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HSCI 216 > Lecture 2 > Flashcards

Lecture 2 Flashcards

1
Q

what is evolution?

A

A change in the genetic make up of a population between cohorts

  • evolution is based on heredity
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2
Q

what is heredity?

A

transmission of genetic information from one generation to the next

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3
Q

How does genetic information transfer between people and cells?

A

people: meiosis - sex cells
cells: mitosis

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4
Q

how does genetic material change across generations?

A

variations like:
- mutation
- meiosis: recombination and separation of homologous chromosome pairs into different gametes
- sex

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5
Q

what does genetic variability explain?

A

can explain part of the phenotypic variability we observe in our anatomy and physiology

  • this is one of the main sources of variability that becomes the substrate for the process of natural selection
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6
Q

percentages

A
  • 3.2 billion nucleotide base pairs in haploid human genome
  • 99.5% of all nucleotide bases are exactly the same in all humans
  • 98.8% are identical between humans and chimpanzees
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7
Q

Not all bp’s (base pairs) are created equal

A
  • exons (translated into final mature RNA - protein)
  • Introns (not translated into RNA but important for gene expression)
  • regulatory sequences

Thus, the relevance of a substitution in a SNP depends on its location within the genome (whether it is in an exon, intron, or regulatory region) and whether it is synonymous (does not change the amino acid) or non-synonymous (changes the amino acid)

  • some sections of DNA originally considered intergenic “junk” are now known to be have some roles as promoters and regulatory, enhancers, spacers, and centromes

Genes are not as “self-contained” as originally thought

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8
Q

strains that BRED TRUE for some traits. When self-fertilised, they only produced offspring with traits identical to those of the P1 (parent generation).

That can only happen if?

A

the plants are homozygous for those traits

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9
Q

Mendel’s work

A
  • Mendel chose to study the garden pea
  • Developed true-breeding strains for various traits by self-fertilizing plants for several generations
  • He also conducted dihybrid crosses, where he studied the inheritance of two traits simultaneously

What he found:
- each organism carries two alleles for each trait, one inherited from each parent
- Found that traits controlled by different genes are inherited independently of one another. The inheritance of one trait does not influence the inheritance of another trait, only if the genes are located on different chromosomes.
- He discovered that some alleles are dominant and can mask the expression of recessive alleles

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10
Q

Mendel’s conclusion

A
  • the member of each pair of factors (alleles), that control for the expression of each trait, separated into different haploid gametes
  • traits (coded by genes located on different chromosomes) are inherited independently
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11
Q

Mendel’s work with dominant alleles - those that mask recessive ones led to a critical realisation

A

phenotype does NOT equal genotype

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12
Q

what is phenotype?

A
  • Phenotype is the combination of their observable characteristics or traits.
  • Phenotype is merely influenced by genotype.
  • Environmental factors can also affect phenotype
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13
Q

what is genotype?

A
  • Genotype is the combination of alleles that they possess for a specific gene.
  • Genotype is directly inherited from its parents

Examples: hair colour, eye colour, height

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14
Q

what is the polygenic traits?

A

traits affected by more than one gene
- human skin, hair, and eye colour

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15
Q

what is the pleiotropic genes?

A

genes that affect more than one trait
- red hair genes associated with resistance to sedation

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16
Q

gene-environment interactions

A

traits influenced by both, genes and the environment
- height and body size (polygenic traits heavily influenced by the environment)

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17
Q

what are other traits that do not follow Mendelian laws?

A

Linked traits - genes that are closely located on a chromosome:
- do not separate independently
- do not follow a Mendelian pattern of inheritance
- linkage disequilibrium (This means that the presence of one allele can provide information about the presence of another allele)
- have a low recombination rate

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18
Q

what is modern synthetic theory of evolution?

A

A way of formalising and thinking mathematically modelling the processes and forces of evolution

  • Darwins theory of evolution through natural selection + menders studies of the principles of inheritance = modern synthetic theory of evolution
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19
Q

The modern synthesis: evolution definition

A

change in allele frequencies in a population over time

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20
Q

The modern synthesis: allele definition

A

an alternative form of a gene for a given locus

21
Q

The modern synthesis: population definition

A

a group of potentially interbreeding individuals

22
Q

The modern synthesis: gene pool definition

A

the entire set of alleles present in a population

23
Q

When does evolution not happen?

A

equilibrium = no evolution = no change in allele frequencies in a population (from generation to generation)

conditions:
- no mutation
- no gene flow
- no natural selection
- large population size
- random mating

24
Q

Forces driving evolution

A

natural selection and genetic drift
- reduces or redistributes variation at population level

non random mating
- redistributes variation at population level

gene flow
- introduces or redistributes variation at population level

25
Q

Natural selection

A
  • selection acts on individuals, but effects are seen at population level
  • the alleles of individual who reproduce more (dying less helps at that) increase in frequency over time relative to others and this CHANGE is evolution by natural selection
26
Q

what is genetic drift?

A

a random change in allele frequencies from one generation to the next based on “sampling” effect

Example:
Genetic bottlenecks - population is reduced and then expands again
- resulting pop may not have the diversity or frequency of the initial population
Founder Effect - small subset of a larger population colonizes a new area but said subset is not representative of the allele frequencies present in parent population

27
Q

Summary of the factors that can prevent realisation HW equilibrium

A
  • mating rules or sexual selection
  • small populations
  • geography
  • natural selection
  • genetic drift
  • gene flow
  • mutation

allele distribution of contemporary populations

28
Q

monogenic disease (single genes)

A

Monogenic diseases are genetic disorders that are caused by mutations in a single gene. These diseases can be inherited in a straightforward manner, following Mendelian inheritance patterns.

usually not very common:
- maintained by recurrent mutations
- deleterious effects tend to be post reproductive
- deleterious effects tend to be only expressed in the homozygous and/or provide an advantage in the heterozygous

29
Q

why disease causing genes have not been eliminated? (balancing selection)

A

“balancing selection”: favours a balance between alleles

  • some alleles (or sets of them) confer different advantages and disadvantages (e.g. Cancer)
  • heterozygote advanatge (e.g. hemoglobin, a mutation linked to sickle cell anemia)
  • frequency-dependent selection
  • unstable environments
30
Q

Example of balancing selection

A

Sickle cell anaemia: hemoglobin mutation (HgbS) leads to rigid, sickle shape erythrocites

Sickle cell anaemia: the HgbS affects circulation in the homozygous making them extremely sensitive to oxygen deprivation, causing kidney failure, chest, back and abdominal pain and short life expectancy

The HgbS mutation has not been eliminated by NS
- The frequency of this HgbS can reach 40% in some populations

This phenomenon was only understood after developing a gene frequency map of the HgbS allele
- which allowed scientists to see that the frequency of this allele coincided with the frequency of malaria

But HgbS in heterozygotes (HgbA/HgbS) provides resistance against malaria

HgbA/HgbS has a normal life expectancy and is resistant to malaria which in some regions of the world kills a large number of people each year

31
Q

why disease causing genes have not been eliminated? (polygenic traits)

A

diseases associated with multiple genes (polygenic traits)

  • common diseases depending on various genes each associated with an increased risk for the disease
  • in some cases disease results from a specific combination of particular alleles
  • or specific alleles in particular environments (e.g. obesity)
32
Q

why else could genes associated with negative health have not been eliminated? (males)

A

genes associated with multiple traits (pleiotropic traits)

Males:
- high testosterone costs: aggression, higher blood pressure, more likely to engage on risk taking behaviours (alcohol, smoke, high risk sports)
- high testosterone benefits: drive, libido, muscle development

33
Q

American society of human genetics statement

A
  • genetics demonstrate that humans cannot be divided into biologically distinct subcategories of race
  • inaccurate to claim genetics as the determinative factor in human strengths or outcomes due to education, environment, wealth, and health
  • no factual basis for attempts to define communities or regions of people with “good” or “bad” genes
34
Q

What are “good genes”?

A
  • We have some alleles that will work well in our environment and others that may not work that well
  • Some alleles may be beneficial when present in a heterozygous state (having two different alleles for a gene).
  • For example, individuals with one normal hemoglobin allele and one sickle cell allele (HgbA and HgbS) have a survival advantage in malaria-endemic regions, as they are less susceptible to severe malaria while avoiding the full effects of sickle cell disease
35
Q

How can anyone tell a group of people from a given State that they have “good genes”?

A

The individual “adaptive value” of each gene can only be evaluated in the context of:
- how it works with other genes in any given genome
- how the whole genome interacts with its environment

36
Q

Why did the American Society of Human Genetics react?

A

the “good genes” argument is often used as part of racist discourses

37
Q

Is there such thing as “human races”?

A

No, biologically speaking there is not such thing as human races

Current “popular/traditional” race attributions do not correspond to geographic patterns of human genetic variation

38
Q

No set of alleles define all individuals grouped in traditional racialised categories such as Asians, Caucasians, Blacks, etc.

A

Those whose ancestors have lived for many generations in a particular geographic region tend to share some phenotypic similarities BUT those shared phenotypic traits not always depend exclusively on genetics and those geographic regions tend to be small

  • about 85% of genetic variation measured is found within “racialised” groups and only 15% of that variation is found among groups. So the majority of between-group variation is local, not racial
39
Q

Within small regions people tend to look like each other and often somewhat like people in neighbouring groups

A
  • genetic similarities
  • similar environmental contexts
  • lead to similar phenotypic expressions that change gradually in time and space, resulting in clines
40
Q

what are clines?

A

A geographical gradient in a particular trait across a species

  • Traits exhibit a smooth transition from one form to another over distance
  • Traits that can show clinal variation include physical characteristics (such as skin color, height, or body shape), genetic traits (like allele frequencies), and even behavioral traits. For instance, in humans, skin pigmentation often changes gradually from darker shades near the equator to lighter shades in northern latitudes.
41
Q

Health inequities often attributed to genetic differences between groups

A
  • races assumed as valid biological entities and health differences assumed to be mainly based on genetic differences between races

Those assumptions are not always tested

  • most models highlight supposed biological vulnerabilities for groups that, interestingly, have been colonized/marginalized
  • while other factors such as the effects of marginalisation and inequities are often overlooked or poorly addressed
42
Q

Racial-Genetic Determinism in a cancer survival study (Albain et al 2009)

A

Compared black and white patients survival following a cancer diagnosis (n=20,000 patients, 12% were black) 35 clinical trials

  • all patients received same treatment
  • SES controlled by zip code
  • black patients had worse survival rates for breast, ovarian, and prostate cancer

Authors concludes that observed differences MIGHT be due to “inherited genetic differences across races”, yet no genetic data was collected only information about race

Variation in exposures to environmental factors known to affects cancer survival such as:
- reproductive history
- residential segregation
- access to nutrition
- discrimination

43
Q

What did the Globe and Mail report? (despite Albans et al 2009)

A

a ground breaking new study suggests that survival gap for at least some types of cancer may also be rooted in biology. In other words, blacks may inherit certain genetic traits that make cancers especially lethal

  • despite the study not including genetic data and not including a definition of race
44
Q

How does race become biology?

A
  • using of simplistic conceptions of human biology
  • equating race to genetics
  • ignoring socioeconomic factors
  • minimising the role of lived experiences, marginalisation, discrimination and environmental exposures
45
Q

How can racism and stigma affect development and health outcomes?

A

differences in social environment including access to housing quality, exposures to pathogens, nutrition, appropriate health care, etc.

Model adapted from Gravel (2009) which explains this

Examples:
- low birth weight baby (LBW) risks increased 34% for Californian women with Arabic names during the 6 months following 9/11, a phenomenon not observed in other groups for that period
- The increased risk of LBW has been argued to be a consequence of stigma as it is often observed among moms exposed to high racial discrimination
- potential intergenerational effects: as LBW can be to developmental problems and poor scholastic and later, work achievements, which in turn affects their parental abilities perpetuating the effects of the original exposures across multiple generations

46
Q

Are there genetic basis for disease?

A
  • There are alleles linked disease-risks.
  • Individual alleles, however, explain only a small fraction of heritable variation in complex diseases such as heart disease, obesity, cancer or stroke
  • Complex diseases, depend on multiple alleles and the interactions between those alleles and environmental factors
47
Q

Summary of this lecture

A
  • “race” is argued to not be a meaningful biological concept
  • Different alleles associated with negative health outcomes (often simpler rather than complex ones) can be more frequent in some geographical regions than others
  • Those geographical regions do not necessarily overlap with traditional race categories
  • Race is argued to be a social construct linked to stigma, marginalisation, bullying and ostracism
  • To attribute negative health outcomes to some genetic trait specific alleles need to be identified. Then their prevalence in a particular group should be assessed and environmental factors such as stigma and marginalisation evaluated.
48
Q

Complete the statement put forward by the American society of Human Genetics regarding the “Good Genes” comment

A

We urge societal attention to address profound educational, economic and health inequalities that fuel differences in human experience and well-being. By pursuing all of these activities, humans can celebrate our common heritage as one people, embrace and fully tap our valued diversity, and help achieve optimal life outcomes for all.

HSCI 216 (10 decks)

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