Human Genetics Chapters 4–6 Vocabulary Flashcards

Comprehensive vocabulary flashcards covering Mendel's laws, extensions of Mendelian inheritance, and sex-linked traits based on Chapters 4–6 Human Genetics notes.

Single-Gene Disease

An inherited disorder caused by mutations or variants in a single gene (also known as a Mendelian disorder).

Penetrance

The proportion of individuals with a specific genetic variant who express the associated phenotype or disease symptoms.

De Novo Mutation

A new genetic mutation that arises in an individual during gamete formation or early development rather than being inherited from a parent.

Mode of Inheritance

The pattern by which a genetic disease or trait is transmitted from one generation to the next.

Trio Testing

Genetic sequencing involving an affected child, the child's mother, and the child's father to determine whether mutations are inherited or de novo.

Homozygous

Having two identical copies of the same allele for a specific gene (e.g., $TT$ or $tt$).

Heterozygous

Having two different alleles for a specific gene (e.g., $Tt$).

Dominant Allele

An allele whose associated phenotype is expressed in a heterozygote, masking the presence of a recessive allele under a simple Mendelian pattern.

Recessive Allele

An allele whose associated phenotype is not expressed in a heterozygote and requires two copies to be phenotypic.

Genotype

The specific allele combination or genetic makeup of an organism (e.g., $Tt$).

Phenotype

The observable physical, physiological, or functional trait or condition of an organism.

Carrier

An individual who is typically an unaffected heterozygote carrying a recessive disease-causing allele (e.g., $Cc$ in a recessive disorder).

Law of Segregation

Mendel's first law stating that allele pairs separate during meiosis into distinct haploid gametes, restoring diploidy upon fertilization.

Testcross

A genetic cross between an individual displaying a dominant phenotype and a homozygous recessive individual to determine the unknown genotype.

Law of Independent Assortment

Mendel's second law stating that the inheritance of one gene does not influence the chance of inheriting another gene when located on different chromosomes.

Product Rule

A probability rule used to calculate the chance of independent events occurring together: $P(A \text{ and } B) = P(A) \times P(B)$.

Sum Rule

A probability rule used to calculate the chance of mutually exclusive outcomes occurring: $P(A \text{ or } B) = P(A) + P(B)$.

Pedigree

A diagram using standardized symbols to represent family relationships and trace the inheritance of traits across generations.

Consanguinity

Mating between close blood relatives, which increases the likelihood that offspring inherit two copies of a rare recessive allele.

Lethal Allele

An allele genotype that causes death before reproduction, removing an expected progeny class and altering observed offspring ratios (e.g., $2:1$ ratio in achondroplasia).

Compound Heterozygote

An individual who carries two different mutant or variant alleles for the same autosomal gene.

Incomplete Dominance

An inheritance pattern in which the heterozygote exhibits a phenotype intermediate between those of the two homozygotes (e.g., familial hypercholesterolemia).

Codominance

An inheritance pattern in which both allele products and phenotypes are distinctly and fully expressed in the heterozygote (e.g., ABO blood type $I^A I^B$).

Epistasis

A phenomenon where an allele or gene at one locus masks or modifies the phenotypic expression of a gene at a different locus.

Expressivity

The degree, extent, or severity of phenotype expression among individuals carrying a specific genotype.

Pleiotropy

A genetic occurrence where a single gene or mutation affects multiple seemingly unrelated phenotypic traits or body systems (e.g., Marfan syndrome).

Phenocopy

An environmentally caused trait that mimics the phenotype of an inherited genetic condition (e.g., thalidomide-induced phocomelia).

Genetic Heterogeneity

A scenario where different genes or different mutations produce the exact same clinical phenotype or disease.

Locus Heterogeneity

A form of genetic heterogeneity where mutations in completely different genes produce the same disease phenotype.

Allelic Heterogeneity

A form of genetic heterogeneity where different mutations within the same gene cause the same condition.

Heteroplasmy

The presence of a mixture of normal and mutant mitochondrial DNA ($\text{mtDNA}$) within a single cell or tissue.

Homoplasmy

A cellular state in which all copies of mitochondrial DNA ($\text{mtDNA}$) are essentially identical (all wild-type or all mutant).

Linked Genes

Genes located close together on the same chromosome that tend to be inherited together and do not assort independently.

Recombination Frequency

The proportion of recombinant gametes or offspring resulting from crossing over between linked genes ($1\% \text{ recombination} = 1\text{ cM}$ or $1\text{ map unit}$).

Haplotype

A set of linked genetic markers or alleles located physically close together on the same chromosome segment that are inherited together.

LOD Score

A logarithm of odds score evaluating the statistical likelihood of genetic linkage versus chance; a score of $3$ or higher signifies linkage.

Heterogametic Sex

The sex possessing two different sex chromosomes (e.g., human males with $XY$).

Homogametic Sex

The sex possessing two of the same sex chromosome (e.g., human females with $XX$).

SRY Gene

The sex-determining region Y gene located on the Y chromosome encoding a transcription factor that initiates testis development.

Pseudoautosomal Regions (PARs)

Homologous sequences shared at the ends of X and Y chromosomes that allow them to pair and recombine during male meiosis.

Hemizygous

Having only one copy of a gene or chromosome region in an otherwise diploid organism, such as males for X-linked genes.

Sex-Limited Trait

A trait governed by genes present in both sexes but expressed in only one sex due to physiological or anatomical differences (e.g., beard growth).

Sex-Influenced Trait

A trait whose expression differs between males and females because an allele acts dominant in one sex and recessive in the other (e.g., pattern baldness).

X Inactivation

An epigenetic mechanism in female mammals where one X chromosome is randomly inactivated in each embryonic cell to balance X-linked gene dosage.

Barr Body

The condensed, inactive X chromosome visible as a dark-staining structure in the cell nuclei of female mammals.

Manifesting Heterozygote

A female carrier of an X-linked recessive trait who expresses disease symptoms because favorable X inactivation left the mutant allele active in key tissues.

Genomic Imprinting

An epigenetic process in which methyl groups silence genes in a sex-dependent pattern, causing expression to depend on parent-of-origin.

Parent-of-Origin Effect

A phenotypic outcome where disease severity or manifestation depends on which parent passed down the genetic alteration (e.g., Prader-Willi vs. Angelman syndrome).

Anticipation

A genetic phenomenon where a disorder manifests at an earlier age and with increased severity in successive generations as a repeat expansion mutation grows.

Spermatogenesis

The continuous process of male gamete creation that starts at puberty and involves ongoing cell divisions, allowing paternal-age mutations to accumulate.

Oogenesis

The process of female gamete formation that begins before birth, arrests in meiosis I for decades, and resumes for individual oocytes after puberty.