Genetic Vulnerability and X-Linked Disorders 🧬

X-Linked Disorders

Males are genetically more vulnerable to genetic disorders than females due to the presence of X-linked disorders, which are carried on the X chromosome.

• For males, they only have one X chromosome (XY), so if they inherit an X with a genetic disorder, they are more likely to express the disorder.

• For females, they have two X chromosomes (XX), so even if one X has a genetic disorder, the other X can compensate for it, making it less likely for the disorder to be expressed.

Examples of X-Linked Disorders

• Hemophilia: a bleeding disorder where the blood doesn't clot properly

• Color Blindness: a condition where a person has difficulty perceiving certain colors

• Duchenne's Muscular Dystrophy: a genetic disorder that causes muscle weakness and degeneration

• Learning Disabilities: such as dyslexia

• Vision Disorders: such as the one that causes blindness by the age of 25

Ratio of Male to Female Fetuses at Conception

The ratio changes due to the presence of genetic disorders that may cause miscarriage or stillbirth.

Genetic Expression

Just because a gene is present, it doesn't necessarily mean it will be expressed. This can be due to various reasons, such as:

• Blocking genes: one gene may block the expression of another gene

• Polygenic traits: multiple genes are involved in the expression of a single trait

• Regulator genes: genes that control the interactions of other genes and direct genetic expression

Epigenetics and Regulator Genes

Regulator genes play a crucial role in epigenetics, controlling the interactions of other genes and directing genetic expression. Research in this area focuses on understanding how genes are turned on and off, and how to potentially prevent genetic disorders from being expressed.## Dominant and Recessive Genes 🧬

In the study of genetics, particularly in cancer research, understanding dominant and recessive genes is crucial.

What are dominant and recessive genes?

A dominant gene is a gene that will always be expressed if an individual has one copy of the gene. A recessive gene is a gene that will only be expressed if an individual has two copies of the gene.

Example: Eye Color

Let's take eye color as an example. Brown eyes (B) are a dominant gene, and blue eyes (b) are a recessive gene.

• BB or Bb: brown eyes (because the dominant gene B is present)

• bb: blue eyes (because there are two copies of the recessive gene b)

Carriers

A carrier is an individual who has a recessive gene that is not visible in their phenotype (physical appearance) but can still pass it on to their offspring.

Punnett Square

A Punnett square is a tool used to predict the probability of certain traits being passed down to offspring.

Inheritance of Recessive Traits

• Two parents with brown eyes (Bb) can have a child with blue eyes (bb) if they both pass on their recessive gene.

• An individual with brown eyes (Bb) can be a carrier of the recessive gene for blue eyes.

Identical and Fraternal Twins 👶

Identical Twins (Monozygotic)

• Definition: Identical twins are formed from a single fertilized egg that splits and develops into two separate embryos.

• Characteristics:

  • Always the same sex

  • Same genotype (DNA)

  • May have slight differences in phenotype due to environmental influences

Fraternal Twins (Dizygotic)

• Definition: Fraternal twins are formed from two separate eggs that are fertilized by two separate sperm.

• Characteristics:

  • Can be same or opposite sex

  • Genetically similar to any other siblings

  • More common than identical twins

  • Occurrence varies by ethnicity and age## Multiple Births and Prenatal Development 🤰

Identical and Fraternal Twins

• Identical twins (monozygotic): form from a single fertilized egg (zygote) that splits and develops into two separate embryos

• Fraternal twins (dizygotic): form from two separate eggs, each fertilized by a different sperm

Factors Influencing Multiple Births

• Ethnicity: some ethnic groups have a higher incidence of multiple births

• Age: older women are more likely to have multiple births

• Fertility treatments: certain fertility drugs can increase the chances of multiple births

Prenatal Development Periods

Vulnerability During Prenatal Development

• Miscarriage: can occur due to genetic problems, chromosomal abnormalities, or errors during rapid cell division

• 67% of zygotes do not survive the germinal period

• 20% of embryos are miscarried or spontaneously aborted during the embryonic period

• 5% of fetuses are miscarried before viability (22 weeks)

Germinal Period (1-14 days)

• Rapid cell division and growth

• Development of placenta and umbilical cord

• Monozygotic twins form during this period

Embryonic Period (3-8 weeks)

• Cells start to differentiate

• Beginning of brain development with the primitive streak

• Folic acid is essential for neural tube formation and brain and spinal cord development

Importance of Folic Acid

• Prevents neural tube defects

• Essential for brain and spinal cord development

• Campaigns to increase folic acid in food supply to reduce neural tube defects## Folic Acid and Prenatal Development 🧬

Folic acid is a crucial vitamin that plays a significant role in preventing neural tube defects during pregnancy. Neural tube defects occur when the brain and spinal cord do not form properly. To combat this, folic acid is often added to foods such as orange juice, breakfast cereal, and bread, which is indicated by the label "enriched with folic acid."

Embryonic Development (3-8 weeks)

During this stage, the head starts to take shape, and the eyes, ears, nose, and mouth begin to form. The heart starts to pulsate, and extremities such as arms and legs begin to develop. Initially, fingers and toes are webbed, but this disappears towards the end of this stage.

Here are some key features of embryonic development:

• The head starts to take shape

• Eyes, ears, nose, and mouth begin to form

• Heart starts to pulsate

• Extremities (arms and legs) begin to develop

• Fingers and toes are initially webbed

Prenatal Tests

Prenatal tests are crucial during pregnancy to identify and prevent potential issues. Some common tests include:

Glucose Testing

Glucose testing is used to detect gestational diabetes, a condition where the body struggles to handle sugar levels during pregnancy. The test involves drinking a sugary drink and then testing blood sugar levels. If diagnosed, gestational diabetes can be managed through diet control and, in some cases, insulin treatment.

Fetal Development (9-40 weeks)

During this period, the brain continues to grow and develop, with significant growth between 4-6 months. The saccharin organs form, and differences in brain organization occur due to hormone levels. The eyes and lungs are the last organs to mature.

Here are some key features of fetal development:

• Brain growth and development

• Saccharin organs form

• Differences in brain organization occur due to hormone levels

• Eyes and lungs are the last organs to mature## Fetal Development during Pregnancy 🤰

Growth Range

The growth range of a fetus during pregnancy is influenced by:

• Genetics: The parents' size and genetic makeup can affect the baby's size.

• Nutrition: The mother's nutrition during pregnancy affects the baby's growth.

• Environmental factors: External factors, such as exposure to teratogens, can impact fetal development.

Brain Development

Preneural Brain Development

• At 25 days, the neural tube forms and eventually develops into the brain and spinal cord.

• By 9 months, the brain has a more developed structure, but it's still not fully developed until around 3 years after birth.

Fetal Brain Development Stages

Fetal Positioning and Delivery

• Head down positioning: Crucial for a safe delivery, as the head is larger compared to the rest of the body.

• Reason for head-first delivery: Prevents the baby from getting stuck in the pelvic cavity.

• Manual turning: A procedure performed by doctors to turn the baby's head down if necessary.

Age of Viability

• Age of viability: The age at which a premature infant might survive with adequate medical care.

• Chances of survival: Decrease with earlier birth and are influenced by lung development.

Chromosomal Anomalies

Types of Chromosomal Anomalies

• Too many chromosomes: Having an extra chromosome on a certain pair (e.g., Trisomy 21, which causes Down syndrome).

• Too few chromosomes: Missing a chromosome on a certain pair.

• Damaged chromosomes: Chromosomes damaged during cell division.

Risk Factors

• Older parents: More likely to have a child with chromosomal abnormalities due to issues with DNA in sperm or egg cells.## Genetic Disorders 🧬

Genetic disorders can be categorized into two types: single gene disorders and polygenic disorders. Single gene disorders are caused by a mutation in a single gene, while polygenic disorders involve multiple genes.

Genotype vs. Phenotype

Examples of Genetic Disorders

• Huntington's disease

• Cystic fibrosis

• Thalassemia

• Sickle cell disease

• Fragile X syndrome

These disorders are caused by dominant or recessive genes. Recessive disorders are more common and require an individual to inherit the gene from both parents.

Spontaneous Mutations

Spontaneous mutations occur when a genetic error occurs during DNA replication, often due to exposure to teratogens such as radiation or pollution. The frequency of chromosomal issues increases with advanced maternal age (over 35) or paternal age (over 40).

Recessive Disorders and Evolution

Researchers believe that some recessive disorders may have originated because being a carrier of the disorder provided protection against another disorder. For example, sickle cell trait may have originated in regions with high malaria prevalence, as carriers of the trait have an increased immunity to malaria.

Preterm and Small for Gestational Age 🤰

Definitions

Preterm babies are often small due to premature birth, while SGA babies may experience prenatal development issues due to teratogens or maternal health factors.

Risk Factors for Low Birth Weight

• Chronic maternal health issues

• Experimental illnesses during pregnancy

• Maternal use of tobacco products or certain prescription medications

• Maternal illegal drug use

• Low socioeconomic status

• Lack of prenatal care

• Chronic stress

Consequences of Low Birth Weight

• Delayed developmental milestones (e.g. sitting, crawling, walking, reading)

• Increased crying due to immature nervous system

• Attention issues

• Behavioral issues

• Language delays

• Learning disabilities

• Neurological delays

• Smaller brain volume

These consequences can persist throughout childhood and into adulthood, highlighting the importance of addressing low birth weight.