Genetics and Meiosis: Key Concepts for Biology

gene

A segment of DNA that codes for a particular trait or protein — Ex: gene for eye color

homologous chromosomes

A pair of chromosomes (one from each parent) that have the same size, shape, and carry genes for the same traits — but may have different versions (alleles) of those genes

gamete

A reproductive sex cell (haploid) — Ex: sperm cell or egg cell

haploid

A cell containing only one set of chromosomes (n); half the full chromosome number — Ex: human sperm or egg cell — 23 chromosomes

diploid

A cell containing two complete sets of chromosomes (2n) — Ex: human somatic (body) cell — 46 chromosomes

meiosis

Cell division that produces 4 haploid gametes from one diploid cell; involves two divisions (Meiosis I and II)

crossing over

The exchange of genetic material between non-sister chromatids of homologous chromosomes during Prophase I; increases genetic diversity

trait

A characteristic that can be passed from parent to offspring — Ex: hair color, eye color, height

heredity

The passing of traits from parents to offspring

genetics

The branch of biology that studies heredity and variation in organisms

allele

Different versions of the same gene — Ex: allele for brown eyes vs. allele for blue eyes

dominant

An allele that masks the expression of the recessive allele; expressed when at least one copy is present — Written as a capital letter (e.g., B)

recessive

An allele whose expression is masked by the dominant allele; only expressed when two copies are present — Written as a lowercase letter (e.g., b)

homozygous

Having two identical alleles for a trait (either both dominant or both recessive) — Ex: BB or bb

heterozygous

Having two different alleles for a trait (one dominant, one recessive) — Ex: Bb

genotype

The genetic makeup of an organism; the alleles it carries — Ex: Bb, BB, bb

phenotype

The physical, observable expression of a genotype — Ex: brown eyes, tall height

law of segregation

Mendel's law stating that allele pairs separate during gamete formation; each gamete receives only one allele per gene

hybrid

An organism that is heterozygous for a trait (has two different alleles)

law of independent assortment

Mendel's law stating that genes for different traits are sorted independently during gamete formation (genes on different chromosomes)

true-breeding

An organism that always produces offspring with the same trait when self-pollinated (homozygous) — Ex: pea plants that always produce purple flowers

self-pollinating

A plant that can fertilize itself using its own pollen — Ex: Mendel's pea plants in controlled crosses

cross-pollinating

Transferring pollen from one plant to another to combine traits — Ex: Mendel crossing tall plants with short plants

P Generation

The parental generation; the original organisms crossed in a genetic experiment — Ex: true-breeding parents in Mendel's pea experiments

F₁ Generation

First filial generation; the offspring of the P generation cross — Ex: all tall plants when crossing true-breeding tall × short

F₂ Generation

Second filial generation; offspring produced by crossing or self-pollinating the F₁ generation — Ex: 3:1 ratio of tall to short plants

genetic recombination

The reshuffling of genes into new combinations through crossing over and independent assortment during meiosis

linked genes

Genes located on the same chromosome that tend to be inherited together

independent assortment

Process by which chromosomes independently sort into gametes, producing more possible genetic combinations

somatic cells

All body cells other than sperm or egg cells; they are diploid (2n)

Interphase (S Phase)

DNA replication occurs — chromosomes are copied before meiosis begins

Prophase I

Homologous chromosomes pair up forming tetrads; crossing over occurs; spindle fibers reach out to tetrads; nuclear membrane disappears

Metaphase I

Homologous chromosome pairs (tetrads) line up in the middle of the cell

Anaphase I

Homologous chromosomes separate and move to opposite ends of the cell (sister chromatids remain attached)

Telophase I

Chromosomes reach opposite sides; nuclear membrane begins to reform

Cytokinesis I

Cytoplasm divides to form two new cells (2n → n); each cell is haploid but chromosomes are still doubled

Prophase II

Spindle fibers start to reach out to centromeres of sister chromatids; nuclear membrane disappears in each cell

Metaphase II

Individual chromosomes (not pairs) line up in the middle of each cell

Anaphase II

Sister chromatids separate and move to opposite ends of each cell

Telophase II

Sister chromatids reach opposite sides; nuclear envelope reforms around each set of chromatids

Cytokinesis II

Cytoplasm divides again — forming 4 haploid daughter cells (gametes)

Interphase II

Brief resting stage between the two meiotic divisions; NO additional DNA replication occurs

Number of divisions

Mitosis: 1 cell division | Meiosis: 2 cell divisions

Daughter cells produced

Mitosis: 2 identical daughter cells | Meiosis: 4 genetically unique daughter cells

Chromosome number

Mitosis: chromosome number maintained (2n → 2n) | Meiosis: chromosome number halved (2n → n)

Genetic variation

Mitosis: no recombination — daughter cells identical to parent | Meiosis: crossing over and independent assortment create variation

Purpose

Mitosis: growth and asexual reproduction | Meiosis: production of gametes for sexual reproduction

Daughter cell identity

Mitosis: daughter cells are the same as the parent cell | Meiosis: daughter cells are NOT the same as the parent cell

Synapsis

Mitosis: does NOT occur | Meiosis: homologous chromosomes pair up (synapsis) in Prophase I

Produces gametes?

Mitosis: No | Meiosis: Yes

Chromosomes duplicated?

Both Mitosis AND Meiosis — chromosomes duplicate during Interphase (S Phase) before division

Where it occurs

Mitosis: all body (somatic) cells | Meiosis: ONLY in reproductive organs (gonads) to produce sperm or eggs

Homologous chr. behavior

Mitosis: homologous chromosomes do NOT pair up | Meiosis: homologous chromosomes pair up as tetrads in Meiosis I

Ploidy of result

Mitosis: diploid (2n) | Meiosis: haploid (n)

Associated with

Mitosis: asexual reproduction and growth | Meiosis: sexual reproduction and genetic diversity

Human diploid number

46 chromosomes (2n = 46) in all somatic body cells

Human haploid number

23 chromosomes (n = 23) in all gametes (sperm and egg)

Chromosomes after Meiosis I

23 chromosomes per cell (still consists of 2 chromatids each)

Chromosomes after Meiosis II

23 chromosomes per cell — now truly haploid single-chromatid chromosomes

Possible gamete combinations

Human chromosomes can combine in over 8 million possible ways (2²³) to form one sperm or egg

Fertilization combinations

When sperm and egg fuse, there are over 64 trillion possible chromosomal combinations — which is why we are all genetically unique

Sex chromosomes

Pair 23 — X and Y; females are XX, males are XY; all other 22 pairs are autosomes

Fertilization

When haploid sperm (23 chr.) and haploid egg (23 chr.) fuse → diploid zygote (46 chr.)

Where meiosis occurs in males

Males do NOT enter meiosis until after puberty; not all sperm enter at once — continuous production throughout life

Where meiosis occurs in females

Females are born with all future eggs (oocytes) stuck in Prophase I; one (or more) complete meiosis each month after puberty in response to hormones

Why siblings look different

Crossing over and independent assortment during meiosis increase genetic variation, giving up to 8 million possible gamete combinations per parent

Autosome pairs

Pairs 1-22 contain genes unrelated to sex determination; pair 23 = sex chromosomes

Genes passed per parent

Each parent has 2 copies of each gene and passes along only 1 copy to offspring

Tetrad

Structure formed when homologous chromosomes pair up in Prophase I; consists of 4 chromatids (2 pairs of sister chromatids)

Result of meiosis

4 haploid cells that are all genetically unique (not identical to each other or the parent)

Interphase II note

There is NO DNA replication between Meiosis I and Meiosis II — this is a key difference from the lead-up to mitosis