Genetics - Unit Ass. 1

A

Allele – A variant form of a gene at a specific locus on a chromosome. Organisms inherit two alleles for each gene, one from each parent.

Autosomal – Refers to genes located on autosomes (non-sex chromosomes). Humans have 22 pairs of autosomes.

Barr Bodies – Inactivated X chromosomes found in female mammalian cells, ensuring dosage compensation between males and females.

Blending Inheritance – An outdated theory that offspring traits result from a smooth mix of parental traits, rather than distinct gene inheritance.

C

Centimorgan (cM) – A unit of genetic map distance representing a 1% recombination frequency between two loci.

Centromere – The region of a chromosome where sister chromatids are joined and where spindle fibers attach during cell division.

Chiasma – The physical site where homologous chromosomes exchange genetic material during crossing over in meiosis.

Chi-square Test – A statistical test used in genetics to compare observed and expected ratios (e.g., Mendelian inheritance) and determine if differences are due to chance.

  • Degrees of Freedom (df) – Calculated as number of categories−1\text{number of categories} - 1number of categories−1.

  • Critical Values – Values from a chi-square distribution table used to determine statistical significance.

Chromatids

  • Sister Chromatids – Identical copies of a chromosome, connected at the centromere.

  • Non-sister Chromatids – Chromatids from homologous chromosomes, which can exchange genetic material during crossing over.

Chromosome – A DNA molecule containing genetic information; humans have 46 chromosomes (23 pairs).

Coefficient of Coincidence – A measure of how often double crossovers occur compared to expected frequency, calculated as Observed DCOExpected DCO\frac{\text{Observed DCO}}{\text{Expected DCO}}Expected DCOObserved DCO​.

Cohesin – A protein that holds sister chromatids together until they separate during mitosis or meiosis.

Complementation – A genetic test where mutations in different genes are combined to see if they restore a wild-type phenotype.

Crossing Over – The exchange of genetic material between homologous chromosomes during meiosis, increasing genetic diversity.

D

Diploid (2n) – A cell containing two sets of chromosomes, one from each parent (e.g., human somatic cells, 46 chromosomes).

Dominance

  • Complete Dominance – One allele completely masks the effect of another.

  • Incomplete (Partial) Dominance – Heterozygotes show an intermediate phenotype (e.g., red + white flower = pink).

  • Co-dominance – Both alleles are fully expressed in heterozygotes (e.g., ABO blood type).

Dosage Compensation – Mechanism that equalizes gene expression between sexes, such as X-inactivation in mammals.

Double Crossover (DCO) – Two crossover events between homologous chromosomes during meiosis.

E

Epistasis – When one gene affects or masks the expression of another gene (e.g., coat color in labrador retrievers).

F

Forked-line Diagram – A method used to predict genetic outcomes for multiple traits using branching pathways instead of a Punnett square.

G

Gamete – A haploid reproductive cell (sperm or egg) carrying one set of chromosomes.

Gene – A DNA sequence that codes for a protein or functional RNA.

Gene-environment Interaction – When environmental factors influence gene expression and phenotypic traits (e.g., temperature-sensitive fur color in Siamese cats).

Genetic Linkage – The tendency of genes located close together on a chromosome to be inherited together.

Genotype – The genetic makeup of an organism, represented by allele combinations (e.g., AA, Aa, or aa).

Germline – The lineage of cells that lead to gametes; mutations in germline cells are heritable.

H

Haploid (1n) – A cell with one set of chromosomes (e.g., gametes, 23 chromosomes in humans).

Haplosufficiency – When a single functional allele is enough to maintain a normal phenotype.

Hemizygous – Having only one allele for a gene, as in males for X-linked genes (XY).

Homozygous and Heterozygous

  • Homozygous – Having two identical alleles (AA or aa).

  • Heterozygous – Having two different alleles (Aa).

I

Incomplete Penetrance – When individuals with a genotype do not always express the expected phenotype.

Interference – A phenomenon where one crossover event reduces the likelihood of another occurring nearby.

K

Kinetochore – A protein complex on the centromere where spindle fibers attach during cell division.

M

Map Unit (m.u.) – Another term for a centimorgan (cM), used to measure genetic distances.

Mendel’s Laws

  • Law of Segregation – Each individual has two alleles for a gene, and they separate during gamete formation.

  • Law of Independent Assortment – Genes on different chromosomes assort independently during meiosis.

Meiosis and Mitosis

  • Meiosis – Cell division that reduces chromosome number by half, producing gametes.

  • Mitosis – Cell division producing identical daughter cells.

Monohybrid and Dihybrid Crosses –

  • Monohybrid Cross – Cross involving one gene (Aa × Aa).

  • Dihybrid Cross – Cross involving two genes (AaBb × AaBb).

Multiplication and Addition Rule (Probability)

  • Multiplication Rule – Probability of independent events occurring together (AND rule).

  • Addition Rule – Probability of mutually exclusive events (OR rule).

N

Mutation – A change in DNA sequence that may affect gene function.

Mutant – An organism or allele carrying a mutation.

Non-disjunction – Failure of chromosomes to separate properly during meiosis or mitosis, leading to aneuploidy.

P

Pangenesis – A discredited hypothesis that genetic information is passed through body cells rather than gametes.

Parental vs. Recombinant –

  • Parental Gametes – Gametes with the same allele combination as parents.

  • Recombinant Gametes – Gametes with new allele combinations due to crossing over.

Particulate Theory – Mendel’s concept that traits are inherited as discrete units (genes), not blended.

Pedigree – A chart used to analyze inheritance patterns.

Phase of Linked Genes – Whether linked genes are in cis (same chromosome) or trans (opposite chromosomes).

Phenotype – The observable characteristics of an organism.

Pleiotropy – When one gene influences multiple phenotypic traits.

Punnett Square – A diagram used to predict genetic crosses.

Q

Quantitative Traits – Traits influenced by multiple genes (polygenic), such as height or skin color.

R

Recessive – An allele that is only expressed in the homozygous state.

Reciprocal Cross – A genetic cross where male and female traits are reversed to test for sex-linked inheritance.

Recombination – The exchange of genetic material during crossing over.

Recombination Frequency – The percentage of recombinant offspring, used to estimate gene distance.

S

Single Crossover (SCO) – A crossover event between homologous chromosomes.

Sister Chromatid – Identical copies of a chromosome formed during replication.

Syntenic – Genes located on the same chromosome.

T–W

Test Cross – A cross with a homozygous recessive individual to determine genotype.

Three-point Test Cross – A genetic cross used to determine gene order and map distances.

True Breeding – Organisms that consistently pass the same traits to offspring.

Variable Expressivity – When individuals with the same genotype show different phenotypic intensities.

Wild Type – The most common phenotype or allele in a population.

X-linked Traits

X-inactivation – The process of silencing one X chromosome in female mammals.

Xist Gene – A gene responsible for X-inactivation.