Download Chapter 9 - Advanced Biology

Chapter 9
Sexual Reproduction
Genetically Unique

A single couple can potentially create
over 70 trillion different zygotes!

All possible egg chromosome
combinations X possible sperm
chromosome combinations
How do you get so many
combinations?

Meiosis
Results in 4 genetically unique gametes
 Females = eggs (oogenesis)

◦ Starts before birth
◦ Ends around age 50

Males = sperm (spermatogenesis)
◦ Does not begin until puberty
◦ Continues through rest of life
The Basics

2 major functions
◦ Reduces chromosome number
 ½ from each parent
◦ Shuffles the chromosome “deck”
 Which chromosomes combine to form each unique
½ set

Fertilization
◦ Fusion of 1 egg and 1 sperm
◦ Restores chromosome number
◦ Provides further shuffling
Homologous Chromosomes
How do you decide which ½ of 46
chromosomes gets separated?
 Mom’s ½ and Dad’s ½ have to match

Chromosomes come in pairs =
homologous chromosomes
 The ½ set found in each gamete = ½ of
each pair

The Pairs
Most pairs = autosomes (regular
chromosomes)
 The XX or XY pair = sex chromosomes

If a cell has a complete set with pairs
matched up = diploid number (2n)
 Gamete with ½ of each pair = haploid
number (n)

What do the pairs have in common?
Size
 Shape
 Centromere location
 Banding pattern when stained
 Same genes in same order

◦ Genes are same (ex. Eye color)
◦ Version of the gene can vary (blue, brown)
 Versions of a gene = alleles
What’s a Life Cycle?
All of the reproductive events from one
generation to the next
 Involves mitosis (growth and
development)
 Involves meiosis (oogenesis or
spermatogenesis)

◦ Cutting the chromosome # in half makes it so
that 2 parents can produce an offspring that is
genetically unique, but still maintains the
overall chromosome # for that species
Overview of Meiosis
A lot like mitosis, but the whole process
happens twice
 Still starts with DNA replication

◦ So, the cell has to divide 2x in order to cut
the chromosome # in half

One main difference: synapsis
◦ Meiosis I: homologous chromosomes “find”
eachother
◦ Form a tetrad
◦ Swap genes & further increase diversity
Crossing Over
Happens during synapsis
 Creates new combinations of genes that
have never existed before
 The chromosomes in you from your mom
and dad get mixed up together
 So, the chromosome you pass on to your
kid will be a blend of their grandparents’
chromosomes

Meiosis I
Tetrads form
 Crossing over occurs
 Homologous chromosomes separate
 Ends with 2 haploid cells (n)
BUT…
 Sister chromatids still joined at the
centromere
 Copies still need to be separated

Meiosis II
A lot like mitosis
 Duplicated chromosomes line up
individually along equator
 Centromeres break
 Copies are separated


Ends with 4 haploid cells (n)
Caveats

Meiosis makes 4 gametes
◦ 4 equal sperm cells
◦ 1 egg cell and 3 polar bodies

Meiosis only happens at special times and
in special tissues
◦ Puberty
 Males vs. females
◦ Ovaries and testes
Abnormal Chromosome Inheritance
When tetrads get stuck together—
nondisjunction
 Egg or sperm can end up with an extra
chromosome or a missing chromosome
 Down Syndrome (trisomy 21)
 Turner Syndrome (monosomy of X
chromosome)
 Klinefelter Syndrome (trisomy XXY)

Red
Algae