Genetics and Cell Division: Key Concepts for Biology

Chromosome vs chromatid

Chromosome = full DNA structure; chromatid = one copy of a duplicated chromosome

Sister chromatids vs homologous chromosomes

Sister chromatids = identical copies; homologous = same genes, different alleles

Centromere vs centrosome

Centromere = holds chromatids together; centrosome = organizes spindle fibers

Phase where chromosomes condense and spindle forms

Prophase

Phase where chromosomes line up in middle

Metaphase

Phase where sister chromatids separate

Anaphase

Phase where nuclei reform

Telophase

Key event of metaphase

Chromosomes align at metaphase plate

Key event of anaphase

Chromatids pulled apart

Result of mitosis

2 identical diploid cells

Key difference meiosis I vs II

Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids

What happens in prophase I

Synapsis + crossing over

What happens in metaphase I

Homologous pairs align randomly

What happens in anaphase I

Homologous chromosomes separate

What happens in meiosis II

Sister chromatids separate

Result of meiosis

4 genetically different haploid cells

Stage where DNA replicates

S phase

Stage with most cell activity

G1

Stage preparing for division

G2

Binary fission

Prokaryotic division producing 2 identical cells (no mitosis)

Animal cytokinesis

Cleavage furrow

Plant cytokinesis

Cell plate

Effect of losing cell cycle control

Uncontrolled division → cancer

Cancer cells

Ignore checkpoints and continue dividing

Autosomal recessive pedigree clue

Trait can skip generations

Autosomal dominant pedigree clue

Trait appears every generation

X-linked recessive clue

More males affected

Autosomal recessive genotype

aa = affected

Autosomal dominant genotype

A_ = affected

X-linked recessive male

X^aY = affected

X-linked recessive female

X^AX^a = carrier

Testcross purpose

Determine unknown genotype using recessive cross

Monohybrid cross result (Aa x Aa)

1:2:1 genotype ratio

Dihybrid cross result

9:3:3:1 phenotype ratio

ABO inheritance

Multiple alleles + codominance

Type AB

I^AI^B

Type O

ii

Asexual reproduction

One parent, identical offspring

Sexual reproduction

Two parents, genetic variation

Genetic variation sources

Crossing over, independent assortment, random fertilization

Law of segregation

Alleles separate in meiosis I

Law of independent assortment

Genes assort independently in metaphase I

Complete dominance

Dominant allele fully masks recessive

Incomplete dominance

Heterozygote shows intermediate

Codominance

Both alleles expressed

Multiple alleles example

ABO blood group

Pleiotropy

One gene affects multiple traits

Recessive epistasis

One gene masks another (9:3:4 ratio)

Polygenic inheritance

Multiple genes control trait

Linked genes

Genes close together inherited together

Gene distance effect

Closer genes = less recombination

X-inactivation

One X chromosome turned off in females

Nondisjunction

Chromosomes fail to separate → abnormal number