ISC 12>UNIT 3> GENETICS> 1.ELEMENTS OF HEREDITY AND VARIATION
SCOPE OF SYLLABUS
Principles of inheritance and variation
- Explanation of the terms heredity and variation;
- Mendel's Principles of inheritance; reasons for Mendel's success;
- Definition of 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 using Punnett square.
- Incomplete dominance with examples from plants (snapdragon - Antirrhinum) and co-dominance in human blood group, multiple alleles – e.g. blood groups,
- Polygenic inheritance with one example of inheritance of skin colour in humans (students should be taught examples from human genetics through pedigree charts. They should be able to interpret the patterns of inheritance by analysis of pedigree chart).
- Biological importance of Mendelism.
- Pleiotropy with reference to the example of Phenylketonuria (PKU) in human beings and starch synthesis in pea seeds.
- Chromosomal theory of inheritance; autosomes and sex chromosomes (sex determination in humans, fruit fly, birds, honey bees and grasshopper),
- Sex-linked inheritance - with reference to Drosophila (colour of body-yellow and brown; and colour of eyes-red and white), and man (haemophilia and colour blindness),
- Definition and significance of linkage and crossing over.
- Mutation: spontaneous, induced, gene (point – transition, transversion and frame-shift);
- Chromosomal aberration: euploidy and aneuploidy;
- Human genetic disorders: phenylketonuria, thalassaemia, colour blindness, sickle cell anaemia;
- Chromosomal disorders: Down’s syndrome, Klinefelter’s syndrome, Turner’s syndrome.
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.
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 or in F1 generation 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 or in F1 generation is called dominant.
Recessive- the trait that remains hidden in the heterozygote is called recessive.
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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
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The contrasting characters selected by Mendel
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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.
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
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