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Chromosomes Exist in Homologous Pairs There are 46 chromosomes in a human somatic (body) cell. These chromosomes exist in 23 homologous pairs The two homologs carry genes controlling the same inherited traits (chromosome theory of heredity) Although each homolog may have a different version (allele) for the gene, the locus is the same for each

Autosomes v. Sex Chromosomes Only the first 22 of the 23 chromosome pairs are definite homologs. (The autosomes) The 23 rd pair contains the sex chromosomes (X/Y). Females have two Xs; Males have an X and Y Only small portions of X and Y are homologous

Gametes Have a Single Set of Chromosomes A cell with two homologous chromosomes is diploid (2n) Gametes (egg & sperm) are haploid (1n) because they only have a single set of chromosomes Fusion of the haploid cells produces a diploid zygote during fertilization

Meiosis Reduces the Chromosome Number from Diploid to Haploid Often referred to as “reduction division” meiosis is a cell division that forms haploid gametes in diploid organisms Many stages closely resemble corresponding stages in mitosis Preceded by duplication of chromosomes, followed by two consecutive divisions, Meiosis I and II Animation

Meiosis I v. Meiosis II

Meiosis in Males v. Females

Meiosis Increases Genetic Variation: Independent Assortment Total # of chromosome combos = 2 n n = # chromosome pairs For humans 2 23 or ~8 million combos

Meiosis Increases Genetic Variation: Crossing Over & Genetic Recombination Animation

Linkage & Gene-Mapping Genes that are located close together have a lower frequency of crossing over By evaluating relative frequencies, geneticists can map the locations of genes on chromosomes

Creating Karyotypes

Non-Disjunction: Meiosis I v. Meiosis II

Turner’s Syndrome

Klinefelter’s Syndrome

Jacob’s Syndrome/Super Male (XYY) Acne Agression Tall stature Superior muscle strength Reduced muscle coordination

Triple X/Super female Syndrome (XXX) No unique physical features apart from tall stature. It may be asymptomatic and diagnosed incidentally There is an increased incidence of motor and cognitive impairment, and psychological problems Fertility of affected females is normal There is a slight increased risk of offspring with chromosomal abnormalities

Down Syndrome (Trisomy 21)

Patau Syndrome (Trisomy 13) Least common trisomy More female fetuses survive to birth Severe heart & kidney defects

Edward’s Syndrome (Trisomy 18) 25% die before within first month, only 10% survive past first year Growth deficiency Feeding difficulties Breathing difficulties Developmental delays Kidney malformations Structural heart defects

Chromosomal Mutations