Download Gametes Have a Single Set of Chromosomes

(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