Genetics and Sex Traits: Chromosomes, Disorders, and Linkage

Chromosomes

Humans have 23 pairs; XX = female, XY = male; X = big, many genes; Y = small, few genes.

Sex-Linked Traits

Traits on sex chromosomes; X-linked: on X; Boys get them more (only 1 X); Y-linked: on Y; Only dad → son.

X-Linked Recessive Pattern

Girls need 2 recessive genes; Boys need 1; Shows up in more males; Can skip generations; Affected dad → all daughters (carriers); Affected mom → all sons.

Common X-Linked Disorders

Colorblindness, Hemophilia, Duchenne muscular dystrophy, Fragile X syndrome.

X-Linked Dominant

Rare; Affected dad → all daughters, no sons; Example: IP (Incontinentia Pigmenti).

Gene Linkage

Genes on the same chromosome stay together; No crossing over → 2 gametes; Crossing over → 4 gametes; Farther apart = more crossing over.

Chromosome Number Problems

Caused by nondisjunction (meiosis fail); Trisomy: extra chromosome; Monosomy: missing chromosome.

Examples of Chromosome Number Problems

Down syndrome (trisomy 21), Turner (XO), Klinefelter (XXY), Jacobs (XYY).

Chromosome Structural Problems

Deletion: piece missing; Duplication: piece copied; Translocation: pieces swap; Inversion: piece flips.