Principles of Inheritance and Variation

Vocabulary flashcards covering the principles of inheritance and variation, including Mendelian genetics, chromosomal theory, and genetic disorders based on the lecture notes.

Genetics

The branch of biology that deals with the inheritance as well as the variation of characters from parents to offspring.

Inheritance

The process by which characters are passed on from parent to progeny; it is the basis of heredity.

Variation

The degree by which progeny differ from their parents.

James Dewey Watson

An American scientist born in Chicago in 1928 who proposed the complementary double-helical configuration for DNA in March 1953 alongside Francis Crick.

Francis Harry Compton Crick

A British physicist born in 1916 who proposed the double-helical structure for DNA and the replication scheme in 1953.

Gregor Mendel

A scientist who conducted hybridisation experiments on garden peas for seven years ($1856-1863$) and proposed the laws of inheritance in living organisms.

True-breeding line

A line that, having undergone continuous self-pollination, shows stable trait inheritance and expression for several generations.

Factors

The term used by Mendel to describe discrete units, now known as genes, which are passed down unchanged from parent to offspring through gametes.

Alleles

Genes which code for a pair of contrasting traits; they are slightly different forms of the same gene.

Homozygous

A condition in which the allelic pair of genes for a particular trait are identical, such as $TT$ or $tt$.

Heterozygous

A condition where an organism contains a pair of alleles that express contrasting traits, such as $Tt$.

Genotype

The genetic makeup of an organism, represented by allelic symbols like $TT$, $Tt$, or $tt$.

Phenotype

The observable physical characteristics of an organism, such as being tall or dwarf.

Dominant Factor

One member of a dissimilar pair of factors that dominates the other and is expressed in the $F_1$ generation.

Recessive Factor

In a pair of dissimilar factors, the one that is suppressed and not seen at the $F_1$ stage but may reappear in the $F_2$ generation.

Monohybrid cross

A genetic cross between parents that differ in only one character, such as height ($TT \times tt$).

Punnett Square

A graphical representation developed by Reginald C. Punnett to calculate the probability of all possible genotypes of offspring in a genetic cross.

Test cross

A cross where an organism showing a dominant phenotype (whose genotype is unknown) is crossed with its recessive parent to determine the unknown genotype.

Law of Dominance

States that characters are controlled by discrete units called factors which occur in pairs; in a dissimilar pair, one factor dominates the other.

Law of Segregation

States that alleles do not show blending and segregate during gamete formation so that a gamete receives only one of the two factors.

Incomplete Dominance

A form of inheritance where the $F_1$ phenotype is intermediate between the two parents, as seen in flower color of the snapdragon ($Antirrhinum \text{ sp.}$).

Co-dominance

A condition in which both alleles in a heterozygote find expression, such as in the ABO blood grouping in human beings where both $I^A$ and $I^B$ express their own sugars.

Multiple Alleles

A condition where more than two alleles govern the same character, which can only be observed through population studies, such as ABO blood groups.

Law of Independent Assortment

States that when two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair of characters.

Chromosomal Theory of Inheritance

Proposed by Walter Sutton and Theodore Boveri, it argues that the pairing and separation of chromosomes lead to the segregation of a pair of factors they carry.

Linkage

A term coined by Thomas Hunt Morgan to describe the physical association of genes on the same chromosome.

Recombination

A term coined by Thomas Hunt Morgan to describe the generation of non-parental gene combinations.

Polygenic inheritance

Traits controlled by three or more genes where the phenotype reflects the additive contribution of each allele, such as human skin color.

Pleiotropy

A phenomenon where a single gene can exhibit multiple phenotypic expressions, such as the mutation causing phenylketonuria in humans.

Male heterogamety

A sex determination system where males produce two different types of gametes, such as $XO$ or $XY$ types found in humans and Drosophila.

Female heterogamety

A sex determination system where females produce two different types of gametes (e.g., $Z$ and $W$ chromosomes in birds).

Haplodiploid sex-determination

A system found in honey bees where females are diploid ($32$ chromosomes) and males (drones) are haploid ($16$ chromosomes).

Mutation

A phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and phenotype of an organism.

Point mutation

A mutation arising due to a change in a single base pair of DNA, such as in sickle cell anemia.

Pedigree Analysis

The study of the family history regarding the inheritance of a particular trait over several generations represented in a family tree.

Aneuploidy

The gain or loss of a chromosome(s) due to the failure of segregation of chromatids during the cell division cycle.

Polyploidy

An increase in a whole set of chromosomes in an organism due to failure of cytokinesis after the telophase stage of cell division.

Down’s Syndrome

A chromosomal disorder caused by the presence of an additional copy of chromosome number $21$ (trisomy of $21$), characterized by short stature and mental retardation.

Klinefelter’s Syndrome

A genetic disorder caused by the presence of an additional copy of X-chromosome resulting in a karyotype of $47, XXY$, characterized by sterile individuals with feminine expressions.

Turner’s Syndrome

A chromosomal disorder caused by the absence of one X chromosome ($45$ with $X0$), leading to sterile females with rudimentary ovaries.