Heredity

1
Heredity

• Chapter 12, 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 R (red) and R (white).
Heterozygotes (RR) have pink flowers. Unlike the
case in complete dominance, the phenotypes show
us which plants are heterozygous.
5
mother
In humans, a gene affecting hair texture (curly,
wavy, straight) shows incomplete dominance.
C2
C1
eggs
C2
C1
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 (R) or white (R) hairs.
Heterozygotes (RR) 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?
9
Solving single-gene (monohybrid) crosses with
incomplete dominance.
One hair color in cattle is controlled by a gene
that produces red (R) or white (R) hairs.
Heterozygotes (RR) 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?
10
Sex-Linked Traits

• In humans, women have two X chromosomes while men
  have an X and a Y.
• 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.

11
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?
If a boy has an X-linked trait, which parent did
he inherit the trait from?
12
female parent
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.
eggs
X2
X1
Xm
X1
Xm
X2
male parent
Xm
female offspring
Y
sperm
Xm
Y
X1
Y
X2
Y
13
This diagram illustrates a cross for an X-linked
trait in fruit flies. Red eyes are dominant,
white are recessive.
female parent
r
R
Xr
XR
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
XR
Y
14
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
15
Solving X-linked crosses.
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?
16
Solving X-linked crosses.
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?
17
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.

18
A and B alleles produce A and B proteins on the
surface of red blood cells. The O allele produces
neither of these proteins.
19
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)

20
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.
21
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.

22
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.
23
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?
24
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.
25
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.
26
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.
27
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.