Genesis and meiosis

Genetic carriers

A heterozygous individual bearing an unexpressed defective gene. Sentence: genetic carrier pertains to an organism carrying an unexpressed defective gene for a recessive trait but when mated with another carrier can produce a homozygous offspring that expresses the recessive trait.

Crossing over

A process occurring during meiosis wherein two chromosomes pair up and exchange segments of their genetic material. Sentence: Crossing over is important because it results in new combinations of genes that are different from either parent, contributing to genetic diversity.

Autosome

Any chromosome not considered as a sex chromosome, or is not involved in sex determination. It occurs in pairs in somatic cells and singly in sex cells (gametes). Sentence: In sex cells like egg cell and sperm cell where chromosomes occur singly (total=23), 22 of them are autosomes while the remaining one is a sex chromosome (either X or Y chromosome).

Multiple alleles

Typically, there are only two alleles for a gene in a diploid organism. The presence of multiple alleles is best illustrated by the ABO blood group system and the HLA genes. Sentence: Multiple alleles are seen in the blood groups.

Probability

(1) A measure of the likelihood of a statement or a theoretical expectation is correct Sentence: Probability is used to measure the chances or likelihood of an event to occur, a hypothesis being correct, or a scientific prediction being true. In biology, it is used in predicting the outcome of a genetic cross or of a random experiment.

Mutation

(1) A permanent, heritable change in the nucleotide sequence in a gene or a chromosome; the process in which such a change occurs in a gene or in a chromosome. Sentence: In genetics, mutation may be small scale (affecting the nucleotide sequence of a gene) or large scale (involving a change in the chromosome). It may arise from faulty deletions, insertions, or exchanges of nucleotides in the genetic material, as caused by exposure to ultraviolet or ionizing radiation, chemical mutagens, viruses, etc. Such a change may result in the creation of a new character or trait.

Heterozygous

(1) Of, or pertaining to an individual (or a condition in a cell or an organism) containing two different alleles for a particular trait. (2) Having dissimilar alleles that code for the same gene or trait. Sentence: A heterozygous Dominant trait for PKU would be Pp.

Phenotype

(1) The physical appearance or biochemical characteristic of an organism as a result of the interaction of its genotype and the environment. (2) The expression of a particular trait, for example, skin color, height, behavior, etc., according to the individual's genetic makeup and environment. Sentence: Phenotypes result from the expression of the genes of an organisms as well as the influence of environmental factors and random variation. The interaction between these factors have often been represented by the following relationship: genotype + environment + random variation → phenotype.

Allele

(genetics) One member of a pair (or any of the series) of genes occupying a specific spot on a chromosome (called locus) that controls the same trait. Sentence: For example, a pair of alleles controlling the same trait, i.e. eye color: one allele codes for blue eyes, another allele for brown eyes. In humans, simple traits such as eye color may be caused by the interaction of only one pair of alleles. But for complex traits, such as height, they are usually caused by the interactions of series of alleles. Some alleles are dominant over other alleles, as in the case of heterozygous pairings (where paired alleles are different, in contrast to homozygous pairings where alleles are the same). In the above example, since the alleles code for different eye colors they are heterozygous

Recessive

(genetics) Of, or pertaining to, a gene (or allele) whose phenotypic expression is masked by a dominant gene (or allele). Sentence: In genetics, a recessive gene or allele is one in which the effect is not tangible, or is masked by the effects of the dominant gene. The recessive trait may be expressed when the recessive genes are in homozygous condition or when the dominant gene is not present. That happens when an organism inherits a pair of recessive genes from its parents.

Homozygous

(1) Of, or pertaining to an individual (or a condition in a cell or an organism) containing two copies of the same allele for a particular trait located at similar positions (loci) on paired chromosomes. Sentence: A 'homozygous' organism for a particular trait is described to possess either a pair of dominant alleles (e.g. AA) or a pair of recessive alleles (e.g. aa).

Karyotype

(genetics) The characterization of the chromosome complement of a species (such as the shape, type, number, etc. of chromosomes). Sentence: The karyotype of an organism is usually displayed in photomicrographs wherein chromosomes are arranged in homologous pairs, and in descending order of size and relative position of the centromere. Karyotype is used to study chromosomal aberrations, cellular function, or taxonomic relationships, or to gather information about past evolutionary events.

Pedigree

2) (genetics) A diagram showing the lineage or genealogy of an individual and all the direct ancestors, usually to analyze or follow the inheritance of trait. Sentence: pedigree can be used to determine the Mendelian inheritance of a genetic trait, especially familial diseases, across several generations of a family.

Hybrid

A complex formed by joining two complementary strands of nucleic acids Sentence: *Single cross hybrid - first generation of offspring resulting from a cross between pure bred parents *Double cross hybrid - offspring resulting from a cross between two hybrids of single cross *Three-way cross hybrid - offspring from a cross between a single cross hybrid and an inbred line *Triple cross hybrid - offspring resulting from the crossing of two different three-way cross hybrids *Population hybrid - offspring resulting from the crossing of plants or animals in a population with another population. For instance, a cross between difference races

Codominance

A condition in which the alleles of a gene pair in a heterozygote are fully expressed thereby resulting in offspring with a phenotype that is neither dominant nor recessive. Sentence: A typical example showing codominance is the ABO blood group system. For instance, a person having A allele and B allele will have a blood type AB because both the A and B alleles are codominant with each other.

Incomplete dominance

A kind of dominance occurring in heterozygotes in which the dominant allele is only partially expressed, and usually resulting in an offspring with an intermediate phenotype. Sentence: In incomplete dominance, a heterozygous organism carrying two alleles wherein one is dominant and the other one is recessive, (e.g. Aa), the dominant allele will only be partially expressed. Hence, the heterozygote (Aa) will have an intermediate phenotype. A typical example is the color of the flower in which R symbolizes the dominant allele for red pigment and r is the recessive allele for no pigment. In incomplete dominance, the heterozygous plant carrying both alleles, Rr, will not be able to produce enough red pigment (since the dominant allele is only partially expressed) and therefore will appear pink.

Gamete

A reproductive cell or sex cell contains the haploid set of chromosomes, e.g. spermatozoon or sperm cell (male reproductive cell) and egg cell or ovum (female reproductive cell). (2) A mature haploid reproductive cell as produced by gametogenesis, a process involving meiosis (3) A cell that fuses with another reproductive cell of the opposite sex during fertilization forming a zygote that develops into a new individual. Sentence: In certain organisms, like humans, there are two morphologically distinct types of gametes: the male gamete (sperm cell) which is generally smaller and motile and the female gamete (ovum) which is larger and nonmotile. This condition is called anisogamy, or heterogamy, wherein the female and male gametes are of different sizes. Isogamy, in contrast, is the condition wherein both sexes are of the same size.

Genotype

A set of alleles that determines the expression of a particular characteristic or trait. Sentence: The genotype refers to the entire set of genes in a cell, an organism, or an individual. A gene for a particular character or trait may exist in two allelic forms; one is dominant (e.g. A) and the other is recessive (e.g. a). Based on this, there could be three possible genotypes for a particular character: AA (homozygous dominant), Aa (heterozygous), and aa (homozygous recessive).

Punnett square

A tool that helps to show all possible allelic combinations of gametes in a cross of parents with known genotypes in order to predict the probability of their offspring possessing certain sets of alleles. Sentence: Punnett squares are typically applied in monohybrid crossess and dihybrid crosses in which the theoretical outcomes are based on the assumptions of segregation and independent assortment of alleles (according to Mendelian inheritance).

Sex chromosome

A type of chromosome in the genome that is involved in the determination of the sex as well as the development of sexual characteristics in an organism. It occurs in pairs in somatic cells while singly in sex cells (gametes). Sentence: A somatic cell contains a genome comprised of paired chromosomes: the autosomes and sex chromosomes. In humans, there are two forms of sex chromosomes: the X chromosome and Y chromosome. A pair of X and Y results in a male while a combination of X and X results in a female. This XX/XY sex-determination system is one of the most familiar sex-determination systems and is applicable in human beings and most other mammals. In this system, the sex of an organism is determined by the sex chromosomes since these chromosomes bear the genes that control the development of reproductive organs and other sexual characteristics of an organism.

F1 and F2 generation

F1- (genetics) The first filial generation, which is comprised of offspring(s) resulting from a cross between strains of distinct genotypes. Sentence:The F1 generation is the generation resulting immediately from a cross of the first set of parents. F2- (genetics) The second filial generation, which is comprised of offspring(s) resulting from a cross of the members of F1 generation. Sentence: The F2 generation is the result of a cross between two F1 individuals (from F1 generation).

Homologous chromosome pair

Homologous chromosomes are made up of chromosome pairs of approximately the same length, centromere position, and staining pattern, for genes with the same corresponding loci. One homologous chromosome is inherited from the organism's mother; the other is inherited from the organism's father. Sentence: n a biological cell, a chromosome pairs with another chromosome during meiosis. This pairing (synapsis) happens between two chromosomes that are homologous, i.e. chromosomes having the same genes at the same loci but possibly different alleles. For example, two chromosomes may have genes encoding eye color, but one may code for brown eyes, the other for blue. A human cell contains 23 pairs of homologous chromosomes: 22 of them are homologous non-sex chromosomes (or autosomes) and 1 homologous pair of sex chromosomes. In females, the homologous sex chromosomes are 2 X's; in males the X and Y chromosomes.

Dominant

Of, or pertaining to) An allele or a gene that is expressed in an organism's phenotype, masking the effect of the recessive allele or gene when present. Sentence: In genetics, the dominant is the one that determines the phenotype of an organism. Its effects are readily recognized than the effects of the recessive. Usually, a dominant allele is symbolized with a capital letter and the recessive allele a small letter, for example: Hh (where H refers to the dominant allele while h to the recessive allele).

Gene

The fundamental, physical, and functional unit of heredity. Sentence: It is because a gene is a segment of DNA (on a specific site on a chromosome) that is responsible for the physical and inheritable characteristics or phenotype of an organism. It also specifies the structure of a protein, and an RNA molecule.