Chapters 1-5 Definition (chad)

Genetics

Study of heredity and variation in organisms

Chromosomal Theory of Inheritance

Genes are carried on chromosomes, and their behavior during meiosis explains inheritance

Epigenesis

Theory that an organism develops from a fertilized egg through developmental events (contrasts preformation)

Preformation

Old theory that a fertilized egg contains a miniature adult (homunculus)

Natural Selection

Process where individuals with advantageous traits survive and reproduce more successfully

Descent with Modification

Darwin’s idea that species evolve over time from common ancestors

Transformation (Griffith)

Process by which genetic material is transferred between bacteria, altering phenotype

DNA as Genetic Material

Discovered by Avery, MacLeod, and McCarty (1944) — DNA, not protein, carries hereditary information

Double Helix

Model of DNA structure proposed by Watson and Crick (1953)

Mitosis

Type of cell division that produces two genetically identical daughter cells

Meiosis

Type of cell division that produces four genetically diverse haploid gametes

Homologous Chromosomes

Chromosome pairs, one from each parent, that are similar in shape, size, and gene content

Sister Chromatids

Two identical copies of a chromosome connected at the centromere

Centromere

Region where sister chromatids are joined and spindle fibers attach

Crossing Over

Exchange of genetic material between nonsister chromatids during Prophase I of meiosis

Independent Assortment

Random distribution of homologous chromosomes during Metaphase I of meiosis

Synapsis

Pairing of homologous chromosomes during Prophase I

Tetrad

Structure of four chromatids (two homologous chromosomes, each with two sister chromatids)

Gamete

Haploid reproductive cell (sperm or egg)

Polar Body

Small nonfunctional cell produced during oogenesis

Allele

Alternative form of a gene

Genotype

Genetic makeup of an organism (the alleles present)

Phenotype

Observable traits of an organism

Homozygous

Genotype with two identical alleles (AA or aa)

Heterozygous

Genotype with two different alleles (Aa)

Dominant Allele

Allele that masks the expression of a recessive allele in a heterozygote

Recessive Allele

Allele whose expression is masked in a heterozygote

Law of Segregation

Mendel’s principle that alleles separate during gamete formation

Law of Independent Assortment

Mendel’s principle that different gene pairs assort independently in gametes

Testcross

Cross used to determine an unknown genotype by mating with a homozygous recessive

Monohybrid Cross

Cross involving one trait (Aa × Aa)

Dihybrid Cross

Cross involving two traits (AaBb × AaBb)

Chi-Square Test

Statistical test used to compare observed vs. expected genetic ratios

Incomplete Dominance

When the heterozygote phenotype is intermediate (blending) between two homozygotes

Codominance

When both alleles in a heterozygote are fully expressed (e.g., AB blood type)

Multiple Alleles

When more than two allele forms exist for a gene (e.g., ABO blood group)

Epistasis

Interaction between genes where one masks or modifies the effect of another

Complementation

When two recessive mutations in different genes restore a wild-type phenotype when combined

Lethal Allele

Allele that causes death when present in certain genotypes

Pleiotropy

Single gene affects multiple traits (e.g., Marfan syndrome)

Penetrance

Proportion of individuals with a genotype who actually express the expected phenotype

Expressivity

Degree to which a genotype is expressed in an individual

Sex Determination

Process that directs development into male or female

SRY Gene

“Sex-determining Region on the Y” gene that initiates male development

Pseudoautosomal Region (PAR)

Regions of homology on X and Y chromosomes that allow pairing during meiosis

X-Inactivation

Random silencing of one X chromosome in female mammals to balance dosage

Lyon Hypothesis

Proposes that one X chromosome is randomly inactivated in female cells early in development

Barr Body

Inactivated X chromosome visible in the nucleus of female cells

Dosage Compensation

Mechanism that balances expression of X-linked genes between males and females

Turner Syndrome

Disorder with karyotype 45,X (sterile female, short stature)

Klinefelter Syndrome

Disorder with karyotype 47,XXY (male, tall, sterile)

Trisomy X

Disorder with karyotype 47,XXX (phenotypically normal female, sometimes reduced IQ)

Double-Y Syndrome

Disorder with karyotype 47,XYY (tall male, sometimes antisocial behavior)

Haplodiploidy

Sex-determination system in bees/ants/wasps where males are haploid and females are diploid

Temperature-Dependent Sex Determination

Sex determined by environmental temperature (seen in reptiles like turtles and crocodiles)

Xist Gene

Gene required for X inactivation