Human Genetics Exam #1 (Chapter 4 and 5)

Autosomal dominant inheritance

Pattern of inheritance where a single-gene trait affects both sexes and appears in every generation

Autosomal recessive inheritance

Pattern of inheritance where a single-gene trait affects both sexes and can skip generations through carriers

True-breeding

Description of organisms that consistently yield offspring with the exact same trait as the parent

Hybrid

Individual that inherits a different gene variant or allele from each parent

Monohybrid cross

Genetic cross following the transmission of a single trait between two hybrid parents

Dominant trait

Trait or allele that is visibly expressed and masks another allele

Recessive trait

Trait or allele whose expression is masked in the presence of a dominant allele

Law of segregation

Mendel's first law stating that two alleles of a gene separate during gamete formation and rejoin at fertilization

Homozygous

Having two identical alleles for a specific gene

Heterozygous

Having two different alleles for a specific gene

Genotype

The specific allele combination present in an organism

Phenotype

The outward observable expression or physical characteristic of an allele combination

Wild type

The most common phenotype or allele for a gene in a population

Mutant phenotype

Variant allele expression that arises when a gene undergoes a mutation

Test cross

Crossing an individual of unknown genotype showing a dominant trait with a homozygous recessive individual

OCA2 gene

Gene that confers eye color intensity by controlling melanin synthesis

HERC2 gene

Gene that controls the expression of the OCA2 gene to determine eye color

Loss of function mutation

Recessive mutation leading to a severe reduction or total lack of normal protein activity

Gain of function mutation

Dominant mutation leading to an altered or toxic new protein activity

Law of independent assortment

Mendel's second law stating that the inheritance of one gene does not influence the chance of inheriting another gene on a different chromosome

Product rule

Mathematical principle stating that the probability of simultaneous independent events equals the product of their individual probabilities

Branch diagram

Visual tool used to track and analyze multiple independent gene loci simultaneously

Pedigree

Symbolic diagram depicting family relationships and the transmission of inherited traits across generations

Proband

The individual who first brings a family to medical attention and prompts a pedigree analysis

Consanguinity

Matings between individuals closely related by blood that increase the risk of inheriting identical recessive alleles

Polydactyly

Autosomal dominant physical trait characterized by having extra fingers or toes

Albinism

Autosomal recessive condition resulting from a deficiency in melanin production

Conditional probability

Risk calculation that adjusts inheritance likelihood by excluding specific ruled-out genotypes

Trio genome sequencing

DNA sequencing of an affected child and both unaffected parents to identify inherited or spontaneous mutations

De novo mutation

Spontaneous new dominant mutation originating in a germ cell or child rather than being inherited

Lethal genotype

Mutant allele combination causing embryonic or early death that eliminates an expected progeny class

Achondroplastic dwarfism

Autosomal dominant condition caused by an FGFR3 gain-of-function mutation where homozygous dominant genotypes are embryonic lethal

Compound heterozygote

Individual carrying two different mutant recessive alleles for the exact same gene

Incomplete dominance

Inheritance pattern where a heterozygote displays a phenotype intermediate between the two homozygotes

Familial hypercholesterolemia

Incompletely dominant disorder where heterozygotes have half the normal liver LDL receptors and homozygotes completely lack them

Codominance

Inheritance pattern where both alleles in a heterozygote are fully and distinctly expressed

ABO blood groups

Cell surface glycoprotein system demonstrating codominance between IA and IB while both are dominant over i

Epistasis

Phenomenon where the expression of one gene masks or interferes with the phenotype of another gene

Bombay phenotype

Epistatic trait where homozygosity for the recessive h allele prevents red blood cell antigen expression resulting in type O phenotype

Penetrance

The percentage or probability that an individual with a specific genotype will express the associated phenotype

Incomplete penetrance

Situation where some individuals carrying a disease-causing genotype do not exhibit any clinical symptoms

Variable expressivity

Variation in the severity or degree of physical expression of a phenotype among individuals with the same genotype

Pleiotropy

Phenomenon where a single gene controls multiple different body functions or causes a wide variety of symptoms

Marfan syndrome

Pleiotropic connective tissue disorder caused by a fibrillin gene defect affecting eyes skeleton and the aorta

Genetic heterogeneity

Situation where mutations in completely different genes produce identical disease phenotypes

Phenocopy

Environmentally caused condition that produces symptoms and recurrence patterns similar to an inherited trait

Mitochondrial DNA (mtDNA)

Circular organelle genome containing 37 genes that are strictly maternally inherited

Features of mtDNA

Organellar genome that lacks crossing over

Heteroplasmy

Cellular state where a cell or oocyte contains a mixture of both mutant and wild-type mitochondrial DNA copies

Linkage

Phenomenon where genes located near each other on the same chromosome are inherited together rather than assorting independently

Recombination

Process of creating new combinations of alleles on chromosomes through crossing over in prophase I of meiosis

Parental chromosomes

Chromosomes that maintain the original allele combination inherited from the parents

Recombinant chromosomes

Chromosomes carrying newly rearranged allele combinations resulting from crossing over

Cis configuration

Allele arrangement where two dominant or two recessive alleles for linked genes are located on the same chromosome

Trans configuration

Allele arrangement where one dominant and one recessive allele for linked genes are located on each chromosome

Linkage map

Diagram showing the relative order and distances between genes on a chromosome based on recombination frequencies

Centimorgan (cM)

Standard unit of distance on a linkage map corresponding to a one percent recombination frequency

Linkage disequilibrium (LD)

Nonrandom association between nearby DNA sequences that are inherited together more often than predicted by chance

Haplotype

Set of closely linked genes or genetic markers inherited together as a unit without crossing over

Genetic markers

DNA sequence landmarks of known locations used to track nearby disease-causing genes in families

LOD score

Statistical logarithm of the odds score where a value of 3 or higher signifies strong evidence for genetic linkage