BIOL 100: Topic 12 P1 - Genetics and Inheritance

Genetics

The study of how traits are inherited through the actions of genes

Gene

A segment of DNA that codes for a specific protein or function

Locus

The specific physical location of a gene on a chromosome

Alleles

Different versions of a gene that may result in different traits

Homozygous

Two identical alleles for a given gene

Heterozygous

Two different alleles for a given gene

Genotype

The genetic makeup of an organism (combination of alleles)

Phenotype

The physical or observable traits of an organism

Chromosome

A structure of DNA and proteins carrying genetic information

Somatic Cell

A diploid body cell that is not involved in reproduction

Gamete

A haploid reproductive cell (sperm or egg)

Diploid (2n)

A cell with two sets of chromosomes, one from each parent

Haploid (n)

A cell with only one set of chromosomes (e.g., gametes)

Fertilization

The fusion of two haploid gametes to form a diploid zygote

Zygote

A diploid cell formed by the union of two gametes

Homologous Chromosomes

Chromosomes of the same type, same genes at the same loci, one from each parent

Karyotype

A display of the chromosome pairs of a cell arranged by size and shape

Sex Chromosomes

Chromosomes involved in determining sex (X and Y in humans)

Autosomes

Non-sex chromosomes that occur in homologous pairs in diploid cells

Meiosis

A two-division process that reduces chromosome number by half and creates four haploid gametes

Purpose of Meiosis

To reduce the chromosome number and produce genetically unique gametes

Interphase (before meiosis)

DNA is replicated during the S phase so each chromosome has two sister chromatids

Prophase I

Homologous chromosomes pair up and undergo crossing over; nuclear envelope breaks down

Synapsis

The pairing of homologous chromosomes during prophase I

Tetrad

A group of four chromatids formed by the synapsis of two homologous chromosomes

Crossing Over

Exchange of genetic material between non-sister chromatids in a tetrad

Chiasmata

The visible site of crossing over between chromatids

Metaphase I

Homologous chromosomes line up along the metaphase plate in pairs

Independent Assortment

Random orientation of chromosome pairs during metaphase I; contributes to genetic variation

Anaphase I

Homologous chromosomes separate and move toward opposite poles

Telophase I

Two haploid nuclei form, each with chromosomes composed of sister chromatids

Cytokinesis I

Division of cytoplasm resulting in two haploid daughter cells

Interkinesis

Short rest phase between meiosis I and II; no DNA replication occurs

Prophase II

New spindles form in each haploid cell; chromosomes begin to move

Metaphase II

Chromosomes align along the metaphase plate in each haploid cell

Anaphase II

Sister chromatids are pulled apart to opposite poles of the cell

Telophase II

Nuclei form around the chromosomes at each pole; chromosomes decondense

Cytokinesis II

Cytoplasm divides producing four haploid genetically distinct gametes

Final Result of Meiosis

Four non-identical haploid gametes each with half the chromosome number

Genetic Variation in Meiosis

Occurs through crossing over (prophase I) and independent assortment (metaphase I)

Differences Between Meiosis and Mitosis

Meiosis has two divisions, produces haploid cells, and increases genetic variation; mitosis is one division producing identical diploid cells