Inheritance and Genetic Diseases - MCB L5 Flashcards
How is genetic variation generated?
Mutation, diploid cells, homologous recombination, independent assortment
Name the stages of meiosis
Meiosis 1: Prophase I, Metaphase I, Anaphase I, Telophase I, cell division. Meiosis 2: Prophase II, metaphase II, anaphase II, telophase II, cell division
When does homologous recombination occur?
Long prophase of meiosis I
When does independent assortment occur?
During cell division in meiosis I
Describe, at molecular and enzymatic level, how homologous recombination occurs?
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.
What comes first, recombination or independent assortment?
Recombination
Non-disjunction
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.
Trisomy 21
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.
Why is trisomy usually fatal?
Because genes and thus proteins from all 3 chromosomes are expressed so toxic.
Why is trisomy 21 not fatal?
Because 21 is small and does not contain many genes that are toxic if expressed from 3 chromosomes instead of 2
Gene dosage
The number of copies of a particular gene present in a genome
Gene dosage effect
The dramatic effects observed when the number of copies of a gene is not what is normal
Monosomy is always fatal, except on one chromosome. What is the one exception? Why?
Monosomy X because men have only one X so genes on X have evolved to be present in a single copy in men.
X-inactivation
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.
Why are XXY and XXX sex chromosome syndromes often undiagnosed?
Because only one X will be activated so not many symptoms.
Monosomy X
Turner Syndrome. Female as no Y chromosome. Infertile as gametes have incorrect chromosome numbers. Pubertal failure. Normal intelligence.
With age, what happens to spindle fibres and cohesins? What effect does this have on chances of non-disjunction?
With age, cohesins stronger and fibres weaker so greater chance of non-disjunction so more likely to get trisomies.
Oocyte
A cell in ovary that can undergo meiotic division to make ovum
Example of autosomal recessive disease
Cystic Fibrosis
Example of an autosomal ‘incomplete’ recessive disease
Sickle Cell Disease
What is meant by an ‘incomplete’ recessive disease
The heterozygous carrier still feels a small effect of the mutation
Example of autosomal dominant disease
Huntington’s disease
Example of X-linked recessive disease
Haemophilia A
Haemophilia A
Lack of blood clotting, leading to bleeding, easy bruising and increased risk of haemorrhages and greater risk for surgery/trauma.
