Meiosis: Meiosis is the process of cell division that is unique to germ cells. It produces haploid eggs and sperm from diploid progenitors. It occurs in two stages, Meiosis I and Meiosis II, each of which goes through the four main stages of mitosis, although
in a different way that allows for genetically variable daughter cells. Meiosis I: During prophase I, the chromosomes condense as the nuclear envelope breaks down. By this point all of the cell's genetic material has replicated, so each chromosome is an
X-shape consisting of two identical chromatids. Also during this phase, homologous pairs of chromosomes line up and undergo crossing over - homologous sections of the chromosomes swap. This process increases the genetic diversity of the haploid cells
that are produced at the end of meiosis. As with mitosis, the centrosomes and spindle fiber structures form during prophase I.
Unlike mitosis, where the chromatids are separated, in meiosis I each chromosome is separated from its homologous chromosome. In metaphase I each chromosome will "join" with its homologous chromosome (forming a tetrad) and align across the
centerline of the cell. Each chromosome is separated from its homologue during anaphase I, and during telophase I and cytokinesis the cell divides completely, forming two diploid daughter cells with differing DNA.
Some cells go into a rest stage, sometimes known as interphase II, after meiosis I is complete. Often, the nuclear envelope reforms and chromosomes uncondense prior to meiosis II. Meiosis II: Meiosis II is more similar to mitosis, except that the parent
cells are diploid instead of tetraploidal (which is how the mitosis parent is after replication during interphase). Each cell divides the same way as in mitosis, with the chromosomes splitting at their centromeres. Note that because crossing over during prophase
I occurs independently on each chromatid, the four daughter cells produced as a result of meiosis II are typically all genetically different. Post-Meiosis: In spermatogenesis, typically all of the daughter cells can become viable sperm. In contrast, during
oogenesis only one of the four daughter cells will become a viable egg cell. The egg cell will receive most of the cytoplasm and organelles, while the remaining three daughter cells become shr unken polar bodies. Most of the organelles brought by the sperm
cell are destroyed after fertilization. Incidentally, this is what allows matrilineal heredity to be traced via mitochondrial DNA.
Genetic Disorders : Genetic disorders are inherited medical conditions caused by abnormalities in the DNA. There are a variety of types of genetic disorders, and some are rarer than others. They are typically caused by mutations in specific genes, deletion
of genes, or a person having an additional chromosome. While these genes can be known as disease-causing genes, the abnormality of a gene is the cause of the disorder. One of the most common genetic disorders is known as trisomy 21, or Down
Syndrome. An individual with this disorder has a third copy of chromosome 21. Cystic fibrosis is also a genetic disorder, caused by mutation in a protein known as CFTR. Even color blindness is a genetic disorder, caused by a mutation on the X
chromosome. Polysomy and Monosomy: Polysomy is when an individual has more than two copies of a particular chromosome. Most often this is a trisomy, such as trisomy 21 (Down
Syndrome) - affected individuals have three copies of chromosome 21. Monosomy is when an individual has one copy of a chromosome rather than the normal two copies. The only known
monosomy in which individuals survive to birth is Turner's Syndrome, a monosomy of the sex chromosomes in which affected individuals are females that have a single X chromosome.
Addition, Deletion, Translocation: Addition, deletion and translocation are all different forms of mutations. Addition (also known as insertion or an insertion mutation) is the addition of
nucleotides into a DNA sequence. Additions can range in size from one base pair to entire sections of chromosomes being added in the wrong place. Deletion is a similar concept, but with the
removal of nucleotides. In deletion, a part of a chromosome or DNA sequence is lost during replication. Any number of nucleotides can be deleted, though small deletions are typically less
dangerous. Large deletions can be fatal, and some can result in various genetic disorders such as Williams syndrome.Translocation is when the parts of a chromosome are rearranged,
occasionally resulting in a genetic disorder. Translocation can be balanced or unbalanced, with unbalanced translocation resulting in missing or extra genes. There are multiple forms of
translocation, but the most common is reciprocal translocation. This occurs when two parts of two chromosomes swap places, resulting in genes changing locations and occasionally gene fusion.
Balanced translocation occurring during meiosis typically doesn't result in any visible symptoms, though in about 6% of cases it can result in autism or congenital abnormalities. However,
translocation occurring in somatic cells during mitosis can result in various forms of cancer. Translocation can also result in infertility, or in specific cases a form of Down syndrome.
Karyotypes : A karyotype is a chart that shows each chromosome. Each karyotype displays 23 pairs of chromosomes, including the X/Y chromosomes. Every pair is assigned a number (except
for the sex chromosomes; they are always referred to as the X and Y chromosomes). Some genetic disorders can be detected by analyzing the number of chromosomes and/or the sex
chromosomes. The gender of the individual can also be deduced from looking at the sex chromosomes. If there is an X and a Y, the individual is a male. A female has two X chromosomes and
no Y chromosome. A karyotype is created by stopping cells in cell division and staining the chromosomes, then observing them under a light microscope. Karyotypes can be used to diagnose
genetic diseases - most often polysomy or monosomy, but also some types of deletion and addition in certain chromosomes. For example, a karyotype can reveal a third chromosome
21, resulting in Down syndrome. It can also reveal Turner syndrome (45, X), a disorder that results in females with one X chromosome, and Klinefelter's syndrome (47, XXY), in
which a man has two X chromosomes and one Y chromosome.
Sex determination In humans, the male and female share 22 of the 23 pairs of chromosomes in each body cell. The 23rd pair is known as the sex chromosomes because it
determines the sex of the individual. In the male, the sex chromosome consists of an X and a Y chromosome(XY) while the pair in females consists of two X chromosomes(XX). The
male is the one who determines the sex of the child and the female gives an X to all eggs while the male randomly produces about 50% X sperm and 50% Y sperm. In rare cases,
through nondisjunction, a person will have three sex chromosomes. If they have three X (XXX) chromosomes, they are female. If they have even one Y chromosome (XXY), they are
male. Although they will show more feminine qualities, any person who has a Y chromosome is considered a male. Other types of sex chromosome polysomy, as well as one
monosomy (X), have been known to occur, though more rarely.
Common Genetic Disorders: Several genetic disorders appear frequently on tests as examples. While this event typically does not deal with the specifics of each disorder, it may be
useful to know the inheritance patterns of some common disorders:
●Cystic Fibrosis - Autosomal recessive - C ystic fibrosis (CF) involves the production of mucus that is much thicker and more sticky than usual. It mainly affects the
lungs and digestive system. CF is a hereditary condition that occurs in a child when both parents have the defective gene. There is no cure, but good nutrition and taking steps to thin
mucus and improve mucus expectoration can help. In autosomal recessive inheritance, a genetic condition occurs when one variant is present on both alleles (copies) of a given gene.
If any affected founding daughter has 2 unaffected parents the disease must be autosomal recessive. An affected individual must inherit a recessive allele from both parents, so both
parents must have an allele.
●Down Syndrome - Trisomy of chromosome 21 - Down Syndrome is a condition in which a person has an extra chromosome.Typically, a baby is born with 46
chromosomes. Babies with Down syndrome have an extra copy of one of these chromosomes, chromosome 21. A medical term for having an extra copy of a chromosome is ‘trisomy.’
Down syndrome is also referred to as Trisomy 21. This extra copy changes how the baby’s body and brain develop, which can cause both mental and physical challenges for the baby.
Down Syndrome is usually associated with physical growth delays, mild to moderate intellectual disability, and characteristic facial features. Most cases of Down syndrome are not
inherited , but occur as random events during the formation of reproductive cells (eggs and sperm).
●Hemophilia - Sex-linked (X-chromosome) recessive - Hemophilia is a rare disorder in which your blood doesn't clot normally because it lacks sufficient blood-clotting
proteins (clotting factors). If you have hemophilia, you may bleed for a longer time after an injury than you would if your blood clotted normally. Small cuts usually aren't much of a
problem. If you have a severe deficiency of the clotting factor protein, the greater health concern is deep bleeding inside your body, especially in your knees, ankles and elbows. That
internal bleeding can damage your organs and tissues, and may be life-threatening. Hemophilia is a genetic disorder. Treatment includes regular replacement of the specific clotting
factor that is reduced. In the most common types of hemophilia, the faulty gene is located on the X chromosome. Everyone has two sex chromosomes, one from each parent. A female inherits an X chromosome from her mother and an X
chromosome from her father. A male inherits an X chromosome from his mother and a Y chromosome from his father. This means that hemophilia almost always occurs in boys and is passed from mother to son through one of the mother's
genes. Most women with the defective gene are simply carriers and experience no signs or symptoms of hemophilia. But some carriers can experience bleeding symptoms if their clotting factors are moderately decreased.
●Polydactyly - Autosomal dominant - Polydactyly is a deformity in which the hand has one or more extra fingers in any of three places of the hand: On the small finger side — most common (ulnar), On the thumb side, also called thumb
duplication — less common (radial), In the middle of the hand — least common (central). This condition is one of the most common congenital hand defects, affecting about one out of every 500 to 1,000 babies. Usually, only one of a child’s
hands is affected. African-American children are more likely to have an extra little finger, while Asians and Caucasians are more likely to have an extra thumb. Polydactyly. Polydactyly is an inherited condition in which a person has extra fingers
or toes. It is caused by a dominant allele of a gene. This means it can be passed on by just one allele from one parent if they have the disorder.
●Red-green color blindness - Sex-linked (X chromosome) recessive - If you have Red-green color blindness, you may have difficulty seeing different shades of red, green, and yellow. A person with “normal” color vision can see all
combinations of the three primary colors — red, blue, and green — in their true form. Deuteranopia is a type of red-g reen color blindness characterized by the inability to distinguish red and green pigments. Protanopia is another type of
red-green color deficiency. Both are primarily caused by recessive genes in the X chromosome. Your ability to see colors is dependent on three genes: OPN1LW , OPN1MW , and OPN1SW . These genes produce instructions for making
pigments that contribute to your retina’s light receptor cells, which are located in the back of your eye. Light receptor cells can be broken down into two parts: Cones and Rods. Both cones and rods transmit signals to the brain to help produce
vision. Cones provide vision for bright light, which includes color vision, while rods are used for dim light conditions. Red-green color vision deficiencies occur when there are defects with the OPN1LW (red pigment cone) and OPN1MW
(green pigment) genes. These affect the way that color wavelengths are detected by the cones in your retina. Research suggests that deuteranopia is most common in men and in those of Northern European descent. It’s also estimated that
red-green color vision deficiencies occur in 1 out of 12 men and 1 out of 200 women. Currently, there’s no cure or treatment option available for deuteranopia. The g enes that can give you red-green color blindness are passed down on the X
chromosome. Since it's passed down on the X chromosome, red-green color blindness is more common in men. This is because: Males have only 1 X chromosome, from their mother.
●Sickle-cell anemia - Autosomal recessive - Sickle cell anemia is an inherited red blood cell disorder in which there aren't enough healthy red blood cells to carry oxygen throughout your body. Normally, the flexible, round red blood cells
move easily through blood vessels. In sickle cell anemia, the red blood is shaped like sickles or crescent moons. The cells die early, leaving a shortage of healthy red blood cells (sickle cell anemia), and can block blood flow causing pain (sickle
cell crisis). Infections, pain, and fatigue are symptoms of sickle cell disease. Treatments include medications, blood transfusions, and rarely a bone-marrow transplant. Treatment can help, but this condition can't be cured. Sickle cell anemia is
inherited in an autosomal recessive pattern, which means that both copies of the gene in each cell have mutations . The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they
typically do not show signs and symptoms of the condition.
●Tay-Sach's Disease - Autosomal recessive degenerative disorder in Ashkenazi Jews - Tay–Sachs disease is a genetic disorder in which a fatty substance results in the destruction of nerve cells in the brain and spinal cord. The most common
form is infantile Tay–Sachs disease which becomes apparent around three to six months of age, with the baby losing the ability to turn over, sit, or crawl. Symptoms progress until they lead to death, often around age four. Treatment can help
but there is no cure for Tay-Sachs disease. Tay-Sachs disease is typically found in people with certain ancestry, such as Eastern European Jews. Symptoms progress until they lead to death, often around age four. Tay-Sachs is inherited in an
autosomal recessive pattern, which means that both copies of the gene in each cell have mutations . The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show
signs and symptoms of the condition.
●Duchenne Muscular Dystrophy - Sex-linked (X-chromosome) recessive (X-LINKED RECESSIVE) RARE: An inherited disorder of progressive muscular weakness, typically in boys. Many people with muscular dystrophy have Duchenne
syndrome. Girls can be carriers and mildly affected, but the disease typically affects boys.Symptoms include frequent falls, trouble getting up or running, waddling gait, big calves, and learning disabilities.There is no cure, but physical therapy
and medications, such as corticosteroids, can help control symptoms and improve quality of life. Muscle weakness usually begins around the age of four, and worsens quickly. Muscle loss typically occurs first in the thighs and pelvis followed
by the arms.
●Edwards' Syndrome - Trisomy of chromosome 18 Also Called: Trisomy 18 VERY RARE: A genetic disorder caused by the presence of a third copy of all or par t of chromosome 18. Many parts of the body are affected. Babies are often
born small and have heart defects. A condition that causes severe developmental delays due to an extra chromosome 18 (third copy of chromosome 18) A first trimester screening that includes a blood test and ultrasound offers early
information about a baby's risk of having it. A second trimester blood test called a quad screen can also detect it. Symptoms include low birth weight, small abnormally shaped head, and birth defects in organs that are often life threatening.
Edwards syndrome has no treatment and is usually fatal before birth or within the first year of life.
●Huntington's Disease - Autosomal dominant RARE: Huntington's disease (HD) is a fatal genetic disorder that causes the progressive breakdown of nerve cells in the brain. It deteriorates a person's physical and mental abilities usually
during their prime working years and has no cure. An inherited condition in which nerve cells in the brain break down over time. It typically starts in a person's 30s or 40s. Usually, Huntington's disease results in progressive movement,
thinking (cognitive), and psychiatric symptoms. No cure exists, but drugs, physical therapy, and talk therapy can help manage some symptoms. Muscular : abnormality walking, increased muscle activity, involuntary movements, problems with
coordination, loss of muscle, or muscle spasms Cognitive : amnesia, delusion, lack of concentration, mental confusion, slowness in activity, or difficulty thinking and understanding. Behavioral : compulsive behavior, fidgeting, irritability, or
lack of restraint Psychological : delirium, depression, hallucination, or paranoia Mood : anxiety, apathy, or mood swings Also common : difficulty speaking, memory loss, tremor, or weight loss
●Klinefelter Syndrome - XXY sex chromosomes, assigned male RARE: Genetic condition in which a boy is born with an extra X chromosome. Instead of the typical XY chromosomes in men, they have XXY, so this condition is sometimes
called XXY syndrome. Men with Klinefelter usually don't know they have it until they run into problems trying to have a child. Klinefelter syndrome isn't inherited, but rather occurs only as a result of a random genetic error after conception.
Males born with Klinefelter syndrome may have low testosterone and reduced muscle mass, facial hair, and body hair. Most males with this condition produce little or no sperm. Treatment may include testosterone replacement and fertility
treatment..
●Patau Syndrome - Trisomy of chromosome 13 Also called: Trisomy 13 VERY RARE: A syndrome caused by a chromosomal abnormality, in which some or all of the cells of the body contain extra genetic material from chromosome 13.
The extra genetic material disrupts normal development, causing multiple and complex organ defects. Prenatal testing can detect Patau syndrome during pregnancy. Patau syndrome causes severe intellectual disability and physical defects. Most
infants with this condition don't live past their first week of life. Treatment varies from child to child and focuses on relieving symptoms and managing complications. Patau syndrome causes severe intellectual disability and physical defects.
Developmental: abnormally small head, birth defect with intestinal organs outside of body, failure to thrive, or low birth weight. Also common: cleft lip and cleft palate, episodes of no breathing, flaccid muscles, having extra fingers or toes,
heart murmur, intellectual disability, low-set ears, microphthalmia, seizures, or underside of foot is convex.
●Turner Syndrome - Monosomy X, assigned female Also called: gonadal dysgenesis RARE: A condition that affects only females, results when one of the X chromosomes (sex chromosomes) is missing or partially missing. Turner syndrome
can cause a variety of medical and developmental problems, including short height, failure of the ovaries to develop and heart defects. A chromosomal disorder in which a female is born with only one X chromosome. Turner syndrome results
from a missing or incomplete sex chromosome. Symptoms include short stature, delayed puberty, infertility, heart defects, and certain learning disabilities. Treatment involves hormone therapy. Fertility treatment may be necessary for women
who want to become pregnant.
