Module 2

Question 1

genes

Answer 1

genes are the building blocks, blueprints, or ingredients of an organism

Question 2

phenotype

Answer 2

If genes are the building blocks, blueprints, or ingredients of an organism, the phenotype is the end product: what the organism ends up looking like and how it ends up behaving.

isn’t a one-to-one relationship between genotypes and phenotypes

phenotype also depends on the environment in which the organism develops, because environmental triggers can turn genes on or off, and they can also create or strengthen connections between cells, like neurons.

Question 3

alleles

Answer 3

are located on the same place of the same pair of chromosomes/two versions of the same gene.

One reason our genes don’t always translate into a clear phenotype

homozygous alleles: exact same version from each parent.

heterozygous alleles: different version from each parent

Question 4

complete dominance

Answer 4

In complete dominance, one of the versions of the gene is dominant, meaning that it will be expressed if present in one or both alleles, and the other version is recessive, meaning that it will only be expressed if both alleles include the recessive version, because the dominant version can’t take over.

Question 5

co-dominance

Answer 5

one version isn’t dominant over the other, but rather both are expressed if present.

In the case of homozygous alleles, we would still get the same situations because there is only one version that can be expressed.

In the case of heterozygous alleles though, the creature may have some patches of blue and some patches of yellow, because both types of alleles would be expressed.

Question 6

incomplete dominance

Answer 6

one version is slightly but not fully dominant over the other.

In the case of homozygous alleles would produce the same effects.

In the case of heterozygous alleles, the organism might display fur that is a colour in between blue and yellow, like some shade of green, because blue is slightly but not fully dominant over yellow.

This means that yellow would affect the phenotype somewhat, instead of being completely silenced like in complete dominance.

Question 7

polygenic inheritance

Answer 7

In reality, most physical traits and probably all or most behavioural and psychological traits involve many more genes

it’s important to remember that almost all psychological and behavioural traits are genetically influenced to some degree. Traits like intelligence and personality have a fairly strong genetic influence

Question 8

Autosomes and genetic disorders

Answer 8

Autosomes are the 22 out of 23 chromosome pairs that you have that aren’t sex chromosomes.

Genetic disorders can be passed on either through a dominant allele or a recessive allele, although those passed on through recessive alleles are much more common

Question 9

autosomal dominant disorders

Answer 9

everyone who inherited at least one dominant allele would have the disorder.

those genes don’t tend to get passed on, so the genes die with the affected individuals

the gene causing the disorder is dominant, each child has a 50% chance of developing it, depending on which version they received from the affected parent.

Question 10

autosomal recessive disorder

Answer 10

people can be carriers of the disorder but not actually have it, meaning they can still pass on the genes for the disorder.

When the gene causing the disorder is recessive, each child has a 25% chance of developing it, because they would need to receive the recessive version from both parents.

On the other hand, they also have a 50% chance of being a carrier of the disorder.

Question 11

Huntington’s disease

Answer 11

autosomal dominant trait, where people experience brain and muscle deterioration starting in middle age. This may also be why it has continued to be passed on in the population despite being a dominant trait: people aren’t affected until middle age, which is usually after they have had children and already passed on their genes.

Question 12

Cystic fibrosis

Answer 12

autosomal recessive disorder, which causes an abnormal production of thick mucus that can affect various organs, including the lungs. The average life expectancy is 40 to 50 years old, but usually with worsening disability over the lifespan.

Question 13

Tay-Sachs disease

Answer 13

autosomal recessive disorder, which causes brain degeneration and is usually fatal before the age of 5. These disorders only persist in the population because people can be carriers, since those who actually develop the disorder are either less likely or completely unable to have children.

Question 14

Sex-linked inherited disorders

Answer 14

he X version is larger and contains many more genes: about 900 compared to the Y chromosome, which only contains about 55 genes.

This means that males only have one copy of most of the genes that are contained on the X chromosome, whereas females have two.

This also means that most sex-linked genetic disorders are located on the X chromosome.

Question 15

X-linked dominant disorders

Answer 15

children would have a 50% chance of inheriting the disorder if their mom had it, since she always passes on one of her two X chromosomes.

if their dad had it but not their mom? It would be 100% of girls, but 0% of boys, since the child would be a female if she received the affected X chromosome from her dad but a male if he received the unaffected Y chromosome.

Question 16

X-linked dominant disorder example

Answer 16

fragile X syndrome, which can cause intellectual disability as well as certain physical characteristics. Males tend to be more severely affected.

Question 17

X-linked recessive disorders

Answer 17

males are more likely to inherit the disorder, since they only have one copy of the X chromosome.

This means that if they inherit the recessive allele on their one X chromosome, they will have the disorder because there is no unaffected, dominant copy to take over.

For girls, the only way they would inherit the disorder is if they received the disordered recessive allele from both parents, which means their dad would have to have the disorder, and their mom would have to be a carrier.

If they only receive it from one parent, they would simply be unaffected carriers.

Question 18

X-linked recessive disorder example

Answer 18

hemophilia, where blood doesn’t clot very fast so people can experience a lot of bleeding from things like cuts or surgery.

Question 19

Other genetic disorders result not from specific genes, but from an abnormal number of chromosomes

Answer 19

Any number other than the normal 46.

This usually occurs by chance because of abnormal eggs, and sometimes abnormal sperm.

genetically abnormal fertilized eggs are actually pretty common, but most of the time, they are miscarried.

Some develop fully though and will result in a disorder that depends on which chromosome was missing or added.

Question 20

Down syndrome

Answer 20

Is called trisomy 21 because individuals with this syndrome have an extra copy of the 21st chromosome: three instead of two.

Individuals with Down syndrome have characteristic facial and physical features, usually mild to moderate mental or social impairments, and often serious medical issues like heart defects.

Question 21

Turner syndrome

Answer 21

comes from a missing or partly missing sex chromosome. People with Turner syndrome are females who only have one full X chromosome.

The symptoms can vary but can include short height, certain physical characteristics like a wide or weblike neck, and problems with the ovaries, which usually means that the woman is infertile and may not even go through puberty.

As with the trisomy 21 form of Down syndrome, Turner syndrome usually results randomly from sperm or eggs that haven’t divided properly to form 23 chromosomes.

Question 22

Nature vs nurture in terms of genetics and environment

Answer 22

genetic influences are referred to as the influence of “nature”, while environmental influences correspond to “nurture”.

Question 23

methylation

Answer 23

Normally, DNA is wound tightly around structures called histones.

In order to be expressed, the DNA has to be unwound so that it’s accessible.

Environmental factors can add methyl molecules that keep the DNA tightly packed around the histones, which effectively keeps gene expression turned off.

This process is called methylation.

Question 24

acetylation

Answer 24

Acetylation happens when acetyl groups are added, which loosens up the DNA from its tight packing around the histones, and allows genes to be expressed.

Question 25

Epigenetics

Answer 25

Is the study of changes in gene expression, rather than in the actual genes themselves, which people can also inherit.

When genes are turned on or off, the change isn’t necessarily permanent, but it can last a long time, even through generations.

This is why the environmental experiences of your grandparents can affect the way that your own genes are expressed.

Question 26

reaction range

Answer 26

it’s best to think of genetic predispositions as specifying a reaction range: the range of phenotypes that can result from a specific genotype because of how these different genes can be turned on or off.

Question 27

gene-environment correlations.

Answer 27

There are three ways that genes are thought to influence variations in the environment, and these are called gene-environment correlations.

All of this is to say that some aspects of our environments are non-random, and they aren’t independent from our genetics: our genes and those of our relatives affect what we experience in the world.

Question 28

passive gene-environment correlations

Answer 28

which is when your parents’ genes affect the environment that you’re raised in.

Your biological parents have certain genetic predispositions.

The main thing to point out is that those environmental experiences aren’t random: they were caused by your parents’ genetic predispositions.

Question 29

Evocative gene-environment correlations

Answer 29

are when your own genes influence how other people treat you or interact with you.

the cause of a specific enrichment is your own genetic predispositions.

Question 30

active gene-environment correlations, or niche- picking.

Answer 30

Like evocative effects, these have to do with your own genetic predispositions rather than those of your parents.

But niche-picking reflects how your own genetic predispositions influence the environment that you choose for yourself and the activities you choose to engage in.

Question 31

Gene-environment effects over time

Answer 31

The influence of these three effects also changes over development.

When you’re little, you don’t have a lot of control over which environments you put yourself in, so active gene-environment effects don’t play a large role.

Rather, your parents choose for you, and so passive effects have a lot more influence early on in development.

As you grow up, your parents’ control over your environment gradually declines, while your own control gradually increases, leaving basically zero influence of passive effects by the time you’re an independent adult but a high influence of active effects.

Evocative effects exert a pretty constant influence throughout your life, because your traits will always influence how people treat you.

Question 32

Monozygotic twins, or identical twins

Answer 32

share pretty close to 100% of the same genes.

This is because monozygotic twins come from one fertilized egg, which has divided completely in two at some point very early in development: mono means one, and a zygote is a fertilized egg.

Question 33

Dizygotic twins, or fraternal twins

Answer 33

share about 50% of their genes and are as genetically similar as two siblings would be if their mom was pregnant with them at different times - they just happen to have shared a prenatal environment.

Dizygotic means two zygotes, because this type of twin results from two different eggs that were fertilized with two different sperm.

Most of the time a woman only releases one egg, but sometimes she can release more than one, and each released egg then has a chance to become fertilized.

Question 34

shared environmental factors in twin studies

Answer 34

which are any experiences that are common to a particular family

could include parenting style, the socioeconomic status of the family, or how often parents read to their children

best way is to isolate the right variable, just like we did to test heritability.

This time though, we want to isolate the shared environment.

To do that, we can compare identical twins who were raised together, and compare them to identical twins raised apart.

This way, genetic relatedness is controlled, and the two types of pairs only differ in their family environment.

If the twins raised together are more similar than the twins raised apart, then family environment probably contributes to that trait.

Question 35

non- shared environmental factors in twin studies

Answer 35

which are experiences that are specific to certain individuals but not their twin or sibling.

are things like the fact that even twins can have different friends, teachers, or hobbies.

are hard to test directly because they’re unique to each individual.

Question 36

Studies that want to examine the genetic contribution of a trait (heritability)

Answer 36

try to compare pairs that are as similar as possible in terms of the environments they are exposed to, like the prenatal environment and the family environment.

On the other hand, the pairs vary in terms of how genetically related they are.

If the pair who is more genetically related is more similar to one another than the pair who is less related, we can assume that the trait is partially heritable, or passed on through genes.

In twin studies, we assume that both types of twins share very similar environments, so if identical twins are still more similar, it’s probably because of their genetics.

Question 37

adoption studies

Answer 37

Another common way of estimating heritability and environmental factors.

Comparing an adopted child to their biological parents as well as their adoptive parents.

If children are more similar to their biological parents, then genes probably make a significant contribution to a trait, whereas if they’re more similar to their adoptive parents, then shared environmental factors probably contribute a lot.

Question 38

Twin studies and adoption studies can help us calculate three things

Answer 38

heritability, shared environmental factors, and non-shared environmental factors.

These studies try to explain how much of the differences between people can be explained by each reason?

Proportions vary from 0 to 1, but they’re kind of like percentages.

If the heritability coefficient is equal to 0.4, this means that 40% of the differences in people’s scores on a trait can be explained by genetics.

researchers first calculate heritability and the contribution of the shared environment.

Whatever proportion is leftover is assumed to be due to the non-shared environment so that all the numbers add up to 1, or 100%.

Question 39

Misconceptions about heritability

Answer 39

heritability explains the differences between people, NOT how much of an individual’s trait is due to genetics. _{So if IQ is up to 80% heritable, this means that 80% of the differences between people can be due to genes, but NOT that 80% of a person’s IQ is due to their genes.}

This is why heritability estimates aren’t static – they depend on the population that you test, and they can change depending on whether you’re testing young children vs. adults or even people with high vs. low socioeconomic status.

Heritability also shouldn’t be thought of as a purely genetic influence. Remember passive, evocative, and active gene-environment correlations?

You can’t really disentangle the effect of environmental experiences that result from biological predispositions from those biological predispositions themselves: there really is no “pure” genetic effect. So when we’re estimating heritability, we’re including those experiences that resulted because of genetics.