Human Genetics Flashcards
What are the main obstacles to Human Genetic Analysis
– Incomplete family records
– Small number of progeny
– Uncontrolled environment
Despite these obstacles, many human genetic traits have been described.
What are four examples of dominant traits in human genetics?
- Achondroplasia (dwarfism)
- Congenital night blindness
- Widow’s peak
- Woolly hair
What are four examples of recessive traits in human genetics?
- Albinism (lack of pigment)
- Cystic fibrosis (a respiratory disorder)
- Sickle-cell disease (a hemoglobin disorder)
- Tay-Sachs disease (a lipid storage disorder)
What is meant by an “obligate carrier”?
A person that carries a gene but does not exhibit the trait, aka the phenotype.
What is meant by an “Asymptomatic carrier”?
A person that carries a gene that is unaffected by it at a certain point in time but may later exhibit the traits or phenotypes of this gene.
What are probands?
The first affected family members coming to the attention of geneticists
What does consanguinity mean?
Incest
What is a human pedigree chart?
The family tree of a family that shows the different conventions leading up to a present generation
What is each generation of a human pedigree identified as?
A roman numeral
In a human pedigree what are family members identified as?
Arabic (normal) number (1, 2, 3, 4, etc)
In a human pedigree there are white and red squares, circles and diamonds, what are they?
These shapes identify what gender the family member is and the colours identify whether that family member has a specific trait or not. For example, the squares are males, the circles are females and the diamonds are unknown. White shapes are family members without a specific trait and red shapes are family members with the specific trait.
What does it mean in a human pedigree when there is a horizontal line through a shape?
This means that the family member is deceased.
How do you know which family members are older than others?
The family members highest in the chart are usually the parents of family members lower down and if there are siblings then they will be listed from oldest to youngest, for example, if a couple, labelled 1 and 2, had three children they would be numbered 3, 4 and 5 with 3 being the oldest and five being the youngest.
In a human pedigree what would an obligate carrier look like?
An obligate carrier would be a white shape with a small red circle in the middle
In a human pedigree what would an Asymptomatic carrier look like?
An Asymptomatic carrier would be a white shape with a vertical line running through the middle of it.
in a human pedigree, what does it mean when there is a number in the middle of a shape?
If there is a number in the middles of a shape this means that there are multiple people represented by this shape. For example, if there is a circle that has a five in the middle of it, this means that there are five females represented by this shape.
How would a proband be represented in a human pedigree?
A proband would be represented by red shape with a “P” either next to or inside the shape.
If the family history of a person is unknown, how is it represented in a human pedigree?
If the family history of a person is unkown, like if they are adopted, then the shape that represents them would have a question mark in the middle of it.
How is an adopted family member represented if their biological parents are known in a human pedigree?
An adopted family member would be represented by a shape enclosed in square brackets. The adoptive parents will be linked to the child with a dotted line while the biological parents will be linked with a solid line.
In a human pedigree how are different kinds of twins represented?
https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing
What are Autosomal Dominant Traits
A disease or genetic disorder that is passed down as a dominant gene and will almost always be expressed.
- Usually appear with equal frequency in both sexes
- Do not usually skip generations (but this depends on penetrance)
- Unaffected individuals usually do not transmit the trait
What are Autosomal Recessive Traits
A disease or genetic disorder that is passed down but is not always shows as it is a recessive gene.
- Appear in both sexes
- Skip generations
- Are more common in families who are consanguineous
What are X-linked dominant genes?
These are genes which contain a deformity or disease which only appear in the x chromosome:
- Both males and females are affected
- More females than males
- Never passed from father to son
- All daughters of affected fathers have trait
What is X-inactivation?
A process by which one of the copies of the X chromosome is inactivated in some female mammals. The inactive X chromosome is silenced by it being packaged in such a way that it has a transcriptionally inactive structure called heterochromatin.
Why is x-inactivation a problem?
X-inactivation blurs the distinction between dominant and recessive X linked conditions
What is the mosaic expression?
Where each cell expresses either the normal or abnormal allele, but not both
When does x-inactivation occur?
Inactivation of X chromosome occurs randomly in each cell early in development
What effect does x-inactivation have on the body?
It has an averaging effect and can lead to intermediate phenotypes
- May be clinically normal but biochemically abnormal
- Eg. Haemophilia
Why are manifesting heterozygotes (females because they are heterozygous for x) more affected by x-inactivation?
They can unluckily express the abnormal allele in most cells by chance and may be quite severely affected even in recessive X-linked conditions
What are Y-linked traits?
- Appears only in males
- Passed on from fathers to all sons
- Apart from “maleness” and some disorders of male sexual dysfunction there are no known Y-linked “diseases”
What is the MELAS mutation?
MELAS = Mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke
Why does the MELAS mutation occur in some mitochondria
Mitochondria independently divide and exhibit heteroplasmy (may be multiple distinct variety of mtDNA in mitochondria of the same cell)
