Classical Genetics Flashcards
(64 cards)
1
Q
Alleles
A
Different versions of the same gene
2
Q
Why were Mendel’s experiments successful?
A
- He chose a suitable organism, Pisum sativum
- Pea varieties available with various contrasting traits
- Could ‘cross’ pea plants with contrasting traits to assess inheritance patterns
- Demonstrated good experimental technique & scientific rigour
- The genetics were ‘simple’
3
Q
Monohybrid Cross
A
The inheritance of a single gene (monogenic) trait
* One pair of contrasting traits (e.g. wrinkled and round seeds)
* Parental lines are pure bred for a different trait or phenotype
* F1 is a ‘monohybrid’ or heterozygote for a single gene trait
3:1 F2 phenotype ratio on
average for each trait
4
Q
Law of Segregation
A
“During the formation of gametes (eggs
and sperm) the paired hereditary factors (i.e. alleles)
segregate (separate) from each other so that each
gamete receives one or other of the pair
with equal chance”
5
Q
What is dominance?
A
- The phenotypes expressed by heterozygotes
- Dominance and recessive describe the relationship between the alleles of a gene
o i.e. intra-gene action or allele interaction - Alleles are formed by mutation
o Wild-type allele = most common form of the gene in the population
6
Q
Dihybrid Cross
A
Inheritance of two independent monogenic traits
* Parental lines are pure bred for two traits
* F1 is heterozygous for both traits (i.e. genes)
* F1 is self-fertilised
* Mendel’s results allowed him to formulate a second law
The dihybrid cross is a combination of two independent monohybrid crosses
each giving a 3:1 phenotypic ratio = 9:3:3:1
7
Q
Law of Independent Assortment
A
“During the formation of gametes the paired
hereditary factors (i.e. alleles of a gene)
segregate independently from other pairs
(i.e. alleles of another genes at a ≠ loci)”
8
Q
Trihybrid Cross
A
Used to confirm independent assortment
9
Q
Mitosis
A
- Cell division, which leads to the formation of identical daughter cells
o i.e. growth and repair of tissue in eukaryotes
10
Q
Meiosis
A
- Cell division, which leads to the formation of genetically unique gametes
o i.e. eggs/sperm - Cross-overs can occur to allow recombination between chromosomes
11
Q
Stages of Mitosis
A
Prophase
ProMetaphase
Metaphase
Anaphase
Telophase
Cytokinesis
12
Q
Prophase
A
- breakdown of nuclear membrane
- spindle fibres appear
- chromosomes condense
13
Q
Prometaphase
A
- spindle fibres attach to chromosomes
- chromosomes condense
14
Q
Metaphase
A
- chromosomes align
15
Q
Anaphase
A
- centromeres divide
- sister chromatids move to opposite poles
16
Q
Telophase
A
- nuclear membrane reforms
- chromosomes decondense
- spindle fibres disappear
17
Q
Cytokinesis
A
- cytoplasm divides
- parent cell becomes 2 daughter cells with identical genetic information
18
Q
Mitosis and Cancer
A
- Mutations can cause
breakdown of checkpoints - Leads to unchecked divisions
19
Q
Meiosis
A
- Cell division, leading to the
formation of genetically
unique gametes (eggs/sperm) - 1 diploid mother cell divides
into 4 haploid gametes
20
Q
Meiosis I
A
Homologous pairs
of chromosomes align at
metaplate and separate
(independent assortment,
segregation)
21
Q
Meiosis II
A
Sister chromatids
align at metaplate and
separate (much like mitosis)
22
Q
Bivalent
A
paired homologous chromosomes
23
Q
tetrad
A
4 chromatids
24
Q
Stages of Meiosis I
A
Interphase
Early and Late Prophase I
Metaphase I
Anaphase I
Telophase I and cytokinesis
25
Stages of Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II and cytokinesis
26
Linkage and recombination
Probability of recombination increases with larger distances
between loci (i.e. greater inter-locus distance)
Linkage can be
detected through a
two-point test cross
27
Recombination frequency
* 0 - 50% RF Syntenic loci (linked)
* 50% RF Syntenic loci (not-linked)
Same chromosome, but far apart so crossing-over (recombination) assorts
genes independently
* 50% RF Non-syntenic loci
Different chromosomes just affected by independent assortment
28
Test Crossing
Parental classes will be highest
frequency as no recombination
Recombinant classes result
from single (SCO) or double
crossovers (DCO)
29
Crossing Over
Occurs during Prophase I of meiosis
* All four chromatids of tetrad shown
* F1 is heterozygous for three genes to be mapped
* Vertical lines represent crossover intervals:
o Single Cross Over (SCO) between D and F occur at i
o SCO between F and E occur at ii
o Double Cross Over (DCO) can occur (at both i & ii)
30
Locating
* Loci on same chromosome are syntenic
(e.g. yellow body & singed bristles on X)
* Loci on different chromosomes are
non-syntenic
(e.g. yellow body & dumpy wings, 2)
31
31
Recessive Lethal Genes
The dominant yellow allele of the Agouti
gene (Ay) encodes for a yellow coat, but
carries a recessive lethal mutation
Therefore a 2:1 ratio
for yellow : black
instead of 3:1
32
Incomplete Dominance
The heterozygote exhibits a novel phenotype
intermediate of the parental homozygotes
e.g. Flower colour of snapdragons, Antirrhinum
Therefore a 1:2:1
ratio for red : pink : white
instead of 3:1
33
Co-dominance
Two alleles encode different gene products. Heterozygote expresses both
alleles and thus phenotypic trait shown by each homozygote
e.g. Coat colour in cattle
Therefore a 1:2:1
ratio for
red : roan : white
instead of 3:1
34
Multiple Alleles
Populations vary in the allele frequencies
ABO allele frequencies in native human populations
Throughout the world:
* A allele - 21% individuals
* B allele - 16% individuals
* O genotype (neither A nor B allele) - 63%
35
Influencing Inheritance
Inheritance can be affected by:
Intra-locus gene action or allele interaction
* Complete Dominance (i.e. haplo-sufficiency)
* Incomplete Dominance
* Co-Dominance
Inter-locus gene action or gene interaction
* Genes act on different pathways (e.g. 𝑅 & 𝑌 in peas)
* Genes act on the same pathway – Epistasis
Modifies Mendelian ratios
Modifies Mendelian ratios
36
2 Genes : 1 Trait
When genes are not linked, demonstrate complete dominance,
and act independently (i.e. in different pathways), expect a
9:3:3:1 ratio
e.g. Skin colour in corn-snakes
* One gene determines
orange pigment
* Other determines black
pigment
Epistasis = an interaction between 2 genes whereby alleles
at one (epistatic) gene masks the phenotypic expression of
alleles at the other (hypostatic) gene
37
Penetrance
% of individuals exhibiting phenotype associated with their allele
38
Expressivity
degree to which a given allele is expressed at the phenotypic level
39
Discrete/Discontinuous traits
(occur in distinct categories)
* Trait is present or not (e.g. cystic fibrosis, Huntington’s disease)
* Follows Mendelian inheritance (usually single genes with complete
dominance)
40
Continuous traits
(phenotypes vary along a continuum)
* Individuals differ by small degrees (e.g. hair color, eye color, skin color, height)
* Polygenic quantitative or multifactorial inheritance (genes act additively)
41
Pleiotropic effects
When one gene affects >1 phenotype
E.g. Marfan syndrome
* Mutations in gene coding for
connective tissue protein fibrillin
* Fibrillin widespread in body and
causes problems in the Eye, aorta,
bones etc
42
Karyotype
o Number of chromosomes
o Chromosome length
o Position of the centromeres
o Banding pattern
o Differences between the sex chromosomes
43
Aneuploidy
* Some gametes have wrong number of copies of a particular chromosome
* Caused by non-disjunction during meiosis involving only individual chromosomes
* Generally deleterious causing abnormality and/or death
* Chromosome number can be > or < than that of wt
* Nomenclature based on the number of copies of the specific chromosome in the aneuploid state:
o Monosomy, one copy instead of two (2n-1)
o Trisomy, 3 copies instead of two (2n+1)
o Nullsomy, no copy instead of two (2n-2)
44
Aneuploidy
Nullisomy
* Generally Fatal
Monosomy
* Generally Fatal, but viable examples:
o Turner’s syndrome (X0)
Trisomy
* Can result in abnormality or death, but viable examples:
o Downs syndrome (Trisomy 21)
o Klinefelter syndrome (XXY)
Trisomy 21: 1 / 650 1000
* Down Syndrome – often have some level of learning disability. Some
more independent than others (e.g. job). Others need regular care (NHS)
Trisomy 18: 1 / 6000 8000
* Edwards Syndrome – most die before or shortly after being born (NHS)
Trisomy 16: >1% of pregnancies
* Usually results in spontaneous miscarriage
Trisomy 13: 1 / 8000 12000
* Patau Syndrome – often results in miscarriage, stillbirth or death > birth
Trisomy associated with
miscarriage, birth defects,
mental retardation and
shortened lifespan
44
Causes of aneuploidy
Non-disjunction occurs due to
problems with meiotic spindle
* Homologous chromosomes
may not separate properly
during meiosis 1
* Sister chromatids may fail to
separate during meiosis 2
too many or too few
chromosomes in daughter cells
Non-disjunction
during Meiosis I
Non-disjunction
during Meiosis II
45
Klinefelter syndrome
General symptoms of Klinefelter syndrome include:
* Reduced fertility
* Weaker muscles, reduced strength
* Reduced facial hair
* Developmental delay
* Learning difficulty
Standard diagnostic through karyotype analysis
46
Chromosomal Rearrangements
Chromosome abnormalities caused by chromosomal rearrangement
Chromosomal Rearrangements occur due to breakage of chromosomes:
* Deletion
* Duplication
* Inversion
* translocation
47
Centromere Positions
Telocentric
* Centromere at end of chromosome (not found in humans)
Acrocentric
* The p arm is very short, but is present (e.g. Y)
* These are involved in Robertsonian Translocations
Submetacentric
* The arms’ lengths are unequal
Metacentric
* Centromere is in the middle (e.g. X)
48
Chromosomal rearrangements
* Deletion
o A portion of the chromosome is missing or deleted
* Duplication
o A portion of the chromosome is duplicated, resulting in extra genetic material
* Inversion
o A portion of the chromosome has broken off, inverted, and reattached
* Translocation
o A portion of one chromosome is transferred to another chromosome
49
Deletion/Duplication
A portion of the chromosome has deleted or duplicated
* May be harmless to the individual
* But if disrupts a gene can cause problems
* Larger the segment of a chromosome lost or duplicated more likely
to cause phenotypic abnormalities
50
Inversion
A portion of the chromosome has broken off, turned upside down and
reattached, therefore the genetic material is inverted
* May be harmless to the individual
* But if disrupts a gene can cause problems
* Associated with fertility issues
o Increased risk of producing unbalanced gametes from recombination within
inverted chromosome segment
51
Paracentric Inversion
- does not include centromere
- chromosome break not around centromere
- reinserted piece of DNA
52
Pericentric Inversion
- includes centromere
- chromosome breaks around centromere
- reinserted piece of DNA with centromere
- can change length of chromosome arm
53
Translocation
Portion of one chromosome is exchanged with a portion from another
* Reciprocal translocation
o Segments from two different chromosomes have been exchanged
o Usually harmless but may increase formation of inviable gametes
* Robertsonian translocation
o An entire chromosome has attached to another at the centromere
o In humans, only occur with chromosomes 13, 14, 15, 21 & 22, loss of small segment
54
Polyploidy
possession of > 2 complete sets of chromosomes
o Caused by non-disjunction during mitosis or meiosis
o Heritable condition
o Polyploid organisms are well-adapted to their environments
o High incidence in some taxa
54
Autopolyploidy
caused by
non-disjunction during meiosis
and self-fertilisation
55
Autoallopolyploidy
caused by non-disjunction during meiosis & fertilisation by another species
56
Epigenetics
* Chemical reactions that switch parts of the genome off and on
* Epigenetic regulation modulates gene expression without altering
the DNA sequence
* Facilitates rapid adjustments to dynamically changing environment
* Formation of an epigenetic memory
57
Transcription regulation (Epigenetics)
A transcription factor molecule binds DNA at a binding site and regulates the production of protein from a gene
* DNA methylation / Histone
modification determines
‘tightness’ of chromatin structure
* Chromatin status controls
access of transcription
machinery (DNA mRNA)
* RNA interference regulates
amount of mRNA produced
58
DNA Methylation (Epigenetics)
* Methylation occur at CpG islands
* C is methylated
* Methylation of CpG islands
can lead to the ‘silencing’ of
genes (i.e. not transcribed)
59
Histone Modification
* Histone modifications (e.g.
methylations, acetylations, etc.)
can tighten (condense)
chromatin structure
* Condensed chromatin
(heterochromatin) blocks
Transcription Factors (TF) from
accessing DNA
60
miRNA
* MicroRNA (miRNA) produced
by cell to regulate gene’s
translation
* If miRNA is perfect match
mRNA degradation
* If miRNA is imperfect match
translational repression
61
mtDNA Inheritance
* Nuclear DNA is inherited from both parents / from all ancestors
* mtDNA is only passed down from mother’s lineage
o Only egg supplies mitochondria to fertilised zygote
* Mitochondrial genome extremely small compared to the
nuclear genome
* Despite small size, mitochondrial DNA mutations important
cause for inherited disease
* Several mtDNA copies in each individual cell
* Acquired mtDNA mutations implicated in ageing & age-
related disease, e.g. diabetes
