Inheritance, Genes and Chromosomes

Flashcards covering Mendelian laws, allele and gene interactions, chromosomal inheritance, and extra-nuclear genes based on the lecture transcript.

Mendel’s first law (Law of Segregation)

States that two copies of a gene segregate during gamete formation such that each gamete receives only one member of a pair.

Mendel’s second law (Law of Independent Assortment)

States that alleles of different genes assort independently during gamete formation.

Theory of particulate inheritance

The theory whereby each determinant for a characteristic had a physically distinct nature and that determinants remained intact after fertilization.

Theory of blending inheritance

The assumption that hereditary determinants blend in the offspring and, once blended, could never be separated.

Character

An observable physical feature, such as flower color.

Trait

A particular form of a character, such as purple flowers or white flowers.

True-breeding

Describes a trait that is the only one present for many generations; these strains are isolated by inbreeding and selection.

P generation

The parental generation.

F1 generation

The first filial generation, consisting of the resulting offspring from a cross of the parental (P) generation.

F2 generation

The second filial generation, produced if $F_1$ plants self-pollinate.

Monohybrid cross

A cross between individuals that involves one pair of contrasting traits.

Recessive trait

A trait that disappears in the $F_1$ generation and reappears in the $F_2$ generation.

Dominant trait

The trait that appears in the $F_1$ generation.

Particulate theory

Mendel’s proposal that heritable units are discrete particles, now known to be sequences of DNA carried on chromosomes.

Genome

The totality of all genes in an organism.

Alleles

Different forms of a gene that produce different traits.

Gene

The fundamental unit of inheritance; short sequences of DNA that reside at a particular locus on a chromosome.

Homozygous

Having two copies of the same allele (e.g., $ss$).

Heterozygous

Having two different alleles for a particular gene (e.g., $Ss$).

Phenotype

The physical appearance of an organism, such as spherical seeds.

Genotype

The genetic makeup of an organism (e.g., $Ss$).

Locus

A particular site on a chromosome where a gene resides.

Test cross

A method to determine whether an individual showing a dominant trait is homozygous or heterozygous by crossing it with a homozygous recessive individual.

Dihybrid cross

A cross between individuals that are identical double heterozygotes for two characters.

Recombinant phenotypes

Phenotypes resulting from independent assortment that differ from those of the parental generation.

Multiplication rule

A rule of probability stating that the probability of two independent events happening together is found by multiplying the probabilities of the individual events (e.g., $1/2 \times 1/2 = 1/4$).

Addition rule

A rule of probability stating that the probability of an event that can occur in two different ways is the sum of the individual probabilities.

Pedigrees

Family trees that show the occurrence of phenotypes in several generations of related individuals.

Mutation

Rare, stable, inherited changes in the genetic material through which different alleles arise.

Wild type

The allele present in most of the population.

Polymorphic

A locus with a wild-type allele present less than $99\%$ of the time.

Incomplete dominance

A condition where some alleles are neither dominant nor recessive, resulting in a heterozygote with an intermediate phenotype.

Codominance

A condition where two alleles at one locus produce phenotypes that are both present in the heterozygote, such as in the ABO blood group system.

Pleiotropic

Describes a single allele that has multiple phenotypic effects, such as the allele for coloration in Siamese cats also resulting in crossed eyes.

Epistasis

The phenomenon where the phenotypic expression of one gene is influenced by another gene, such as coat color in Labrador retrievers.

Penetrance

The proportion of individuals with a certain genotype that show the expected phenotype.

Expressivity

The degree to which a genotype is expressed in an individual.

Quantitative variation

Variation in which phenotypes vary continuously over a range rather than in discrete, qualitative categories; usually due to both genes and environment.

Quantitative trait loci

The genes that together determine complex, continuously varying characters.

Linked genes

Genes inherited together because their loci are on the same chromosome.

Linkage group

All the loci located on a specific chromosome.

Crossing over

The process by which genes on the same chromosome recombine during prophase I of meiosis.

Autosomes

All chromosomes in an organism that are not sex chromosomes.

Sex chromosomes

A pair of chromosomes that determine the sex of an individual (e.g., XX for female mammals, XY for male mammals).

Nondisjunction

When a pair of sister chromosomes fail to separate in meiosis I, or a pair of sister chromatids fail to separate in meiosis II.

Aneuploidy

A condition resulting from nondisjunction where there is an abnormal number of chromosomes.

Hemizygous

Having only one copy of a gene, such as genes on the X chromosome in human males.

Sex-linked inheritance

Inheritance patterns governed by loci specifically on the sex chromosomes.

Cytoplasmic inheritance

The transmission of genes located in organelles like mitochondria and plastids, which are typically inherited from the mother.