DEV2004 - Chapter 2 Heredity and Genetic Basics

Heredity and Genetic Basics

• Heredity significantly shapes individual development through genetic contributions known as nature.
• Genes are sequences of nucleotides that instruct protein creation, influencing cell structure and function.
• Humans possess an estimated 20,500 genes located on 46 chromosomes (23 pairs: 22 autosomes and 1 pair of sex chromosomes).
• Mitosis duplicates chromosomes for body cells; meiosis creates gametes (sperm and ova) that contain half the genetic material (23 chromosomes each).
• The chance of producing genetically identical offspring (not twins) is one in trillions.

Genotypes and Phenotypes

• Genotype is the total genetic information inherited; phenotype is the expressed traits.
• Inheritance can be homozygous (same gene version from both parents) or heterozygous (different versions).
• Dominant genes express traits regardless of the allele; recessive genes only show in the homozygous condition.
• Genetic variations are called alleles, and many traits are polygenic, influenced by multiple genes.
• Incomplete dominance occurs when dominant genes do not completely suppress recessive counterparts, as seen in sickle cell disease.

Twin Formation

• Identical twins (monozygotic) arise from a single fertilized egg that splits, sharing the same genotype and phenotype.
• Fraternal twins (dizygotic) result from separate fertilization of two eggs and share genetic similarities akin to regular siblings.
• Older mothers and those using fertility treatments are more likely to conceive dizygotic twins.

Genetic Disorders

• Genetic disorders can be dominant or recessive, with various levels of severity.
• Dominant disorders like Huntington's Disease are generally serious but may not manifest until midlife.
• Recessive disorders like cystic fibrosis and sickle-cell anemia often result in more fatalities as many carriers are unaware of their condition.
• Sex-linked disorders are more prevalent in males due to their single X chromosome, making conditions like hemophilia and color-blindness more impactful on males.

Chromosomal Abnormalities

• Chromosomal abnormalities stem from incorrect chromosomal counts, often due to maternal age, leading to conditions like Down syndrome (Trisomy 21).
• Down syndrome presents intellectual disabilities and distinct physical traits, with variable severity.
• Other chromosomal abnormalities include Trisomy 13 and Trisomy 18, which are often fatal.
• Turner syndrome (XO) and Klinefelter syndrome (XXY) are notable sex-linked disorders affecting females and males respectively, influencing physical and cognitive characteristics.

Genetic Counseling

• Genetic counseling helps individuals understand potential genetic conditions based on family history, ethnic backgrounds, and personal medical history.
• Patients learn about the likelihood of genetic conditions impacting themselves or their offspring.

Behavioral Genetics

• Behavioral genetics studies the relationship between genetics and environmental influences on behavior, often simplifying the nature/nurture debate.
• Three types of genotype-environment correlations illustrate interactions between inherited traits and environmental factors: passive, evocative, and active.
• Epigenetics explores how environmental factors modify gene expression, influencing traits and susceptibilities even among genetically identical twins.

Human Genome Project

• Launched in 1990, the Human Genome Project sequenced the human genome, completing the task in April 2003.
• The project cataloged thousands of disease-associated genes, aiding in medical treatments and understanding genetic variations across populations.### Pharmacogenomics and HGP
• Pharmacogenomic tests assess responses to drugs in conditions like breast cancer, lung cancer, and HIV using data from HapMap.
• Future goals of the Human Genome Project (HGP) include identifying genetic markers for 50 major cancer types and improving drug efficacy.
• Ethical considerations are essential to the HGP, addressing ownership of genetic information and its impact on healthcare policies.

Prenatal Development Overview

• Prenatal development consists of three periods: germinal, embryonic, and fetal.

Germinal Period

• Lasts about 14 days from conception to implantation in the uterus.
• Fertilization usually occurs in the upper fallopian tube, forming a zygote.
• The zygote undergoes mitosis, leading to the formation of a blastocyst after five days.
• Around 50-75% of blastocysts fail to implant successfully in the uterine wall.
• Fewer than half of zygotes survive the first two weeks, often due to issues with genetic material or implantation.

Embryonic Period

• Begins at the third week post-implantation, with the blastocyst termed an embryo.
• Key developments include the formation of blood vessels, and the placenta connects the mother and embryo.
• Growth occurs head-to-tail (cephalocaudal) and midline outward (proximodistal).
• Major structures form, including a beating heart by the fourth week; gills and a tail are present initially but disappear later.
• This stage is critical as exposure to harmful substances can lead to damage, often before pregnancy is recognized.

Fetal Period

• From the ninth week until birth, the organism is classified as a fetus.
• By the third month, all body parts, including external genitalia, develop.
• Major development includes hair, nails, and the digestive system prior to birth.
• Age of viability occurs around 24 weeks; survival outside the womb is limited before this time.
• Neurons mostly develop by 24 weeks, with key brain functions beginning to emerge, including sensory perception.
• The fetus prepares for birth by exercising muscles and gaining weight, reaching approximately 6 pounds and 18.5 inches by 36 weeks.

Prenatal Brain Development

• Begins in the third gestational week with the emergence of stem cells in the neural plate.
• Neural tube formation occurs within the first month, creating the brain's ventricles and spinal canal.
• Neurogenesis is largely complete by five months, although hippocampal neuron development continues throughout life.
• Growth persists post-birth, with neuron connections (dendrites and axons) forming neural networks over the years.

Teratogens

• Teratology studies factors that lead to birth defects during prenatal development.
• Teratogens, such as maternal diseases, pollutants, drugs, and alcohol, significantly affect fetal health.

Factors Influencing Prenatal Risks

• Timing of exposure is critical; certain developmental stages are more vulnerable to damage from teratogens.
• The amount of exposure can influence the severity of potential damage.
• Co-exposure to multiple teratogens increases risk compared to singular exposure.
• Genetic make-up can affect susceptibility to teratogenic effects.
• Males may be more adversely affected by teratogens due to genetic differences.

Alcohol as a Teratogen

• Alcohol is notably the leading preventable course of intellectual disabilities.
• Impact includes neurocognitive and behavioral issues, and severe cases may result in fetal death or Fetal Alcohol Spectrum Disorders (FASD).
• Fetal Alcohol Syndrome (FAS) presents with distinctive facial features and cognitive deficits, persisting into adulthood.
• Binge drinking significantly increases the likelihood of FASD.

Tobacco Use During Pregnancy

• Over 7% of pregnant women smoked in 2016, exposing fetuses to teratogenic risks.
• Smoking prevalence varies among different pregnancy stages, with many women quitting before the last trimester.

Fetal Alcohol Spectrum Disorder Facial Features

• Below-average head size, smaller eye openings, low nasal bridge, and thin upper lip are common characteristics of FAS in children.