Heredity

Chapter 4 

Heredity

1. What is Heredity?

Offspring resemble their parents because genes in chromosomes carry hereditary traits.

Study of genes = Genetics.

Chromosomes → DNA → Genes → Traits

Sex chromosomes determine sex (XY = male, XX = female).

2. DNA & RNA

DNA (Deoxyribonucleic acid)

Double-stranded, found in nucleus (eukaryotes), cytoplasm (prokaryotes).

Made of nucleotides = sugar + phosphate + nitrogen base (A, T, G, C)

Function: Store and transmit hereditary information

RNA (Ribonucleic acid)

Single-stranded, mainly in cytoplasm

Sugar = ribose; Bases = A, U, G, C

Types: mRNA, tRNA, rRNA

Function: Helps synthesize proteins

3. Chromosomes

Chromatin fibers in the nucleus → condense into chromosomes during cell division

Made of DNA + protein

Structure: 2 sister chromatids joined at centromere

Gene = small DNA segment coding a trait

Types:

Autosomes (22 pairs) → Body traits

Sex chromosomes (1 pair) → Male XY, Female XX

Humans:

Somatic cell = 46 chromosomes (2n)

Gamete = 23 chromosomes (n)

4. Cell Division

A. Mitosis (Somatic cells)

1 diploid cell → 2 identical diploid daughter cells

Phases: Karyokinesis (nucleus divides) + Cytokinesis (cytoplasm divides)

Function: Growth, repair, asexual reproduction, maintaining genetic stability

B. Meiosis (Gametes)

1 diploid cell → 4 haploid daughter cells (gametes)

Chromosome number halved (2n → n)

Crossing over creates variation

Function: Sexual reproduction, genetic variation, evolution

5. Fertilization & Sex Determination

Male gamete (sperm: 22+X or 22+Y) + Female gamete (ovum: 22+X) → Zygote (44)

Zygote undergoes mitosis → develops into full organism

Sex determination in humans:

Chance of having son or daughter = 50%