Study Guide Answers Bio A
Genetics and Pedigrees
Identify the purpose and “set up” of a test cross.
• Purpose: Is a dominant
phenotype is homozygous or
heterozygous ?(Use with
complete dominance)
• Setup: cross the dominant
phenotype (Tt or TT (second
allele is unknown) with a
purebred recessive (tt).
• Offspring born with a
recessive trait indicates a
heterozygous genotype.
T
t
t
?
List and differentiate between the Law of Segregation
and the Law of Independent assortment. What type
of chromosomes do each refer to?
• Law of Segregation refers to a pair of genes on homologous
chromosomes (you don’t know which allele you will get).
You get one or the other, but not both.
• Law of Independent assortment says that the random
allele you get from any gene is independent of the allele
you will get from another gene. If you get small height, that
doesn’t mean you will get a tongue that curls. So the “set
of chromosomes” (or genes) you get will be totally
random. (not always true for alleles on the same
chromosome).
How do current day molecular genetics support
Mendel’s conclusions? Based on his laws, what
were Mendels three conclusions? Does present day
molecular biology support his Laws? Explain which
inheritance patterns support his claims and which
do not.
• Mendels conclusions are his three laws
(dominance, segregation, independent
assortment)
• His laws are true in the case of complete
dominance and for genes located on different
chromosomes (or far apart on the same
chromosome)
(Continued) How do current day molecular genetics
support Mendel’s conclusions? Based on his laws,
what were Mendels three conclusions? Does
present day molecular biology support his Laws?
Explain which inheritance patterns support his
claims and which do not
• His laws don’t hold up for co-dominance,
incomplete dominance, sex-linked, polygenic,
and multiple allele
• His laws don’t hold up for genes that are
linked (found close together on the same
chromosome)
What are three things we know now, that Mendel
did not know?
What inheritance patterns do we now know?
• Alleles on genes can be A) codominant or
B)incompletely dominant (more inheritance
patterns than dominant)
• C) Some genes are on the same chromosome and
can be linked.
• All inheritance not due to complete dominance
patterns; we also have co-dominance, incomplete
dominance, sex-linked, polygenic, and multiple
allele
Explain why the Punnett square does not tell you
what each of your four children would be but is
simply a visual that shows the random assortment
and recombination of genes during meiosis and
fertilization
• Shows the likelihood of a genotype for any given child.
What you actually get could be different.
• The alleles of the parents show the possible alleles for
what the gamete (egg or sperm) will contain after
meiosis. The combinations inside the box represent
what the zygote will contain after fertilization.
• For each child, the same probabilities exist, each time a
child is conceived.
Be able to do Punnett Square word problems
• Recognize: heterozygous, homozygous dominant, be
able to identify the genotype of a phenotype,
• Be able to calculate percentages and ratios (labeling all
of them).
• Know what the heterozygous will look like in
codominance (both traits seen) and incomplete
dominance (blending).
• Know that X linked means the male gets ONE allele.
Incomplete dominance example
• A true breeding tall
black rose is crossed
with a true
breeding short
black rose. Show a
punnet square and
identify the
genotypes and
phenotypes that
would result (for
height) if height
was incompletely
dominant
T
T
t Tt
Tt
t Tt
Tt
All offspring would be
medium and heterozygous
Co-dominance example
Assume a codominant
inheritance pattern.
CB
A blue and white flower is
crossed with a pure blue
flower.
What is the probability of
getting a blue flower?
A white flower?
CW
CB CB C B CB CW
CB CB C B CB CW
50% blue, 0% white
Dihybrid test cross
• In the small ones only one for each trait. In a 16
square, it would show 2 (one for each trait)
• You combine the letters for each trait
– SsTt or SSTT, the offspring would have four letters.
– If you do test cross (and you have a heterozygous)
and cross with all recessive you would get 25%
totally recessive for both characteristics
– If you do a test cross and you have a homozygous
dominant, all offspring would be heterozygous.
Dihybrid test cross
• Create two test crosses, one for each
characteristic. For each characteristic
T
?
T
t
t
t
t
?
Dihybrid Blue, Horned bird
• Mother : HhBb
•
HB, Hb, hB, hb
• Father : hhbb
– hb, hb, hb, hb
• OR Father HHBB
• HB, HB, HB, HB
Horned, Blue
no horn, white
Horn, Blue
HB
Hb
hB
hb
HB HHBB HHBb HhBB HhBb
HB HHBB HHBb HhBB HhBb
CBCB CBCW
HB HHBB HHBb HhBB HhBb
CBCB CBCW
HB HHBB HHBb HhBB HhBb
HB
hb
hb
hb
hb
Hb
HhBb Hhbb
HhBb Hhbb
HhBb Hhbb
HhBb Hhbb
hB
hb
hhBb hhbb
hhBb hhbb
CBCB CBCW
hhBb hhbb
CBCB CBCW
hhBb hhbb
What is a sex-linked trait?
• Any gene that is on either the X or Y
chromosome is considered “sex linked”.
• A gene that is on only the X chromosome is Xlinked. All genes that we have talked about
that are on a sex chromosome are on the X
(hemophilia, colorblindness)
X linked show up more in males because:
• A male has only one X, while females have two.
• Since everything we studied on the X is a
recessive disease, a female needs two XhXh and a
male only needs one XhY allele to have the
disease.
• A son can inherit the disease only from his
mother. (he always gets the X from mom.
• A daughter who has the disease MUST have a
father that has the disease, and a mother that
carries the allele.
Linked genes
• Linked genes are genes that are on the same
chromosome (this means whatever alleles are
on that chromosome are transferred together
when homologous chromosomes separate)
• When crossing over occurs, alleles are
exchanged between the two homologous
chromosomes. This means that alleles that
were linked become “unlinked”.
When a pedigree shows an autosomal dominant
disease, what key inheritance patterns do you see in
the pedigree?
• Doesn’t skip generations
(no hidden alleles)
• A child MUST have a
parent that has the
disease
• If both parents have the
disease, children CAN be
born without the
disease
Examples: Achondroplasia
(Dwarfism)
Huntingdon’s disease
When a pedigree shows an X-linked disease, what
key inheritance patterns do you see in the pedigree?
• If offspring have the disease,
they are almost always males.
• A son has only ONE allele that
dictates the disease. Since it is
on the X chromosome it MUST
come from his mother
• If a girl has the disease her
father MUST have the disease.
• All carriers are female
Autosomal recessive
• Parents can both carry the
allele, but not have the
disease, but the offspring will
inherit. Can be carried
Cystic fibrosis
• Since it is NOT on the X
chromosome, both males and
females get this disease
equally.
• If both parents have the
disease, ALL offspring will
have the disease.
Tay Sachs
Sickle cell anemia (sort of )
Polygenic Characteristic
• A characteristic that ADDs together the effect of
several genes In pigment, the more dominant
alleles, the darker the skin. Distribution (normal
curve, or continuous)
• Example: skin color, 3 genes (6 alleles).
– No dominants  albino
– One dominant  light skin, blonde hair
– Five dominants dark brown eyes, skin, hair
– Six dominant  v dk br eyes, skin, hair black.
Polygenic
Normal distribution OR
Continuous distribution OR
Normal curve
Polygenic Characteristic
• A characteristic that ADDs together the effects of
each dominant allele. Distribution normal curve or
continuous)
• Example: height, 3 genes (6 alleles).
– No dominants  short (under 5 ft
– One dominant  5’
– Five dominants 6’ 2
– Six dominant  Tall 6’ 5” or taller
Multiple allele characteristic
• More than two alleles for each gene , BUT,
each person has only two of the possibilities
• Multiple allele is ONE gene with more than
two alleles
• Polygenic is more than one gene each with
two alleles
• Example: Blood type alleles IA, IB, i
•
A and B are dominant to i (O)
Some families have a “baldness” allele that is Xlinked. Is it possible for a female offspring to show
this phenotype? Create a Punnett square that
provides evidence to support your answer.
• If the father has the baldness gene XbY and
the mother is a carrier XBXb, this can happen.
• But the both parents must have the gene and
the father MUST have the disease (or the
phenotype )..baldness is really not a disease.
•
Research indicates that there are several kinds of baldness, and some
types are inherited from the father!!!!!