Transmission Genetics

Each list begins with basic conceptual vocabulary you need to know for MCAT questions and proceeds to advanced terms that might appear in context in MCAT passages. The terms are links to Wikipedia articles.
An allele is a viable DNA coding that occupies a given locus on a chromosome.
The genotype describes the genetic constitution of an individual, that is the specific allelic makeup of an individual, usually with reference to a specific character under consideration.
The phenotype describes the ourward physical appearance or manifestation of a trait in an organism, as opposed to its genotype.
Gregor Mendel
Gregor Mendel (1822 - 1884) was an Austrian Augustinian priest and scientist often called the father of modern genetics for his study of the inheritance of traits in pea plants.
A locus is a fixed position on a chromosome.
Genetic fingerprinting
Genetic fingerprinting is a technique used to distinguish between individuals of the same species using only samples of their DNA, exploiting highly variable repeating sequences called minisatellites.
Nondisjunction is the failure of chromosome pairs to separate properly during meiosis or mitosis.
Aneuploidy is a change in the number of chromosomes that can lead to a chromosomal disorder.
Dominance relationship
A dominance relationship refers to how the alleles for a locus interact to produce a phenotype.
Punnett square
The Punnett square is a diagram used by biologists to determine the probability of an offspring having a particular genotype.
Dihybrid cross
A dihybrid cross is a cross between two individuals identically heterozygous at two loci.
Monohybrid cross
A monohybrid cross is a cross between individuals who are identically heterozygous at one locus.
Pedigree chart
A pedigree chart is a chart which shows the known phenotypes for an organism and its ancestors.
Widow's peak
A widow's peak is a descending V-shaped point in the middle of the hairline. This is a dominant genetic trait.
The recessive trait albinism is a form of hypopigmentary congenital disorder, characterized by a lack of melanin pigment in the eyes, skin and hair. It is a recessive genetically inherited trait.
Dimples are visible indentations of the skin, a dominant genetic trait which shows on some people's cheeks when they smile.
Freckles are small colored spots of melanin on the exposed skin. Having them is genetic and is related to the presence of the melanocortin-1 receptor MC1R gene variant, which is a dominant trait.
Test cross
A test cross crosses a homozygous recessive with an individual with an unknown genotype.
A trait is a distinct phenotypic character of an organism that may be inherited, environmentally determined or somewhere in between.
Wild type
The wild type is the typical form of an organism, strain, gene, or characteristic as it occurs in nature.
F1 hybrid
F1 hybrid is a term used in genetics and selective breeding to describe the first filial generation offspring resulting from a cross mating of distinctly different parental types.
Genetic linkage
Genetic linkage occurs when particular genetic loci or alleles for genes are inherited jointly because the loci are on the same chromosome and thus tend to segregate together during meiosis.
Down syndrome
Down syndrome or trisomy 21 is a genetic disorder caused by the presence of all or part of an extra 21st chromosome.
Turner syndrome
Turner syndrome encompasses several chromosomal abnormalities, of which monosomy X is the most common.
Klinefelter's syndrome
Klinefelter's syndrome or XXY syndrome is a condition caused by a chromosome aneuploidy in which affected males have an extra X sex chromosome.
An autosome is a non-sex chromosome.
Sex linkage
Sex linkage is the phenotypic expression of an allele that is related to the gender of the individual and is found on the sex chromosomes.
XY sex-determination system
The XY sex-determination system is the sex-determination system found in humans, most other mammals, and some insects.
Heritability is the proportion of phenotypic variation in a population that is attributable to genetic variation among individuals.
Zygosity describes the similarity or dissimilarity of DNA between homologous chromosomes at a specific allelic position or gene.
A minisatellite is a section of DNA that consists of a short series of 10-100 bases. These types of sequences occur at more than 1000 locations in the human genome.
Chromosome 21
Chromosome 21 is the smallest human chromosome.
Triple X syndrome
Triple X syndrome is a form of chromosomal variation characterized by the presence of an extra X chromosome in each cell of a human female.
A phene is an individual characteristic or trait which can be possessed by an organism, such as eye colour or height, or any other observable characteristic.
Complete linkage
Complete linkage is defined as the state in which two loci are so close together that alleles of these loci are virtually never separated by crossing over.
True breeding organism
A true breeding organism is an organism having certain biological traits which are passed on to all subsequent generations when bred with another organism of the same type for the same traits.
Barr body
A Barr body is the inactive X chromosome in a female cell.
Penetrance is a term used in genetics describing the proportion of individuals carrying a particular variation of a gene that also express the particular trait.
Phenylthiocarbamide, also known as PTC, or phenylthiourea, is an organic compound which either tastes very bitter, or virtually tasteless, depending on the genetic makeup of the taster.
Sexadactyly or hexadactyly is dominant genetic condition in which a person has six fingers on one or both hands, or six toes on one or both feet.
Cri du chat
Cri du chat syndrome is due to a partial deletion of the short arm of chromosome number 5.
Norms of reaction
A norm of reaction describes the pattern of phenotypic expression of a single genotype across a range of environments.
Pleiotropy occurs when a single gene influences multiple phenotypic traits.
A phenome is the set of all phenotypes expressed by a cell, tissue, organ, organism, or species.
Complementation refers to a relationship between two different strains of an organism which both have homozygous recessive mutations that produce the same phenotype.
Identity by type
Alleles have identity by type if they have the same phenotypic effect.
XYY syndrome
XYY syndrome is an aneuploidy of the sex chromosomes in which a human male receives an extra Y chromosome.
Brachydactyly is an inherited, usually dominant trait, which leads to shortness of the fingers and toes relative to the length of other long bones of the body.
Mitochondrial Eve
Mitochondrial Eve is the name given by researchers to the woman who is defined as the matrilineal most recent common ancestor for all living humans.
X-inactivation (also called lyonization) is a process by which one of the two copies of the X chromosome present in female mammals is inactivated.
Polygenic inheritance
Polygenic inheritance, also known as quantitative or multifactorial inheritance refers to inheritance of a phenotypic characteristic that is attributable to two or more genes and their interaction with the environment.
A haplogroup is a large group of haplotypes, which are series of alleles at specific locations on a chromosome.
A haplotype is a combination of alleles at multiple linked loci that are transmitted together.
The term heterosis, also known as hybrid vigor or outbreeding enhancement, describes the increased strength of different characteristics in hybrids.
Y-chromosomal Adam
Y-chromosomal Adam is the patrilineal human most recent common ancestor from whom all Y chromosomes in living men are descended.
Mosaicism denotes the presence of two populations of cells with different genotypes in one individual
Human mitochondrial DNA haplogroup
A human mitochondrial DNA haplogroup is a large group of haplotypes defined by differences in human mitochondrial DNA.
Haploinsufficiency occurs when a diploid organism only has a single functional copy of a gene and the single functional copy of the gene does not produce enough of a gene product to bring about a wild-type condition.
Null allele
A null allele is a mutant copy of a gene that completely lacks that gene's normal function.
Quantitative trait locus
Quantitative trait loci are stretches of DNA that are closely linked to the genes that underlie a trait in question.
Canalization is a measure of the ability of a genotype to produce the same phenotype regardless of variability of its environment.
Hypostatic gene
A hypostatic gene is one whose phenotype is masked by the expression of an allele at a separate locus, in an epistasis event.
Human Y-chromosome DNA haplogroups
Human Y-chromosome DNA haplogroups are large groups of haplotypes defined by differences in the non-recombining portions of DNA from the Y chromosome.
Edwards syndrome
Trisomy 18 or Edwards Syndrome is the most common trisomy after Down Syndrome.
Patau syndrome
Patau syndrome is a syndrome in which a patient has an additional chromosome 13.
Malecot's method of coancestry
Malecot's coancestry coefficient refers to an indirect measure of genetic similarity of two individuals.
Ousiotype is a term defined in segregation analysis to describe whether a phenotype is due to common factors, rare or both.
X0 sex-determination system
The X0 sex-determination system is a system that grasshoppers, crickets, roaches, and some other insects use to determine the sex of their offspring.
ZW sex-determination system
The ZW sex-determination system is a system that birds, some fish, and some insects (including butterflies and moths) use to determine the sex of their offspring.
Haplodiploid sex-determination system
The haplodiploid sex-determination system determines the sex of the offspring of many Hymenopterans (bees, ants, and wasps), and coleopterans (bark beetles).
Robertsonian translocation
Robertsonian translocation is a common form of chromosomal rearrangement that occurs in the five acrocentric human chromosome pairs, namely 13, 14, 15, 21, and 22.
The Genographic Project
The Genographic Project is a five-year genetic study that aims to map historical human migration patterns by collecting and analyzing DNA samples from hundreds of thousands of people across five continents.
Uniparental disomy
Uniparental disomy occurs when a person receives two copies of a chromosome, or part of a chromosome, from one parent and no copies from the other parent.

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