Notes-Class 9-Science-Chapter-16-Heredity and Variations-Maharashtra Board

Structure of chromosomes :

• Each chromosome is made up of DNA
• The proteins present on the chromosomes are histones. DNA is present along with histones in the chromosomes.
• They are seen only at the time of cell division. They appear dumbbell-shaped.
• There is ‘primary constriction’ or centromere on each chromosome. This centromere divides the chromosome into two arms. The short arm is called ‘p’ arm while the long arm is called ‘q’ arm.
• Based on the position of centromere, chromosomes are categorized into four types, viz. metacentric, sub-metacentric, acrocentric and telocentric.
• Based on their functions the chromosomes can be somatic (body chromosomes) and sex chromosomes (those that determine the sex). Somatic chromosomes can be homologous or heterologous chromosomes.

1. Metacentric :

• Structure : The centromere is exactly at the mid-point in this chromosome.
• Pattern : This chromosome looks like the English letter ‘V’.
• Arm : The arms of this chromosome are equal in length.

2. Sub-metacentric :

• Structure : The centromere is somewhere near the mid-point in this chromosome
• Pattern : It looks like English letter ‘L’.
• Arm : One arm is slightly shorter than the other.

3. Acrocentric :

• Structure : The centromere is near one end of this chromosome
• Pattern : It looks like the English letter ‘j’.
• Arm : One arm is much smaller than other.

4. Telocentric :

• Structure : The centromere is right at  the end of this chromosome.
• Pattern : This chromosome look like the English letter ‘i’.
• Arm : This chromosome consists of only one arm.

Types of chromosomes according to their function :

• Homologous chromosomes : If the pair consists of similar chromosomes by shape and organization, they are called homologous chromosomes.
• Heterologous chromosomes : If the chromosomal pairs are not similar they are called heterologous chromosomes.
• Sex chromosomes or allosomes : The chromosomal pair that decides the sex of the sexually reproducing organisms.
• Somatic chromosomes or autosomes : Chromosomes that decide the body characters other than sex.

Functions of DNA:

• DNA is called master molecule as it controls the functioning, growth and reproduction of the cell.
• The two main functions of the DNA are protein synthesis and the transmission of hereditary characters.
• Genes are actually the sequence of nucleotide in the DNA molecule. They are transmitted from parental generation to the offspring through chromosomes. In this way DNA is responsible for the genetic characters of the organisms.

Structure of DNA :

• Watson and Crick proposed the model for DNA structure in 1953.
• According DNA structure proposed by Watson and Crick, the DNA molecule is a double helix.
• Each strand of this helix is made up of nucleotides. Each nucleotide is made up of a phosphoric acid, a deoxyribose sugar and a nitrogenous base.

• Nitrogenous bases are of two types, viz. purines and pyrimidines. The purines are of two types, viz. adenine and guanine. Pyrimidines are of two types, viz, cytosine and thymine.
• The helices have hydrogen bonds with opposite sides. The adenine always pairs with thymine with double hydrogen bonds while cytosine always pairs with guanine with triple hydrogen bond.
• The deoxyribose sugar and the phosphoric acid alternate with each other and are arranged on the outer side of each helix.
• The genes in the form of nucleotide sequence are located on the DNA molecule which is transmitted through parental DNA to offspring.

Human Genome Project : In 1990, this project was together undertaken by all the geneticists of the world. In June 2000.

• Depending upon the findings of this project, scientists confirmed that the number of genes in the human genome is about 20,000 to 30,000.
• Later, scientists discovered the genomic sequence of many microorganisms.
• Due to research in genomics, disease causing genes can be identified.
• If disease causing genes are identified, genetic diseases can be diagnosed and properly treated.

Types of RNA according to their function :

• Ribosomal RNA (rRNA) : The molecule of RNA which is a component of the ribosome organelle is called a ribosomal RNA. Ribosomes helps in function of protein synthesis.
• Messenger RNA (mRNA) : Carries the information of protein synthesis from genes i.e. DNA chain in the cell nucleus to ribosomes in the cytoplasm which produce the proteins, is called messenger RNA.
• Transfer RNA (tRNA) : The RNA molecule which, according to the message of the mRNA carries the amino acid up to the ribosomes is called transfer RNA.

Seven mutually contrasting visible characteristics :

When the characteristics are sent to the next generation, they follow certain rules. The Mendel’s laws are based on these findings. Mendel’s laws are said to be the foundations of the modern genetics.

Monohybrid cross : Cross between two pea plants with only one pair of contrasting characters is called monohybrid cross.

• Parental generation (P1) consisted of tall and dwarf plants. These plants were crossed with each other.
• After crossing these plants the F1 generation is obtained. F is for filial. All the F1 offspring were tall and none of them was dwarf.
• Mendel called tallness as dominant character while dwarf is said to be recessive character as it was not showing its presence.
• Mendel concluded that the factors responsible for inheritance of characteristics are present in pairs. Now these factors are called genes.
• Dominant characters are written in capital alphabets while recessive ones are denoted by small letters.

The experiment and its results are shown in the Punnet Square.

• E.g. - TT Pair of genes of one parent tt - Pair of genes of another parent.
• TT and tt separate at the time of gamete formation.

Gametes of P1 generation : TT = T and T,  tt = t and t

First filial generation : In this experiment, Mendel observed that all plants of first filial generation (F₁) were tall. Even when one of the parents was dwarf, none of the offspring were dwarf. This observation made Mendel to give the principle of dominance and recessiveness.

• Tall is the dominant character whereas dwarf is the recessive character.
• Character of tallness hides the expression of dwarfness.
• If the factor for tallness is absent then only dwarfness is expressed.

Tallness or dwarfness is the external appearance. This is called a phenotype, whereas the genetic constitution is called a genotype.

• Phenotype : External appearance.
• Genotype: The factors or genes present in the offspring.

F₂ filial generation formed by : Self cross between F₁ plants.

The F₂ generations show following gametes

• Tt=T and t; Tt = T and t.

The genotypic ratio : In F₂ the cross shows following genotypic ratio.

1TT : 2Tt : 1tt

1 Homozygous dominant : 2 heterozygous : 1 homozygous recessive

• Phenotypic ratio= 3 tall plants: 1 dwarf plant are visible in F₂ progeny.

Phenotypic ratio = 3 : 1

Mendel’s figures : Total plants = 929, Tall plants = 705, Dwarf plants = 224

Mendel’s dihybrid cross : When two pairs of contrasting characteristics are under consideration, then it is called dihybrid cross.

Round and yellow seed bearing plant was crossed with green and wrinkled seed bearing plant to have a dihybrid cross. Round and yellow are dominant characters. Green and wrinkled are recessive characters.

Parental generation P₁ :

First parent : RR (Round seeds) + YY (Yellow seeds)

Second parent : rr (Wrinkled seeds) + yy(Green coloured seeds)

Gametes from RRYY parent = RY

Gametes from rryy parent = ry

F₁ generation

Genotype of these plants was RrYy, their phenotype was producing rounded-yellow seeds.

F₂ generation : Plants of the F1 generation of dihybrid cross produce four types of gametes RY, Ry, rY, ry. of these gametes, RY and ry are similar to those of the P₁ generation.

F₁ first filial generation :

Phenotype : All the plants formed show round and yellow seeds.

Genotype : All are heterozygous RrYy.

The offspring obtained through such a cross is called dihybrid.

The plants with RrYy genes produced four different kinds of combinations of gametes.

These are RY, Ry, rY and ry.

RY, Ry, rY and ry

F2 Second filial generation :

Phenotypes :

• 9 Round and yellow seeds
• 3 Round and green seeds
• 3 Wrinkled and yellow seeds
• 1 Wrinkled and green seeds
• Total phenotypic ratio : 9 : 3 : 3 : 1

Difference between Monohybrid cross and Dihybrid cross :

Difference between Phenotype and Genotype :

Disorders due to chromosomal abnormalities : Due to numerical changes

1. Down syndrome : Total chromosomes 47 (extra chromosome with the 21st pair)

Causes of disorder : Trisomy 21 (46+1, Trisomy of 21st Chromosome)

• This disorder is characterised by the presence of 47 chromosomes.
• This disorder have one extra chromosome with the 21st pair in every cell of their body.
• Therefore they have 47 chromosomes instead of 46.

Symptoms :  Children suffering from Down’s syndrome are usually mentally retarded and have a short lifespan. Other symptoms are,

• Mental Retardation
• Short lifespan
• Life expectancy
• Short height
• Short wide neck
• Flat nose
• Short fingers
• Scanty hair
• Single horizontal
• Crease on palm

2. Turner syndrome : Total chromosomes 45 (44+X)

Causes of disorder  :

• Abnormality in sex chromosome number (Monosomy of X chromosome)

• Instead of the normal 44+XX condition, Turner syndrome show a 44+X condition.

Symptoms :

• One X Chromosomes less
• Sterile females.
• Improper growth of reproductive Organs
• Mental retardation.

3. Klinefelter syndrome Total chromosomes 47 (44+XXY)

Causes of disorder  :

• Abnormality in sex chromosome number

Symptoms :

• Extra X chromosome in the cells
• Sexually sterile males
• Under developed reproductive organs

Monogenic disorders : Disorders or diseases occurring due to mutation in any single gene into a defective one are called monogenic disorders.

• There are approximately 4000 different monogenic disorders.
• Every gene has a sequence to synthesize a definite product such as an enzyme but the defective gene cannot do so. Either the essential enzyme is not synthesized or it is synthesized in deficient amount.
• Due to this condition some metabolic reactions does not occur in the body and unwanted toxic substances may accumulate within. These disorders are therefore known as inborn errors of metabolism.
• This abnormal metabolism may lead to death at a tender age.
• Examples of such disorders are  Hutchinson’s disease, Tay-Sachs disease, galactosaemia, phenylketonuria, sickle cell anaemia, cystic fibrosis, albinism, haemophilia, night blindness, etc.

Symptoms of Sickle-cell anaemia :

• Swelling of hands and legs,
• Pain in joints, severe general body aches,
• Frequent colds and cough,
• Constant low grade fever,
• Exhaustion,
• Pale face,
• Low haemoglobin content.

Diagnosis of sickle-cell anaemia : ‘Solubility Test’ ‘and Electrophoresis’ is performed to detect sickle cell anaemia.

Remedies for sickle cell anaemia :

• Marriages between the carriers or sufferers should not take place.
• A tablet of folic acid daily should be taken by a patient of sickle cell anaemia.

 Mitochondrial disorders :

• The mitochondrial DNA is inherited from mother only.
• If the mitochondrial DNA is mutated, then the disorders produced are called mitochondrial disorders.
• These disorders are thus transmitted from mother only. E.g. Leber’s hereditary optic neuropathy.

Polygenic disorders : (Disorders due to  mutations in multiple genes)

• Mutations in more than one gene at the same time cause polygenic disorder.
• Severe foetal disorders caused for the effects of environment on the foetus E.g. cleft lip, cleft palate, constricted stomach, spina bifida (a defect of the spinal cord), etc. are polygenic disorders. Diabetes, blood pressure, heart disorders, asthma, obesity are also polygenic disorders.
• Polygenic disorders do not strictly follow. Mendel’s principles of heredity. They arise from a complex interaction between environment, life style and defects in several genes.

Inter-relationship between tobacco addiction and cancer :

Tobacco is either smoked or chewed. Any kind of tobacco product is carcinogenic.

Nicotine in the tobacco is the main carcinogenic agent.

Harmful effects of smoking :

• adverse effects on digestion
• instability and trembling of fingers
• dry cough causing sleeplessness
• shortening of life span
• chronic bronchitis
• cancer of the lungs, mouth, larynx (voice box), pharynx, pancreas, urinary bladder, etc.
• central and peripheral nervous system affected
• arteriosclerosis and hypertension
• visual disorders or tremors

Harmful chemicals in the tobacco smoke :

• pyridine
• ammonia
• aldehyde furfural
• carbon monoxide
• nicotine
• sulphur dioxide
• minute carbon particles