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The Study of
Inheritance
Gregor Mendel – The
Father of Genetics
• Austrian monk
• Published ideas in 1866
• Worked with garden peas
• Determined basic genetics
principles.
Garden Peas
• Self pollinating so
Mendel didn’t have to
worry about bees and
wind, etc.
• Traits are contrasting
• Easily controlled
• Can be eaten at end
Mendelian Genetics
Some terminology:
parental strains (P1 or F0)-- originally crossed organisms
F1 generation-- offspring of the F0 generation (parents)
F2 generation-- offspring of F1 generation crossed to itself
reciprocal cross-- switching the phenotype of the male and female parents
dominant-- phenotype visible in the F1 generation -----a trait that masks
another
recessive-- trait which reappears in the F2 generation after self cross ---a hidden trait
Mendelian Genetics
gene-- hereditary element that gets segregated
allele-- particular form of a gene
can a single gene have multiple alleles?
homozygous-- organism carrying two copies of the same allele
heterozygous- organism carrying different alleles of the same gene
genotype-- genetic makeup of an organism (what alleles it contains)
phenotype-- observable properties of an organism (characteristic)
Traits studied by Mendel
round
wrinkled
Crosses by Mendel:
round
wrinkled
All round
¾ round
¼ wrinkled
Round
Wrinkled seeds
Simple Steps for Solving
Genetics Problems:
Determine what the question is asking
1. Write a key with information given:
KEY: TT = tongue roller
Tt = tongue roller
tt = nontongue roller
Use letters given or make up your own – be
sure to include all possible genotypes and
the phenotypes they produce.
Keep the key handy – I must be able to find it
easily
Ex:
2. Determine what the cross is: read
carefully and figure out which two
genotypes are being crossed:
– Ex – if the problem states that a
heterozygous man is married to a woman
who can’t roll her tongue, the cross
would be:
Tt x tt
• If the problem states that the P1
generation is one homozygous
dominant crossed with a
homozygous recessive, the cross
would be:
TT x tt
3. Draw a Punnett Square
Write the cross _____ x _____
Ex: Tt x Tt
Determine which gametes will be possible by each
parent. The gametes of the individual on the left go
down the side of the square, those of the parent on
the right, go across the top of the square:
T
T
t
t
NOW YOU CAN FIGURE
OUT THE POSSIBLE
TYPES OF OFFSPRING
THEY MAY HAVE
T
t
T
TT
Tt
t
Tt
tt
Move each gamete over or down
and then make pairs of gametes to
show what genes will be expected to
be in the offspring.
4. Now you can show the genotypic
and phenotypic ratios
• First, do the genotypic ratio – GR
– Count how many of the offspring will
have each genotype
• Ex – GR = 1TT:2Tt:1tt
• Then do the phenotypic ratio – PR
• Ex – PR = 3 tongue-rollers:1 nontongue roller
Possible Questions
Regarding a Cross:
• Can they have a child who is a
tongue roller?
• Can they have a heterozygous
tongue-rolling child?
• Can they have a child who is a
homozygous tongue roller?
Other types of problems
•
You might be given the offspring and asked to
determine the possible cross:
–
Ex – If two brown dogs had a litter of puppies that
included 3 brown and 1 white, what are the most
likely genotypes of their parents?
1.
Write a key
2.
Write the cross, leave genotypes blank
3.
Write the kinds of offspring
4. Then figure out what the parents were
Key: brown is dominant so use B = brown and b = white
BB = brown
______ x _______
Bb = brown
3 brown
3 brown:1white ratio
bb = white
1 white
3 B_ : 1 bb
The bb got one b from the mother, the other from the father, so each parent must have a
small b. Since both parents are brown, they must also have a B, making them both Bb
Monohybrid and Dihybrid Crosses
• The previous examples were monohybrid
crosses. We were interested in one
characteristic only.
• In a dihybrid cross we will be following two
characteristics at the same time.
Garden pea example
• We have two characteristics we’d like to
follow, flower color and plant height.
• Each characteristic is controlled by a pair of
alleles, one dominant and the other
recessive:
F – purple flowers, dominant
f – white flowers, recessive
T – tall plants, dominant
t – dwarf plants, recessive
• Each gene is located on a different type of
chromosome.
FfTt x FfTt
list gametes from one parent here
list
gametes
from
other
parent
here
FfTt x FfTt
FT
FT
Ft
fT
ft
Ft
fT
ft
FfTt x FfTt
FT
FT
Ft
fT
ft
FFTT
Ft
fT
ft
genotype of diploid offspring
FfTt x FfTt
FT
Ft
fT
ft
FT
FFTT
FFTt
FfTT
FfTt
Ft
FFTt
FFtt
FfTt
Fftt
fT
FfTT
FfTt
ffTT
ffTt
ft
FfTt
Fftt
ffTt
fftt
F f T t x F f T t Genotypes
1/16
2/16
1/16
2/16
4/16
2/16
FFTT
FFTt
FFtt
FfTT
FfTt
Fftt
FfTt x FfTt
FT
Ft
fT
ft
FT
FFTT
FFTt
FfTT
FfTt
Ft
FFTt
FFtt
FfTt
Fftt
fT
FfTT
FfTt
ffTT
ffTt
ft
FfTt
Fftt
ffTt
fftt
F f T t x F f T t Genotypes
1/16
2/16
1/16
2/16
4/16
2/16
1/16
2/16
1/16
FFTT
FFTt
FFtt
FfTT
FfTt
Fftt
ffTT
ffTt
fftt
FfTt x FfTt
FT
Ft
fT
ft
FT
FFTT
FFTt
FfTT
FfTt
Ft
FFTt
FFtt
FfTt
Fftt
fT
FfTT
FfTt
ffTT
ffTt
ft
FfTt
Fftt
ffTt
fftt
F f T t x F f T t Phenotypes
1/16
2/16
1/16
2/16
4/16
2/16
1/16
2/16
1/16
FFTT
FFTt
FFtt
FfTT
FfTt
Fftt
ffTT
ffTt
fftt
9/16 Purple flowers and
Tall plants
F f T t x F f T t Phenotypes
1/16
2/16
1/16
2/16
4/16
2/16
1/16
2/16
1/16
FFTT
FFTt
FFtt
FfTT
FfTt
Fftt
ffTT
ffTt
fftt
3/16 Purple flowers and
dwarf plants
F f T t x F f T t Phenotypes
1/16
2/16
1/16
2/16
4/16
2/16
1/16
2/16
1/16
FFTT
FFTt
FFtt
FfTT
FfTt
Fftt
ffTT
ffTt
fftt
3/16 white flowers and
Tall plants
F f T t x F f T t Phenotypes
1/16
2/16
1/16
2/16
4/16
2/16
1/16
2/16
1/16
FFTT
FFTt
FFtt
FfTT
FfTt
Fftt
ffTT
ffTt
fftt
1/16 white flowers and
dwarf plants
Non-Mendelian Genetics
• Codominance – two alleles and are both
dominant; both express themselves, in
an unchanged fashion, whenever present
• Multiple Alleles – more than two alleles are
present for a gene
Incomplete
dominance
• Neither allele is dominant and when
hybrid (heterozygous/both alleles present), the
phenotype is mixed.
Incomplete dominance
Heterozygotes
Intermediate in
phenotype
Incomplete Dominance
• Red, pink and white flower color in
snapdragons
R – red flowers, incomplete dominant
W– white flowers, incomplete dominant
Mate a red flowered plant with a white and
all the offspring have pink flowers,
RW
What would you get from a cross of two
pink flowered plants?
Solving Incomplete Dominance
Problems
• Since neither gene is dominant, use two
different capital letters in your key:
Ex: Key RR = red
Cross = RW x RW
RW = pink
R
W
WW = white
R
W
GR =
PR =
Codominance
• In codominance, both traits are dominant at
the same time.
• Example: A roan cow has both red and
white hairs at the same time
• Genotypes:
RR=red cow
WW=white cow
RW= roan cow (some red hairs, some white hairs)
Sex-Linked traits :
gene is found on X-chromosome, not the Y
• Genes for colorblindness
and hemophilia are
common in humans
• Females have two genes
for this trait
• Males have only one gene –
so they have no chance
to be a carrier
Are you colorblind?
Can you see the shapes?
Sex-linked Traits – hemophilia
pedigree in royal families of Europe
Current
royalty in
England
Russian family
before
revolution
Pedigree analysis
Sex-Linked Traits – the gene for
that trait is on the
X-chromosome, not on the
Y-chromosome
• Ex: in humans, both colorblindness and
hemophilia are sex-linked
• Key for colorblindness:
XBXB = normal visioned female
XBXb = normal visioned female
XbXb = colorblind female
XBY = normal visioned male
XbY = colorblind male
Punnett Square for
colorblindness:
Cross = XBXB x XbY
Xb
XB
XB
Y
• Would you expect any of their
children to be colorblind?
• What happens when the carrier
daughters grow up and marry
men who have normal vision?
– Do that cross at your seats….Can
they have colorblind children?
Colorblind daughters?
Colorblind sons?
Multiple Alleles
• Some traits are controlled by more than two
alleles that can be inherited. Each person
only inherits two of those alleles – one from
each parent.
• This is the case with human blood types:
– There are 3 alleles in the population:
• IA allele
Incompletely dominant to
B
• I allele each other
• i allele – recessive to the
other alleles
A Blood test will show which
type you are:
ABO Blood type crosses:
• The key is complicated:
–
–
–
–
IAIA or IAi = type A
IBIB or IBi = type B
IAIB = type AB
ii = type O
– How could we simplify this key?
Polygenic Traits
• Traits that are controlled by more than one
pair of alleles
• For example, human skin color is controlled
by several pairs of alleles.
Let capital letters represent alleles that
produce lots of pigment and lower case
letters produce less pigment.
Assume we have three pairs of alleles.
If we mate two individuals both with the
genotypes AaBbCc, 27 different genotypes are
possible with 7 different phenotypes.
AaBbCC
AABBCC
6
AABbCc
aaBBCc
aaBbCc
AaBBCc
AaBbCc
AabbCc
AaBbCC
AabbCC
AaBbcc
AaBBCC
AABBcc
AaBBcc
aabbCC
aabbCc
AABbCC
AAbbCC
AAbbCc
aaBBcc
aaBbcc
AABBBc
aaBBCC
AABbcc
AAbbcc
Aabbcc
aabbcc
3
2
1
0
5
4
Increasing number of alleles for dark skin
Fetal diagnosis