Download Mendel II Notes - Brookwood High School

Mendel II Notes
CP Biology
Ms. Morrison
Independent Assortment
 Mendel questioned whether two different
genes segregated alleles separately or if
alleles linked
 Crossed two true-breeding plants –
round, yellow peas (RRYY) with wrinkled,
green peas (rryy)
 All F1 generation was round, yellow (RrYy)
 Did not show if genes segregated
independently
Further Experimentation
 Performed F1 cross to determine if genes
segregated independently
 F2 plants:




Round and yellow
Wrinkled and green
Round and green
Wrinkled and yellow
 If had not segregated independently, then all
offspring would have been round and yellow
OR wrinkled and green
Principle of Independent
Assortment
Genes for different traits can
segregate independently during the
formation of gametes (genes for
different traits do NOT influence
each other’s inheritance).
Summary of Mendel’s
Principles
1. Inheritance of characteristics determined by
genes which are passed from parents to
offspring.
2. When two or more forms of a gene exist,
some are dominant and some are recessive.
3. Sexually reproducing adult organisms have
two copies of each gene – one from each
parent. These copies are segregated during
gamete formation.
4. Alleles for different genes usually segregate
independently of one another.
Beyond Mendel
 Mendel’s work was important to
understanding genetics, but important
exceptions to his principles
 Some alleles are neither dominant or
recessive
 Many traits controlled by multiple alleles
or multiple genes
Incomplete Dominance
 One allele not completely dominant and
other is not completely recessive
 Heterozygous phenotype is blend of two
homozygous phenotypes
 Use different letters and capitalize them
when doing Punnett squares
Incomplete Dominance
Example
Four o’clock plants – red (RR) flowers crossed
with white (WW) flowers produces offspring
with pink flowers (RW)
R
R
W
RW
RW
W
RW
RW
Codominance
 Both alleles contribute to the phenotype
of the organism
 Both alleles expressed – NOT blended
 Ex. Cattle have alleles for red hair and
white hair, cattle with both alleles are
roan – a pinkish brown color because
they have mix of red and white hairs
Multiple Alleles
 Genes with more than two alleles
 Each individual still only has two alleles
 Ex. Coat color in rabbits – four alleles
and can produce four different coat colors
(full color, chinchilla, Himalayan, or
albino)
 Ex. Human blood types (A, B, AB, and O)
Polygenic Traits
 Traits controlled by several genes (not
just one gene)
 Ex. Fruit flies – eye color controlled by
three genes
 Ex. Humans – skin color controlled by
four genes
 Produces wide range of phenotypes