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GED > Genes > Flashcards

Genes Flashcards

1
Q

what is a gene?

A

a part of a chromosome involved in the transcription of DNA to RNA

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2
Q

what are the 4 stages in the cell cycle?

A

G1 - first growth phase
S - DNA synthesis and replication phase
G2 - second growth phase
cytokinesis - actual division of the cell

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3
Q

what is the function of independent assortment?

A

to generate different combinations of chromosomes in genes

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4
Q

what is the function of crossing over?

A

to generate different combinations of alles in chromosomes

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5
Q

what are some features of a monohybrid cross?

A
  • includes one gene locus with two alleles
  • 3:1 phenotypic ratio in F2 generation
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6
Q

what are some features of a dihybrid cross?

A
  • invovles two genes
  • 9:3:3:1 phenotypic ratio in the F2 generation
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7
Q

what are different forms of genes known as?

A

alleles

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8
Q

true or fale: gain of function mutations are dominant, and loss of function mutations are recessive

A

true

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9
Q

what is incomplete dominance?

A

when a heterozygous phenotype is intermediate between the two homozygote phenotypes

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10
Q

what is codominance?

A

when heterozygotes show phenotype of both alleles
- e.g. blood group AB has both type A and type B characteristics

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11
Q

what is pleiotropy?

A

when one gene may influence more than one trait

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12
Q

what are lethal alleles?

A

alleles that cause an organism to die only when present in a homozygous condition
- e.g. yellow allele in mice being dominant for coat colour, but recessive for lethality
- Ay = yellow gene - if mouse has genotype Ay A, it will be born with yellow coat
- if mouse has genotype Ay Ay, it will not survive

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13
Q

what is phenotypic variability?

A

the phenomena when individuals with the same genotype show different phenotypes

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14
Q

what is penetrance?

A

the measure of the % of individuals with a given genotype who expressed the expected phenotype
- e.g. take a group of 5 individuals with the same genotype, but only 3 of them express the expected phenotype; phenotype is therefore 60% penetrant

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15
Q

what is expressivity?

A

the measure of the extent to which a given genotype is expressed at the phenotypic level
e.g. take a group of 5 individuals with the same genotype, and all 5 express the phenotype. penetrance is 100%. however, all individuals express the phenotype only to a certain extent. e.g. one individual will have very very light blonde hair, whereas the other 4 will have varying shades of blonde hair.

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16
Q

what is epistasis?

A

the interaction between two or more genes to control a single phenotype
e.g. the allele of one gene can mask the phenotypic effect of an allele of another gene

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17
Q

what phenotypic ratio arises in the case of recessive epistasis?

A

9:4:3

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18
Q

what phenotypic ratio arises in the case of dominant epistasis?

A

12:3:1

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19
Q

what is epigenetics?

A

“heritable changes in gene expression that do not involve alterations of the DNA sequence”

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20
Q

how can epigenetics regulate gene expression?

A
  • environmental factors can switch on expression of genes (e.g. red and green gene)
  • some genes that are expressed are transient, and this gene is not expressed in daughter cells
  • some gene expressions persist throughout generations, meaning an epigenetic effect has occured
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21
Q

How can DNA methylation or histone modification in general affect gene expression?

A
  • DNA methyl. and histone mods. can alter chromatin structure
  • chromatin structure affects gene expression
  • this means that the altered chromatin structure can be passed on to daughter cells
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22
Q

what is paternal imprinting?

A
  • when the paternal allele is imprinted and silenced by epigenetic tags
  • this means that the maternal allele is preferentially expressed in the embryo
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23
Q

what is maternal imprinting?

A
  • when the maternal allele is imprinted and silenced by epigenetic tags
  • this means that the paternal allele is preferentially expressed in the embryo
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24
Q

what is the function of genomic imprinting?

A
  • the genetic conflict hypothesis - conflict between interest of maternal + paternal genes in the foetus
  • father may only be related to a subset of foetuses (as female may mate with others) and wishes to increase the survival chances of his offspring by promoting their growth
  • mother equally related to all offspring and wishes to divide resources equally between all of them
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25
Q

what is an autosome?

A

any chromosome that isn’t a sex chromosome

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26
Q

how can it be that an individual with XX chromosomes is a male?

A
  • only one gene on the Y chromosome is required - the SRY (sex-determining region on Y)
  • translocation of SRY to the X chromosome causes the individual the be male
  • the same thing can occur if the SRY is mutated on the Y chromosome, leading to XY females
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27
Q

how does the SRY gene confer maleness?

A

SRY = DNA binding protein (transcription factor)
- regulates expression of genes necessary for testis formation
- SRY expression changes the indifferent gonad to the testis

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28
Q

what is a gyandromorph?

A
  • a sexual mosaic (normally just in birds)
  • cells on the right side of the body express ZW (female chromosome set)
  • cells on the left side of the body express ZZ (male chromosome set)
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29
Q

what is sex-linked inheritance?

A
  • usually involves genes located on the X chromosome (X-linkage)
30
Q

what is a barr body?

A
  • dosage compensation for female X-chromosomes
  • female mammals have double the gene dosage of X-linked genes
  • one X chromosome in each female cell becomes inactivated
  • this inactivated X chromosome is a barr body
31
Q

what is the purpose of gene maps?

A
  • identifying genes responsible for diseases or traits
  • compare genome organisation between organisms
32
Q

What does it mean to say that females are the homogametic sex and males are the heterogametic sex?

A

Females have XX, males have XY

33
Q

What is the function of Giemsa in human cytogenetic mapping?

A

Cause G-banding on human chromosomes to make dark (G-dark) and light (G-light) bands

34
Q

What is Genetic Linkage?

A

Genes on the same chromosome may violate Mendel’s second law of independent assortment as they are linked
- alleles of genes on the same chromosome will segregate together in gametes during meiosis UNLESS crossing over occurs to separate them (crossing over occurs at random positions along chromosome)

35
Q

How do you calculate frequency of recombinant genes & how does it relate to distance?

A

No. Of recombinant gametes / total number of gametes (e.g. 4 recombinant gametes / 16 total gametes = 25% recombinant)

The frequency of recombinant gametes is proportional to the frequency of crossing-over ( and therefore distance apart on the same chromosome)

36
Q

What do recombination frequencies tell us? (<50% and >50%)

A
  • > 50% = genes on different chromosomes (ind. assortment) or genes far apart on same chromosome
  • <50% = genes linked on same chromosome - smaller the RF, closer the genes
37
Q

How is genetic distance (recombination frequency) calculated?

A

RF = ( no.of recombinant progeny x 100 ) / total number of progeny

Unit = centimorgans

38
Q

When doing a pedigree diagram, what does generation skipping mean?

A

The allele is recessive

39
Q

What is genetic heterogeneity?

A

Having mutations in different genes that give the same disease
- e.g. at least 3 genes causing familial early-onset Alzheimer’s disease

40
Q

What makes doing a pedigree diagram difficult?

A
  • incomplete penetrance
  • genetic heterogeneity
  • non-paternity / misattributed paternity
41
Q

What are some commonly used DNA markers?

A
  1. Short tandem repeats (STRs)
  2. Single nucleotide polymorphisms (SNPs)
42
Q

What are short tandem repeats? (STRs)

A
  • microsatellite repeats
  • tandem repeats of a short sequence
  • usually noncoding sequences
43
Q

What are single nucleotide polymorphisms? (SNPs)

A
  • single nucleotide differences between individuals
  • e.g. one nucleotide difference will be found between individuals in every 1000 nucleotides
44
Q

What is the principle of linkage analysis with SNPs?

A
  • disease causes mutation
  • mutation occurs in base closest to SNP1, and further downstream are SNPs 2 and 3
  • crossing over occurs between the mutant base and SNPs (linkage)
  • SNP2 shows linkage in some pedigrees as it is relatively far away from mutation, whereas SNP1 shows linkage in all pedigrees as it is closest to mutation
  • SNPs used as markers to determine position of mutation
45
Q

What is positional cloning of the disease gene?

A

selecting ‘candidate genes’ in the region of the chromosome and looking for disease-associated mutations in each candidate gene

-genetic cluedo

46
Q

How does next generation sequencing (NGS) aid with disease gene discovery?

A
  • sequence whole genomes or whole exomes from unaffected and affected members of a pedigree
  • identify rare variants shared by affected members
  • identify causative variants in candidate genes + investigate the functional consequences of the causative variants
47
Q

What is aneuploidy?

A

A chromosome abnormality where the number of chromosomes is abnormal

48
Q

What is the normal human karyotype? (Complete set of chromosomes)

A

46 chromosomes, XY = males, XX = females

49
Q

What is monosomy?

A

Loss of a single chromosome

50
Q

What is trisomy?

A

Gain of one extra chromosome

51
Q

What is tetrasomy?

A

Gain of an extra pair of chromosomes

52
Q

How can aneuploidy arise?

A

Mistakes in chromosome segregation (non-disjunction) during meiosis can give aneuploid gametes

53
Q

How is prenatal screening for Down syndrome performed?

A
  • pregnant women offered ‘combined test’ to identify pregancies with higher risk of aneuploidy (only shows RISK)
  • chorionic villus sampling (CVS) used to sample foetal cells
  • karyotype of foetal cells determined
54
Q

What is genetic mosaicism?

A

Where individuals have a mixture of normal and trisomy cells
- can be another cause for Down syndrome

55
Q

What is Turner syndrome a result of?

A

monosomy

56
Q

What are some syndromes caused by Trisomy?

A
  • Klinefelter syndrome
  • triplo-X
  • XYY syndrome
57
Q

What is the correlation between maternal age and incidence of trisomy?

A

As age increases, so does risk of trisomy in women - leads to increased frequency of miscarriage

58
Q

At what phase does 95% of maternal non-disjunction occur to induce trisomy?

A
  • prophase 1
  • caused by a premature loss of cohesion in prophase 1, which gives 2 univalents that segregate independently - gives aneuploid gametes
59
Q

What types of genes are found in mitochondrial genomes?

A
  • mtDNA
  • contains structural genes for proteins involved w/ oxidative phosphorylation
60
Q

What types of genes are found in the chloroplast genome?

A
  • cpDNA
  • contains structural genes for proteins involved in photosynthesis
61
Q

How does maternal inheritance of mtDNA work?

A
  • in mammals, paternal mitochondria + other sperm components destroyed after fertilisation
  • females transmit traits of extranuclear genes to all offspring
  • males never transmit traits of extranuclear genes
62
Q

What are some differences between Mendelian and complex traits?

A

Mendelian
- single gene w/ high penetrance
- predictable mode of inheritance
- simple relationship between genotype and phenotype

Complex
- multiple genes w/ low individual penetrance
- familial clustering - inheritance not predictable
- complex relationship between genotype + phenotype
- often affected strongly by environment

63
Q

What is an example of genetic and environmental influences on a complex trait?

A

Type 2 diabetes
Genetic - if both parents affected, risk increased by 75%
Environmental - if BMI is >30% (obesity), risk increases

64
Q

True or false: complex genes can be continuous or discontinuous?

A

True

65
Q

How is total phenotypic variance calculated?

A

Vp = Vg + Ve

Vp = total phenotypic variance
Vg = genetic variance
Ve = environmental variance

66
Q

What does it mean to have a high heritability?

A
  • Genetic differences in a population explain a high proportion of the phenotypic variation
  • i.e. genes affect the phenotype more

Low heritability = more easily affected by the environment

67
Q

What is concordance in relation to twins?

A

Concordance = probability that if one twin is affected by a trait, the other twin is also affected
- concordance = 1 (one twin is affected, other twin is always affected)
- concordance = 0.6 (one twin is affected, other twin has 60% chance of being affected)

Difference in concordance between MZ and DZ twins can be used to estimate heritability

68
Q

Why are disease-associated alleles common?

A
  • late-onset diseases have little effect on fitness (weak purifying selection)
  • alleles that were advantageous or neutral in the past may confer disease susceptibility in modern society
  • disease causing alleles can be maintained at high frequency by balancing selection
69
Q

What is Genome wide association studies (GWAS)?

A
  • a population-level approach to disease gene mapping
  • help to identify disease associated SNPs
70
Q

What is a haplotype ?

A
  • closely linked SNPs on the same chromosome that are usually inherited together in ‘haplotype blocks’
  • gaps between haplotype blocks are areas where crossing over occurs preferentially
  • crossing over never occurs within haplotype blocks
71
Q

What is linkage disequilibrium?

A

The co-inheritance of SNPs in a haplotype block

72
Q

What does linkage equilibrium mean for haplotype frequency?

A
  • each haplotype is present at frequency expected from allele
  • e.g. with genes A, a, B, and b, there are equal frequencies of AB, aB, Ab, and ab

GED (3 decks)

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