Human Genetics

Question 1

What are the main obstacles to Human Genetic Analysis

Answer 1

– Incomplete family records
– Small number of progeny
– Uncontrolled environment
Despite these obstacles, many human genetic traits have been described.

Question 2

What are four examples of dominant traits in human genetics?

Answer 2

• Achondroplasia (dwarfism)
• Congenital night blindness
• Widow’s peak
• Woolly hair

Question 3

What are four examples of recessive traits in human genetics?

Answer 3

• Albinism (lack of pigment)
• Cystic fibrosis (a respiratory disorder)
• Sickle-cell disease (a hemoglobin disorder)
• Tay-Sachs disease (a lipid storage disorder)

Question 4

What is meant by an “obligate carrier”?

Answer 4

A person that carries a gene but does not exhibit the trait, aka the phenotype.

Question 5

What is meant by an “Asymptomatic carrier”?

Answer 5

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.

Question 6

What are probands?

Answer 6

The first affected family members coming to the attention of geneticists

Question 7

What does consanguinity mean?

Answer 7

Incest

Question 8

What is a human pedigree chart?

Answer 8

The family tree of a family that shows the different conventions leading up to a present generation

Question 9

What is each generation of a human pedigree identified as?

Answer 9

A roman numeral

Question 10

In a human pedigree what are family members identified as?

Answer 10

Arabic (normal) number (1, 2, 3, 4, etc)

Question 11

In a human pedigree there are white and red squares, circles and diamonds, what are they?

Answer 11

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.

Question 12

What does it mean in a human pedigree when there is a horizontal line through a shape?

Answer 12

This means that the family member is deceased.

Question 13

How do you know which family members are older than others?

Answer 13

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.

Question 14

In a human pedigree what would an obligate carrier look like?

Answer 14

An obligate carrier would be a white shape with a small red circle in the middle

Question 15

In a human pedigree what would an Asymptomatic carrier look like?

Answer 15

An Asymptomatic carrier would be a white shape with a vertical line running through the middle of it.

Question 16

in a human pedigree, what does it mean when there is a number in the middle of a shape?

Answer 16

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.

Question 17

How would a proband be represented in a human pedigree?

Answer 17

A proband would be represented by red shape with a “P” either next to or inside the shape.

Question 18

If the family history of a person is unknown, how is it represented in a human pedigree?

Answer 18

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.

Question 19

How is an adopted family member represented if their biological parents are known in a human pedigree?

Answer 19

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.

Question 20

In a human pedigree how are different kinds of twins represented?

Answer 20

https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing

Question 21

What are Autosomal Dominant Traits

Answer 21

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

Question 22

What are Autosomal Recessive Traits

Answer 22

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

Question 23

What are X-linked dominant genes?

Answer 23

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

Question 24

What is X-inactivation?

Answer 24

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.

Question 25

Why is x-inactivation a problem?

Answer 25

X-inactivation blurs the distinction between dominant and recessive X linked conditions

Question 26

What is the mosaic expression?

Answer 26

Where each cell expresses either the normal or abnormal allele, but not both

Question 27

When does x-inactivation occur?

Answer 27

Inactivation of X chromosome occurs randomly in each cell early in development

Question 28

What effect does x-inactivation have on the body?

Answer 28

It has an averaging effect and can lead to intermediate phenotypes

• May be clinically normal but biochemically abnormal
• Eg. Haemophilia

Question 29

Why are manifesting heterozygotes (females because they are heterozygous for x) more affected by x-inactivation?

Answer 29

They can unluckily express the abnormal allele in most cells by chance and may be quite severely affected even in recessive X-linked conditions

Question 30

What are Y-linked traits?

Answer 30

• 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”

Question 31

What is the MELAS mutation?

Answer 31

MELAS = Mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke

Question 32

Why does the MELAS mutation occur in some mitochondria

Answer 32

Mitochondria independently divide and exhibit heteroplasmy (may be multiple distinct variety of mtDNA in mitochondria of the same cell)