Chapter 24

Genetics

The study of the inheritance of characteristics.

Genome

A complete set of genetic instructions in a person’s cells.

Exome

The part of the genome that consists of protein-making genes, accounting for <2% of the 3.2 billion DNA bases.

Chromosome 7

One of the structures that organizes and stores DNA, containing important genes for normal bodily functions and health.

Cystic fibrosis

A genetic disorder caused by defective chloride ion channels in cell membranes, leading to thick mucus.

Karyotype

A chart displaying the 23 chromosome pairs in size and order.

Alleles

Various forms of a gene that differ in DNA sequence.

Homozygous

Having two identical alleles of a particular gene (e.g., AA or aa).

Heterozygous

Having two different alleles for a gene (e.g., Aa).

Genotype

The particular combination of alleles for a specific gene.

Phenotype

The appearance of a trait or health condition that results from gene expression.

Punnett Square

A table used to predict probabilities of genotypes in offspring for specific traits.

Pedigree

A diagram showing family relationships and known genotypes and phenotypes for each family member.

Mendelian Inheritance

Inheritance regulated in a dominant/recessive manner.

Albinism

An autosomal recessive disorder characterized by the absence of melanin production.

Huntington’s Disease

An autosomal dominant disorder that leads to loss of coordination and behavioral changes.

Dominant Allele

An allele that masks the expression of a recessive allele in heterozygotes.

Recessive Allele

An allele that may be masked by a dominant allele in heterozygotes.

Codominance

A phenomenon where both alleles are fully expressed in a heterozygous individual.

Incomplete Dominance

A situation where the phenotype of a heterozygote is intermediate between those of the homozygotes.

Polygenic Traits

Traits influenced by multiple genes that contribute small effects to the final result.

Pleiotropy

A single gene that contributes to multiple traits or shows multiple effects.

Sex-linked Traits

Traits coded for by genes on a sex chromosome, mainly the X chromosome.

X-linked Traits

Traits transmitted on the X chromosome; males express these traits with one copy.

Y-linked Traits

Traits transmitted on the Y chromosome, associated with male characteristics.

Intersex Conditions

Conditions resulting from irregularities in the biochemical pathway initiated by the SRY transcription factor.

Penetrance

Refers to whether a person expresses a phenotype at all.

Expressivity

Refers to the degree or intensity of a phenotype.

Genetic Heterogeneity

Occurs when the same phenotype can be caused by different genes.

Sex-limited Trait

Affects a structure or function present in only males or only females.

Sex-influenced Inheritance

An allele that is dominant in one sex and recessive in the other.

Multifactorial Inheritance

Traits affected by both genetic and environmental factors.

Aneuploidy

Presence of an abnormal number of chromosomes in a cell, caused by nondisjunction during meiosis.

Trisomy

Having an extra chromosome, such as in Down Syndrome (Trisomy 21).

Monosomy

Having one less than the normal number of chromosomes.

Turner’s Syndrome

A sex chromosome aneuploidy leading to delayed sexual development and infertility.

Klinefelter’s Syndrome

A genetic condition in males resulting from an extra X chromosome (XXY).

Jacob’s Syndrome

A genetic condition in males characterized by an extra Y chromosome (XYY).

5-Alpha Reductase Deficiency

A condition preventing formation of penis in XY individuals, leading to female identification at birth.

Androgen Insensitivity Syndrome (AIS)

A condition where a mutation leads to insensitivity to androgens in XY individuals, resulting in a female appearance.

Codominance Example

Both A and B alleles are expressed in a heterozygous individual leading to blood type AB.

Incomplete Dominance Example

Sickle cell anemia where the heterozygous genotype has a mild trait (sickle cell trait).

Familial Hypercholesterolemia Example

A disorder affecting cholesterol processing with variable effects based on genotype.

Autosomal Recessive Example

Cystic fibrosis, an autosomal recessive disorder characterized by defective chloride channels.

Autosomal Dominant Example

Huntington’s Disease, where only one copy of the mutant allele causes the disorder.

Phenotype Size Variation Example

Height, as a polygenic trait influenced by multiple genes.

Blood Clotting Disorders Example

Different types of bleeding disorders caused by mutations in any clotting enzyme genes.

Sex-linked Trait Examples

Red-green colorblindness and hemophilia are X-linked genetic conditions.

Mendelian Extensions Types

Includes codominance, incomplete dominance, multiple alleles, and sex-linked inheritance.

Chromosomal Rearangements

Changes in the structure of chromosomes that may interfere with vital genes.

Chromosome Disorders

Changes to the normal number of chromosomes affecting health and development.

Extreme Case of Aneuploidy

Polyploidy, involving an extra set of chromosomes from abnormal gamete formation.

X Chromosome Contribution to Traits

Males are more frequently affected by X-linked traits due to having one X chromosome.

Incomplete Penetrance Example

Not everyone with the allele for polydactyly shows the trait.

Variable Expressivity Example

Among those showing polydactyly, the number and placement of extra digits can vary.

Normal Chromosome Configuration

Euploid cells have a normal number of chromosomes (2n = 46).

Nondisjunction Definition

A meiotic error leading to uneven separation of chromosomes.

Penetrance Concept

Complete penetrance means all with an allele express the trait; incomplete means some do not.

Expressivity Scope

People with the same genotype may show the trait to varying extents.

Chromosomal Number in Down Syndrome

Involves Trisomy 21, the most common autosomal aneuploid condition.

XYY Condition Effects

Jacob’s syndrome resulting in tall stature, tremors, and behavioral issues.

Habitual Characteristic of Turner’s Syndrome

Delayed sexual development and infertility as a result of X0 chromosomal configuration.

SRY Gene Irregular Expression

Can lead to phenotypic male from an XX chromosomal configuration due to crossing over.

Chromosomal Abnormalities Consequences

May lead to miscarriages or developmental abnormalities depending on the affected chromosomes.

Major Consequence of Polyploidy

Most drastic disruption in chromosome number, often leading to embryonic death.

Microscopic Structure Displayed in Karyotype

23 pairs of chromosomes, organized by size and structure.

Sickle Cell Anemia Genotype Types

SS is normal, ss has the disease, and Ss is a carrier with mild symptoms.

Pleiotropic Effect Example

Marfan syndrome showing different symptoms due to a defect in a single gene.

ABO Blood Type Example of Codominance

Both A and B alleles are expressed leading to AB blood type in heterozygous individuals.

Characteristics of Females with X-linked Traits

Require two copies of the recessive allele to express the trait.

Sex-influenced Trait Example

Baldness is dominant in males but recessive in females.

Pedigree Diagram Importance

Illustrates family relationships and known genetic traits within family members.

Autosomal Recessive Trait Expression

Requires two copies of the allele for the trait to be expressed.

Impact of Environmental Factors on Traits

Some traits like height can be influenced by both genetics and environment.

Causes of Genetic Heterogeneity

Different genes can lead to the same observable trait or condition.

Nondisjunction Resulting Condition

Aneuploidy, including conditions like Down syndrome and Turner’s syndrome.

Recessive Trait in Heterozygous Males

Only needs one copy of the recessive allele to express X-linked traits.

Effects of Turner’s Syndrome

Often results in infertility and delayed sexual development.

Sickle Cell Trait Population Advantage

Provides a survival advantage against malaria to heterozygous individuals.

Detection of Chromosomal Abnormalities

Karyotyping can identify chromosomal disorders and arrangements.

Health Risks of Autosomal Aneuploidy

Typically results in disorders like Down syndrome, causing cognitive deficits.

Genetic Testing Purpose

To identify carriers and affected individuals in genetic conditions.

X-linked Recessive Inheritance Pattern

Males express the trait and females are carriers unless homozygous recessive.

Down Syndrome Additional Risks

Associated with increased risk of congenital heart defects and other conditions.

Characteristics of Klinefelter's syndrome

Includes sexual underdevelopment and possibly normal intelligence despite XXY configuration.

Example of Multiple Alleles in Blood Types

The existence of A, B, and O alleles affecting blood type traits.

Common Autosomal Disorders Related to Chromosomal Abnormalities

Down syndrome and Turner syndrome are examples of conditions with prominent effects.

Intersex Variation due to SRY Expression

Can result in both male and female anatomy based on genetic anomalies.

The Role of Androgens in Sexual Development

Influence on the development of male internal and external reproductive structures.

Impact of Genetic Variability on Medical Phenotypes

How identical genotypes can lead to diverse phenotypic expressions.

Healthcare Implications of Genetic Testing

Social and ethical considerations become important for individuals and families.

Penetrance's Relevance to Genetic Counseling

Critical for assessing risk and probabilities of trait expression in families.

Research Focus in Genetic Studies

Understanding genetic contributions to complex traits and disorders.

Long-term Effects of Genetic Disorders

Chronic implications for health, wellness, and psychological well-being.

Importance of Genetic Counseling

Facilitates informed decisions about testing, treatment, and risk management.

Educational Focus in Genetics

Promoting understanding of inheritance patterns and their implications.

Core Concept of Mendelian Genetics

The principles outlining dominant and recessive allele interactions.

Sex-linked Inheritance Exploration

Study of how traits associated with sex chromosomes are transmitted.

Predicting Offspring Genotypes

Using tools like Punnett squares to visualize inheritance patterns.

Understanding of Multifactorial Conditions

How various genes and environmental factors interact to influence health outcomes.