Genetics and Prenatal Development

Biological/Evolutionary Theories

Evolutionary theories propose that the genetic and physiological processes that underlie human development changed gradually over time through genetic mutation and natural selection

• focuses on how genetics and traits are passed on over time (including differences and similarities)

• scientists are discovering the complex role that inheritance plays in human health and the related importance of early intervention

  • helps mitigate inherited conditions/risks

Genetic Foundations of Development

Chromosome: 1 of 46 molecules that contain 23 pairs of tight strands of DNA

DNA: molecular structure inside cells that contains genetic information responsible for the development and function of an organism

Gene: basic unit of heredity that occupies a specific location on a chromosome.

• ~20,000 - 25,000 genes within our chromosomes

Genome: complete set of genetic info in an organism

Sex Cells and Reproduction

Gamete: an organism’s reproductive cells (aka sex cells)

• female gametes are called ova or egg cells

• male gametes are called sperm

• gametes are haploid cells as each cell caries only one copy of each chromosome

Zygote: a single celled organism resulting from a fertilized egg

Genetic Inheritance

• traits and characteristics are inherited in predictable ways

• most of humans have 23 pairs of chromosomes

• the genes within each chromosome can be expressed in different forms: alleles

Genetic Inheritance: Definitions

Alleles: one of two or more different forms of the same gene at the same place on a chromosome

Homozygous: a chromosomal pair consisting of 2 identical alleles

Heterozygous: chromosomal pair consisting of 2 different alleles

Genotype Vs. Phenotype

Types of Genetic Dominance

Incomplete Dominance: both genes are expressed in the phenotype

Co-dominance: both alleles of a heterozygote lack the dominant and recessive relationship

Polygenic Inheritance

Occurs when a trait is a function of inheritance of many genes, such as with height, intelligence and temperament

Multifactorial Inheritance

Outcomes resulting from a combination of multiple genetic and environmental factors, rather than a single gene or environmental cause.

Mitochondrial Inheritance

Aka: maternal inheritance

• the pattern where mitochondrial DNA (mtDNA) is exclusively from the biological mother, not the father

• mitochondria in the egg contains the inherited mtDNA, while paternal sperm mitochondria are destroyed or diluted after fertilization

Epigenetics

The study of how DNA interacts with a multitude of smaller molecules found within cells which can activate and deactivate genes

• epigenetic factors are pivotal in development across the lifespan

• helps us understand how some things develop

Assisted Human Reproduction (AHR): Fertility Medications

• increase the chance of ovulation

• stimulate the amount of eggs

AHR: Artificial Insemination

• take sample of semen and artificially inseminate it into the uterus

• increases chance of conception

• used for surrogates or single individuals

AHR: Invitro Ferilization (IVF)

• eggs are fertilized and developed in the lab, then implanted into the uterus (depending on where in the menstruation cycle)

• requires fertility medication

• rules controlling how many embryo can be implanted at once

Pregnancy: 1st Trimester

• 0-3 months; 0-13 weeks

• begins when the zygote implants in the uterus (womb)

• zygote sends out chemical messages that cause the women’s menstrual periods to stop

  • some chemicals are excreted in urine, making it possible to diagnose pregnancy

• cervix thickens and secretes mucus that protects the embryo from harmful organisms that might enter thru the vagina

• uterus shifts position and puts pressure on the woman’s bladder

• morning sickness: feelings of nausea, vomiting

Pregnancy: 1st Trimester Prenatal Care

• prenatal care is critical because the baby’s organs form during the first 8 weeks

• can identify maternal conditions (eg. STDs)

• early prenatal care is also important to the mother’s health (eg. ectopic pregnancy)

Pregnancy: 2nd Trimester

• 3-6 months, 14-27 weeks

• morning sickness disappears

• increase in appetite

• weight gain and uterus expands to accommodate growing fetus (begins to “show)

• begins to feel fetus’ movement ~16-18 weeks

• risk of miscarriage drops

Pregnancy: 2nd Trimester Prenatal Care

• doctors monitor both mom and baby’s vital functions and keeps track of growth in the womb

• ultrasound tests

• sex can be determined by ~week 12

• monthly urine tests for gestational diabetes (could lead to premature labor)

Pregnancy: 3rd Trimester

• 6-9 months; 28-42 weeks

• weight gain and abdominal enlargement

• breasts may begin to secrete a substance (colostrum) in preparation for nursing

• feel more emotionally connected to the fetus

Pregnancy: 3rd Trimester Prenatal Care

• ultrasounds produce increasingly clear images (eg. hiccuping, thumb-sucking)

• visits are once a week

• monitoring of blood pressure (could lead to toxemia of pregnancy which causes stroke)

Placenta

• temporary organ connecting uterus to embryo via umbilical cord

  • only needed for pregnancy

  • each fraternal twin has its own placenta

  • identical twins may or may not share a placenta

    • if they do, there’s the risk of one twin taking more resources

  • the safest twins would be individual sacs and placentas

Ectopic Pregnancy

• fertilized egg implanted in the fallopian tube instead of uterus

• early surgical removal of the zygote is critical to the woman’s future ability to have children

Labelling Twins A and B

• see which one is closer to the cervix (most likely to come out first) and which is further

• helps with prenatal care, knowing which one or both to track

Prenatal Development

• change occurs rapidly 

• cephalocaudal pattern: development proceeds from head down

• proximodistal pattern: development happens in an orderly way from the center of the body outward to the extremities

Sex Differences: Physical Maturation

• male fetuses are larger and grow more rapidly

  • newborn boys are typically longer and heavier

• female infants are 1-2 weeks ahead in bone development at birth

• female superioirity in skeletal development persists thru childhood and early adolescence, allowing girls to acquire many coordinated movements and motor skills earlier than boys

• the gap between sexes gets wider every year until the mid-teens, when boys catch up and surpass girls in physical coordination

Sex Differences: Problems in Prenatal Development

• boys are more vulnerable to all kinds of prenatal problems

• male fetuses seem to be more sensitive to maternal drug use, malnutrition and stress

• female fetus placentas are larger and better developed

Teratogens

Environmental factors that cause damage to prenatal development

• critical periods: certain body systems are especially sensitive to teratogens (in both embryonic and fetal stages)

  • a particular body structure will not form properly

• dose: might not always be dentrimental, however we don’t know the dose amount that switches from harmless to harmful 

  • including intensity and duration

• individual differences: fetuses vary widely in their susceptibility to teratogens

  • arises from genes that moderate or black the effects of some harmful substances

Additional Factors Influencing Prenatal Outcomes

• maternal nutrition: eg. need to adjust diet to meet caloric requirement or consume folic acid

• maternal illness: sometimes prevents the mom from taking medications for fetal health

• paternal + other social factors

• chromosome and genetic factors

• maternal age: younger → physically better, older: socio+cognitively better

• emotional well-being: may be harder for mom to take care of her own needs

• prenatal care (incl. barriers): not always accessible to everyone

Chromosomal Abnormalities

• results from errors during cell reproduction, meiosis, mitosis or damage caused afterward

  • eg. trisomy 21

• sex chromosome abnormalities affect the number or structure of X and Y chromosomes

• sex linked disorders: caused by genes on the X-chromosome

• autosomal disorders: a disease caused by whole or partial change in a gene/genes on autosomes

  • can be caused by single or multiple genes and/or environmental factors 

Predicting and Detecting Genetic Disorders: Genetic Counselling

• construct a family history of heritable disorders

• blood tests can be performed to detect chromosomal abnormalities and/or dominant and recessive genes for various disorders

• known as carrier screening

  • non invasive

  • done to look for something specific

• ppl referred to genetic counsellors: have relative with genetic condition, difficulty conceiving, women over 35, couples from various ethnic groups

Prenatal Screening vs Prenatal Testing

Screening: provides information about the likelihood of a genetic or chromosome disorder

• eg. “increased/lower/normal risk for…”

• not a diagnosis

Testing: provides confirmation (diagnosis) of a genetic or chromosome disorder

Enhanced First-Trimester Screening (eFTS)

• provides info about the chance of certain chromosomal differences

• performed between 11w - 13 w gestation

• involves an NT ultrasound + blood work

• screen POSITIVE: a higher than normal chance of fetus having one of the conditions (not a diagnosis)

• screen NEGATIVE: lower than normal chance of the fetus having one of the conditions

Non-Invasive Prenatal Testing (NIPT)

• should actually be called non-invasive prenatal screening; NIPS

  • not diagnosis

• provides info about the chance of fetus having certain chromosome abnormalities

  • 13, 18, 21 and sex chromosomes

• blood test to analyze cell-free DNA from maternal blood sample (mixture of fetal and maternal DNA)

• performed at or after 9 weeks gestation

  • needs to be enough fetal DNA circulating

• usually more accurate than eFTS

• time consuming

Invasive Diagnostic Testing

• optional testing provides definitive answer abt specific genetic and/or chromosome differences

• small but serious risks (eg. miscarriage, infection)

Typically offered if:

• someone is screen positive/high risk on prenatal screening

• anatomical abnormalities are discovered on ultrasounds

Invasive Diagnostic Testing: Types

Chorionic Villus Sampling (CVS):

• collecting certain part of placenta that contains fetal DNA

Amniocentesis (amnio):

• takes amnio fluid which contains fetal cells

• risk of rupturing sac (no more protection), forcing the fetus to be delivered

Potential Steps after Prenatal Diagnosis

• decisions to make

• prenatal treatments (eg. medications, fetal surgery)

• seeking support

• preparation for remainder of pregnancy

• preparation for birth (eg. change birth plan)

• preparation for newborn care (eg. baby needs oxygen)

Labor/Delivery/Postpartum

Stage 1: cervix relaxes, causing it to dilate and thin out

• canal is made

• no specific timing

Stage 2: uterine contractions increase in strength and the infant is delivered

Stage 3: placenta is expelled

Where are babies born?

98% born in hospital

2% born at home: must be low risk and close enough to the hospital just in case

APGAR SCore

• assessment of the newborn

• done immediately after birth, then 5-10 mins later again

• low score at both times: need to intervene