Definitions
Allele: is a variant form of a given gene. Sometimes, different alleles can result in different observ-
able phenotypic traits, such as different pigmentation, or the ABO blood type.
Albino: Albinism in humans is a congenital disorder characterized by the complete or partial absence
of pigment in the skin, hair and eyes. Albinism is associated with several vision defects, such as
photophobia, nystagmus, and amblyopia. Lack of skin pigmentation makes for more susceptibility
to sunburn and skin cancers.
Anaphase: Anaphase is the fourth phase of mitosis, the process that separates the duplicated genetic
material carried in the nucleus of a parent cell into two identical daughter cells.
Autosme: Any chromosome not considered as a sex chromosome or is not involved in sex determi-
nation. It occurs in pairs in somatic cells and singly in sex cells.
Auxotrophy: is the inability of an organism to synthesize a particular organic compound required
for its growth.
Inbreeding: is a term in genetics, meaning the crossing (mating) of closely related animals or plants.
Self-fertilisation in plants is the most extreme kind of inbreeding. It is quite widespread in plants
which carry both male and female flowers on the same plant.
Crossing over: A process occurring during meiosis wherein homologous chromosomes pair up and
exchange segments of their genetic material. Chromosomal crossover occurs when homologous
chromosomes exchange genetic material.
Dihybrid crossing: In the name "Dihybrid cross", the "di" indicates that there are two traits involved
(e.g., R and Y), the "hybrid" means that each trait has two different alleles (e.g., R and r, or Y and
y), and "cross" means that there are two individuals (usually a mother and father) who are combining
or "crossing" their genetic information. Dihybrid cross is a cross between two different lines (varie-
ties, strains) that differ in two observed traits. In the Mendelian sense, between the alleles of both
these loci there is a relationship of complete dominance - recessive. In the example pictured to the
right, RRYY/rryy parents result in F1 offspring that are heterozygous for both R and Y (RrYy).
Diploid: that is a cell or an organism with two sets of chromosomes.
Dominant: an allele or a gene that is expressed in an organism’s phenotype, masking the effect of
the recessive allele or gene when present. ("the most common trait in a population").
Epistasis: Epistasis is a type of polygenic interaction where one gene controls the phenotype of
another gene for a trait. Both genes have an influence on the physical appearance of the trait, but the
one that shows epistasis masks the effect of the other. The important thing to remember about epi-
stasis is that the epistatic gene has no effect on the genotype, or genetic composition, of the gene
being controlled. It affects only the phenotype.
Dominant epistasis: Dominant epistasis happens when the dominant allele of one gene masks the
expression of all alleles of another gene. If an organism inherits one or two copies of the dominant
allele, it will have the trait.
Phenotype: is the composite of an organism's observable characteristics or traits, such as its mor-
phology, development, biochemical or physiological properties, behavior, and products of behavior.
1/13
, A phenotype results from the expression of an organism's genetic code, its genotype, as well as the
influence of environmental factors and the interactions between the two.
Gamete: a mature sexual reproductive cell, as a sperm or egg, that unites with another cell to form
a new organism.
Genome: In terms of modern molecular biology and genetics, a genome is the genetic material of an
organism. It consists of DNA (or RNA in RNA viruses). The genome includes both the genes (the
coding regions) and the noncoding DNA, as well as the genetic material of the mitochondria and
chloroplasts.
Genotype: the genotype is the part of the genetic makeup of a cell, and therefore of an organism or
individual, which determines one of its characteristics (phenotype).
Gene: A gene is a sequence of DNA or RNA which codes for a molecule that has a function. During
gene expression, the DNA is first copied into RNA. The RNA can be directly functional or be the
intermediate template for a protein that performs a function.
Gene mutation: a mutation is the permanent alteration of the nucleotide sequence of the genome of
an organism, virus, or extrachromosomal DNA or other genetic elements.
Haploid: a cell (especially gametes or germ cells) containing half of the number of homologous
chromosomes in somatic cells.
Hemizygote: is a diploid individual with only one allele at a given locus or gene instead of typical
two. The men are physiologically hemizygous for the genes located on X chromosome with the ex-
ception of the genes within the pseudoautosomal regions of chromosome X as their homologous
copies are present on chromosome Y.
Homologous chromosome: A couple of homologous chromosomes, or homologs, are a set of one
maternal and one paternal chromosome that pair up with each other inside a cell during meiosis.
Homologs have the same genes in the same loci where they provide points along each chromosome
which enable a pair of chromosomes to align correctly with each other before separating during mei-
osis.
Homozygote: an organism with identical pairs of genes (or alleles) for a specific trait. If both of the
two gametes (sex cells) that fuse during fertilization carry the same form of the gene for a specific
trait, the organism is said to be homozygous for that trait. In a heterozygous organism, or heterozy-
gote, the genes for a specific trait are different.
Intermediate inheritance: the trait from dad mixes with the trait from mom and the child gets an
intermediate appearance. This type of inheritance, or passing down of traits from parent to offspring,
is called intermediate inheritance, also known as incomplete dominance.
Codominant: A trait resulting from an allele that is independently and equally expressed along with
the other. (AB0 blood types).
Chromosome: is a DNA molecule with part or all of the genetic material (genome) of an organism.
Most eukaryotic chromosomes include packaging proteins which, aided by chaperone proteins, bind
to and condense the DNA molecule to prevent the DNA from becoming an unmanageable tangle.
Lethal mutation: A type of mutation in which the effect(s) can result in the death or reduce signifi-
cantly the expected longevity of an organism carrying the mutation.
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Allele: is a variant form of a given gene. Sometimes, different alleles can result in different observ-
able phenotypic traits, such as different pigmentation, or the ABO blood type.
Albino: Albinism in humans is a congenital disorder characterized by the complete or partial absence
of pigment in the skin, hair and eyes. Albinism is associated with several vision defects, such as
photophobia, nystagmus, and amblyopia. Lack of skin pigmentation makes for more susceptibility
to sunburn and skin cancers.
Anaphase: Anaphase is the fourth phase of mitosis, the process that separates the duplicated genetic
material carried in the nucleus of a parent cell into two identical daughter cells.
Autosme: Any chromosome not considered as a sex chromosome or is not involved in sex determi-
nation. It occurs in pairs in somatic cells and singly in sex cells.
Auxotrophy: is the inability of an organism to synthesize a particular organic compound required
for its growth.
Inbreeding: is a term in genetics, meaning the crossing (mating) of closely related animals or plants.
Self-fertilisation in plants is the most extreme kind of inbreeding. It is quite widespread in plants
which carry both male and female flowers on the same plant.
Crossing over: A process occurring during meiosis wherein homologous chromosomes pair up and
exchange segments of their genetic material. Chromosomal crossover occurs when homologous
chromosomes exchange genetic material.
Dihybrid crossing: In the name "Dihybrid cross", the "di" indicates that there are two traits involved
(e.g., R and Y), the "hybrid" means that each trait has two different alleles (e.g., R and r, or Y and
y), and "cross" means that there are two individuals (usually a mother and father) who are combining
or "crossing" their genetic information. Dihybrid cross is a cross between two different lines (varie-
ties, strains) that differ in two observed traits. In the Mendelian sense, between the alleles of both
these loci there is a relationship of complete dominance - recessive. In the example pictured to the
right, RRYY/rryy parents result in F1 offspring that are heterozygous for both R and Y (RrYy).
Diploid: that is a cell or an organism with two sets of chromosomes.
Dominant: an allele or a gene that is expressed in an organism’s phenotype, masking the effect of
the recessive allele or gene when present. ("the most common trait in a population").
Epistasis: Epistasis is a type of polygenic interaction where one gene controls the phenotype of
another gene for a trait. Both genes have an influence on the physical appearance of the trait, but the
one that shows epistasis masks the effect of the other. The important thing to remember about epi-
stasis is that the epistatic gene has no effect on the genotype, or genetic composition, of the gene
being controlled. It affects only the phenotype.
Dominant epistasis: Dominant epistasis happens when the dominant allele of one gene masks the
expression of all alleles of another gene. If an organism inherits one or two copies of the dominant
allele, it will have the trait.
Phenotype: is the composite of an organism's observable characteristics or traits, such as its mor-
phology, development, biochemical or physiological properties, behavior, and products of behavior.
1/13
, A phenotype results from the expression of an organism's genetic code, its genotype, as well as the
influence of environmental factors and the interactions between the two.
Gamete: a mature sexual reproductive cell, as a sperm or egg, that unites with another cell to form
a new organism.
Genome: In terms of modern molecular biology and genetics, a genome is the genetic material of an
organism. It consists of DNA (or RNA in RNA viruses). The genome includes both the genes (the
coding regions) and the noncoding DNA, as well as the genetic material of the mitochondria and
chloroplasts.
Genotype: the genotype is the part of the genetic makeup of a cell, and therefore of an organism or
individual, which determines one of its characteristics (phenotype).
Gene: A gene is a sequence of DNA or RNA which codes for a molecule that has a function. During
gene expression, the DNA is first copied into RNA. The RNA can be directly functional or be the
intermediate template for a protein that performs a function.
Gene mutation: a mutation is the permanent alteration of the nucleotide sequence of the genome of
an organism, virus, or extrachromosomal DNA or other genetic elements.
Haploid: a cell (especially gametes or germ cells) containing half of the number of homologous
chromosomes in somatic cells.
Hemizygote: is a diploid individual with only one allele at a given locus or gene instead of typical
two. The men are physiologically hemizygous for the genes located on X chromosome with the ex-
ception of the genes within the pseudoautosomal regions of chromosome X as their homologous
copies are present on chromosome Y.
Homologous chromosome: A couple of homologous chromosomes, or homologs, are a set of one
maternal and one paternal chromosome that pair up with each other inside a cell during meiosis.
Homologs have the same genes in the same loci where they provide points along each chromosome
which enable a pair of chromosomes to align correctly with each other before separating during mei-
osis.
Homozygote: an organism with identical pairs of genes (or alleles) for a specific trait. If both of the
two gametes (sex cells) that fuse during fertilization carry the same form of the gene for a specific
trait, the organism is said to be homozygous for that trait. In a heterozygous organism, or heterozy-
gote, the genes for a specific trait are different.
Intermediate inheritance: the trait from dad mixes with the trait from mom and the child gets an
intermediate appearance. This type of inheritance, or passing down of traits from parent to offspring,
is called intermediate inheritance, also known as incomplete dominance.
Codominant: A trait resulting from an allele that is independently and equally expressed along with
the other. (AB0 blood types).
Chromosome: is a DNA molecule with part or all of the genetic material (genome) of an organism.
Most eukaryotic chromosomes include packaging proteins which, aided by chaperone proteins, bind
to and condense the DNA molecule to prevent the DNA from becoming an unmanageable tangle.
Lethal mutation: A type of mutation in which the effect(s) can result in the death or reduce signifi-
cantly the expected longevity of an organism carrying the mutation.
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