Basic Concept of Allele, Homologous Chromosomes,Sister & Non-Sister Chromatids

For detailed description on gene and allele you can watch this video as well
https://youtu.be/H-XEzXqXqVU

Alleles are alternate pairs of genes. alleles occur in pairs in Mendelian genetics. Alleles can be both in dominant state, both in recessive state,one dominant one recessive. Character is the visible thing. For example plant height is a character. The varieties of character are called as traits.. For example- height is of two types tall n dwarf. Tall n dwarf are traits. So varities if genes are alleles n varities of character are traits. Character is due to gene and traits are due to alleles.
There are 23 pairs of homologous chromosomes. Each pair has one from paternal side and one from maternal side. When fertilization occurs an egg with 23 chromosomes fuses with a sperm with 23 chromosomes resulting in 46 number chromosome zygote.
Genotype is the pattern of alleles of an individual. It is combination of alleles for a particular gene or locus.
Phenotype is the physical observable characters or appearance of an individual. For example- TT is the genotype and tall is its phenotype.
When meiosis occurs,in S phase after replication, every chromosome has two chromatids each. These chromatids are called as sister chromatids. They have same DNA. Homologous chromosomes also have same DNA. But sister chromatids are more similar to one another. Two chromosomes which are homologous have less similarity than that of two sister chromatids.
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Видео Basic Concept of Allele, Homologous Chromosomes,Sister & Non-Sister Chromatids канала Shail's Biology Nest