Heredity

1
Heredity

• Chapter 10, part 2

2
Beyond Mendels Laws

• Not all traits are controlled by single genes
  with dominant and recessive alleles. Other
  patterns of heredity involve
• Incomplete dominance
• Sex-linked traits
• Multiple alleles
• Multiple genes (most traits involve this)
• Chromosomal abnormalities

3
Incomplete Dominance

• In incomplete dominance and co-dominance, both
  alleles are expressed in the phenotype.
• incomplete dominance two traits appear to blend
  in the heterozygotes.
• co-dominance both traits appear in the
  heterozygotes.

4
These snapdragons have two alleles controlling
flower color R1 (red) and R2 (white).
Heterozygotes (R1 R2) have pink flowers. Unlike
the case in complete dominance, the phenotypes
show us which plants are heterozygous.
R1 R1
R2 R2
R1 R2
R1 R2
R1 R2
R1 R2
R1 R1
R2 R2
5
mother
In humans, a gene affecting hair texture (curly,
wavy, straight) shows incomplete dominance.
C2
C1
C2
C1
eggs
father
C1
C1
C1
C1
C2
sperm
C2
C1
C2
C1
C2
C2
C2
6
The golden palomino horse is a cross between a
white and a brown horse. This is another example
of incomplete dominance the colors appear to
blend in the horses hairs.
7
The red roan horse has both white and red-brown
hairs, while the blue roan has both white and
gray hairs. The coat colors of both parents are
expressed in the hairs. This is co-dominance.
At the gene level, incomplete dominance and
co-dominance are the same in both cases, both
alleles are expressed in the heterozygote. The
only difference is at the phenotype level.
8
Solving single-gene (monohybrid) crosses with
incomplete dominance.
One hair color in cattle is controlled by a gene
that produces red (R1) or white (R2) hairs.
Heterozygotes (R1 R2) are roan. a. What color
would the offspring of a red bull and a white cow
be? b. What are the phenotypic ratios of a cross
between a white cow and a roan bull?
Both parents are homozygous. What gametes can
they produce?
A red bull (R1R1) produces these gametes
A white cow (R2R2) produces these gametes
R1

R2
R1R2
All offspring are roan and heterozygous
9
Solving single-gene (monohybrid) crosses with
incomplete dominance.
One hair color in cattle is controlled by a gene
that produces red (R1) or white (R2) hairs.
Heterozygotes (R1 R2) are roan. a. What color
would the offspring of a red bull and a white cow
be? b. What are the phenotypic ratios of a cross
between a white cow and a roan bull?
As always begin with the gametes each parent can
produce.
A roan bull (R1R2) produces these gametes
A white cow (R2R2) produces these gametes
R1
R2
R2
R1R2
R2R2
1/2 of offspring are roan 1/2 are white
10
W O R K T O G E T H E R

• Pure-breeding red radishes (R1R1) crossed with
  pure-breeding white radishes (R2R2) produce
  purple radishes.
• What is the genotype of the purple radishes?
• What are the phenotypic ratios of a cross between
  a purple radish and a white radish?

11
Melvin takes pollen from a plant with red flowers
and pollinates the flowers on a plant with white
flowers. If color in these flowers shows
incomplete dominance, what will the offspring
look like?

1. All red
2. All white
3. All pink
4. Red or white, but not pink
5. Red, white, or pink

12
Sex Chromosomes

• In humans, genetic sex is usually determined by
  the sex chromosomes.
• Typically, women have two X chromosomes, while
  men have an X and a Y (exception the rare XY
  female, due to non-expression of the sry gene on
  the y chromosome during fetal development)

Gender, however, is a complex issue, so saying
XXfemale, XY male is an oversimplification.
13
A Karyotype is an image of the replicated
chromosomes in prophase. All chromosomes have
matching homologues except the 23rd pair in males.
Homologous chromosomes
Paired sister chromatids
The X and Y chromosome each have their own unique
genes.
To make a karyotype, a photo of a cell in
prophase is cut apart and the homologues are
carefully matched.
14
Sex-linked Traits

• Traits that are carried on the sex chromosomes
  will show different genotypic and phenotypic
  rations in men and women.
• The X chromosome has many genes, while the Y has
  only a few, so there are many more X-linked
  traits than Y-linked traits.

15
Women pass their X chromosomes to their children.
Men can contribute either an X or a Y.
Which parent determines the sex of the child?
Mother or father?
Can men be carriers of a recessive X-linked trait?
No!
If a boy has an X-linked trait, which parent did
he inherit the trait from?
Mother
16
When determining the outcome of a cross that
involves an X-linked trait, we have to take into
account how the two sex chromosomes are
distributed in the offspring.
female parent
XA
Xa
eggs
Xa
XA
XA
XA
XA
Xa
male parent
Nettie Stevens was one of the first researchers
to discover the patterns of X-linked inheritance.
XA
female offspring
sperm
XA
Y
Y
XA
Y
Xa
Y
male offspring
17
This diagram illustrates a cross for an X-linked
trait in fruit flies. Red eyes are dominant,
white are recessive.
female parent
r
R
Red eyed carrier
Xr
XR
r
R
eggs
Xr
XR
R
R
r
R
R
all the F2 females have red eyes
male parent
XR
XR
XR
Xr
XR
female offspring
sperm
R
r
half the F2 males have red eyes, half have white
eyes
Y
XR
Y
Red eyed
Y
Y
XR
Xr
male offspring
18
Where does a boy inherit his X chromosome from?

1. His mother
2. His father
3. Equal chances that it could come from either
   parent.

19
Red-green color blindness is an X-linked trait.
Charts such as these are used to diagnose
red-green color blindness. They look very
different for those with normal vision and those
with RG-color blindness.
Normal RG color-blind
A 29 70
B 45 nothing
C abstract 5
D 26 nothing
20
W O R K T O G E T H E R
I have one brother who is red-green color-blind.
Where did he inherit the color-blind gene from?
Mom or Dad? What are the odds that I am a
carrier?
21
Solving X-linked crosses.
In sex-linked problems, we must track the gene
and the chromosome that carries it.
Red-green color-blindness is X-linked. If a man
and woman with normal vision have a color-blind
son a. What are the genotypes of the parents? b.
What are the odds of having another color-blind
son? c. What are the odds of having a color-blind
daughter?
An X with a normal allele is shown like this
An X with an affected allele is shown like this
Xb
XB
The father is not color-blind so must not have an
affected allele. His genotype is
The mother is not color-blind, but can be a
carrier. Her genotype must be
XB Y
XB Xb
22
Solving X-linked crosses.
To set up the Punnet square, keep the chromosome
and its allele together as one unit.
Red-green color-blindness is X-linked. If a man
and woman with normal vision have a color-blind
son a. What are the genotypes of the parents? b.
What are the odds of having another color-blind
son? c. What are the odds of having a color-blind
daughter?
Father can make these gametes
XB
Y
Mother can make these gametes


XB
XB XB
XB Y
Xb
XB Xb
Xb Y
Notice there are both girls and boys here, so we
have to express the odds separately for each
23
W O R K T O G E T H E R

• Hemophilia is an X-linked trait. If a man who is
  normal for the blood-clotting protein marries a
  woman who is a carrier, what are the odds that a
  daughter of theirs will have hemophilia? What are
  the odds that a son will have this disorder?
• What would the phenotypes of the parents have to
  be to produce a daughter with hemophilia?

24
W O R K T O G E T H E R

• Adeline is red-green color blind. Her husband,
  Gaston, has normal vision. Theyve come to you, a
  genetic counselor, to find out what the chances
  are that any of their children will inherit
  Adelines red-green color blindness. The chances
  for girls will be different from the chances that
  boys will inherit your red-green color
  blindness, you say. They look puzzled and ask
  why that is? Create a Punnet square to help them
  understand.

25
Odin Orange Male
Licorice Black Male
In cats, one coat color gene on the X chromosome
has two alleles orange and black.
Sprocket Calico (orange and black) Female
26

• Cats also have X and Y chromosomes. In cats, one
  coat color gene is on the X chromosome. One
  allele for this gene produces black fur. The
  other allele produces orange fur. Use this to
  explain why male cats can be orange OR black, but
  normally only females can be calico or tortoise
  shell (orange AND black).

W O R K T O G E T H E R
27
In humans, whose gamete determines the (genetic)
sex of the child?

1. Moms
2. Dads
3. Equal odds that it could be either
4. No one knows

28
Duschennes muscular dystrophy (DMD) is a
devastating neurological disorder that often
causes death before the teen years. It is caused
by a recessive allele carried on the X
chromosome. Is it likely that a girl could be
born with this disorder?

1. Yes, if her mother is a carrier.
2. Yes, if both parents are carriers.
3. No, shed have to have a father with DMD, which
   is nearly impossible.
4. No, girls never show X-linked traits.

29
Multiple Alleles

• Human blood type (A, B, AB, and O) is determined
  by a gene that has three alleles.
• A and B are co-dominant
• O is recessive to both
• Though there are three alleles, each person still
  has only two copies of the ABO gene.

30
A and B alleles produce A and B proteins on the
surface of red blood cells. The O allele produces
neither of these proteins.
31
W O R K T O G E T H E R

• Edmund has type B blood. His father had type AB,
  while his mother had type O. His wife, Francesca,
  has type A blood. Both of her parents had type
  AB. What possible blood types can Edmund and
  Francescas children have?

32
W O R K T O G E T H E R

• Barney has type AB blood. Betty has type A blood.
  What blood types can their children have? What
  types can they NOT have?
• Bernadine has type B blood. Her husband, Rupert,
  has type A blood. Their daughter, Opal, has type
  O blood. Assuming no infidelities, how did this
  happen?

33
W O R K T O G E T H E R

• Orson and his wife, Laurita, just had a child.
  But Orson is not pleased. He has his suspicions
  about his neighbor, Horatio, and suspects Horatio
  and Laurita have been a little too friendly. He
  orders blood tests done and finds that he has
  Type A blood. Laurita is also Type A. But the
  baby is Type O. Horatio gallantly offers to have
  his blood tested, and finds he has Type AB. Who
  can and who cannot be the father of this child?

34
Genevieve had a blood test done to find out her
blood type and learned that she inherited an A
allele from her father and a B allele from her
mother. What is her blood type?

1. A
2. B
3. AB
4. O

35
Agnes decides to give blood for the first time.
Shes looking forward to getting her blood donor
card so that she can find out what her blood type
is. Both of my parents are type AB, she says,
so I know my blood type cant be

1. A
2. B
3. AB
4. O

36
Multiple Genes

• Most human traits are the result of multiple
  genes.
• In some cases (such as skin and hair color),
  there are multiple copies of the same gene (such
  as the melanin gene).
• In many others, there are many different genes
  controlling a trait, and the environment may
  affect how a trait is expressed. (Example human
  height)

37
Human skin color is controlled by at least three
melanin-producing genes, which are incompletely
dominant.
eggs
sperm
This massive Punnet square shows a cross between
two people who are heterozygous for all three
genes.
38
Chromosomal Abnormalities

• Chromosomal abnormalities include
• nondisjunctions (failure of chromatids to
  separate during meiosis)
• deletions of parts of chromosomes
• Most chromosomal abnormalities cause cell death,
  but a few are survivable.

39
Cri-du-chat syndrome is caused by a deletion of a
large segment of chromosome 5. Cri-du-chat
children often have small head circumference and
are severely cognitively challenged. Some may
have heart defects, muscular or skeletal
problems, or vision problems.
40
Trisomy 21 results in Down Syndrome. Using what
you know about meiosis, explain how a fertilized
human egg cell can end up with three copies of
chromosome 21. If a person with Down Syndrome
planned to have a child, could the child inherit
Down Syndrome?
41
Nondisjunction of the sex chromosomes is more
often survivable than nondisjunctions of somatic
chromosomes. As long as the fetus has at least
one X chromosome, it can survive.
42
Klinefelter syndrome produces an XXY male. At
puberty, Klinefelter males fail to fully develop
secondary sex characteristics. Men with this
syndrome may or may not be sterile. They have a
tendency to gain weight easily and their muscle
mass is underdeveloped, but mental function is
usually normal.
43
Turner syndrome occurs when a girl inherits only
one X chromosome. Turner children are often
short, and may show swelling in the hands and
feet. Some have heart defects, but most are
cognitively normal. Hormone therapy at puberty
can help Turner girls grow to normal height and
develop secondary sex characteristics.
44
W O R K T O G E T H E R

• Use what you know about meiosis and the sex
  chromosomes to explain
• how a boy could be born with two X chromosomes.
• how a girl could be born with only one X
  chromosome.
• why there are no Turner boys.

45
Recap

• Traits inherited by classic Mendelian genetics
  are by far in the minority.
• Incomplete dominance, sex linkage, and multiple
  alleles involve single-gene traits that show
  unique patterns of inheritance.
• Most traits involve multiple genes and
  gene-environment interactions.