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A condition in which an extra copy of a chromosome is present in the cell nuclei, causing developmental abnormalities.  Trisomy 13 (Patau syndrome)  trisomy 21 (Down syndrome)  trisomy 18 (Edward syndrome)

 Reductional division- the child gets half of its chromosomes from each parent  Provides genetic diversity.  Separation of chromosomes follows law of independent assortment where each gamete receives a selection of parental chromosomes. The chance of getting same chromosomes in two gametes is approximately 1 in 8 million.  Because of crossing over each chromatid receives DNA derived from both parental homologous chromosomes referred to as gene shuffling

 It is the process of nuclear division that occurs during the final stage of gamete formation.  It has two cell divisions- meiosis 1 which is reductional division and meiosis 2 which is equational division resulting in 4 daughter cells.  Prophase 1 is longer and has 5 stages-leptotene, zygotene, pachytene, diplotene and diakinesis  The chromosomal complement is reduced to haploid set of chromosomes (n-23)  Duration to complete one meiosis varies from few days to years

 A karyotype refers to a full set of chromosomes from an individual arranged according to length, position of centromere, banding pattern.  Karyotype is written as total number of chromosomes followed by sex chromosomes  Normal male karyotype: 46,XY, normal female karyotype: 46,XX  Chromosomes are classified into 7 groups and pasted accordingly- Group A- 1, 2, 3; group B- 4, 5; group C- 6-12, X; group D- 13, 14, 15; group E- 16, 17, 18; group F- 19, 20; group G- 21, 22, Y

 An autosome is a chromosome not involved in sex determination and is numbered.  Humans have 22 pairs of autosomes and one pair of sex chromosomes (the X and Y)  Autosomal dominant genes are expressed in heterozygous state.  Autosomal recessive genes are expressed in homozygous state.

 A genome is an organism's complete set of DNA, including all of its genes.  Each genome contains all of the information needed to build and maintain that organism.  In humans, a copy of the entire genome—more than 3 billion DNA base pairs—is contained in all cells that have a nucleus.  The genome includes both the genes and the non-coding sequences of the DNA/RNA  The Human Genome Project was an international research effort to determine the sequence of the human genome and identify the genes that it contains.

 A gene that is phenotypically expressed in the homozygous state but has its expression masked in the presence of a dominant gene is called recessive gene.  The person with one normal and one recessive gene will not express any abnormal features and is called as carrier.  The recessive genes are expressed in the individuals of same generation leading to horizontal transmission.  Examples- Cystic fibrosis, Alkaptonuria, Albinism