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Human Genetics Changes in Chromosomes. Other than crossing over, changes can occur in the actual chromosomes code that can have profound affects on the possible outcomes!!! Changes will usually occur spontaneously when a cell becomes irradiated or exposed to certain chemicals.
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Other than crossing over, changes can occur in the actual chromosomes code that can have profound affects on the possible outcomes!!! • Changes will usually occur spontaneously when a cell becomes irradiated or exposed to certain chemicals. • There are four main types of changes:- deletion - duplication - inversion -translocation
Deletion – is when a portion of the chromosome is lost or removed due to irradiation, viruses or various chemicals. These pieces coded for genes, so when they are lost so is the genetic trait it coded for. • Example when a piece of chromosome #5 is lost, the child is born with a mentally handicapped and a different facial appearance.
Inversion – occurs when a piece of the chromosome becomes free momentarily before being reinserted in the reverse order. This completely changes the genes that this chromosome coded for. Example - Autism is believed to be linked to a chromosomal inversion.
Duplication – is when multiple copies of a gene sequence occur. For the most part this can have no affect on a human, but in some cases to many repeats can be detrimental.
Example – Duplication on the X chromosome may become too excessive and lead to a condition known as fragile X syndrome. Most people have about 29 repeats of this particular region, but a person with fragile X syndrome will have around 700 repeats.
Translocation – occurs when part of one chromosome changes places with part of a non-homologous chromosome.
Example – If part of chromosome 14 exchanges places with #8 then cancer can occur. • Down syndrome – linked to translocation between chromosome 14 & 21. • A type of Leukemia has been traced to translocation between #22 and #9.
Many other defects occur due to Nondisjunction • Nondisjunction – is the result of chromosomes not separating during meiosis. As a result the chromosomes will either have too many or not enough chromosomes. • inheriting an extra chromosome is referred to as trisomy • inheriting only one chromosome is referred to as monosomy
Human embryos with either condition rarely survive to birth. Many miscarriages can be linked to these conditions. • Down Syndrome – is probably one of the most common nondisjunction syndromes. It occurs when an individual receives 3 copies of chromosome 21.
-Symptoms include: mild to moderate mental impairment and a thick tongue that can create speech defects. Skeleton may not develop properly resulting in a short stocky body type.
Turner Syndrome – results when a person only has one X sex chromosome. The woman will have external female genitalia, but will lack ovaries. They are therefore infertile and not mature sexually. Other defects include heart, kidney and skeletal defects. • A single Y chromosome individual is not possible. This embryo would not survive where they would be lacking vital genetic information.
Klinefelter syndrome – occurs when an extra X chromosome occurs in a male (XXY). This individual will have immature sex organs and will not grow facial hair. They are also likely to develop some breasts. XXX females do not show any at all symptoms.
Jacobs syndrome – occurs in males with an extra Y (XYY). These individuals show speech and reading problems and have persistent acne. A study once found that there seemed to be an extremely high occurrence of this condition amongst prisoners compared to the rest of society.
The Human • 46 Chromosomes that occur in 23 pairs of homologous chromosomes. • 1 pair of sex chromosomes (X and Y) • 22 pairs of autosomes Autosome – non-sex determining chromosomes, responsible for containing the remaining traits of the human being. Each chromosome contains anywhere from hundreds to thousands of genes for particular traits.
Autosomal Recessive Inheritance • Disorders that are carried on the autosomes and are not related to the sex of the individual.- Tay-Sachs disease – a disease where the body lacks the ability of producing a vital enzyme within the lysosomes of the nervous system. These individuals are normal at birth, but by 8 months the lysosomes rupture and break down the brain cells. By their 1st birthday they will usually be blind, mentally handicapped, and display little muscle activity. Most die before the age of 5.
A.R.I.’s cont. • Phenylketonuria (PKU) – this condition again affects children. In PKU an enzyme that converts phenylalanine to tyrosine is defective or missing. A child with PKU will breakdown phenylalanine abnormally creating products that damage the nervous system. Luckily there is a routine test and treatment in place for babies with this condition. If the condition is not detected the baby with become severely mentally handicapped within a few months.
A.R.I.’s cont • Albinism – A genetic condition where the hair, skin and eyes have no pigment. A normal individual is capable of producing different colours in our bodies due to a varying amounts of the brown pigment called melanin. Where an albino individual lacks this pigment they lack the ability to tan, thereby lacking the ability to naturally protect their bodies from the sun’s powerful UV rays.
Codominant Inheritance • Sickle Cell Anemia – where an individual carries 2 different copies of an allele, but both are observed. Affected individuals have a defect in the hemoglobin of their red blood cells, causing an irregular shape that can clog up capillaries and lead to blood clots. These individuals tend to lack energy, suffer from various illnesses and are in constant pain. • This recessive allele is believed to have originated from Africa.
Heterozygous Advantage • Until recently homozygous recessive individuals never lived to adulthood. Therefore the presence of the allele should have decreasing each generation. However in some African regions nearly half of an entire population would be heterozygous for the condition. • How could this be possible????
The answer • Malaria – Yes, the answer was found while studying the leading cause of illness and death in Africa. It appeared that children that were heterozygous for the sickle cell gene were less likely to contract malaria and survive to adulthood. • Heterozygous Advantage – where an individual with two different alleles for the same trait have a better chance of survival.
Autosomal Dominant Inheritance • Due to the behaviour of dominant traits in Mendelian genetics, we can trace dominant disorders 2 ways:- since both heterozygous and homozygous individuals show a trait, the trait should be seen in every generation.- if one parent is heterozygous and crosses with a homozygous recessive individual, then the trait should still be present 50% of the time.
Although rare they do exist! • Some can be caused be random mutations of a gene sequence. • In most cases the condition does not become prevalent until the individual has already conceived and potentially passed on the gene to their offspring.
Two examples Progeria – a rare disorder that causes an individual to age rapidly. It occurs in newborns at a rate of 1 in 8 million, and does not run on families. It is therefore linked to a rare random mutation. Huntington’s Disease – a lethal disorder where the brain progressively deteriorates over time. Symptoms generally begin around age 35, after the individual has already had children.
Incomplete Dominance • A condition where having even one copy of the affected gene (dominant or recessive) leads to the condition.- Familial hypercholesterolemia (FH) – is a condition that affects heterozygotes (1:500).The cell produces less receptors for LDL (lipids) that are required to take these lipids into the cell. Without them these lipids build up in the arteries and lead to a heart attack or stroke by around the age of 35. Homozygous individuals for this recessive condition can die of a heart attack by the age of 2
X-linked Inheritance • Red-green colourblindness is a sex linked condition due to the allele for detecting the red- green pigment is located on the X chromosome. • This explains why it is so much more common in men (8%) than women (0.04%).- For a woman to be colourblind, her father had to be colourblind and her mother had to be colourblind or a carrier.- For a man to be colourblind only the mother had to possess the allele.
Analysis of Human Genetics • Two techniques have been used to examine human genetics:- Karyotypes- Pedigrees
The Human Karyotype • The human karyotype is an illustration or photograph of the chromosomes in the nucleus of a somatic cell in an organism. • Creating a Karyotype involves growing cells and stopping the division process during the metaphase stage. The chromosomes are then separated, stained and photographed. The chromosomes are then cut out and arranged in pairs according to size, shape and appearance.
Pedigrees • As described earlier, a pedigree shows the genetic relationships between individuals in a family, • Through many years of tracing a family history and applying this data to Mendelian genetics, scientists can determine if a condition is dominant, recessive, autosomal, or sex-linked. • Pedigrees can also be used to predict the possible inheritance of a disorder.Ex. Page 560 shows a pedigree tracing Hemophilia throughout Queen Victoria’s family.
Do Page 559 – Mini Lab Karyotype • Page 561 – Solve the Case of the Caped Murderer • Pg 562 - #1, 7, 9, 10
