Heredity class 10 notes

 

HEREDITY

Heredity: It refers to the transmission of characters or traits from the parents to their offspring. Heredity is the continuity of features from one generation to another which are present in fertilized egg or zygote. The zygote develops into an organism of a particular type only.

Genetics: It is the branch of biology which deals with heredity and variation. Genetics is to help our understanding of heredity by knowing how offspring inherit characteristics from their parents.

Variation: It means the differences in the characters or traits among the individuals of a species. Variations occur during reproduction both because of error in DNA copying and as a result of sexual reproduction. Variations contribute to evolution.
There are two types of variation- somatic variation and gametic variation.

Importance of variations

·         It is the basis of the heredity.

·         It is the basis of the evolution also.

·         It increases the chances of the survival of the organism according to the changing environment.

Causes of variation

·      The most common causes of variations are mutation, recombination and random mating.

·      Recombination or crossing over is one of the important reasons for variation. It is a exchange of chromosome segment at the time of gamete formation.

·      Interaction of genes with environmental changes (adaptations).

Mendel and his contribution in Genetics

G.J. Mendel started his work on Pisum sativum (garden pea). He was known as Father of genetics.

He had chosen seven pair of contrasting character-

The reason of choosing garden pea for experiment was-

·         The flowers of this plant are bisexual. ( hermaphrodite)

·         They are self-pollinating, and thus, self and cross pollination can easily be performed. 

• Easy to obtain pure breed plant through self-fertilization

·         The different physical characteristics were easy to recognise and study. 

·         They have a shorter life span and are the plants are easier to maintain.

 

Main reasons for the success of Gregor Mendel: 

·         1. Mendel concentrated in one or few characters at a time.

·         2. He made controlled crosses and kept careful numerical records of the results.

·         3. He suggested 'factors' as a cause of characters.

·         4. The experimental material Pisum sativum was a wise choice.

Some important terms

1. Chromosomes are long thread-like structures present in the nucleus of a cell which contain hereditary information of the cell in the form of genes.

2. DNA is a chemical in the chromosome which carries the traits in a coded form.

3. Gene is the part of a chromosome which controls a specific biological function.

4. Contrasting characters: A pair of visible charactes such as tall and dwarf, white and violet flowers, round and wrinkled seeds, green and yellow seeds etc.

5. Dominant trait: The character which expresses itself in a (Ft) generation is dominant trait. Example : Tallness is a dominant character in pea plant.

6. Recessive trait: The character which does not express itself but is present in a generation is recessive trait. Ex. dwarfism in the pea plant.

7. Homozygous: A condition in which both the genes of same type are present for example; an organism has both the genes for tallness it is expressed as TT and genes for dwarfness are written as tt.

8. Heterozygous: A condition in which both the genes are of different types for example; an organism has genes Tt it means it has a gene for tallness and the other for dwarfness only tall character is expressed.

9. Genotype: It is genetic make up of an individual for example; A pure tall plant is expressed as TT and hybrid tall as Tt.

10. Phenotype: It is external appearance of the organism for example; a plant having Tt composition will appear tall although it has gene for dwarfness.

11. Homologous pair of characters are those in which one member is contributed by the father and the other member by the mother and both have genes for the same character at the same position.

12. Monohybrid Cross

When one pair of contrasting characters was taken to cross two pea plants, it is known as monohybrid cross.

Fig.1. depicts the monohybrid cross between true breeding yellow pod and true breeding green pod. All the pods obtained were green in colour. The offspring’s obtained are known as F1 progeny or First filial generation.

Example:-

In case of heterozygous condition,

In the above figure, the parents are heterozygous,

 Phenotype:  3 purple flower and 1 white flower

Genotypically:  1 homozygous dominant (BB), 2 heterozygous dominant (Bb) and 1 homozygous recessive (bb).

Mendel Laws

Law of Dominance:  This law states that in a heterozygous condition, the allele whose characters are expressed over the other allele is called the dominant allele and the characters of this dominant allele are called dominant characters. The characters that appear in the F1 generation are called as dominant characters. The recessive characters appear in the F2 generation.

Law of Segregation:  This law states that when two traits come together in one hybrid pair, the two characters do not mix with each other and are independent of each other. Each gamete receives one of the two alleles during meiosis of the chromosome.

Law of Independent Assortment: Each pair of alleles segregates independently of other pairs of alleles during gamete formation As the individual heredity factors assort independently, different traits get equal opportunity to occur together.

It means different genes and their alleles are inherited independently within sexually reproducing organisms. During meiosis, chromosomes are separated into multiple gametes. Genes linked on a chromosome can rearrange themselves through the process of crossing-over. Therefore, each gene is inherited independently.  

Mendel developed the Law of Independent Assortment after breeding two different pea plants with two different characteristics;

 He bred plants with yellow, round peas with plants that had wrinkled, green peas. Since yellow and round were dominant over wrinkled and green, all the offspring had yellow, round peas.

But, when this first generation was crossbred with each other in a dihybrid cross, there was a lot of variation in the second generation. Peas were no longer either just yellow and round or green and wrinkled; some were green and round, while some were yellow and wrinkled. Furthermore, the offspring showed their characteristics in a ratio of 9:3:3:1. Nine were round and yellow, three were round and green, three were wrinkled and yellow, and one was wrinkled and green. This ratio stayed the same even when hundreds of dihybrids were crossed.

This occurred because each of the parent plants only gave their offspring one allele and because yellow and round were dominant traits and masked the green and/or wrinkled traits in certain individual plants. The diagram below depicts Mendel’s dihybrid cross.

Mendel’s experiment showed that the alleles for round or wrinkled peas were inherited independently from the alleles for yellow or green peas since the plants were not just round and yellow or green and wrinkled. We now know that they exist on different chromosomes, which allows them to be mixed up during the process of meiosis.

Sex determination in human beings: 

In human beings, all chromosomes are not paired. 22 chromosomes are paired but one pair called sex chromosome is odd in not having a perfect pair in males. Females havea perfect pair both represented by XX. On the other hand males have a normal sized X but the other is short called Y so it is shown as XY. All gametes or ova formed by the homogenetic female are similar i.e. have X chromosome. Males heterogenetic form two types of sperms i.e. half with X chromosome and the other half with Y chromosome. Sex of the baby will depend on fertilization. There are two possibilities :

Autosomes: Those chromosomes which do not play any role in sex determination.

Sex chromosomes: Those chromosomes which play a role in determining sex of the newborn.

·         If the sperm having X chromosome fertilizes with ovum with X chromosome then the baby will have XX chromosome and it will be female.

·         If the sperm having Y chromosome fertilizes with ovum with X chromosome then the baby will have XY chromosomes and it will be male.

 

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