Genetics: Chromosomes, Genes, Genotypes & Phenotypes

Comprehensive vocabulary flashcards covering the fundamental concepts of genetics, chromosome structure, meiosis, Mendelian and non-Mendelian inheritance, and pedigree analysis.

Gene

A sequence of DNA bases located on a chromosome that codes for a specific protein.

DNA

A universal code consisting of the bases A, T, C, and G used by all organisms to build genetic instructions.

Allele

Different versions or forms of the same gene found at the same locus on homologous chromosomes.

Locus

The specific position or spot where a gene sits on a chromosome.

Genome

The complete set of genetic instructions, including all the DNA, of an organism.

Diploid ($2n$)

Cells containing two copies of each chromosome, typically found in somatic (body) cells.

Haploid ($n$)

Cells containing one copy of each chromosome, typically found in gametes like sperm and eggs.

Histones

Proteins that DNA is packaged with to form chromosomes.

Nucleosome

A core of histone proteins with DNA wrapped around it.

Chromatin

The combination of DNA and histone proteins in a less-condensed form seen during interphase.

Supercoiling

The further tight coiling of chromatin fibre to condense it into a visible chromosome before cell division.

Chromatid

One of two identical copies of a chromosome joined at the centromere, formed after DNA replication.

Metacentric

A chromosome where the centromere is in the middle, making the $p$ and $q$ arms equal in length.

Submetacentric

A chromosome where the centromere is towards one end and the $q$ arm is approximately twice the length of the $p$ arm.

Acrocentric

A chromosome where the centromere is very close to one end, resulting in very short $p$ arms.

Telocentric

A chromosome where the centromere is at the very tip and no $p$ arm is present.

Homologous chromosomes

Paired chromosomes that carry the same genes at the same loci, with one inherited from each parent.

Autosomes

Non-sex chromosomes; in humans, these are numbered 1 through 22.

Sex chromosomes (allosomes)

The 23rd pair of chromosomes (X and Y in mammals) that determines biological sex.

Karyotype

An organized arrangement of an individual's chromosomes by size and shape into homologous pairs, used to identify abnormalities.

Translocation

A chromosomal change where a segment of one chromosome breaks off and joins a different, non-homologous chromosome.

Non-disjunction

The failure of homologous chromosomes or sister chromatids to separate properly during meiosis, leading to gametes with extra or missing chromosomes.

Monosomy

A condition where one chromosome of a pair is missing; usually non-viable in humans except for Turner syndrome ($XO$).

Trisomy

A condition where there is an extra whole chromosome, such as Down syndrome (trisomy 21).

Meiosis

A two-stage division process that reduces the chromosome number by half to produce four genetically unique haploid gametes.

Synapsis

The process during Prophase I where homologous chromosomes pair up side by side.

Crossing over

A major source of genetic variation occurring in Prophase I where chromatids exchange segments of genetic material.

Independent (random) assortment

The random orientation and lining up of homologous pairs along the cell equator during Metaphase I.

Homozygous

Having two identical alleles at a specific locus (e.g., $TT$ or $tt$).

Heterozygous

Having two different alleles at a specific locus (e.g., $Tt$).

Genotype

The specific combination of alleles an individual carries at a particular locus (e.g., $Yy$).

Phenotype

The visible or observable expression of a genotype, which can be influenced by environmental factors.

Dominant allele

An allele that produces its phenotype even if the individual has only one copy.

Recessive allele

An allele that only produces its phenotype if the individual has two copies.

Carrier

A heterozygous individual who carries a recessive allele for a trait but does not express the recessive phenotype.

Co-dominance

A pattern of inheritance where both alleles of a heterozygous individual are fully and separately expressed in the phenotype.

Incomplete (partial) dominance

A pattern of inheritance where the heterozygote has a phenotype that is an intermediate blend of the two homozygous phenotypes.

Multiple alleles

A situation where more than two alleles for a gene exist within a population, such as the ABO blood group system.

Epigenetics

Factors and mechanisms, like DNA methylation, that influence gene expression and phenotype without changing the underlying DNA sequence.

Huntington's disease

A condition caused by a dominant lethal allele on chromosome 4 characterized by more than 36 $CAG$ repeats.

Monohybrid cross

A genetic cross involving the alleles of only one gene.

Test cross

A method to determine an unknown genotype by crossing an individual showing the dominant phenotype with a homozygous recessive individual.

Dihybrid cross

A genetic cross involving the alleles of two different genes at once.

Unlinked genes

Genes located on non-homologous chromosomes that assort independently into gametes.

Linked genes

Genes located on the same chromosome that tend to be inherited together as a unit.

Parental (non-crossover) gametes

Gametes that contain the same combination of alleles as found in the original parental cells.

Recombinant (crossover) gametes

Gametes containing new combinations of alleles produced by crossing over during meiosis.

Map unit

A unit of genetic distance where $1\%$ recombinant offspring is approximately equal to 1 map unit.

Pedigree chart

A symbolic diagram representing a family's genetic lineage used to track the inheritance of specific traits.

Hemizygous

A condition in which only one copy of a gene or chromosome is present, such as X-linked genes in males ($XY$).