(8.12-8.24). Chromosomes Exist in Homologous Pairs There are 46 chromosomes in a human somatic (body) cell. These chromosomes exist in 23 homologous pairs.

(8.12-8.24)

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 23rd 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 = 2n •n = # chromosome pairs•For humans 223 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