Download Genetics

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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
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:
1.
Determine what the question is
asking
2. 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:
3. 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
4. Draw a Punnett Square
Write the cross _____ x _____
Ex: Tt x Tt
Determine which sex cells will be possible by each
parent. The sex cells 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.
5. 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
Incomplete
dominance
• Also called
co-dominance.
• Neither allele is
dominant and when
hybrid, the phenotype
is mixed.
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 =
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
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