Unit 5: Heredity

Heredity

The passing of genetic information from one generation to the next.

Chromosome

A DNA molecule packaged with proteins (mainly histones) that carries many genes.

Gene

A DNA sequence that influences a trait.

Locus

The specific position of a gene on a chromosome.

Diploid (2n)

Having two sets of chromosomes (typically one set from each parent).

Haploid (n)

Having one set of chromosomes; typical of gametes.

Gamete

A haploid sex cell (sperm or egg) that contributes one chromosome set to a zygote.

Zygote

The diploid cell formed when two haploid gametes fuse during fertilization.

Homologous chromosomes

A maternal and paternal chromosome pair with the same genes in the same locations but potentially different alleles.

Allele

A different version of the same gene.

Sister chromatids

Identical DNA copies of a replicated chromosome, attached to each other at the centromere.

Centromere

The chromosome region where sister chromatids are attached and where they separate during division.

Meiosis

A cell division process that produces haploid gametes from diploid cells; includes two divisions after one DNA replication.

Meiosis I

The first meiotic division (reduction division) in which homologous chromosomes separate.

Meiosis II

The second meiotic division, similar to mitosis, in which sister chromatids separate.

Synapsis

Pairing of homologous chromosomes side-by-side in prophase I of meiosis.

Tetrad (bivalent)

The paired homologous chromosomes during synapsis; contains four chromatids total.

Crossing over

Exchange of corresponding DNA segments between non-sister chromatids in prophase I, creating recombinant chromatids.

Independent assortment

Random orientation of homologous pairs in metaphase I, producing different combinations of maternal and paternal chromosomes in gametes.

Random fertilization

Genetic variation produced because any sperm can fuse with any egg, multiplying possible allele combinations.

Mutation

A change in DNA (or chromosome-level change) that creates new alleles; the ultimate source of new alleles.

Genotype

The allele combination an organism carries for a gene.

Phenotype

The observable trait, influenced by genotype and often the environment.

Homozygous

Having two identical alleles for a gene.

Heterozygous

Having two different alleles for a gene.

Dominant allele

An allele whose phenotype appears in the heterozygote (in complete dominance scenarios).

Recessive allele

An allele whose phenotype typically appears only when two copies are present (in complete dominance scenarios).

Law of Dominance

In Mendel’s peas, one allele can mask the effect of another in the heterozygote; traits do not necessarily blend.

Law of Segregation

Each gamete receives only one allele for each gene because homologous chromosomes separate during meiosis.

Law of Independent Assortment (Mendel)

Alleles of different genes can mix and match in gametes when genes are unlinked (on different chromosomes or far apart).

Testcross

A cross between an individual with a dominant phenotype but unknown genotype and a homozygous recessive individual to reveal the unknown genotype.

Incomplete dominance

A dominance pattern where the heterozygote phenotype is intermediate between the two homozygotes; heterozygote crosses often show a 1:2:1 phenotype ratio.

Codominance

A dominance pattern where both alleles are fully expressed in the heterozygote (e.g., AB blood type shows both A and B antigens).

Multiple alleles

A situation where a gene has more than two allele forms in a population (though each individual still has only two alleles).

Epistasis

A gene interaction where one gene affects the expression of another gene, often by acting upstream in a pathway, modifying expected ratios.

Polygenic inheritance

Inheritance in which many genes contribute to a single trait, often producing continuous variation (a range of phenotypes).

Pleiotropy

When one gene affects multiple traits, producing multiple phenotypic effects from a single allele.

Penetrance

The proportion of individuals with a given genotype who actually show the associated phenotype.

Expressivity

The degree or intensity of a phenotype among individuals who express the same genotype.

Genetic linkage

The tendency of genes close together on the same chromosome to be inherited together, deviating from independent assortment.

Recombination frequency

The percent of offspring that are recombinants: (recombinant offspring ÷ total offspring) × 100; used to infer gene distance.

Chi-square goodness-of-fit test

A statistical test comparing observed offspring counts to expected counts from a genetic model using χ² = Σ((O−E)²/E).

Nondisjunction

Failure of chromosomes to separate properly during meiosis (homologs in meiosis I or sister chromatids in meiosis II), producing abnormal gametes.

Aneuploidy

An abnormal number of particular chromosomes (e.g., 2n+1 or 2n−1) often affecting phenotype via gene dosage changes.

Karyotype

A display of an individual’s chromosomes arranged by size/shape/banding; used to detect large-scale chromosome number changes like aneuploidy.

X-linked gene

A gene located on the X chromosome; recessive X-linked traits are more common in males because they have only one X.

Hemizygous

Having only one copy of a gene in a diploid organism (e.g., males for most X-linked genes: XY).

X-inactivation

Dosage compensation in female mammals where one X chromosome is randomly inactivated early in development, creating mosaic expression.

Pedigree

A family tree diagram tracking a trait across generations to infer inheritance patterns (e.g., autosomal vs X-linked; dominant vs recessive).

Mitochondrial inheritance

Inheritance of mitochondrial DNA, typically through the maternal line because the egg contributes most cytoplasm/organelles; affected mothers can pass to all children.