Human Inheritance: Chromosomes, Disorders, and Evolutionary Patterns

Human Inheritance and Chromosomal Research

Genetic studies utilize various organisms to understand the transmission of traits, but humans present unique challenges compared to model organisms like pea plants and fruit flies.

Comparison of Model Organisms vs. Human Subjects

Pea plants and fruit flies are considered ideal for genetic studies due to specific biological and logistical factors:

• They possess relatively few chromosomes.
• They reproduce at a rapid rate.
• They are easily maintained in laboratory conditions.
• Their use presents few ethical dilemmas.

Humans, however, involve significantly more complex issues for researchers:

• Relatively few human traits follow a simple Mendelian inheritance pattern.
• Ethical constraints prohibit performing test crosses on humans.
• Humans live in highly variable environments and across diverse geographical locations.
• Humans select their own mates rather than being selectively bred for research.
• Humans reproduce at their own discretion regarding timing and frequency.
• Small family sizes often lead to sampling errors in data collection.

The Complexity of Human Traits

Human genetics is rarely determined by a single factor. While single-gene disorders affect approximately $1$ in $100$ individuals, most human traits are polygenic.

• Polygenic Traits: These are traits influenced by multiple genes acting in concert.
• Environmental Influence: Traits can be significantly affected by environmental factors.
• Rarity of Severe Alleles: Alleles that lead to severe genetic disorders are generally rare within the population.

Pedigree Analysis and Historical Records

Because standardized breeding experiments cannot be performed on humans, geneticists rely on historical records to study traits across generations.

• Definition of Pedigree: A pedigree is a chart of family connections that illustrates the appearance of a specific phenotype throughout various generations.
• Utility of Pedigrees: Researchers use these charts to:
  • Determine the probability that a specific trait will recur in future generations.
  • Predict the probability that a genetic disorder will reappear in offspring.

Autosomal Inheritance Patterns

Autosomal traits are those governed by genes located on autosomes (non-sex chromosomes).

Autosomal Dominance

An autosomal dominant trait typically appears in every generation.

• Pattern: The trait appears in both homozygous and heterozygous individuals.
• Probability: When one parent is heterozygous and the other is homozygous recessive, each child has a $50\%$ chance of inheriting the dominant allele and displaying the trait.

Autosomal Recessive Pattern

An autosomal allele is considered recessive if it is expressed only in individuals who are homozygous for that allele.

• Carriers: Individuals who are heterozygous for the allele are known as carriers. They possess the allele but do not display the trait.
• Pattern: Recessive traits may skip generations.
• Probability: If two carriers produce offspring, each child has a $25\%$ chance of being homozygous and expressing the trait.

Summary of Autosomal Abnormalities and Disorders

Autosomal Dominant Disorders

Autosomal Recessive Disorders

Note on Tay-Sachs Prevalence: The allele is carried by $1$ in $300$ of the general population, but frequency increases to $1$ in $30$ among individuals of eastern European Jewish descent.

X-Linked Inheritance Patterns

Traits governed by genes on the sex chromosomes, specifically the X chromosome, follow unique inheritance patterns.

X-Linked Recessive Pattern

• Prevalence in Sexes: These disorders appear more frequently in men than in women. Men ($XY$) possess only one X chromosome, so a single recessive allele will manifest the trait. Women ($XX$) have two X chromosomes and can be heterozygous carriers.
• Transmission: Men transmit X-linked alleles to all of their daughters but never to their sons. Only a mother can pass an X-linked allele to a son.
• Lethality: Most X-linked inheritance patterns observed are recessive because X-linked dominant alleles tend to be lethal in male embryos.

Summary of X-Linked Traits

X-Linked Recessive Patterns

• Androgen Insensitivity Syndrome: $XY$ individual possessing some female traits; results in sterility.
• Red—Green Color Blindness: Inability to distinguish red from green. Most genes for pigment-containing receptors in the eyes are on the X chromosome. Some individuals confuse red and green; others see green as gray but perceive blues and yellows well.
• Hemophilia: Impaired blood clotting ability. Mutations in clotting factors VIII and IX (both on the X chromosome) lead to prolonged bleeding. Internal bleeding is the most serious risk. Historically common in the royal families of Europe and Russia (specifically Hemophilia B).
• Muscular Dystrophies: Progressive loss of muscle function. Duchenne Muscular Dystrophy (DMD) is caused by the absence of the protein dystrophin. Muscle and nerve cells are replaced by fat. It affects $1$ in $3,500$ people, mostly boys. Affected individuals often utilize wheelchairs by age $12$ and may die from heart or respiratory failure before age $30$.
• X-linked Anhidrotic Dysplasia: Characteristics include mosaic skin (patches with or without sweat glands).

X-Linked Dominant Patterns

• Fragile X Syndrome: Intellectual and emotional disability.
• Incontinentia Pigmenti: Abnormalities of the skin, hair, teeth, nails, and eyes, along with neurological problems.

Changes in Chromosome Structure

While mutations are typically small-scale changes in DNA sequences, larger-scale chromosomal alterations can occur. These changes are evolutionary drivers but often result in genetic disorders.

Types of Structural Changes

• Insertions: Large segments of DNA added to a chromosome, often resulting from the activity of transposable elements (DNA segments that move spontaneously within or between chromosomes).
• Duplications: Repeated sections of a chromosome, typically occurring during Prophase I of meiosis.
• Deletions: The loss of a part of a chromosome. In mammals, these are often lethal or cause serious disorders.
• Inversions: A structural rearrangement where a chromosomal segment is reversed. This may not affect the carrier's health but can significantly impact fertility.
• Translocations: A broken piece of a chromosome reattaches in an incorrect location. Most are reciprocal (balanced), where two chromosomes exchange parts. These can also affect fertility.

Evolutionary Impact of Chromosomal Changes

• Speciation: Large-scale changes in chromosomes can lead to the formation of new species.
• Human Chromosome 2: Human somatic cells have $23$ pairs of chromosomes, whereas chimpanzees, gorillas, and orangutans have $24$ pairs. During human evolution, two ancestral chromosomes fused end-to-end to form our Chromosome 2.
• Evolution of the Y Chromosome: Approximately $350$ million years ago ($350\text{ mya}$), the X and Y chromosomes were homologous autosomes. A mutation in one gene interfered with crossing over during meiosis, causing mutations to accumulate separately. Today, the SRY gene on the Y chromosome determines male sex.

Changes in Chromosome Number

Abnormal events during or before meiosis can lead to individuals having an incorrect number of chromosomes.

Primary Conditions

• Polyploidy: Having three or more of each type of chromosome ($3n$, $4n$, etc.). This is lethal in humans but common in flowering plants, some insects, and some fish.
• Aneuploidy: Having too many or too few copies of a specific chromosome. Most autosomal aneuploidy is lethal to embryos.
• Nondisjunction: The failure of sister chromatids or homologous chromosomes to separate during nuclear division. This is the primary cause of changes in chromosome number at fertilization.

Human Chromosome Number Syndromes

Genetic Screening and Diagnosis

Genetic counselors use pedigrees and genotype analysis to estimate the probability of a child inheriting a disorder.

Early Detection and Newborn Screening

• Phenylketonuria (PKU): Newborns are screened for mutations in the gene for phenylalanine hydroxylase. If defective, phenylalanine accumulates and inhibits protein synthesis in the brain, causing severe neurological symptoms. Early detection allows for dietary countermeasures.

Prenatal Diagnosis

More than $30$ conditions can be detected prenatally through various methods:

• Obstetric Sonography: Ultrasound imaging.
• Fetoscopy: Direct visualization of the fetus.
• Amniocentesis: Sampling of the amniotic fluid.
• Chorionic Villus Sampling (CVS): Testing of placental tissue.

Note: Invasive procedures like amniocentesis and CVS carry inherent risks to the fetus.

Application: Evolution of Skin Color

Skin color is a polygenic trait with a genetic basis linked to regional evolution.

• Mechanism: Minor differences in alleles for melanin synthesis and the deposition of melanosomes affect color.
• Biological Balance: Skin color evolved as a balance between the need for Vitamin D production and protection against harmful UV radiation.
• Diversity: Mixed ethnicity creates unique combinations of alleles, leading to a wide spectrum of pigmentation.

Questions & Discussion

• If it becomes possible to easily and inexpensively choose the sex of your child, how will this change the male to female ratio among newborns? Do you think it is ethically correct to select the sex of your children?
• Why do so many people insist that girls cannot be red–green color blind?