Cell division & heredity · Topic 17 of 17

Chromosomes & Non-Mendelian Inheritance

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Biology · chromosomes and inheritance

Humans have 23 chromosome pairs, with XX or XY setting sex; some traits blend (incomplete dominance) or show both alleles (codominance).

01 · SexFather's X or Y sets the sex

02 · Don't confuseBlend = incomplete · both show = codominant

03 · Sex-linkedX-linked recessive: mostly males

Karyotype: chromosomes photographed and lined up in numbered pairs

Nondisjunction: chromosomes fail to separate → gamete with extra or missing one

Multiple alleles: 3+ versions in a population; each person has only 2

X-linked: color blindness, hemophilia; XHXh female = carrier

Chromosome: one long DNA molecule packed with proteins; carries many genes must know

Humans: 46 = 22 pairs of autosomes + 1 pair of sex chromosomes must know

Trisomy = 3 copies (47 total) · monosomy = 1 copy (45) · Down = trisomy 21

XX = female, XY = male — the father's sperm (X or Y) decides must know

ABO: A = IAIA or IAi · B = IBIB or IBi · AB = IAIB · O = ii

X-linked recessive: one copy shows in males (XY) → more common in males

Fathers pass X-linked alleles to all daughters, never to sons

Incomplete or codominance: heterozygote × heterozygote → 1 : 2 : 1 phenotypes

Incomplete dominance: BY × BY → 1 blue : 2 green : 1 yellow

IAi × IBi → 25% each: AB, A, B, O

XhY father × XHXH mother → all daughters carriers, all sons unaffected

Incomplete = blend (pink) · codominance = both show (AB blood) must know

Commonly confused

Show 3 more facts

Y is much smaller than X with far fewer genes

Turner = XO (one X) · Klinefelter = XXY

Nondisjunction risk (e.g. Down syndrome) rises with maternal age

Pearl

Pink flowers blend their parents; AB blood shows both.

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