2.1-2.3 Genetic Foundations
2.1 What are genes, and how are they transmitted from one generation to the next?
Cellular Control Center
Within the trillions of cells in the human body (except red blood cells), there is a control center called the nucleus.
The nucleus contains rod-like structures called chromosomes, which are responsible for storing and transmitting genetic information.
Human chromosomes are organized into 23 matching pairs, with the exception of the XY pair found in males.
Each member of a chromosome pair corresponds to the other in size, shape, and genetic functions.
Genetic information is transmitted from one generation to the next as one chromosome of each pair is inherited from the mother and the other from the father.
The Genetic Code: DNA and Genes
Chromosomes are primarily composed of a chemical substance known as deoxyribonucleic acid, or DNA.
DNA is structured as a long, double-stranded molecule that visually resembles a twisted ladder.
The rungs of this DNA ladder are formed by specific pairs of chemical substances called bases, joined together between the two sides.
The unique sequence of these base pairs provides the entire set of genetic instructions.
Definition of a Gene: A gene is defined as a segment of DNA that occupies a specific position along the length of a chromosome.
Genes can vary considerably in length, potentially spanning from 100 to several thousand base-pair rungs on the DNA ladder.
Types of Genes and Their Functions:
Protein-coding genes: An estimated 21,000 such genes directly influence our body’s characteristics.
These genes transmit instructions for the synthesis of a diverse array of proteins to the cytoplasm, which is the region surrounding the cell nucleus.
Proteins are crucial as they trigger chemical reactions throughout the body and serve as the fundamental biological building blocks of our characteristics.
Regulator genes: An additional 18,000 regulator genes are present, and their function is to modify the instructions conveyed by protein-coding genes, thereby significantly complicating their overall genetic impact.
2.2 Describe various patterns of gene–gene interaction.
Alleles and Phenotypes
Allele: Each form of a gene that occurs at the same place on chromosomes, with one inherited from the mother and one from the father.
Homozygous: When alleles from both parents are alike, the child will display the inherited trait.
Heterozygous: When alleles from both parents differ, relationships between the alleles influence the phenotype (observable characteristics).
Dominant–Recessive Pattern
In many heterozygous pairings, only one allele affects the child's characteristics. It is called dominant; the second allele, which has no effect, is called recessive.
Example: Hair Color
The allele for dark hair is dominant (represented by D).
The allele for blond hair is recessive (represented by b).
A child inheriting a homozygous pair of dominant alleles (DD) or a heterozygous pair (Db) will have dark hair.
Blond hair (bb) can only result from inheriting two recessive alleles.
Carriers: Heterozygous individuals with one recessive allele (Db) can pass that recessive trait to their children, making them carriers.
Impact of Recessive Alleles:
Most recessive alleles (e.g., for blond hair, pattern baldness, nearsightedness) are of little developmental importance.
Some recessive alleles can cause serious disabilities and diseases.
Phenylketonuria (PKU): A Recessive Disorder
A well-known recessive disorder affecting how the body breaks down proteins contained in many foods.
Infants born with two recessive alleles lack an enzyme that converts phenylalanine into tyrosine. Without this enzyme, phenylalanine quickly builds to toxic levels that damage the central nervous system.
By 1 year, infants with PKU suffer from permanent intellectual disability if untreated.
Treatment: All U.S. states require newborn blood tests for PKU. If the disease is found, doctors place the baby on a diet low in phenylalanine.
Outcomes with treatment: Children usually attain an average level of intelligence and have a normal lifespan, though they may show mild deficits in certain cognitive skills (memory, planning, decision-making, problem-solving).
Inheritance: For a child to inherit the condition, each parent must have a recessive allele.
Variability: Regulator genes cause children to vary in the degree to which phenylalanine accumulates in their tissues and in the extent to which they respond to treatment.
Dominant Disorders:
Serious diseases due to dominant alleles are rare because affected children usually develop the disorder and seldom live long enough to reproduce, thus eliminating the harmful dominant allele from the family’s heredity in a single generation.
Exception: Huntington’s Disease: A condition in which the central nervous system degenerates. Its symptoms usually do not appear until age 35 or later, after the person has passed the dominant allele to his or her children.
Incomplete-Dominance Pattern
Occurs in some heterozygous circumstances where the dominant–recessive relationship does not hold completely.
Instead, both alleles are expressed in the phenotype, resulting in a combined trait, or one that is intermediate between the two.
Example: Sickle Cell Trait - A heterozygous condition present in many black Africans.
Sickle Cell Anemia (full form): Occurs when a child inherits two recessive genes. These cause the usually round red blood cells to become sickle (crescent-moon) shaped, especially under low-oxygen conditions. The sickled cells clog blood vessels and block blood flow, causing intense pain, swelling, and tissue damage. North Americans with sickle cell anemia have an average life expectancy of only 55 years.
Heterozygous Individuals (Carriers): Protected from the disease under most circumstances. However, when they experience oxygen deprivation (e.g., at high altitudes or after intense physical exercise), the single recessive allele asserts itself, and a temporary, mild form of the illness occurs.
Adaptive Advantage: The sickle cell allele is common among black Africans because carriers are more resistant to malaria than individuals with two alleles for normal red blood cells. In Africa, these carriers survived and reproduced more frequently, leading the gene to be maintained. In regions with low malaria risk, the frequency of the gene is declining (e.g., only 8% of African Americans are carriers, compared with 20% of black Africans).
X-Linked Pattern
When a harmful allele is carried on the X chromosome, X-linked inheritance applies.
Males are more likely to be affected because their sex chromosomes do not match (XY). The Y chromosome is only about one-third as long and lacks many corresponding genes to override those on the X.
In females, any recessive allele on one X chromosome has a good chance of being suppressed by a dominant allele on the other X.
Example: Hemophilia: A disorder in which the blood fails to clot normally, showing a greater likelihood of inheritance by male children whose mothers carry the abnormal allele.
Male Disadvantage: Rates of miscarriage, infant and childhood deaths, birth defects, learning disabilities, behavior disorders, and intellectual disability all are higher for boys. This is possibly due to the female, with two X chromosomes, benefiting from a greater variety of genes.
Sex Ratio at Birth: Worldwide, about 103 boys are born for every 100 girls, and an even greater number of males are conceived (judging from miscarriage and abortion statistics).
Cultural Influence: In cultures with strong gender-biased attitudes that induce expectant parents to prefer a male child, the male-to-female birth sex ratio is often much larger (e.g., China's historical one-child policy and ultrasound technology led to a ratio of 117 boys for every 100 girls in 2015).
Environmental Influence: In many Western countries, the proportion of male births has declined, possibly due to a rise in stressful living conditions which heighten spontaneous abortions, especially of male fetuses.
Genomic Imprinting
In genomic imprinting, alleles are imprinted, or chemically marked through regulatory processes within the genome, in such a way that one pair member (either the mother’s or the father’s) is activated, regardless of its makeup.
The imprint is often temporary; it may be erased in the next generation and may not occur in all individuals.
Less than 1% of genes are subjected to genomic imprinting, but they significantly impact brain development and physical health.
Disruptions in imprinting are involved in several childhood cancers and Prader-Willi syndrome (intellectual disability and severe obesity).
Imprinting may explain parent-of-origin effects, such as children being more likely to develop diabetes if their father, rather than their mother, suffers from it.
Fragile X Syndrome: Can operate on sex chromosomes and is the most common inherited cause of intellectual disability.
Affects about 1 in 4,000 males and 1 in 6,000 females.
Caused by an abnormal repetition of a sequence of DNA bases on the X chromosome, damaging a particular gene.
In addition to cognitive impairments, most individuals suffer from attention deficits and high anxiety, and 30-35% also have symptoms of autism.
The defective gene at the fragile site is expressed only when it is passed from mother to child. Males are more severely affected because it is X-linked.
Mutation
A mutation is a sudden but permanent change in a segment of DNA.
A mutation may affect only one or two genes, or it may involve many genes, as in chromosomal disorders.
Some mutations occur spontaneously, simply by chance; others are caused by hazardous environmental agents.
Ionizing (high-energy) Radiation: An established cause of mutation. Moderate to high doses over an extended time can impair DNA.
Women who receive repeated doses before conception are more likely to miscarry or give birth to children with hereditary defects.
The incidence of genetic abnormalities is higher in children whose fathers are exposed to radiation in their occupation.
Germline Mutation: Takes place in the cells that give rise to gametes. When the affected individual mates, the defective DNA is passed on to the next generation.
Somatic Mutation: Normal body cells mutate, an event that can occur at any time of life. The DNA defect appears in every cell derived from the affected body cell, eventually becoming widespread enough to cause disease (such as cancer) or disability.
Shows that each of us does not have a single, permanent genotype; genetic makeup of each cell can change over time.
Increases with age, potentially contributing to the age-related rise in disease and to the aging process itself.
Some people harbor a genetic susceptibility that causes certain body cells to mutate easily in the presence of triggering events (e.g., smoking, pollutants, psychological stress).
Impact: Less than 3% of pregnancies result in hereditary abnormalities, but these children account for about 20% of infant deaths and contribute substantially to lifelong impaired physical and mental functioning.
Adaptive Role: Although virtually all studied mutations are harmful, some spontaneous ones (e.g., sickle cell allele in malaria-ridden regions) are necessary and desirable, as they increase genetic variation and help individuals adapt to unexpected environmental challenges.
Polygenic Inheritance
Describes characteristics that vary on a continuum among people, such as height, weight, intelligence, and personality.
These traits are due to polygenic inheritance, in which many genes affect the characteristic in question.
Polygenic inheritance is complex, and much about it is still unknown, making it harder to trace precise patterns of inheritance than for traits determined by single genes.
2.3 Describe major chromosomal abnormalities, and explain how they occur.
Chromosomal defects are a major cause of serious developmental problems, often resulting from mistakes during meiosis when sperm and ovum are formed.
Errors include chromosome pairs not separating properly or a part of a chromosome breaking off.
These errors involve significantly more DNA than single-gene problems, leading to a wider range of physical and mental symptoms.
Down Syndrome
Prevalence: The most common chromosomal disorder, occurring in 1 out of every 700 live births.
Primary Cause (95% of cases): A failure of the twenty-first pair of chromosomes to separate during meiosis, resulting in the individual receiving three chromosome 21s instead of the normal two. This is known as trisomy 21.
Other Forms:
Translocation pattern: An extra broken piece of a twenty-first chromosome is attached to another chromosome.
Mosaic pattern: An error occurs during early prenatal cell duplication, leading to some (but not all) body cells having the defective chromosomal makeup. Symptoms may be less extreme due to less affected genetic material.
Consequences and Symptoms:
Intellectual disability, memory and speech problems, limited vocabulary, and slow motor development.
Brains function in a less coordinated fashion than typical individuals as indicated by electrical brain activity measures.
Distinct physical features: short, stocky build, flattened face, protruding tongue, almond-shaped eyes, and an unusual crease across the palm of the hand (in 50% of cases).
Associated health issues: eye cataracts, hearing loss, and heart and intestinal defects.
Life Expectancy: Due to medical advances, it has increased significantly to about 60 years of age.
Alzheimer's Disease Link: Approximately 70% of affected individuals who live past age 40 show symptoms of Alzheimer’s disease, with genes on chromosome 21 being linked to this disorder.
Early Development and Environmental Influence:
Infants with Down syndrome often smile less readily, show poor eye-to-eye contact, have weak muscle tone, and explore objects less persistently.
Development improves when parents encourage engagement with their surroundings.
Benefits from infant and preschool intervention programs; emotional, social, and motor skills generally improve more than intellectual performance.
Risk Factors:
Maternal Age: The risk of bearing a baby with Down syndrome (and other chromosomal abnormalities) rises dramatically with increased maternal age, although the exact reason why older mothers are more likely to release ova with meiotic errors is unknown.
Paternal Role: In about 5% of cases, the extra genetic material originates with the father. Some studies suggest a role for advanced paternal age, while others show no age effects.
Abnormalities of the Sex Chromosomes
Contrast with Autosomal Disorders:
Disorders of autosomes (non-sex chromosomes) other than Down syndrome usually lead to severe developmental disruption, often resulting in miscarriage or rarely surviving beyond early childhood.
Sex chromosome disorders, in contrast, are often not recognized until adolescence, sometimes presenting as delayed puberty.
Common Problems: Involve the presence of an extra chromosome (either X or Y) or the absence of one X chromosome in females.
Debunked Myths: Research has disproved myths, such as males with XYY syndrome being inherently more aggressive or antisocial.
Cognitive Challenges: Most children with sex chromosome disorders do not suffer from intellectual disability, but experience specific cognitive challenges.
Verbal Difficulties: Common among girls with triple X syndrome (XXX) and boys with Klinefelter syndrome (XXY), both of whom inherit an extra X chromosome. These difficulties often involve reading and vocabulary.
Spatial Relationship Problems: Experienced by girls with Turner syndrome (XO), who are missing an X chromosome. This includes difficulties with drawing, telling right from left, following travel directions, and noticing changes in facial expressions.
Brain Development: Brain-imaging evidence confirms that changes (adding or subtracting) to the usual number of X chromosomes alter the development of specific brain structures, leading to particular intellectual deficits.
2.4 The Sex Cells
Gametes and Conception
New individuals are formed through the combination of two specialized cells called gametes, or sex cells: the sperm (male) and the ovum (female).
Each gamete contains only 23 chromosomes, which is precisely half the number found in a regular human body cell (46 chromosomes).
Gametes are produced through a cell division process known as meiosis, which is responsible for reducing the chromosome count by half.
At conception, when a sperm and an ovum unite, the resulting single cell, termed a zygote, restores the full complement of 46 chromosomes.
Genetic Variability through Meiosis
During meiosis, a critical event occurs where chromosomes pair up and exchange segments, allowing for the recombination of genes.
Following this exchange, chance determines which specific member of each chromosome pair will be distributed to a given gamete. This random assortment further increases genetic diversity.
These complex events make it extremely improbable—approximately 1 in 700 trillion—for non-twin siblings to be genetically identical.
The extensive genetic variability generated by meiosis is highly adaptive, enhancing the likelihood that at least some individuals within a species will possess traits enabling them to survive and thrive in constantly changing environments.
Sex Differences in Gamete Production
Male Gamete Production:
In males, meiosis typically results in the formation of four functional sperm cells.
The precursor cells from which sperm are derived are continuously produced throughout a man's life.
Consequently, a healthy man retains the capacity to father a child at any age after reaching sexual maturity.
Female Gamete Production:
In females, meiosis generally yields only one mature ovum.
Females are born with a finite, but substantial, supply of ova already present in their ovaries.
Approximately 1 to 2 million ova are present at birth.
Around 40,000 remain by adolescence.
An estimated 350 to 450 ova will mature and be released during a woman’s entire childbearing years.
Recent scientific findings suggest a possibility that new ova might arise from ovarian stem cells later in life, though the primary understanding remains that females have a fixed supply.
Sex Determination
Of the 23 pairs of chromosomes, 22 are matching pairs called autosomes (non-sex chromosomes).
The twenty-third pair consists of sex chromosomes: XX in females and XY in males.
The X chromosome is relatively long, containing significant genetic material, while the Y chromosome is short and carries little genetic material.
During gamete formation in males, the X and Y chromosomes separate into different sperm cells.
All gametes (ova) formed in females carry an X chromosome.
Therefore, the sex of the new organism is determined at conception by whether an X-bearing sperm or a Y-bearing sperm fertilizes the ovum.
2.5 Multiple Offspring
Fraternal (Dizygotic) Twins
Result from the release and fertilization of two ova.
Genetically, they are no more alike than ordinary siblings.
Factors increasing the chances of fraternal twins:
Older maternal age.
Fertility drugs.
In vitro fertilization (IVF).
These factors are major causes of the dramatic rise in fraternal twinning and other multiple births in industrialized nations.
Currently account for approximately 1 in every 33 births in the United States.
Identical (Monozygotic) Twins
Occur when a single zygote duplicates and separates into two clusters of cells that develop into two individuals.
Have the same genetic makeup.
Frequency is consistent worldwide: about 1 in every 350 to 400 births.
Environmental influences observed in animal research that can prompt identical twinning include:
Temperature changes.
Variation in oxygen levels.
Late fertilization of the ovum.
Identical twinning rarely runs in families and is likely due to chance rather than heredity when it does.
Developmental Considerations for Multiple Births
Children of single births are often healthier and develop more rapidly than twins during their early years.
Twins are commonly born early (e.g., three weeks before the due date), often requiring special care.
Twins may walk and talk several months later than single-birth infants, possibly due to reduced individual attention, though most catch up by middle childhood.
Parental energies are further strained after the birth of triplets, whose early development is even slower than that of twins.
Complexity of Human Development
Despite having significantly fewer genes than initially estimated (only about twice as many as a worm or a fly), humans exhibit remarkable biological complexity, which is attributed to several factors:
Extensive Protein Diversity: Human genes direct the creation of proteins that can break apart and reassemble in an astonishing variety of ways, resulting in approximately 10 to 20 million distinct proteins, far more than simpler species.
Intricate Cellular Communication: The communication system that operates between the cell nucleus and the cytoplasm, responsible for fine-tuning gene activity, is considerably more intricate in humans than in simpler organisms.
Influence of Environmental Factors on Gene Expression: A broad spectrum of environmental factors within the cell can actively modify gene expression.
Recent research indicates that many of these environmental effects are unique to humans and play a significant role in influencing brain development.
Ultimately, biological events of profound developmental significance are understood to be the complex outcome of the interplay