The Principles of Genetics

Mendelian GeneticsMonohybrid cross

Dihybrid crossPolyhybrid cross

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

Characters studied by Mendel Seven Characters in Garden peas

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

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

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

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

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

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

• 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

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

Cell cycle90%

Replication happens here

23 x 2 23 x 2 x 2