Inheritance and Genetic Diseases - MCB L5

Question 1

How is genetic variation generated?

Answer 1

Mutation, diploid cells, homologous recombination, independent assortment

Question 2

Name the stages of meiosis

Answer 2

Meiosis 1: Prophase I, Metaphase I, Anaphase I, Telophase I, cell division. Meiosis 2: Prophase II, metaphase II, anaphase II, telophase II, cell division

Question 3

When does homologous recombination occur?

Answer 3

Long prophase of meiosis I

Question 4

When does independent assortment occur?

Answer 4

During cell division in meiosis I

Question 5

Describe, at molecular and enzymatic level, how homologous recombination occurs?

Answer 5

Nuclease creates nicks in double helix of DNA in a chromosome so you have a strand of DNA floating about. This is almost identical to strand of DNA floating about from the other homologous chromosome. So they combine to (relatively) the other chromosome.

Question 6

What comes first, recombination or independent assortment?

Answer 6

Recombination

Question 7

Non-disjunction

Answer 7

Chromosomes do not separate in meiosis I or meiosis II. This leads to gametes having either 3 copies of a gene or only one. Often fatal.

Question 8

Trisomy 21

Answer 8

Down’s Syndrome. Non-specific effects because not a mutation of a specific gene. Mental retardation. Broad, flat face, slanting eyes. Congenital heart disease, intestinal blockage, enlarged colon.

Question 9

Why is trisomy usually fatal?

Answer 9

Because genes and thus proteins from all 3 chromosomes are expressed so toxic.

Question 10

Why is trisomy 21 not fatal?

Answer 10

Because 21 is small and does not contain many genes that are toxic if expressed from 3 chromosomes instead of 2

Question 11

Gene dosage

Answer 11

The number of copies of a particular gene present in a genome

Question 12

Gene dosage effect

Answer 12

The dramatic effects observed when the number of copies of a gene is not what is normal

Question 13

Monosomy is always fatal, except on one chromosome. What is the one exception? Why?

Answer 13

Monosomy X because men have only one X so genes on X have evolved to be present in a single copy in men.

Question 14

X-inactivation

Answer 14

Explains how genes on X have evolved to be present as single copy in men and 2 copies in women. Due to tight folding of DNA into heterochromatin for the whole of one X chromosome in females, genes only expressed from one chromosome.

Question 15

Why are XXY and XXX sex chromosome syndromes often undiagnosed?

Answer 15

Because only one X will be activated so not many symptoms.

Question 16

Monosomy X

Answer 16

Turner Syndrome. Female as no Y chromosome. Infertile as gametes have incorrect chromosome numbers. Pubertal failure. Normal intelligence.

Question 17

With age, what happens to spindle fibres and cohesins? What effect does this have on chances of non-disjunction?

Answer 17

With age, cohesins stronger and fibres weaker so greater chance of non-disjunction so more likely to get trisomies.

Question 18

Oocyte

Answer 18

A cell in ovary that can undergo meiotic division to make ovum

Question 19

Example of autosomal recessive disease

Answer 19

Cystic Fibrosis

Question 20

Example of an autosomal ‘incomplete’ recessive disease

Answer 20

Sickle Cell Disease

Question 21

What is meant by an ‘incomplete’ recessive disease

Answer 21

The heterozygous carrier still feels a small effect of the mutation

Question 22

Example of autosomal dominant disease

Answer 22

Huntington’s disease

Question 23

Example of X-linked recessive disease

Answer 23

Haemophilia A

Question 24

Haemophilia A

Answer 24

Lack of blood clotting, leading to bleeding, easy bruising and increased risk of haemorrhages and greater risk for surgery/trauma.

Question 25

Why are Y-linked diseases rare?

Answer 25

A Y-linked disease will affect individual but probably not able to pass to next generation as individual will be infertile.