Genetics

Question 1

Define disability.

Answer 1

A person who has a physical or mental impairment preventing them going about their day to day tasks

Question 2

What’s the technical term for Down syndrome? Why?

Answer 2

Trisomy 21

Due to the presence of a whole or part of an third copy of chromosome 21

Question 3

Give some characteristic features of Down syndrome.

Answer 3

Learning difficulties
Developmental delay
Hypotonia
Short stature
Congenital heart disease
Duodenal atresia
Brachycephaly: flat back of head
Single palmar crease
Upward sloping palpebral fissures: upward sloping eye
Epicanthic folds: skin fold covering inner corner of eye

Question 4

Draw our family tree.

Answer 4

Ensure:
square for male
circle for female
cross out for deceased

Question 5

How is Down syndrome inherited?

Answer 5

Non-disjunction:

• during meiosis, the 2 chromosome 21s don’t split, instead they both go into one gamete
• so when this gamete fertilises with the other there are 3 copies of chromosome 21

Translocation:
- part of chromosome 21 is attached to chromosome 14

Question 6

What is translocation? There are two types.

Answer 6

Robertsonian: when one acrocentric chromosome gets stuck to another one. For example 21 stuck to 14.
When meiosis occurs some of the gametes end up with monosomy or trisomy (draw out picture, p118 textbook)

Reciprocal: when a piece of one chromosome 1 (for eg.)swaps with a piece of chromosome 2. When meiosis occurs some gametes are left with too much of one chromosome and too little of the other

Question 7

Which are the acrocentric chromosomes? What’s different about them?

Answer 7

13, 14, 15, 21, 22

They have very short P arms so get stuck to eachother more easily

Question 8

What is:
trisomy 21
trisomy 18
trisomy 13
45, X
47 XXY?

Answer 8

Down syndrome
Edwards syndrome
Patau syndrome
Turner syndrome
Klinefelter syndrome

Question 9

Clinical features of Edwards syndrome? What is the genetic name?

Answer 9

Trisomy 18

Low birth weight
Small mouth and chin
Overlapping fingers
Rocker bottom feet
Cardiac and renal abnormalities

Question 10

Clinical features of Patau syndrome? What is the genetic name?

Answer 10

Trisomy 13

Structural brain defects
Small eyes
Cleft lip and palate
Cardiac and renal abnormalities

Question 11

Clinical features of Turner syndrome? What is the genetic name?

Answer 11

45, X

Short stature
Webbing of neck
Widely spaced nipples
Congenital heart defects (coarctation of aorta)
Delayed puberty
Ovarian dysgenesis, infertility
Much more

Question 12

Clinical features of Klinefelter syndrome? What is the genetic name?

Answer 12

47, XXY

Many are asymptomatic
Delayed development
Shyness, introverted
Small penis and testes
Little body hair
Infertility

Question 13

Consanguineous couples increase their child’s risk of developing which genetic conditions?

Answer 13

Autosomal recessive

Question 14

Are boys or girls worse affected by X-linked recessive conditions? Why?

Answer 14

Boys

If they have a genetic mutation on their X chromosome, there is no other X chromosome that’s healthy to compensate.

Question 15

If the mother is a carrier of an X-linked recessive genetic disease, what are the odds that:

• a daughter will be a carrier
• a son will be affected?

Explain.

Answer 15

50% chance daughter will be a carrier
There’s a 50% chance mother will pass on the affected X chromosome

50% chance son will be affected, because there’s a 50% mother will pass on affected X chromosome

Question 16

If the father is affected by an X-linked recessive genetic disease, what are the odds that:

• a daughter will be a carrier
• a son will be affected?

Explain.

Answer 16

100% chance the daughter will be a carrier
She inherits one X from her mother and one from father. The father only has one X and it’s affected, so she will carry one affected X chromosome.

0% chance son will be a carrier, because the father only passes his Y chromosome to his son. So the son won’t inherit an affected X chromosome.

Question 17

What’s the difference between X-linked recessive and X-linked dominant?

Answer 17

Dominant, both girls and boys are affected

Recessive, girls are carriers and boys are affected

Question 18

Give some examples of X-linked recessive conditions?

Answer 18

Colour blindness
Duchenne and Becker MD
Fragile X syndrome
Haemophilia A and B

Question 19

What is fragile X syndrome?

Answer 19

X-linked recessive condition.
Causes learning difficulties
Severe in affected males, and mild in carrier females

Question 20

How does mitochondrial inheritance work?

Answer 20

The zygote (fertilised egg) gets its mitochondria from the egg, not the sperm.

So mitochondrial conditions only passed down by an affected mother

Question 21

Would the children of a man affected by a mitochondrial disease be affected themselves?

Answer 21

No because the mother is unaffected and they’ll receive their mitochondria from her. So they’ll be unaffected

Question 22

Official chromosome name for Down’s?

Answer 22

47 XY + 21

XX for female

Question 23

What is the name of the type of inheritance shown whereby the disease is different depending on if the defect is inherited from the mother or the father?
Give an example?

Answer 23

Imprinting

Angelman's (maternal)
Prader Willi (paternal)

Question 24

What’s the inheritance of Angelman’s and Prader Willi?

Answer 24

Imprinting
Problem with chromosome 15
If maternal defect - Angelman’s
If paternal - Prader Willi