Genetics

A gene is a nucleotide sequence situated at a precise location on a chromosome. It carries a piece of hereditary information that influences a specific characteristic.
Alleles are versions of a gene, differing from one to another on a small number of bases.

Different alleles exist because of DNA mutations. Mutations are the only process creating genetic diversity. 
Those mutations can be the result of:

1. Basic DNA polymerase errors during replication
2. DNA alterations following exposition to chemicals or high-energy radiations (X-rays…)

Mutations usually end up with the replacement of one base by another (substitution), adding a few bases within the sequence (insertion) or removing a few bases from the sequence (deletion).

A small difference in a DNA sequence can cause huge consequences to cells and body functions.

People with sickle cell anemia experience chronic anemia and painful episodes.
This genetic disease is caused by a base substitution, meaning a Thymine is replacing an Adenine in the beta globin gene.
This causes different hemoglobin 3D shapes (a Valine replacing a Glutamic acid in sixth position) which leads the hemoglobin of the patients to form long chains.
Those hemoglobin chains give the red blood cells they fill a sickle shape. 
This sickle shape makes it difficult for the red blood cells to navigate through smaller blood vessels leading to the symptoms.

DNA technology had grown fast during the past decades.

Sequencing = it is now possible to unravel the full DNA sequence of a living organism. Different models have been sequenced (E. Coli, Saccharomyces Cerevisiae, Fruit fly, Human…) and species sequence comparisons help understand Life evolution.

Profiling = humans differ not only in the versions of their genes but also in the number of repetitions of small untranscripted base sequences (short tandem repeat). Analyzing those numbers can help you solve a crime (comparing DNA traces on a scene with suspect DNA) or to find your father (paternity test).

Modifying = scientists can modify the DNA of an organism by transferring a gene from the same species, or another one (GMOs).

Cloning = besides natural cloning achieved by asexual reproduction, Humans can clone plants and some animals with the use of DNA technology.

Chromosomes are DNA molecules. 
There is only one circular chromosome in prokaryotic cells, whereas in eukaryotic nuclei, there are linear chromosomes (more than one).

Scientists can study the entire set of chromosomes, or karyotype. The karyotype of an individual gives information about the species it belongs to. 
In Humans, each cell contains 23 pairs of chromosomes (diploid state). The last pair determines the sex of the individual (XX:women and XY:men). As part of the life cycle, individuals produce gametes (spermatozoa or ovum) containing only 23 chromosomes (haploid state) through meiosis.
Meiosis consists of two cell divisions, without DNA replication in between:

1. First division splits each chromosome pair ;
2. Second division splits each double chromosome into two simple chromosomes.

The meiosis process brings genetic variation.
Random fertilization amplifies diversity.

Random positioning of chromosomes pairs in metaphase of the first meiotic division gives many sorting possibilities (2ⁿ, n being the number of chromosomes pairs).
In the first meiotic division, homologous chromosomes pair up during prophase and can exchange a part of their DNA through chromosome crossover. This process gives new combinations of alleles in sexual cells.
Fertilization occurs after the random encounter of two sex cells, which raises the genetic diversity brought by sex reproduction.

Sometimes, chromosomes sorting in meiosis fails and produces gametes with an uneven number of chromosomes. If selected for fertilization, this ends with an abnormal number of chromosomes in the child‘s cells (which can lead to trisomy…).

A genotype consists of allele combination whereas a phenotype mainly depends on dominant alleles only.
In order study genetic inheritance, scientists use crosses in animals and follow human pathologies with pedigree charts.

Crosses in animal = Since Mendel‘s studies on peas, scientists have crossed homozygous individuals with different characteristics to unravel dominant and recessive alleles. The heterozygous descendant (F1) inherits its phenotype from his parent having the dominant allele. With self-fertilization of F1 individuals, you end up with 3:1 ratio in F2 of dominant vs. recessive phenotypes.

Pedigree charts in humans = Data gathered on genetic pathologies in a family can be of interest to clarify its transmission, dominant (Huntington) or recessive (sickle cell); autosomal (cystic fibrosis) or sex related (color blindness).