BIG PICTURE: Meiosis and independent assortment of alleles result in new genetic combinations for the next generation.
1. From Friday's movie on Darwin and Wallace...
- individuals differ slightly in various
characteristics
- survival and reproduction appear
to depend on these characteristics
- some variants are more successful
= Natural Selection
Genetics is the mechanism behind
variation and inherited traits, providing the raw material for evolution.
2. Differences between Mitosis and Meiosis (see handouts
and Fig. 13.7)
a. Mitosis is division of somatic
cells for growth and repair. Meiosis occurs in germ cells for production
of gametes.
b. A key mechanistic difference between
mitosis and meiosis is how the replicated (=duplicated) chromosomes line up
on the metaphase plate during the first division.
c. Mitosis has only a single division; meiosis
involves 2 divisions.
d. Mitosis results in 2 diploid (2n) cells,
each with a genotype identical to the original cell. Meiosis results
in 4 haploid (n) cells, some or all of which become gametes.
e. Follow individual alleles through mitosis
and meiosis, without and with crossing over (recombination) during Prophase
I of meiosis. Understand this thoroughly! (understand Fig. 13.8; practice
by placing alleles on the chromosomes and follow them through meiosis with
altering arrangements at Metaphase I)
3. Terminology (see handout or text glossary)
a. Chromosome, gene, locus, allele
b. Homozygous, heterozygous (different
from homologous or homologues!)
c. Recessive allele, dominant allele
d. Genotype, phenotype
4. Mendelian inheritance
a. Gregor Mendel (1822-1884)
b. Series of controlled breeding experiments
with the garden pea
c. True breeding stocks; followed
7 different inherited traits
d. P, F1, F2
e. Monohybrid cross = cross involving
1 character, followed over 3 generations
i. one
trait not seen in F1, but "reappears" (phenotypically) in F2
(Fig. 14.2)
ii.
Mendel explained results with a "particulate" theory (genes)
iii.
physical appearance of a character (phenotype) = result of genetic constitution (genotype)
f. Mendelian ratios are averages not
absolutes (Table 14.1)
g. Punnett Square to figure genotype
and phenotype ratios (Fig. 14.4)
5. Mendel's Laws and patterns
a. Mendel's Law of Segregation ("1st
Law"): Each individual has 2 alleles for each gene. When gametes
are formed, those alleles segregate and pass into separate gametes.
b. Linked genes (on the same chromosome)
do not segregate independently.
c. Use of probability theory to determine
the expected frequencies of genotypes and phenotypes:
- probability of event 1 AND
event 2 = (prob. event 1) x (prob. event 2)
- probability of event 1
OR event 2 = (prob. event 1) + (prob. event 2)
F1 cross:
Bb x Bb
Probability that an F2 plant is BB is
1/2 x 1/2 = 1/4
= prob. B will segregate from the mother (1/2) AND that
it will segregate from father
(also 1/2)
Probability that an F2 plant is bb is 1/2 x 1/2 = 1/4
Probability that an F2 plant is Bb is (1/2
x 1/2) + (1/2 x 1/2) = 1/2
= prob. B will segregate from mother and b from father
OR b from mother and B
from father
d. Test Cross = a way to test whether
an individual that is phenotypically dominant has a homozygous or heterozygous genotype
(Fig. 14.6)
B_ x bb (unknown genotype crossed
with a homozygous recessive)
e. Independent assortment of alleles - what happens if parents differ at 2 (or more) loci? (Mendel wanted to see if alleles of maternal and paternal origin segregate in groups; Fig. 14.7)
Dihybrid cross: SSYY x ssyy
f. Mendel's Law of Independent Assortment ("2nd Law"): Alleles of different genes assort independently of one another during gamete formation (applies to genes that are on separate chromosomes, not the same chromosome) (Fig. 15.1)
6. Beyond Mendel
a. Many phenotypic characters are
based on >1 gene (eg. human height)
b. The degree of expression of a gene
can change depending on other genes in the individual (eg. Marfan's syndrome)
c. Sex linked traits; interpreting
pedigrees to determine mode of inheritance
d. Non-disjunction
e. Recombination = crossing over:
important to evolution because it puts genes into new combinations. This enormously
increases the number of unique gametes that meiosis can produce, which in turn increases
the range of variation of offspring.