MEIOSIS
SEXUAL REPRODUCTION
• A combination of the genetic material of
two separate individuals
• Humans have 46 chromosomes - 23 pairs
• Humans reproduce by joining sperm and
egg from two parents
• If sperm and egg each had 46
chromosomes, the baby would have 92
• Therefore----- meiosis is the production of
gametes (sperm or eggs) with only 23
• Meiosis has the same stages as mitosis,
but repeated twice each – end result- four
haploid cells (23 chromosomes each). 46
chromosomes = diploid cell.
• Meiosis compared to mitosis
• All chromosomes in a cell have numbers –
1-22 pairs
• Sex chromosomes make the total 23
• They all come in pairs, paternal and
maternal
• Therefore, you received one #1
chromosome from your Mom and the other
from your Dad.
• Each cell MUST have one #1
chromosome and one #2 chromosome
and so on. No more or less than 1.
• Therefore, during meiosis metaphase 1,
homologous pairs match up. The
homologous chromosomes are separated
at anaphase 1.
• Each resulting sperm or egg can have
paternal OR maternal #1 chromosome,
paternal or maternal chromosome #2, etc.
• This is known as independent assortment.
• While the homologous pairs are matched
in metaphase 1, they can trade pieces of
DNA = crossing over
• This creates lots of genetic variation (good
for the human species)
• Crossing over only occurs in meiosis
Nondisjunction
NONDISJUCTION – change in
chromosome #
• Error in Meiosis
• Homologous chromosomes fail to separate
• Leads to abnormal chromosome # (aneuploidy)
– Down syndrome: trisomy 21
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Characteristic facial features
Heart defects, mental retardation
1/700 births
Incidence rises with mother’s age
– Non disjunction of sex chromosomes
• Turner’s syndrome, only one X chromosome
• Female, short stature, sexually immature
– Klinefelter’s syndrome, XXY
• Male, testosterone deficient, sterile
Mendelian genetics
• Genotype (genes), Phenotype, and alleles
of a specific gene
• Capital letters for dominate alleles
• Lowercase letters for recessive alleles
• Every person has two alleles for each trait
• Gametes have only one allele for each
trait
• Genotype of recessive phenotype is
known (rr)
• Genotype of dominant phenotype: two
possible – 1. homozygous dominant (RR),
2. Heterozygous (Rr)
• Test-cross determines genotype of
dominant phenotype – RR X Rr – Punnett
square
• Humans not test-crossed, but can look at
ancestors
• Cystic fibrosis: recessive, absence of
protein causes chloride ion channels to
malfunction – CC, Cc, cc
• Carrier: unaffected, but can pass
recessive allele to offspring
• Do example genetics problems on
handout
• Blood types – multiple alleles – A,O,B
• Sex linked genes – on X&Y chromosomes
• Male (Y) chromosome  all alleles
expressed
• Colorblindness example: carrier mother
gives defective allele to half her sons and
half her daughters
Amniocentesis
• Karyotype shows any chromosomal
abnormalities