Write notes on : (a) recessive lethal genes (b) pleiotropy (c) linkage groups (d) mitochondrial inheritance (e) human karyotype (f) human genome
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RECESSIVE LETHAL GENES ----
A pair of identical alleles that are both present in an organism that ultimately results in death of that organism are referred to as recessive lethal alleles. Though recessive lethals may code for dominant or recessive traits, they are only fatal in the homozygous condition.
PLEIOTROPY ---
Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits. Therefore, a mutation in a pleiotropic gene may have an effect on several traits simultaneously due to the gene coding for a product used by a myriad of cells or different targets that have the same signaling function.
Pleiotropy can arise from several distinct but potentially overlapping mechanisms, such as gene pleiotropy, developmental pleiotropy, and selectional pleiotropy.
An example of pleiotropy is phenylketonuria
LINKAGE GROUPS ---
Linkage group, in genetics, all of the genes on a single chromosome. They are inherited as a group; that is, during cell division they act and move as a unit rather than independently. The existence of linkage groups is the reason some traits do not comply with Mendel’s law of independent assortment (recombination of genes and the traits they control); i.e., the principle applies only if genes are located on different chromosomes. Variation in the gene composition of a chromosome can occur when a chromosome breaks, and the sections join with the partner chromosome if it has broken in the same places. This exchange of genes between chromosomes, called crossing over, usually occurs during meiosis, when the total number of chromosomes is halved.
Sex linkage is the tendency of a characteristic to be linked to one sex. The X chromosome in Drosophila flies and human beings, for example, carries a complete set of genes; the Y chromosome has only a few genes. Eggs of females carry an X chromosome; sperm of males may carry an X or a Y. An egg fertilized by a sperm with an X chromosome results in a female; one fertilized by a sperm with a Y chromosome results in a male. In offspring with the XY chromosome pair, any trait carried by the X chromosome will appear unless there is a corresponding gene (allele) on the Y chromosome.
MITOCHONDRIAL INHERITANCE ---
Mitochondrial inheritance: The inheritance of a trait encoded in the mitochondrial genome. Because of the oddities of mitochondria, mitochondrial inheritance does not obey the classic rules of genetics. Persons with a mitochondrial disease may be male or female but they are always related in the maternal line and no male with the disease can transmit it to his children.
The mitochondria are responsible for energy production. They consist of two sets of membranes, a smooth continuous outer coat and an inner membrane arranged in tubules or in folds that form plate- like double membranes (cristae). The mitochondria are in fact the principal energy source of the cell (thanks to the cytochrome enzymes of terminal electron transport and the enzymes of the citric acid cycle, fatty acid oxidation, and oxidative phosphorylation). The mitochondria convert nutrients into energy as well as doing many other specialized tasks.
HUMAN KARYOTYPE
Karyotyping is the process by which cytogeneticists take photographs of chromosomes in order to determine the chromosome complement of an individual, including the number of chromosomes and any abnormalies.
Karyotypes describe the chromosome count of an organism and what these chromosomes look like under a light microscope. Attention is paid to their length, the position of the centromeres, banding pattern, any differences between the sex chromosomes, and any other physical characteristics The preparation and study of karyotypes is part of cytogenetics.
The study of whole sets of chromosomes is sometimes known as karyology. The chromosomes are depicted (by rearranging a photomicrograph) in a standard format known as a karyogram or idiogram: in pairs, ordered by size and position of centromere for chromosomes of the same size.
The basic number of chromosomes in the somatic cells of an individual or a species is called the somatic number and is designated 2n. In the germ-line (the sex cells) the chromosome number is n (humans: n =23).p28 Thus, in humans 2n = 46.
So, in normal diploid organisms, autosomal chromosomes are present in two copies. There may, or may not, be sex chromosomes. Polyploid cells have multiple copies of chromosomes and haploid cells have single copies.
The study of karyotypes is important for cell biology and genetics, and the results may be used in evolutionary biology (karyosystematics)[5] and medicine. Karyotypes can be used for many purposes; such as to study chromosomal aberrations, cellular function, taxonomic relationships, and to gather information about past evolutionary events.
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