gene #2

proband

The specific individual whose phenotype first brought the family to the attention of the geneticist. In a diagram, this individual is identified by an arrow

obligate carrier

An unaffected individual who must carry a specific allele based on the phenotypes of their parents or offspring.

consanguinity

Mating between biological relatives (e.g., first cousins). In pedigrees, this is indicated by a double horizontal line and often explains the appearance of rare recessive disorders.

autosomal trait

A characteristic controlled by a gene located on one of the 22 pairs of non-sex chromosomes (autosomes).

hemizygous

The state of possessing only a single copy of a gene in an otherwise diploid organism. Human males are hemizygous for most genes on the X and Y chromosomes.

crisscross inheritance

An inheritance pattern where an X-linked recessive trait "crosses" from one sex to the other between generations, such as from an affected grandfather to an affected grandson via a carrier daughter.

penetrance

A population-level measurement defined as the percentage of individuals with a specific genotype who actually display the expected phenotype.

expressivity

An individual-level measurement describing the degree or intensity (the "volume") to which a genotype is physically manifested as a phenotype.

incomplete penetrance

A pattern where an individual carries a disease-associated genotype but appears phenotypically normal, allowing a dominant trait to skip generations.

linked genes

Genes located close together on the same chromosome that tend to be inherited together

recombination frequency (RF)

The proportion of recombinant offspring produced in a genetic cross, usually expressed as a percentage. RF = number of recombinant progeny / total number of progeny × 100

map unit

A unit used to describe genetic distance between genes. One map unit corresponds to a recombination frequency of 1%.

centimorgan (cM)

Another term for a map unit.

genetic distance

The relative distance between genes estimated from recombination frequency.

configuration

The arrangement of linked alleles on homologous chromosomes.

haplotype

The specific combination of alleles present on a single chromosome.

coupling configuration (cis)

Arrangement in which dominant or wild-type alleles are located together on one homolog and recessive alleles are located together on the other homolog.

replusion configuration (trans)

Arrangement in which dominant or wild-type alleles are located on opposite homologs.

polyploidy

possession of more than two complete chromosome sets

aneuplodidy

abnormal chromsome number caused by gain or loss of individual chromsomes

nullisomy

loss of both chromosomes from a homologous pair