Principles of Inheritance and Variation Flashcards

Vocabulary flashcards covering fundamental terms, laws, Mendelian crosses, non-Mendelian inheritance, chromosomal theory, sex determination, mutations, pedigree analysis, and genetic disorders from the lecture notes.

Genetics

The study of inheritance, heredity, and variation of characters, or the study of genes and chromosomes.

Inheritance

The transmission of characters from parents to progeny, which serves as the basis of heredity.

Variation

The difference between parents and offspring.

Character

A heritable feature among parents and offspring, such as eye colour.

Trait

Variants of a character, such as brown eye or blue eye.

Alleles

Alternative forms of a gene, such as $T$ (tall) and $t$ (dwarf) for the character height.

Homozygous

The condition in which a chromosome pair carries similar alleles of a gene, also known as pure line or true breeding.

Heterozygous

The condition in which a chromosome pair carries dissimilar alleles of a gene.

Dominant character

The character which is expressed in heterozygous condition, indicated by a capital letter.

Recessive character

The character which is suppressed in heterozygous condition, indicated by a small letter.

Phenotype

The physical expression of a character.

Genotype

The genetic constitution of a character.

Hybrid

An individual produced by mating of genetically unlike parents.

Gregor Mendel

Known as the Father of Genetics; conducted hybridization experiments on garden peas (Pisum sativum) for 7 years from 1856 to 1863.

Emasculation

The removal of anthers of one plant to avoid self-pollination during artificial cross-pollination experiments.

Monohybrid cross

A cross involving two plants differing in one character pair.

Monohybrid phenotypic ratio

The ratio of physical characteristics in a monohybrid cross F2 generation, which is $3:1$ (3 Tall : 1 Dwarf).

Monohybrid genotypic ratio

The ratio of genetic constitutions in a monohybrid cross F2 generation, which is $1:2:1$ (1 Homozygous tall : 2 Heterozygous tall : 1 Homozygous dwarf).

Back cross

A cross between a hybrid and any of its parents.

Test cross

The crossing of an organism with a dominant phenotype to a recessive individual to determine its unknown genotype, producing a $1:1$ phenotypic ratio for a monohybrid cross.

Law of Dominance

Mendel's First Law stating that characters are controlled by discrete units called factors occurring in pairs, and in a dissimilar pair, one factor dominates over the recessive one.

Law of Segregation

Mendel's Second Law stating that during gamete formation, the factors (alleles) of a character pair segregate from each other so that a gamete receives only one factor.

Dihybrid cross

A genetic cross between two parents differing in 2 pairs of contrasting characters.

Dihybrid F2 phenotypic ratio

The ratio of phenotypic combinations in a dihybrid F2 generation, which is $9:3:3:1$ (9 Round yellow : 3 Round green : 3 Wrinkled yellow : 1 Wrinkled green).

Dihybrid F2 genotypic ratio

The genotypic ratio of a dihybrid F2 generation, represented as $1:2:1:2:4:2:1:2:1$.

Law of Independent Assortment

Mendel's Third Law stating that when more than one pair of characters are involved in a cross, factor pairs independently segregate from the other pair of characters.

Incomplete Dominance

Inheritance in which the offspring shows an intermediate character between two parental characteristics, yielding identical phenotypic and genotypic ratios of $1:2:1$.

Co-dominance

Inheritance in which both alleles of a gene are fully expressed in a hybrid, as seen in human ABO blood grouping.

Multiple Allelism

The presence of more than two alleles of a gene governing the same character within a population, such as alleles $I^A$, $I^B$, and $i$ in ABO blood grouping.

Polygenic Inheritance

Inheritance in which a single trait is controlled by several genes, where each allele has an additive effect and phenotype is influenced by the environment.

Pleiotropy

A phenomenon in which a single gene exhibits multiple phenotypic expressions, usually through its effect on metabolic pathways.

Chromosomal Theory of Inheritance

Proposed by Walter Sutton and Theodore Boveri in 1902, stating that the pairing and separation of a pair of chromosomes leads to the segregation of a pair of factors they carry.

Linkage

The physical association of two or more genes on the same chromosome, causing them to be inherited together without segregating independently.

Recombination

The generation of non-parental gene combinations arising due to independent assortment or crossing over.

Male Heterogamety

A sex determination system where males produce two distinct types of gametes, seen in $XX-XO$ and $XX-XY$ mechanisms.

Female Heterogamety

A sex determination system where females produce two distinct types of gametes, seen in the $ZZ-ZW$ mechanism in birds.

Haplodiploid sex determination system

A mechanism found in honeybees where diploid females ($32$ chromosomes) develop from fertilized eggs and haploid males ($16$ chromosomes) develop from unfertilized eggs via parthenogenesis.

Point mutation

A mutation resulting from a change in a single base pair of DNA, such as in sickle cell anaemia.

Frameshift mutation

A mutation caused by the deletion or insertion of base pairs that alters the reading frame of DNA sequences.

Mutagens

Physical or chemical agents that induce genetic mutations.

Pedigree Analysis

The analysis of genetic traits across several generations of a family represented in a family tree chart.

Mendelian Disorders

Genetic disorders caused by an alteration or mutation in a single gene.

Haemophilia

A sex-linked ($X$-linked) recessive blood disorder in which a protein involved in blood clotting is affected, leading to non-stop bleeding from simple cuts.

Colour blindness

An $X$-linked recessive disorder caused by a defect in red or green retinal cones, resulting in the failure to discriminate between red and green colors.

Sickle cell anaemia

An autosome-linked recessive trait caused by the substitution of Glutamic acid by Valine at the 6th position of the $\beta$-globin chain due to a single base change from GAG to GUG.

Phenylketonuria (PKU)

An autosomal recessive inborn error of metabolism caused by mutation in the gene coding for phenylalanine hydroxylase, leading to mental retardation due to phenylalanine accumulation.

Thalassemia

An autosome-linked recessive blood disease caused by mutation or deletion leading to reduced synthesis of one of the $\alpha$ or $\beta$ globin chains of hemoglobin.

Aneuploidy

The gain or loss of individual chromosomes due to the failure of chromatid segregation during cell division.

Polyploidy

An increase in a whole set of chromosomes resulting from the failure of cytokinesis after the telophase stage of cell division.

Down's syndrome

A chromosomal disorder caused by trisomy of chromosome 21 ($47$ total chromosomes), characterized by short stature, furrowed big tongue, simian crease, and mental retardation.

Klinefelter's syndrome

A chromosomal disorder caused by an additional copy of the X-chromosome in males ($44A + XXY$), leading to overall masculine development with gynaecomastia and sterility.

Turner's syndrome

A chromosomal disorder caused by the absence of one X-chromosome in females ($44A + XO$), leading to sterility, rudimentary ovaries, dwarfism, and webbed neck.