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The Principles of Genetics
Mendelian GeneticsMonohybrid cross
Dihybrid crossPolyhybrid cross
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Definition, Concepts and Topics of Genetics• The word Genetics is derived from the Greek word genetikos, "genitive" combined with genesis, "origin" • Genetics is the most exciting field of biology, it is the science of genes, heredity and variation in living organisms. It is concerned with mechanisms of heredity by which traits “characters” are passed from generation to generation• Genetics also deals with the molecular structure and function of genes, e.g. gene behavior in the context of a cell or organism, patterns of inheritance from parents to offspring, and gene distribution, variation and change in populations• Genes are universal to all living organisms, and are applied to the study of all living systems, from viruses and bacteria, to plants and animals, Special attention is given to crop plants and domestic animals and humans
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Characters studied by Mendel Seven Characters in Garden peas
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Why Garden pea (Pisum sativum) ?
• Garden pea has a short life cycle
• Garden pea has several observable contrasting characters
• Garden pea is a self pollinating plant
• The general features for a good genetic experimental organism are:-
• short life cycle• easy to grow and breed• produce large number of offspring
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The monohybrid cross, Law of segregation Inheritance of stem length in Garden pea
Parents
F1
F2
Tall Dwarf
Tall
3 Tall 1 Dwarf
DD
Dd
DD Dd Dd dd
dd
Phenotype
Genotype
Dominant Recessive
DD = Homozygous dd = HomozygousDd = Heterozygous
Phenotype
Genotype
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Explanation of the law of segregation Inheritance of stem length in Garden pea
♂♀
D d
D DD
Tall
Dd
Tall
d Dd
Tall
dd
Dwarf
3- Tall 1- Dwarf
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Law of independent assortmentInheritance of seed shape and color in Garden pea
Dwarf plants, wrinkled seeds
ddww
Tall plants, Round seeds
DDWWParents
dw DW P. Gametes
Phenotype: Tall plants Round seeds
Genotype: DdWwF1
W DWD
w Dw
W dWd
w dw
F1
Gametes
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Mendel’s Foundations of Genetics
• Genes are found in allelic pairs
• One allele is dominant and the other is recessive
• Law of segregation: Each character is controlled by a pair of (factors) genes that segregate at
meiosis and reunite in the offspring
• Law of independent assortment: Each pair of genes segregate independently of all other pairs during gamete formation
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About Chromosomes• What is a chromosome?• Eukaryotic DNA: multiple linear diploid chromosomes• Number of chromosomes (types) depends on species• Chromosomes replicate
Unwound form of DNA = Chromatin
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• Terms: chromosome vs chromatid chromatin vs condensed chromosome
centromere vs centriole dulipcated chromosome vs
homologous pair
• 2 cell types: 1. somatic cell: body diploid = 2 sets 2. gamete: sex cell M & F haploid = 1 set
• Human genome has 23 types of chromosome 22 autosomes + 1 sex chromosome (#23) human conception = 23M + 23F = 46 chromosomes NOTE: these are NOT stuck together – NOT identical
• How do corresponding M & F chromosomes compare? Same genes in same corresponding positions…but… Are they exactly alike? Alleles: different forms of a gene
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Eukaryotic cell division• 2 ways that Eukaryotic cells divide: 1.
Mitosis: cell division for growth, healing & asexual repro. 1 diploid set 2 identical diploid sets (1 round)
somatic cells only somatic cells genetically identical cloning?
2. Meiosis: cell division (2 rounds) for gamete formation 1 diploid set 2 diploid sets 4 haploid sets NOT identical genetic variability strategy of sexual reproduction
• What is replication? It precedes cell division 46 X 2 = 92
+ replication
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Cell cycle90%
Replication happens here
23 x 2 23 x 2 x 2
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