Genetics: Part I
Mendel and the Gene
Colorblindness
Marfans Syndrome
Polydactyly
Freckles
Cleft chin
Widows peak
2
Mendelian Genetics
What do you remember about
Mendel and his genetics studies
from your first biology course?
What are some of the terms of
genetics that you remember?
3
Can you define these?
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•
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Genotype
Phenotype
Monohybrid
Dominant
Recessive
P1
Homozygous
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•
•
•
•
Heterozygous
Allele
Test cross
F1
F2
4
3
Phenotype
Genotype
Purple
PP
(homozygous)
Purple
Pp
(heterozygous)
1
2
1
Purple
Pp
(heterozygous)
White
pp
(homozygous)
Ratio 3:1
Ratio 1:2:1
1
Figure 14.1
P
p
Curriculum Framework
3. The pattern of inheritance (monohybrid,
dihybrid, sex-linked, and genes linked on the
same homologous chromosome) can often be
predicted from data that gives the parent
genotype/ phenotype and/or the offspring
phenotypes/genotypes.
Mendelian Genetics
• Law of Dominance - if the two alleles at a
locus differ, then one (the dominant allele)
determines the organism’s appearance, and
the other (the recessive allele) has no
noticeable effect on appearance
12
Human Traits with simple dominance:
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Widows peak
Handedness
Free earlobes
Cleft Chin
Dimples
Freckles
PTC testing
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Polydactyly
Bent little finger
Nearsightedness
Mid-digit hair
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Dominant does not mean numerous
Frequency of Dominant
Alleles
• Dominant alleles are
not necessarily more
common in populations
than recessive alleles
• For example, one baby
out of 400 in the United
States is born with
extra fingers or toes
In humans, alleles for dark hair are genetically dominant,
while alleles for light hair are recessive. Which of the
following statements is/are most likely to be correct?
a. Dark hair alleles are more common than light hair
alleles in all areas of Europe.
b. Dark hair alleles are more common than light hair
alleles in southern Europe but not in northern Europe.
c. Dark hair alleles are equally common in all parts of
Europe.
d. Dark hair is dominant to light hair in southern Europe
but recessive to light hair in northern Europe.
Curriculum Framework
b. Segregation and independent assortment of
chromosomes result in genetic variation.
1. Segregation and independent assortment
can be applied to genes that are on different
chromosomes.
2. Genes that are adjacent and close to each
other on the same chromosome tend to move as a
unit; the probability that they will segregate as a
unit is a function of the distance between them.
Mendelian Genetics
• Law of segregation: the two alleles for a
heritable character separate (segregate)
during gamete formation and end up in
different gametes
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Figure 14.7
TECHNIQUE
Dominant phenotype,
unknown genotype:
PP or Pp?
Predictions
If purple-flowered
parent is PP
Sperm
p
p
Recessive phenotype,
known genotype:
pp
or
If purple-flowered
parent is Pp
Sperm
p
p
P
Pp
Eggs
P
Pp
Eggs
P
p
Pp
Pp
RESULTS
All offspring purple
Pp
Pp
pp
pp
or
1/
2
offspring purple and
1/ offspring white
2
Figure 14.8
EXPERIMENT
YYRR
P Generation
yyrr
yr
Gametes YR
F1 Generation
Predictions
YyRr
Hypothesis of
dependent assortment
Hypothesis of
independent assortment
Sperm
or
Predicted
offspring of
F2 generation
1/
Sperm
1/
2
YR
1/
2
2
YR
YyRr
YYRR
Eggs
1/
2
1/
4
YR
4
Yr
4
yR
4
yr
Eggs
yr
YyRr
3/
yyrr
1/
4
YR
1/
4
1/
Yr
4
yR
1/
4
yr
yr
1/
1/
4
1/
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
4
Phenotypic ratio 3:1
1/
9/
16
3/
16
3/
16
1/
16
Phenotypic ratio 9:3:3:1
RESULTS
315
108
101
32
Phenotypic ratio approximately 9:3:3:1
Independent Assortment
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Practice
• In order to solidify your ability to explain the
Laws of Segregation and Independent
Assortment, describe how these laws would
apply as sperm cells are developed in a man
with genotype RrSs.
• Draw an image to support your explanation.
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Sample image
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Imagine crossing a pea heterozygous at the loci for
flower color (white versus purple) and seed color
(yellow versus green) with a second pea homozygous
for flower color (white) and seed color (yellow). What
types of gametes will the first pea produce?
a. two gamete types: white/white and purple/purple
b. two gamete types: white/yellow and purple/green
c. four gamete types: white/yellow, white/green,
purple/yellow, purple/green
d. four gamete types: white/purple,
yellow/green,white/white, and purple/purple