PSYCH 223 - Chapter 2: The Start of Life

lectures from Prof. Vrushali Rao Gumnur; set in progress

What is the breakdown of how genetic information is organized in human cells?

DNA → chromosomes → genes

1. DNA - chemical substance that determines nature and function of each cell

2. Chromosome - rod shaped portions of DNA or strings of genetic material

3. Genes - basic units of genetic information

What genetic information is contained in every human cell?

23 chromosome pairs (46 chromosomes) containing ~25000 genes

What are autosomes vs allosomes?

Autosomes - cells that contain 23 pairs of chromosomes (46 total) → all cells in the body except sex cells

Allosomes - cells that contain 23 chromosomes → gametes (ovum and sperm)

What is mitosis vs meiosis?

Mitosis - the process of cell replication

Meiosis - the process by which gametes (ovum/egg and sperm) form

How does genetic diversity arise?

Processes in meiosis:

• Random sorting of chromosomes - chromosomes randomly sort into gametes

• Gene recombination - paired chromosomes crossing over and exchanging DNA

What monozygotic vs dizygotic twins?

Monozygotic (identical) twins - genetically identical twins resulting from the developing cluster of cells splitting into two after fertilization

Dizygotic (fraternal) twins - non-genetically identical twins resulting from two separate ova being fertilized by two separate sperm

What causes behavioural/developmental diversity among twins?

Monozygotic twins → environment only (genetically identical)

Dizygotic twins → genetic and environmental factors

What trends are seen in rate of multiple births (mostly twins)? What are some factors that may be contributing to this trend?

Rate of multiple births has been increasing since the 80s (~33% in Canada)

• Rate of multiple births can differ by race, ethnicity, nationality

• Increased use of fertility drugs

• Rising average age of mothers

• Assisted human reproduction

How is sex determined in a human child?

23rd pair of chromosomes are sex chromosomes (X or Y), ova always carry X chromosomes and sperm may carry X or Y so sex is always determined by the father’s sperm

Who is Gregor Mendel? What did he discover?

Scientist considered the father of modern genetics → discovered that when two competing traits were present together, only one would be expressed

What are dominant traits vs recessive traits?

Dominant trait - trait is expressed when two competing traits are present (often denoted by a capital letter)

Recessive trait - trait that is not expressed when competing traits are present (often denoted by a lowercase letter)

• (In the image: Y is dominant, y is recessive)

What are alleles?

Variant forms of a gene that occur in pairs

• In the image: Y and y are alleles for the seed colour in pea plants

What is genotype vs phenotype?

Genotype - underlying combination of all genetic material or actual DNA in an individual

Phenotype - set of all the observable traits of an individual

• Genotype and other factors determine phenotype

What is homozygous vs heterozygous?

If the child receives same alleles, they are homozygous for the trait

If the child receives different alleles, they are heterozygous for the trait

What are polygenic traits?

Traits whose inheritance/phenotype is controlled by multiple gene pairs

• Eg. skin colour

What is polygenic inheritance?

The process of inheriting polygenic traits (Eg. skin colour)

Do all traits follow a dominant-recessive relationship?

No, for example, skin colour blood type

What is reaction range?

The degree of the variability in the expression of a trait depending on the environment

What are X-linked genes?

Recessive genes located only on the X chromosome

Who is more at risk of X-linked disorders? Why?

Males are at a higher risk because they only have one X chromosome (no second one to override a recessive allele)

What has gene mapping uncovered about the human genome?

Humans share 99.9% of their genetic sequence

What are some ways that genes can get damaged?

• Cell division

• Spontaneous mutation

• Environmental factors

What are some examples of genetic disorders?

• Fragile X syndrome

• Sickle cell anemia

What is fragile X syndrome?

Genetic disorder that occurs when a gene on the X chromosome is injured (simplified explanation from the slides, more accurate description is that a segment of the gene is duplicated so many times that it creates a fragile site that interferes with protein production)

• Results in mild to moderate intellectual disabilities

What is sickle ell anemia?

Genetic disorder that occurs when an Individual has a mutation that changes shape of red blood cells

• Causes poor appetite and stunted growth. Severe forms causes death

What are some examples of chromosomal disorders?

• Down syndrome

• Klinefelter’s syndrome

• Turner’s syndrome

What is down syndrome?

Chromosomal disorder also known as trisomy 21 (extra chromosome on the 21st pair)

• Causes intellectual disabilities

• 1 in 750 Canadian children are born with this

What is Klinefelter’s syndrome?

Chromosomal disorder that occurs in XXY individuals (males with an extra X chromosome)

• Causes under-developed genitals, experiencing both male and female changes

• 1 in 400 males born with this

What is Turner’s syndrome?

Chromosomal disorder that occurs in X0 individuals (females missing an X chromosome)

• Anatomically female but don’t menstruate or develop breasts at puberty; stunted growth

What is multifactorial transmission?

The process where traits are determined by both genetics and environmental factors

How do we test the relative influence of genes compared to the environment?

Human studies → we can control the environment

Animal studies → we can control both genetics and the environment

How can we use twin studies to test the relative influence of genes compared to the environment?

1. Expose genetically identical individuals to different environments (twin studies with monozygotic twins)

2. Expose genetically different individuals to same environments (twin studies with dizygotic twins)

3. Compare people based on genetic similarity, like parents and children (Eg. higher association between children and biological parents compared to adoptive parents → higher heritability)

How do genes and environment work together when it comes to physical traits?

Even with environmental factors taken into account, genetically similar people are likely to be share some physical characteristics

• Eg. obesity, heart rate, lifespan

How do genes and environment work together when it comes to intelligence?

Related people have greater correspondence in IQ scores (50-80%)

• Age of individuals impacts this link

• Education can play a role

How do genes and environment work together when it comes to personality?

It can have genetic roots but family environment can matter

• Certain personality traits reflect greater contribution of heredity (Eg. extroversion, neuroticism)

slide 14 - watch video

How do genes and environment work together when it comes to psychological disorders?

With the example of schizophrenia: It can result from genetic disposition (abnormal chemical imbalance) but may not manifest itself if not under highly stressful environments

• Other examples: depression, alcoholism, autism

What influences can genes have on our environments?

• Individuals could choose environment to suit genetic abilities

• Parents could provide opportunities based on their parent’s abilities

• Child’s genetics may evoke environmental influences from parents

How many gametes do females vs males have?

• Females are born with 400,000 ova in ovaries, released during ovulation and do not produce any more

• Males produce millions of sperms everyday, stored in the testis

What is fertilization?

The process of the sperm (only takes one) fusing with the ovum to form a zygote

Where does conception occur in the body?

Ova are fertilized in the fallopian tubes and they travel towards the uterus while developing

What are the 3 stages of fetal development?

1. Germinal stage: 0-2 weeks

2. Embryonic stage: 2-8 weeks

3. Fetal stage: 8 weeks to birth

What is the germinal stage? What major changes occur during this period?

First and shortest stage of fetal stage (0-2 weeks)

• Zygote begins to divide into multiple cells and cells begin to specialize

What is the developing human called during the germinal stage?

Blastocyst (after 32 cell divisions)

When does implantation occur?

Around day 12 during the germinal stage

What are some structures formed in the germinal stage?

• Placenta

• Umbilical cord

• Yolk sac

• Amniotic sac

What is the placenta?

Specialized organ that helps in the transfer of materials between the mother and the developing embryo formed during the germinal stage

• Formed from outer cells of the zygote/blastocyst

What is the umbilical cord?

The structure that links the placenta and the developing embryo formed during the germinal stage

• Formed from inner cells of the zygote/blastocyst

What is the yolk sac?

Structure that produces blood cells for the fetus formed in the germinal stage

What is the amniotic sac?

The fluid-filled protective sac formed in the germinal stage

What is the embryonic stage? What major changes occur during this period?

Second stage of fetal development (2-8 weeks) where major organs begin to develop

• Rudimentary eyes, teeth, limbs form

• Neurons starts growing and brain enlarges → brain starts showing weak electrical activity

• Major organs begin to develop

What is the developing human called during the embryonic stage?

Embryo

What are gonads?

Glands that produce sex hormones

• They receive signals from the brain to produce sex hormones which differentiates sex organs in testes or ovaries

What are the three embryonic germ layers?

1. Ectoderm

2. Mesoderm

3. Endoderm

What is the ectoderm?

Outermost embryonic germ layer that gives rise to skin, hair, teeth, and nervous system

What is the mesoderm?

Middle embryonic germ layer that gives rise to musculoskeletal system (muscles, bones), blood, circulatory system, and kidneys

What is the endoderm?

Innermost embryonic germ layer that gives rise to digestive system and respiratory system

What is the fetal stage? What major changes occur during this period?

Final stage of fetal development (8 weeks to birth) where growth and organs are refined

• Substantial increase in length and weight

• Organs more differentiated and start to work

• Eyes can open

• Androgen release leads to emerging sex differences

What is the developing human called during the fetal stage?

Fetus

At what point are fetuses viable?

33% of fetuses have viability by week 22 (fetal stage)

What is neural proliferation? When is it of importance in fetal development?

The dividing of of the neural stem cells to create billions of neurons

• Increases dramatically during fetal stage to strengthen interconnections

What are hemispheres?

The two halves of the brain

What is myelin?

An insulating sheath/material that helps in speeding up neuronal transmission

What are the three main ocmponents of a neuron?

1. Cell body

2. Axon

3. Dendrites/terminal fibres

What is the cell body?

Part of the neuron that contains the nucleus and carries out vital cell functions

What is the axon?

Tail-like extension of the neuron that carries the electrical signal

What are dendrites and terminal fibres?

Protrusions of the neuron that make connections with other neurons

What are some fetal behaviours that have been revealed by ultrasound imaging?

• Thumb sucking

• The fetus is able to hear sounds and feel vibrations

• Shows sleeping and wking patterns

• Prenatal environment can affect development and future behaviours

What evidence do we have that newborns can recognize familiar stimuli from the womb?

1986 study that shows babies responded more (increased sucking response) to “The Cat in the Hat” if the mother had read it during gestation

• Babies also show increased response when the story is read by their mother compared to a stranger’s voice

What is a prenate?

An unborn baby

What does vision look like in prenates?

• Minimal visual experience

• Light sensitivity develops between 17-20 weeks after conception

What evidence shows that prenates have visual experiences/behaviours?

• Holding a flashlight to your pregnant belly could cause the fetus to kick, flutter, or show elevated heart rate

• Premature babies (32 weeks) are also capable of focusing and horizontal/vertical tracking

What evidence shows that prenates have auditory experiences/behaviours?

• 1986 study that shows babies responded more (increased sucking response) to “The Cat in the Hat” if the mother had read it during gestation

• Babies also show increased response to their mother’s voice and language compared to a stranger’s, even more so if the voice is filtered mimicking the prenatal environment

  • Can be argued that babies are capable of operant conditioning → memories are originally formed in prenatal period

What does the sense of touch look like in prenates?

• Touch is the first sense develop (8 weeks), especially in the perioral area which is around the mouth - observed using ultrasound when movement is larger when touched in the perioral area compared to other areas

• Twin shave shown strong reaction to being touched or pushed by the other twin

• Maternal touch to the abdomen can increase movement in the arms, head, and mouth of the fetus at around 32 weeks

What does the sense of taste look like in prenates?

• Amniotic fluid contains variety of flavour

• Fetus has taste buds with adult-like abilities at birth

What evidence shows that prenates have experiences/behaviours relating to taste?

• In an experiment where amniotic sacs were injected with sweet tasting fluid and dyes, prenates ingested more amniotic fluid if it had a sweet taste

  • Fetuses likely prefer sweet tastes

• Babies tend to show preference for prenatal tastes from mother’s diet → cultural preferences for food

• Ultrasounds show babies physically react with facial expressions depending on what the mother consumes (smile muscles activate with sweeter foods, frown muscles activate with bitter foods)

What does the sense of smell look like in prenates?

Amniotic fluids absorbs odours from the mother’s diet (confirmed by obstetricians)

What evidence shows that prenates have experiences/behaviours relating to smell?

• Experiment where cotton pads are dipped in different amniotic fluids, newborns show the strongest preference/reaction to their own mother’s amniotic fluid

• Ultrasounds show babies physically react with facial expressions depending on what the mother consumes (smile muscles activate with sweeter foods, frown muscles activate with bitter foods

What is infertility?

The inability to conceive after 122-18 months of trying

How many couple face infertility?

~15% of all couples

What are some factors that an contribute to infertility?

• Low sperm count and motility

• Hormonal imbalance in women

• Stress, alcohol, and drug use

What are some treatments for infertility?

• Artificial insemination

• In-vitro fertilization (IVF)

  • Donors

  • Surrogate mothers

What is artificial insemination?

Infertility treatment that involves a man’s sperm being directly places into a woman’s reproductive tract

What is in-vitro fertilization (IVF)?

Ova and sperm are removed and fused in a laboratory, then implanted into the uterus or fallopian tubes

• Can utilize donors or surrogate mothers

What is cryoperservation?

Freezing cells to stop biological activity

• Used by IVF labs to preserve embryos

Do all successful IVF implantations lead to successful births?

No → IVF procedures resulted in successful births for 31% of women under 35 yrs and for 15% of women above 40 years in 2015

• IVF success rate drops with age because hormone imbalances developed closer to menopause create uterus more hostile to a growing fetus

What is viability?

The ability for a developing child to survive outside the womb

What is a miscarriage? What occurs and how?

Pregnancy ends before the developing child gains viability

• Results in embryo, placenta, and umbilical detaching

• Can occur due to genetic abnormality, physical injury, or toxins

• 15-20% of pregnancies end in miscarriage

What are the risks/adverse effects miscarriages have on mothers?

Mothers at risk of suffering from depression and anxiety, or even problems with developing attachments/bonds with future children

What is abortion?

The process of voluntarily ending a pregnancy

• May give rise to feelings of relief, grief, or regret; very dependent on the individual

  • Feelings generally don’t last as long as the negative feelings that come with a miscarriage

How can we help predict future inherited disorders?

• Genetic counselling

  • Karyotyping

• Prenatal testing

What is genetic counselling?

Field that deals with helping people tackle issues related to inherited disorders

How does genetic counselling work?

Counsellor perform physical examination of parents and examine family history for patterns of inheritance (Eg X-linked disorders)

• Can help individuals (parents) identify their own susceptibility to future genetic abnormalities (Eg. Huntington’s disease manifests later in life)

What is a karyotype?

Chart of enlarged photos of chromosomes

• Used in genetic counselling

What is prenatal testing?

First trimester screen (11-13 week) involving blood test or imaging to look for signs of a toxic prenatal environment

What are some methods of prenatal testing?

• Ultrasound sonography

• Chorionic villus sampling

• Amniocentesis

What is ultrasound sonography?

Prenatal testing method involving using high frequency sound waves to produce an image of the unborn baby

What is chorionic villus sampling?

Prenatal testing method involving obtaining sample of hairlike material from the placenta to obtain a karyotype

What is amniocentesis?

Prenatal testing method involving fetal cells drawn from the amniotic fluid to obtain a karyotype

What are some practical issue with genetic testing?

• Some things cannot be detected in the prenatal period

• Finding out about susceptibility to future genetic abnormalities can lead to emotional distress and fearmongering

• Only provides a range of probability and can be misguided if personal differences are not taken into account

What are some ethical issues with genetic testing?

• Sex determination tests could be misused and lead to gender discrimination in some cultures

• Sex specific IVF treatments can also lead to the same ethical dilemma

• Can tread dangerously into eugenics territory