ISC 12>UNIT 3> GENETICS> 1.ELEMENTS OF HEREDITY AND VARIATION
SCOPE OF SYLLABUS
Fundamentals of Genetics: concept of alleles: dominant and recessive; phenotype and genotype, homozygous and heterozygous, mono and dihybrid crosses.
Homologous chromosomes, autosomes and sex chromosomes; alleles – dominant and recessive; phenotype; genotype; homozygous; heterozygous, monohybrid and dihybrid crosses; back cross and test cross, definitions to be taught with simple examples.
Homologous chromosomes, autosomes and sex chromosomes; alleles – dominant and recessive; phenotype; genotype; homozygous; heterozygous, monohybrid and dihybrid crosses; back cross and test cross, definitions to be taught with simple examples.
GENETICS
Genetics is the study of the way in which inheritable factors are coded in the cells of an organism, how this coding is maintained during cell reproduction, how it is passed from one generation to the next, and how it is expressed in physical features
HEREDITY AND VARIATION
The transfer of characters in living beings from parent to offspring generation after generation is called HEREDITY.
The difference among individuals of a species is called VARIATION.
HEREDITY AND VARIATION
The transfer of characters in living beings from parent to offspring generation after generation is called HEREDITY.
The difference among individuals of a species is called VARIATION.
Father of Genetics
Gregor John Mendel (1822-84) was an Austrian monk and teacher. He conducted experiments on plant hybridization. using garden pea plant (Pisum sativum). Mendel worked out the patterns of inheritance before any knowledge of DNA, genes and chromosomes were available.
Mendel did not get the recognition for his valuable work in his lifetime.
Mendel’s work was rediscovered by Hugo deVries, Karl Correns and Eric Von Tschermak, (1900) independently. They named this as Mendel’s laws or Mendelism.
W.Bateson found that same laws apply to animals also.
Mendel did not get the recognition for his valuable work in his lifetime.
Mendel’s work was rediscovered by Hugo deVries, Karl Correns and Eric Von Tschermak, (1900) independently. They named this as Mendel’s laws or Mendelism.
W.Bateson found that same laws apply to animals also.
Characters selected by Mendel
The contrasting characters selected by Mendel
- Length of the stem: Tall/ Dwarf
- Position of flowers: Axial/ Apical or terminal
- Colour of flower: Purple/white
- Shape of the seed: Round/ Wrinkled
- Colour of seed: Green/ Yellow
- Colour of the pod: Green/ Yellow
- Shape of the pod: inflated/ Constricted
Reason for Mendel's success
Mendel was to a certain extent lucky as he chose characteristics that are controlled by single genes.
He succeeded where his predecessors did not was due to-
1. His method of working-, he worked with only one character at a time.
2. He carried out experiments to the second and the third generation
3. He built up sufficient amounts of accurate data so that he could come to a scientific conclusion.
4. The performed the crossing techniques with proper care during cross-fertilization. He emasculated the flowers early to avoid self-pollination and bagged the flowers after pollination to avoid the deposition of pollen from other plants.
Why Mendel selected the Pea plant?
1. Pea plants have bisexual flowers.
2. Pea flowers are self-pollinating flowers, thus are mostly pure line as they bear the same character generation after generation.
He succeeded where his predecessors did not was due to-
1. His method of working-, he worked with only one character at a time.
2. He carried out experiments to the second and the third generation
3. He built up sufficient amounts of accurate data so that he could come to a scientific conclusion.
4. The performed the crossing techniques with proper care during cross-fertilization. He emasculated the flowers early to avoid self-pollination and bagged the flowers after pollination to avoid the deposition of pollen from other plants.
Why Mendel selected the Pea plant?
1. Pea plants have bisexual flowers.
2. Pea flowers are self-pollinating flowers, thus are mostly pure line as they bear the same character generation after generation.
Some terminologies
Character- The feature of an individual, hair colour, flower colour, height etc.
Trait- The variant of the character- black or brown hair colour.
Gene- A gene is an ordered sequence of nucleotides located in a particular position on a particular chromosome that encodes a specific functional product. It is the fundamental physical and function units of heredity.
Allele- The alternative forms of a gene on the same locus (the position on the chromosome) is called as alleles.
Genotype- The genetic constitution of an organism.
Phenotype- Observable characteristics of an organism produced by the organism's genotype interacting with the environment.
Homozygous- An individual with both identical alleles at one locus (position).
Heterozygous – An individual with different alleles at a locus (position)
Dominant- the trait expressed in the heterozygote is called dominant.
Recessive- the trait that remains hidden in the heterozygote is called recessive.
Trait- The variant of the character- black or brown hair colour.
Gene- A gene is an ordered sequence of nucleotides located in a particular position on a particular chromosome that encodes a specific functional product. It is the fundamental physical and function units of heredity.
Allele- The alternative forms of a gene on the same locus (the position on the chromosome) is called as alleles.
Genotype- The genetic constitution of an organism.
Phenotype- Observable characteristics of an organism produced by the organism's genotype interacting with the environment.
Homozygous- An individual with both identical alleles at one locus (position).
Heterozygous – An individual with different alleles at a locus (position)
Dominant- the trait expressed in the heterozygote is called dominant.
Recessive- the trait that remains hidden in the heterozygote is called recessive.
Monohybrid inheritance
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Monohybrid inheritance is the study of inheritance of a single characteristic. This involves the inheritance of two alleles of a single gene.
Consider an example of plant height- In pea plant the gene for plant height has two alleles- tall (T) and dwarf (t). In this case there are three possible genotypes for plant height. TT- Homozygous tall Tt- Heterozygous tall tt- Dwarf. To perform a cross, homozygosity or pure line has to be ascertained. (i.e. to ensure that the plants are pure for their trait.) This type of a cross that involves, a single character is called as MONOHYBRID CROSS. Thus a monohybrid cross can be stated as a cross between two parents that breed true for different versions of a single trait. |
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http://www.siskiyous.edu/class/bio1/genetics/monohybrid_v2.html
http://biology.clc.uc.edu/courses/bio105/geneprob.htm