Topic 2: chromosomal basis of inheritance Flashcards
1
Q
Describe genes
A
- Long DNA = many nucleotides
- Each gene = carries info for synthesis of specific protein
- Each chromosome = 1000+ genes
2
Q
Describe homologous chromosomes
A
- Chromosome pairs during meiosis
- Same genes = different alleles
- In pair = 1 from father + 1 from mother
3
Q
Define chromosomal theory of inheritance
A
- Via Morgan’s exeriment with flies
- Genes have loci on chromosomes
- Chromosomes undergo segregation + independant assortment
4
Q
Why are fruit flies good for genetic studies?
A
- Breed at high rate
- Bred every 2 weeks
- Only 4 pairs of chromosomes
5
Q
Describe Morgan’s experiment
A
- Wild/normal = red eyes
- Mutant = white eyes
- Mated male with white + female with red
- F1 = red eyes
- F2 = 3:1 R:W = only males had white
- Determined = white eye mutation allele = X chromosome
6
Q
Describe inheritance of sex-linked genes
A
- X-linked/Y-linked
- Recessive = 2 alleles in females + 1 in males
- Sex linked recessive disorders = more common in males
7
Q
Give X-linked recessive disorders
A
- Haemophilia
- Color blindness
- Duchenne muscular dystrophy
8
Q
Describe the transmission of sex-linked recessive traits for normal female + affected male
A
1) All females = heterozygous
- 1 normal from mom + 1 mutant from dad
2) All male normal = only 1 normal X from 1
9
Q
Describe the transmission of sex-linked recessive traits for heterozygous female + normal male
A
1) 50% female normal 50% heterozygous
2) 50% male normal 50% mutation
10
Q
Describe the transmission of sex-linked recessive traits for heterozygous female + affected male
A
- 50% female heterozygous carriers 50% mutation
- 50% males normal 50% mutation
11
Q
Describe Y-linked genetic disorders
A
- SRY gene = development of testes
- Absence of gene in females = gonards > ovaries
- Abnormalities in gene = XY as female + XX as male
- E.g. Swyer syndrome + XX male syndrome
12
Q
Describe Swyer syndrome
A
- Mutation in SRY = XY females = gonadal dysgenesis
- Inactivation of SRY = normally male have female characteristics
13
Q
Describe XX male syndrome
A
- Translocation of part of Y chromosome + SRY > X chromosome
- X chromosome carries SRY gene = females with male characteristics
14
Q
Types of chromosomal exchange
A
1) Translocation = exchange of chromosomal fragments between non-homologous chromosome = abnormal
2) Genetic recombination = exchange of chromosomal fragments between homologous chromosomes = normal
15
Q
Define color blindness
A
- Decreased ability to perceive color differences
16
Q
Define Duchenne muscular dystrophy
A
- Fatal
- Progressive weakening of muscles + loss of coordination
17
Q
Define haemophilia
A
- Progressive bleeding prolonged following injury
- Clotting factor VIII deficiency
18
Q
Describe X inactivation in female mammals
A
- 1 of 2 X chromosomes in somatic cell = randomly inactivated during embryonic
- Inactive X = condenses into Barr body = into nuclear envelope
- In ovaries = Barr body reactivated in cells = rise to eggs = every female gamete has active X
19
Q
Define mosaicism
A
- Some somatic cells express phenotype of 1 X-linked gene + some cells of the other = from paternal/maternal
- E.g. why calico have patchy fur
20
Q
Explain mosaicism in human females
A
- X-linked mutation = prevents development of sweat glands = hypohidrotic ectodermal dysplasia
- Female heterozygous = patches of normal skin + patches of skin without sweat glands = different cells with different genes
21
Q
Describe gene linkage
A
- Genes near eachother on same chromosomes = inherited together
- Morgan crossed flies different body color + wing size = found inherited together in combination = on same chromosome
- Mendel’s law of independent assortment = not apply to linked genes
22
Q
Describe the results of gene linkage
A
- Complete linkage = 2 phenotypes = 50% + 50%
- Results were = 80% parental phenotypes + 20% non-parental = due to genetic recombination
23
Q
How to calculate recombination frequnecy
A
Number of offspring with recombinant phenotypes / total number of offspring
24
Q
% of recombination frequency for linked genes
A
- <50%
- Completely linked = RF 0%
- Incompletely linked = RF 0-50%
25
Describe unlinked genes
- Frequency of recombination = 50% = 50% new phenotypes observed for any 2 genes on different chromosomes
- Due to independent assortment of chromosomes
26
Describe how distance affects recombination
- More further apart 2 genes = higher probability of crossover occuring = higher chance of separating
- Higher RF = higher chance 2 linked genes separating
- Further away on same chromosome = RF almost 50% = physically linked but genetically unlinked = behave as if found on different chromosomes
27
Types of chromosomal alterations
1) Chromosome number
2) Chromosome structure
28
Describe abnormal chromosome number
- AKA aneuploidy
- Due to gametes with nondisjunction
- Nondisjunction = abnormal separation of homologous pairs during meiosis 1 + sister chromotids in meiosis 2
29
Describe nondisjunction of homologous in meiosis 1
- 1 gamete recives both homologous chromosomes from pair + other gamete recieces none
30
Describe nondisjunction ofsister chromatids in meiosis 2
- Non-separation = half gametes have 1 more/less chromosomes at end of meiosis + other half is normal
- Monosomatic zygote = 1 copy of chromosome
- Trisomic zygote = 3 copies of chromosome
31
Describe alterations of chromosome structures
1) Deletion = removal of chromosomal segment
2) Dupliction = repetition of segment
3) Inversion = reversal of segment
4) Translocation = exchange of segments between non-homologous chromosomes
5) Amplification
32
Types of autosomal disorders due to chromosomal alteration
- Trisomy 21 = Down syndrome
- Trisomy 18 = Edwards syndrome
- Trisomy 13 = Patau syndrome
- Symptoms = birth defects + intellectual disability + shortened life expectancy
33
Describe disorders due to aneuploidy of sex chromosomes
- XXY/XYY males
- XXX/XXXX/XO females
- Klinefelter syndrome = XXY = male with extra X = some female characteristics
- Turner syndrome = XO = only known viable monosomy = sterile
34
Describe disorders caused by structural alteration on chromosomes
- Cri du chat = deletion of chromosome 5 = mentally retarded + cat like cry = die in infancy
- Some cancers = CML = due to reciprocal translocation
35
Exceptions to Mendelian genetics
1) Inheritance of nuclear genes = genomic imprinting
2) Inheritance of genes located outside nucleus = extranuclear/cytoplasmic genes
36
Describe genomic imprinting
- Silencing maternal/paternal alleles at beginning of development
- Silencing = stamping with imprint methylation during gamete production
- Imprinting = add CH3
- Only affects 1% genes
- Critical for embryonic development
37
Give the genes that have an imprinting role
- Igf2 = growth factor = for embryomic development = maternal allele
- BWS = abnormal activation of maternal Igf2 during egg development
- Symptoms overgrowth + increased risk of childhood cancer
38
Describe the inheritance of extranuclear genes
- Inherited maternally = zygote's cytoplasm comes from egg
- Defects in mitochondrial genes = prevent cell naming enough ATP
- E.g. mitochondrial myopathy + Leber's hereditary optic neuropathy
