Download Unit 8 Review B b B BB Bb B Bb bb B bb Bb bb b Bb bb

Unit 8 Review
Complete Dominance
In some cats the trait for brown hair (B) is dominant to the trait for black (b) hair. Two brown
cats are crossed. Most of the offspring have brown hair, and the rest of the offspring are black
(bb). Create a Punnett square showing a cross of two brown cats that would result in some of
the offspring being black.
B
b
B
BB
Bb
B
Bb
bb
If hair color is completely
dominant, the only way that two
brown cats could have a brown
kitten is if both of the parents
were heterozygous for the trait.
Certain kidney diseases are genetic and can be passed down from one generation to the next.
In most cases these ate caused by a dominant allele. What is the chance that a child would
have kidney disease if the father is unaffected and the mother is heterozygous for the disease?
In order for the
father to be
unaffected he
must be
homozygous
recessive (No
copy of the
dominant allele)
B
b
b
Bb
bb
b
Bb
bb
Mom is
heterozygous
There would be a 50% chance
that children of these parents
would have kidney disease.
But what happens if we know the phenotypes of the children, but knot of the parents? Can will
still figure this out? Albinism for example is caused by a recessive allele. What would you
expect the parents genotypes to be if they had 4 children, 3 that are normal and 1 that is albino.
?
?
?
AA
Aa
?
aa
Aa
aa
In order for one of the children to be
albino, both of the parents must be
heterozygous (Aa) {have at one copy
of the albino allele}
A
a
A
AA
Aa
a
aa
Aa
aa
Test Crosses
In many plants the allele for being tall (T) is dominant to the allele for being short (t). if you
have a plant whose phenotype is tall (what we see) but do not know its genotype {remember it
could be (TT) or (Tt)} you would cross it with a homozygous recessive (tt) in what is called a
test cross. If all the plants are tall then the unknown parent is homozygous dominant (TT). If
half the plants are short, then the unknown parent must be heterozygous (Tt).
Homozygous Dominant Unknown Parent
T
T
t
Tt
Tt
t
Tt
Tt
ALL TALL
Heterozygous Unknown Parent
T
t
t
Tt
tt
t
Tt
tt
50% tall
50% short
Incomplete Dominance
In some organisms, however, an individual displays a trait that is intermediate between the two
parents, a condition known as incomplete dominance. For example, when a snapdragon with red
flowers is crossed with a snapdragon with white flowers, a snapdragon with pink flowers is
produced. Neither the red nor the white allele is completely dominant over the other allele.
Cholesterol levels in humans are a condition controlled by alleles that are incompletely
dominant.
Genotype
Expressed Phenotype
NN
Normal cholesterol levels
Nn
Slightly elevated cholesterol levels
nn
Greatly elevated cholesterol levels
If two parents with slightly elevated cholesterol levels (Nn) marry, their children could have
three different levels of cholesterol.
N
N
n
NN
Nn
25% would have normal cholesterol
50% would have slightly elevated levels
N
N
Nn
Tt
nn
Tt
25% would have greatly elevated levels
Dihybrid Crosses
You must be able to read a Punnett square showing a dihybrid cross like the one below.
Purple (P) is dominant to Yellow (p) and Smooth texture (S) is dominant to shrunken (s)
In a cross between parents that are heterozygous for both traits (PpSs) you would expect the
following genotypes.
9 Purple and Smooth
3 Purple and Shrunken
3 Yellow and Smooth
1 Yellow and Shrunken
Sex Linked Inheritance
If a trait is autosomal, it will appear in both sexes equally. Recall that an autosome is a
chromosome other than an X or Y sex chromosome. If a trait is sex-linked, it is usually seen only
in males. A sex-linked trait is a trait whose allele is located on the X chromosome. Most sexlinked traits are recessive. Because males have only one X chromosome, a male who carries a
recessive allele on the X or Y chromosome will exhibit the sex-linked condition. A female who
carries a recessive allele on one X chromosome will not exhibit the condition if there is a
dominant allele on her other X chromosome. She will express the recessive condition only if she
inherits two recessive alleles. Thus, her chances of inheriting and exhibiting a sex-linked
condition are significantly less.
Dominant or Recessive? If the trait is autosomal dominant, every individual with the trait will
have a parent with the trait. If the trait is recessive, an individual with the trait can have one,
two, or neither parent exhibit the trait.
Heterozygous or Homozygous? If individuals with autosomal traits are homozygous dominant
or heterozygous, their phenotype will show the dominant characteristic. If individuals are
homozygous recessive, their phenotype will show the recessive characteristic. Two people who
are heterozygous carriers of a recessive mutation will not show the mutation, but they can
produce children who are homozygous for the recessive allele.
Hemophilia is a sex linked trait in humans that causes the blood to not clot properly What if a
woman is a carrier for hemophilia XXh what would most likely be the patterns of inheritance of
hemophilia in her children.
h
X
X
X
XX
X X
Y
XY
X Y
h
h
Because hemophilia is sex linked and
recessive more of the more of the
male children will be affected than
female.
Pedigree Analysis
Imagine that you want to learn about an inherited trait present in your family. How would you
find out the chances of passing the trait to your children? Geneticists often prepare a pedigree, a
family history that shows how a trait is inherited over several generations. Pedigrees are
particularly helpful if the trait is a genetic disorder and the family members want to know if they
are carriers or if their children might get the disorder. Carriers are individuals who are
heterozygous for an inherited disorder but do not show symptoms of the disorder. Carriers can
pass the allele for the disorder to their offspring.
I
II
III
The gene for the ability to
taste phenylthiocarbamide
(PTC) is a dominant allele
over the non-tasting gene.
Notice that Female A in
generation III has the ability
to taste PTC, while Female B
in the same generation does
not.
How many individuals in
generation II can taste PTC?
The trait in the above pedigree is autosomal and recessive because it appears in both sexes
almost equally. If it were sex linked, it would appear more often in males than in females.
DNA Fingerprinting
Mrs. Brown had baked a birthday cake for her husband
and then left it in a tin on the kitchen table. When she
returned from shopping all that was left in the cake tin
was a few crumbs and a smudge of blood where the
thief had snagged their finger on the sharp edge of the
tin. Below are the DNA fingerprints of her four
children. Which one of the children’s DNA fingerprints
most closely matches the blood stain and therefore is
most likely to be the thief?
The correct answer is John because he shares 9
alleles with the blood stain.
• Bob only shares 7 alleles
• Sue share 4, and
• Lisa shares 4
Important Things To Remember
Genes can be autosomal (all of our body cells) or sex cells (gametes / sperm or eggs). Only
traits carried in the sex cells (gametes/sperm and egg) can be passed down from one generation
to the next. A mutation in a skin cell, blood cell or muscle cell will not be passed down to the
next generation.
Genetic variations can help an organism survive when the environment changes. A mutations
that leads to pesticide resistance might lead an insect to better survive than one that does not
receive the mutated allele.