AP Biology unit 5- Heredity

terms and definitions

Meiosis

A two-step cell division process that reduces chromosome number from diploid to haploid and produces genetically unique gametes.

Homologous Chromosomes

Chromosome pairs with the same genes at the same loci but possibly different alleles.

Crossing Over

Exchange of genetic material between nonsister chromatids during Prophase I, increasing genetic variation.

Chiasmata

The physical locations where crossing over occurs.

Independent Assortment

Random orientation of homologous chromosome pairs during Metaphase I, creating variation.

Random Fertilization

Random pairing of any sperm with any egg, contributing to genetic diversity.

Nondisjunction

Failure of chromosomes or chromatids to separate properly during meiosis.

Aneuploidy

Abnormal number of chromosomes (e.g., trisomy 21).

Mendel’s Law of Segregation

Alleles separate during gamete formation so each gamete gets only one allele.

Mendel’s Law of Independent Assortment

Genes on different chromosomes assort independently during meiosis.

Genotype

The alleles an organism possesses (AA, Aa, aa).

Phenotype

The observable traits of an organism.

Homozygous

Having two identical alleles for a gene.

Heterozygous

Having two different alleles for a gene.

Monohybrid Cross

A cross examining a single trait; expected phenotype ratio 3:1.

Test Cross

Crossing an organism with an unknown genotype with a recessive individual to determine genotype.

Incomplete Dominance

Heterozygotes show an intermediate phenotype.

Codominance

Both alleles are fully expressed in the phenotype (e.g., AB blood type).

Multiple Alleles

A gene with more than two possible alleles (e.g., blood type).

Polygenic Inheritance

A trait influenced by many genes, producing a range of phenotypes.

Pleiotropy

A single gene influencing multiple traits.

Epistasis

One gene affects or masks the expression of another gene.

Linked Genes

Genes located close together on the same chromosome; inherited together.

Recombination Frequency

Percentage of recombinant offspring used to estimate gene distance.

Genetic Map

Diagram showing gene positions based on recombination frequencies.

Sex-Linked Trait

A trait determined by a gene on a sex chromosome, usually the X.

X-Linked Recessive

More common in males; females may be carriers.

Pedigree

A chart showing inheritance of traits across generations.

Chi-Square Test

Statistical test used to compare observed vs. expected genetic ratios.

Chi-Square Formula

χ²

Null Hypothesis (Genetics)

States that observed results match expected ratios.

Map Unit (Centimorgan)

Equivalent to 1% recombination frequency; measures gene distance.