Download Mendel and Heredity (Chapter 8)

Mendel and Heredity
(Chapter 8)
D. Blanck
PLHS Biology
I. Orgins of Genetics:
A. Heredity: the passing
of traits from parents
to offspring
(Characteristic=Trait)
 before DNA and
chromosomes were
discovered, heredity
was a great mystery
B. Gregor Johann Mendel:
1) Austrian monk that
is credited as the “father”
of the scientific study of heredity
2) Experimented with different varieties
of garden peas
a) 1st to develop rules to predict
patterns of heredity
b) heredity provides the basis for:
Genetics –the study of “genes”
c) repeated experiments of T.A. Knight
Mendel extended
Knight’s work by
applying math!
(ratios and proportions)
3. Why did Mendel use garden peas?
a) Peas have clear traits - easy to tell
apart (see table 8-1, pg 163)
b) Easy to control pollination (male
and female parts are in same flower)
1) self-fertilization – flower
fertilizes itself
2) cross-pollination –
transfer of pollen
between plants
c) Easy to grow
C) Mendel’s work with ratios:
1. Mendel’s Experiment
monohybrid cross – only 1 trait
Step 1: Make sure plant is true-breeding
 allow plant line to self-pollinate for
many generations
 results in no variation in traits
Step 2: Cross two “P” generation (parental
generations) plants with contrasting traits
 observe offspring (F1 generation)
 record # of F1 plants with each trait
Step 3: Allow F1 plants to self-pollinate
 observe and count this second
generation of offspring
= F2 generation
2. Mendel’s results:
a) F1 gen. – showed only one form of trait
(ex: purple flowers)
b) F2 gen. – showed both forms of trait
(ex: 705 purple: 224 white)
c) For each of the 7 traits, he found the
same 3:1 ratio!
Mendel’s results for other traits
results2
II. Gene Theory:
A. Mendel’s Hypothesis - “foundation of
genetics”
1. For each trait, an individual has 2 copies
of the gene, one from each parent
2. There are alternative versions of genes
Alleles = alternative forms of a gene
(green seed vs yellow seed)
 1 allele for each gene comes
from each parent
Genotype = set of 2 alleles
ex: GG or gg
(G = green and g= yellow)
Phenotype = observable characteristic
ex: pea appears green or yellow
Homozygous = 2 identical
alleles for a trait
(ex: GG and gg)
Heterozygous = 2 different
alleles for a trait
(ex: Gg or gG)
3. When 2 different alleles occur together,
one may be completely expressed. The
other may have no observable effect on
phenotype
a) Dominant = allele exclusively
expressed:
PP = purple
Pp = purple
b) Recessive = allele NOT expressed
when dominant form is present:
Pp = Purple
pp = white
III. Studying Heredity:
A. Punnett Square – predicts the expected
genotypes of a cross.
• Punnett squares can also deal with multiple or
complex traits.
A = Green
a = absence of Green
(blue)
B = Brown
b = absence of Brown
(blue)
Color Blindness
• An X-Linked Trait
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Codominance or incomplete dominance