (Testis Determining Factor) gene which is responsible for the development of masculine features.
3. (b)
Some common genetic diseases (What are the causes of genetic diseases?)
Common genetic diseases in the population :
Many genetic diseases in man have been discovered. These are mainly because of various mutant genes that are located either in the autosome (autosomal gene) or in the allosome／sex chromosome (sex-linked gene). If the sex-linked gene is present in the X-chromosome or Y-chromosome, it may be called an X-linked gene or Y-linked gene respectively.
For reference only:
The mutation is a natural process of change in gene or DNA sequence. Any physical or chemical agent that causes mutation is called mutagen e.g. X-ray, gamma ray, UV ray, radioactive substance, and many chemicals.
Thalassemia : It is a type of haemolytic anaemia caused by abnormal synthesis of faulty haemoglobin. It was discovered by American doctor Cooley (1925).
Cause:
1. Thalassemia is a genetic disorder caused by autosomal recessive gene.
2. The defects occur due to mutation of genes located in the chromosome 16 and chromosome 11.
3. The mutations cause defective synthesis of oα globin chain or β globin chain of haemoglobin.
4. Thalassemia is generally of three types-Alpha, Beta and Delta.
（Beta thalassemia is called cooley's anaemia)
Symptoms:
1. Synthesis of less haemoglobin or formation of abnormal haemoglobin.
2. Premature destruction of RBC and development of anaemia.
3. Expansion of bone narrow and heart problem.
4. Enlargement of liver-hepatomegaly.
5. Enlargement of spleen-Splenomegaly and increased risk of infection.
6. Skeletal deformation and iron overload.
7. Retardation of body growth.
8. Affected fetus may die inside mother's uterus or the baby may die soon after birth
（jaundice and erythroblastosis).
Haemophilia: It is a clinical condition in which the ability of blood clotting of a person is severely reduced, causing the sufferer to bleed severely even from a slight injury.
It is also known as bleeder's disease. This was first discovered by an American doctor John Otto (1803).
Cause:
1.Haemophilia is caused due to the presence of a recesive sex-linked gene h, located on the chromosome.
2.There are two types of haemophilia-Haemophilia A and Haemophilia B, caused by two mutant genes present on the X-chromosome in man with a distinct gap.
3. Haemophilia A (Royal disease) is caused due to deficiency of a protein for blood clotting known as＂Factor 8” or “Antihaemophilic factor” (AHF). About 80% Haemophilia are of this type.
4. Haemophilia B (Christmas disease) is caused due to deficiency of another blood-clotting protein known as "Factor 9" or Plasma thromboplastin. About 20% of Haemophilia are of this type. It is called Christmas disease according to the name of the first patient-Stephen Christmas.
5. The disease (Haemophilia A) was first discovered in Queen Victoria. Probably the gene was inherited to her from parents or the mutation was developed in herself.
Symptoms :
1. For normal blood clotting, both factor 8 and 9 are needed. Defect or deficiency of any one factor will impair blood clotting. So blood does not clot within 3-8 minutes but it takes hours. As a result, there will be continuous bleeding and the patient may die. There is no permanent cure of this genetic disease but it may be partly managed by blood transfusion.
2. Females may be 'carrier' with one dominant normal gene and one recessive 'h' gene of haemophilia.
3. Haemophilia is found more in males than in females.
C. Colour blindness: It is the defective color vision when someone can not distinguish between certain colors, usually between green and red, and occasionally blue due to the absence of a reduced amount of visual pigments.
First scientific paper about colour blindness was written by John Dalton in 1793. Dalton himself was red-green colour blind. That's why red-green color blindness is sometimes called Daltonism.
Cause:
1. The gene for normal vision is dominant. Red-green color blindness is caused by a recessive gene located on X-chromosome (sex-linked gene) in men.
2. The mutant gene for red blindness is protein and the mutant gene for green blindness is deutan. They are located at the different locus of the X-chromosome.
For reference only: Blue color blindness (often referred to as blue-yellow color blindness) is extremely rare (only 5% of color blind people suffer from it). The responsible gene is present in chromosome 7 (autosome).
Symptoms:
1. Colour blindness is an inheritable disease of the X-linked recessive gene.
2. The cone cells of the eye in these persons fail to distinguish red and green colours.
3. Besides the problem of colour vision, these colourblind persons will have otherwise normal vision for reading, writing etc.
4. Colour blindness is more common in males than in females because males have one X and other Y sex chromosome whereas in human female there are two X-chromosomes.
Thalassemia is an autosomal chromosomal disorder and genetic counseling:
Thalassemia is a type of genetic disorder where hemoglobin is produced in decreased amounts. The decreased amount of hemoglobin in the blood causes anemia, which reduces the oxygen-carrying capacity of the blood. RBC becomes fragile and breaks down easily (hemolysis). To compensate for the loss of RBC, frequent blood transfusion is done.
All the above conditions may result in iron overload, which means excess iron in the body. Excess iron in vital organs increases the risk for liver disease (cirrhosis, cancer), heart attack, diabetes mellitus, osteoarthritis, osteoporosis as well as problem of an endocrine system like hypothyroidism, defective gonads, etc.
Iron overload may be inherited or it may be acquired by numerous blood transfusion, iron injection etc. When a person is receiving blood transfusions on a regular basis, iron may build-up to a toxic level in the body.
Role of genetic counseling in the prevention of thalassemia:
Those persons who contain the gene for thalassemia in recessive condition are called carriers or minors. They have the normal dominant gene. So, they will not suffer from any problem of thalassemia. .If both father and mother are carriers, then in F, generation, the probability of thalassemia maybe 25%.
Before the marriage of any two-man and woman, family history must be studied for genetic counseling to find out any thalassemia carrier in the proposed male and female or their relatives, parents, etc. Two thalassemia carrier should not get married.
There is a method of the blood test for detection "haemoglobin disorders" which can be done before marriage to find thalassemia carrier (thalassemia minor). These premarital precautions will help to prevent the inheritance of thalassemia major (homozygous). Thus the birth of thalassemia babies can be checked that would justify-'prevention is better than cure'.
Thanku so much sir
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