Comprehensive University Lecture Notes on Genetics and Heredity

Mutagens and Chromosomal Abnormalities

Definition of Mutagen: A mutagen is any agent that alters DNA or the structure of DNA chromosomes.
* Examples of mutagens include various forms of radiation and numerous types of chemicals.
Genetic Disorders and Meiosis: Most genetic disorders commonly arise from chromosomes that fail to separate properly during the process of meiosis.
* Disjunction: This is the term for the normal, proper separation of homologous chromosomes.
* Nondisjunction: This occurs when chromosomes fail to separate properly.
Nondisjunction of Sex Chromosomes:
* In normal disjunction, a mother's two X chromosomes split into two different cells during meiosis. If they meet a sperm with an X chromosome, the result is a normal female. If they meet a sperm with a Y chromosome, the result is a normal male.
* If nondisjunction occurs, one cell may receive two X chromosomes while the other receives none.
Triple X Syndrome (Triplo-X):
    * This occurs when an egg receives two X chromosomes and is fertilized by an X-carrying sperm.
    * The conceptus has $47$ chromosomes instead of the standard $46$ (designated as $47, XXX$ ).
    * The individuals develop as females but are usually infertile and may possess mild intellectual impairment.
Turner Syndrome:
    * This occurs when an egg receives no X chromosomes but is fertilized by an X-carrying sperm, resulting in a total of $45$ chromosomes.
    * Characteristics: Individuals develop as female but exhibit few secondary sexual characteristics. Features include poor breast development, underdeveloped reproductive structures, lack of menstruation, potential elbow deformities, and the presence of small brown spots on the skin.
    * Clinical Outcomes: These individuals are sterile, and there is usually some degree of mental retardation involved.
Klinefelter Syndrome:
    * This occurs when an egg receiving two X chromosomes is fertilized by a Y-carrying sperm.
    * Because of the Y chromosome, the individual is male, but they are sterile.
    * Characteristics include underdeveloped testes and generally average intelligence.

Autosomal Nondisjunction and Basic Heredity

Down Syndrome (Trisomy 21): This is the most common version of autosomal nondisjunction.
    * Physical Characteristics:
        * An extra epicanthal fold on the eyes (a feature not present in those without the syndrome).
        * Short stature and stubby fingers.
        * A large tongue.
        * A single palmar simian crease (most people have multiple creases on their palms, but individuals with Down Syndrome often have only one).
    * Cognitive Effects: Mental retardation is typically present.
    * Maternal Age Correlation: The occurrence of Down Syndrome is proportional to the age of the mother. For a mother who is approximately $48$ years old, the chance is $1$ in $9$ .
Defining Heredity: Heredity is the transmission of genetic characteristics from parents to their offspring.
The Karyotype: A karyotype is a chart of the $46$ chromosomes laid out in order of size.
* Chromosome $1$ is the largest chromosome.
* Chromosome $22$ is the smallest chromosome.
Chromosome Count and Distribution:
    * Humans have $23$ pairs of chromosomes total.
    * $22$ pairs are autosomes.
    * $1$ pair consists of sex chromosomes.
Diploid vs. Haploid Cells:
* Diploid Cells ( $2n$ ): These contain $23$ pairs of chromosomes (totaling $46$ ). This applies to almost all cells in the body.
* Haploid Cells ( $n$ ): These contain only $23$ individual chromosomes. These are the germ cells, specifically the sperm and the egg.

Genetic Terminology and Patterns of Inheritance

Locus: This refers to the specific location of a particular gene on a chromosome.
* Example: The gene for sickle cell anemia is located at the "locus" at the very top of chromosome $11$ .
Alleles: Different forms or versions of the same gene found at the same locus.
Dominant and Recessive Alleles:
* Dominant Alleles: Represented by capital letters (e.g., $B$ or $C$ ). These traits take precedence in the phenotype.
* Recessive Alleles: Represented by lowercase letters (e.g., $b$ or $c$ ).
Zygosity:
    * Homozygous Dominant: Inheriting two copies of the dominant allele (e.g., $BB$ ).
    * Homozygous Recessive: Inheriting two copies of the recessive allele (e.g., $bb$ ).
    * Heterozygous: Inheriting one copy of the dominant allele and one copy of the recessive allele (e.g., $Bb$ ).
Genotype vs. Phenotype:
* Genotype: The actual alleles or genes an individual carries (e.g., $BB$ or $Bb$ ).
* Phenotype: The observable trait resulting from the genotype (e.g., purple flowers vs. white flowers).
Punnett Squares and Genetic Counseling: Counselors use Punnett squares to predict offspring outcomes.
* Albinism Example: Albinism is a recessive trait. If two phenotypically normal parents are carriers ( $Aa \times Aa$ ), there is a $25\%$ chance of an offspring having albinism ( $aa$ ), a $50\%$ chance they are carriers ( $Aa$ ), and a $25\%$ chance they are normal/non-carriers ( $AA$ ).
* Cleft Chin: Having a cleft chin is a dominant trait ( $C$ ), while a non-cleft chin is recessive.

Autosomal Dominant Inheritance Examples

Achondroplasia: This form of dwarfism is inherited in an autosomal dominant fashion.
    * Case Study (Little People, Big World): If both parents have achondroplasia and are heterozygous ( $Aa \times Aa$ ):
        * Outcome $AA$ : Usually leads to a severe form that is fatal, often resulting in a miscarriage.
        * Outcome $Aa$ : Achondroplasia ( $50\%$ chance).
        * Outcome $aa$ : Phenotypically normal ( $25\%$ chance).
    * Offspring who are phenotypically normal ( $aa$ ) have no chance of passing the condition to their own children.
Huntington’s Disease: This is a neurological disorder inherited via an autosomal dominant allele.
    * Symptoms: Involuntary movements, nervous system deterioration, and eventually death.
    * Clinical Challenge: The disease typically does not manifest until an individual is in their $30s$ , often after they have already reproduced.
    * Risk: If one parent has the disease ( $Hh$ ) and the other is normal ( $hh$ ), each child has a $50\%$ chance (ratio of $1:1$ ) of inheriting the disease.

Complex Inheritance: Codominance, Incomplete Dominance, and Blood Types

Codominance: This occurs when both alleles are equally expressed in the phenotype.
* Example: Blood types A and B are codominant. If an individual has both alleles, their phenotype is type AB, as neither takes over the other.
Incomplete Dominance: This occurs when the phenotype is a "blend" of the parental traits.
* Example: A red flower and a white flower producing pink offspring.
The ABO Blood Group System:
    * Determined by carbohydrates (sugars) on the surface of blood cells (Carbohydrate A and Carbohydrate B).
    * Blood Type O is recessive to both A and B.
    * Possible Genotypes:
        * Type A: $AA$ or $AO$
        * Type B: $BB$ or $BO$
        * Type AB: $AB$
        * Type O: $OO$
    * Paternity logic: If a parent is Type AB ( $AB$ ) and the other is Type O ( $OO$ ), they can have children with Type A ( $AO$ ) or Type B ( $BO$ ), but they cannot have a Type O child. If the child is Type O, the Type AB individual is not the father.
Rh Factor and Pregnancy:
    * Concerns arise when a mother is Rh-negative and the fetus is Rh-positive.
    * First Pregnancy: Usually safe, but mom and fetal blood mix during delivery, causing the mother to produce antibodies against Rh-positive blood.
    * Future Pregnancies: The mother's body will fight a subsequent Rh-positive fetus.
    * Prevention: The RhoGAM shot is administered to prevent the mother from producing these antibodies.
Sickle Cell Anemia (Incomplete Dominance):
    * Normal red blood cells are discoid. Sickle cells "sickle" or squeeze up in low-oxygen environments.
    * Alleles: $A$ (Normal) and $S$ (Sickle).
    * Genotype $SS$ : Sickle cell disease.
    * Genotype $AS$ (Carriers): Generally normal but exhibit incomplete dominance because their cells are "just off" enough to be resistant to malaria.
    * Malaria Correlation: In regions where malaria is prevalent, the sickle cell carrier gene is more common because carriers survive to reproduce more frequently.

Polygenic Inheritance, Pleiotropy, and Sex Linkage

Polygenic Inheritance: This occurs when many genes affect a single trait. Examples include eye color, skin color (determined by levels of pheomelanin and eumelanin), alcoholism, mental illness, cancer, and heart disease.
Pleiotropy: This occurs when a single gene has multiple phenotypic effects.
* Alkamperemia: A mutation on chromosome $3$ that blocks the breakdown of tyrosine. Effects include darkening of the sclera (white of the eye) and urine that darkens upon standing.
* Sickle cell anemia is also pleiotropic, causing joint pain, circulatory issues, and multiple organ effects.
Sex-Linked Traits:
    * The Y chromosome is smaller and carries the SRY gene but few others; it often lacks a partner gene for those on the X chromosome.
    * Color Blindness: A recessive trait on the X chromosome.
        * Females ( $XX$ ): Need two recessive copies to be color blind. If they have one, they are normal but are "carriers."
        * Males ( $XY$ ): Only need one recessive copy to express the trait because the Y chromosome cannot mask the gene.
        * Example: A color-blind father ( $X^c Y$ ) and a normal mother ( $XX$ ) will have sons who are normal (receiving Y from dad) and daughters who are $100\%$ carriers ( $X^c X$ ).
    * Hemophilia: A sex-linked risk of blood clotting failure.
        * Historical Context: Queen Victoria was a carrier and passed it to royal families throughout Europe. Her granddaughter Alexandra married Czar Nicholas II of Russia. Their son, Alexis, had hemophilia, which contributed to the clinical and political instability of the Russian royal family.
Duchenne Muscular Dystrophy: Another example of a sex-linked trait.

Penetrance, Commonality Myths, and Environment

Penetrance: The percentage of a population with a given genotype that actually expresses the predicted phenotype.
* Example: Polydactyly (extra fingers/toes) is autosomal dominant but has only $65\%$ penetrance. $35\%$ of people with the gene do not show the trait.
Myth of Dominance vs. Frequency: Dominant alleles are not necessarily more common in the gene pool.
* Type O blood is recessive but is the most common.
* Type AB blood is dominant but is the rarest.
Environmental Effects on Genetics:
* Phenylalanine is an amino acid from the diet that enzymes convert into melanin for eye and skin color.
* Babies are often born with bluish-gray eyes because melanin has not yet fully developed. While eye color could theoretically be kept light by withholding phenylalanine, this is dangerous and not recommended.

Questions & Discussion

Query regarding media: "Why is we watching another Cinderella story gagging me right now? Like, I knew who was it. Hello. I am the owner of the floor. And If you're swollen chesty and you hate wearing a You you you don't know that. You have to get the…"
Color Blindness in Class: The lecturer noted that in almost every semester, at least one student, usually male, is color blind due to the frequency of the trait.