Principles of Inheritance and Variation

Comprehensive vocabulary flashcards covering the principles of inheritance and variation, Mendelian genetics, chromosomal theory, mutations, and genetic disorders based on the lecture transcript.

Genetics

Branch of biology in which we study inheritance and variation of characters from parents to progeny.

Inheritance

Process by which characters transfer from parent to progeny.

Variation

The degree of changes between parents and progeny.

Homologous Chromosomes

Chromosomes that have the same shape, size, centromere position, and same loci for genes.

Haploid

An organism or cell having a single set of chromosomes with only one locus for each gene.

Diploid

An organism or cell having two sets of chromosomes and two loci for each gene.

Polyploid

An organism or cell having more than two sets of chromosomes and more than two loci for each gene.

Character

A feature of an individual, such as seed shape, height, or eye color.

Trait

The different variants of a character, such as round and wrinkled seeds, or tall and dwarf height.

Gene

The unit of inheritance; chemically it is a segment of DNA present on a chromosome that controls a character.

Allele

Different molecular forms of a gene; according to Mendel, only two molecular forms are possible for a gene.

Dominant Allele

An allele that is expressed in both homozygous and heterozygous conditions; generally represented by a capital letter.

Recessive Allele

An allele that is expressed only in the homozygous condition; generally represented by a small letter.

Homozygous

A condition where both alleles for a gene are of the same type (pure), such as $TT$ or $tt$.

Heterozygous

A condition where both alleles for a gene are of dissimilar types (impure), such as $Tt$.

Hemizygous

A condition in diploid organisms where only one copy of a gene is present, such as in male sex chromosomes ($XY$); recessive alleles can express in this state in the absence of a dominant allele.

Phenotype

The observable morphological features of an individual, such as tall, dwarf, or round.

Genotype

The genetic complement of an organism, such as $TT$, $Tt$, or $Rr$.

Phenocopy

When two organisms are phenotypically the same but genotypically different because one was influenced by the environment.

Monohybrid Cross

A genetic cross involving one pair of contrasting traits.

Dihybrid Cross

A genetic cross involving two pairs of contrasting traits.

Back Cross

A cross between a progeny and any one of its parents.

Test Cross

A cross between a progeny having a dominant phenotype and a homozygous recessive parent to determine the unknown genotype of the dominant individual.

Out Cross

A cross between a progeny ($F_1$ or $F_2$) and a homozygous dominant parent; used to eliminate recessive traits.

Reciprocal Cross

A cross where the sexes of the parents are reversed compared to a previous cross.

Punnett Square

A graphical representation used to calculate the probability of all possible genotypes of offspring in a genetic cross; given by British geneticist R.C. Punnett.

Law of Dominance

Mendel's first law stating that characters are controlled by discrete units called factors (genes) which occur in pairs; in a dissimilar pair, one is dominant and the other recessive.

Law of Segregation

Also called the Law of Purity of Gametes, it states that two factors of a character segregate during gamete formation so each gamete carries only one factor.

Law of Independent Assortment

Mendel's third law stating that the segregation of one pair of traits is independent of the other pair of traits in a hybrid.

Incomplete Dominance

A situation where the $F_1$ phenotype is intermediate between the dominant and recessive phenotypes, such as pink flowers in Snapdragon ($Antirrhinum$).

Co-dominance

A condition where both traits are equally expressed in the $F_1$ progeny, such as ABO blood groups or coat color in cattle.

Multiple Allelism

When a gene has more than two molecular forms (alleles), observed only in a population, such as the ABO blood group system.

Pleiotropy

A situation where a single gene or gene product regulates multiple phenotypic expressions, such as Phenylketonuria ($PKU$) or Starch synthesis in garden peas.

Lethal Gene

A gene that causes the death of the organism, usually in the embryonic stage, when present in a homozygous condition.

Qualitative Inheritance

Inheritance where a character is controlled by one or two genes and the effect of each allele is not additive; phenotypic expression is discrete.

Polygenic Inheritance

Also known as quantitative inheritance, where a character (like human skin color) is controlled by three or more genes that have an additive effect.

Chromosomal Theory of Inheritance

Proposed by Sutton and Boveri in 1902, stating that chromosomes are the carriers of Mendelian factors and follow the same patterns of segregation and assortment.

Linkage

The physical association of genes on the same chromosome; term given by T.H. Morgan.

Recombination

The generation of non-parental gene combinations due to crossing over during meiosis.

Linkage Group

A group of genes inherited together on a chromosome; the number of linkage groups corresponds to the haploid set of chromosomes ($n$).

Genetic Map

A representation of the distance between genes on a chromosome based on recombination frequency; unit is CentiMorgan ($cM$).

Male Heterogamety

A sex determination system where the male produces two different types of gametes, such as $XY$ in humans or $XO$ in grasshoppers.

Female Heterogamety

A sex determination system where the female produces two different types of gametes, such as $ZW$ in birds or $ZO$ in moths/butterflies.

Barr Body

Deactivated $X$ chromosomes in a cell; the number of Barr bodies is equal to the total number of $X$ chromosomes minus one ($X-1$).

Haplodiploid Sex Determination

A system found in honeybees where males (drones) are haploid ($n=16$) and females (queens/workers) are diploid ($2n=32$).

Parthenogenesis

The process by which an unfertilized egg develops into a male honeybee (drone).

Criss-Cross Inheritance

The pattern where a male transfers his trait to his grandson through his daughter, or a female transfers her trait to her granddaughter through her son.

Mutation

A sudden, discontinuous change in the genotype or phenotype of an organism caused by changes in DNA sequence or chromosomes.

Point Mutation

A gene mutation occurring due to a change in a single base pair, such as in sickle cell anemia.

Frame Shift Mutation

A mutation caused by the insertion or deletion of nucleotides that are not in multiples of three, changing the reading frame.

Aneuploidy

A genomic mutation where changes in chromosome number are not a multiple of the basic set, such as trisomy ($2n+1$) or monosomy ($2n-1$).

Euploidy

A genomic mutation where the chromosome number changes in multiples of the basic set ($n$, $3n$, $4n$, etc.).

Pedigree Analysis

The study of the inheritance of specific traits or disorders across several generations of a family using a chart.

Sickle Cell Anemia

An autosomal recessive disorder caused by a point mutation (substitution) where glutamic acid is replaced by valine at the $6^{th}$ position of the beta-globin chain.

Thalassemia

An autosomal recessive quantitative disorder involving the reduced synthesis of alpha or beta globin chains of hemoglobin.

Down Syndrome

A chromosomal disorder caused by the trisomy of chromosome number $21$, leading to short stature, mental retardation, and a furrowed tongue.

Turner's Syndrome

A chromosomal disorder in females caused by the absence of one $X$ chromosome ($44+XO$), resulting in sterility and underdeveloped feminine characters.

Klinefelter's Syndrome

A chromosomal disorder in males caused by an extra $X$ chromosome ($44+XXY$), leading to sterility, tall stature, and gynaecomastia.