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Genetics Notes Part
II
Sex Determination
O Each cell in your body (somatic cells), except
for gametes, contains 46 chromosomes, or
23 pairs of chromosomes.
O One pair of these chromosomes, the sex
chromosomes, determines an individual's
gender.
O The other 22 pairs of chromosomes are
called autosomes.
The Sex Chromosomes
O There are two types of sex chromosomes—X
and Y.
O X chromosome is larger than the Y
chromosome.
O X chromosome carries a variety of genes
that are necessary for the development of
both females and males.
O Y chromosome mainly has genes that relate
to the development of male characteristics.
O XX - Females – have 44 autosomes (22 pair) plus
2 X chromosomes (44 + XX) – The only possibility
that a female can offer is “X”
O XY - Males – have 44 autosomes (22 pair) plus 1 X
and 1 Y chromosome (44 + 1 X + 1 Y)
O The offspring's gender is determined by the
combination of sex chromosomes in the egg and
sperm cell
O When gametes (sex cells) are formed, the
number of chromosomes reduces by ½
(haploid)
O (22 + X) = gamete of female (egg or ovum)
O (22 + X) or (22 + Y) = gamete of male (sperm)
Only sex chromosomes are shown.
We can use a Punnett Square to determine the possible
sex of the offspring.
Alleles:
XX = female
XY = male
Cross a male and a female = XY x XX
•Father (sperm) determines the sex of the offspring.
Twins
O There are two types of twins:
O Fraternal twins – these individuals arise from and
develop from separate eggs fertilized by separate
sperm. They are not necessarily alike
O Identical twins – These individuals started as a single
egg fertilized by one sperm. After the first division of the
egg, the 2 cells separate and start dividing all over
again. It may result in identical twins, triplets, etc….
*Will have the same genetic code
Sex Linked Traits
O Traits controlled by genes located on the X chromosome
are called sex-linked traits, or X-linked traits.
O The sex-linked trait is represented by writing the allele on
the X chromosome.
Common Sex Linked Traits
O Red-green color blindness
recessive X-linked trait.
About 8 percent of males in the United States have
red-green color blindness.
red-green color blindness is very rare in females.
People who are colorblind
See shades of grey instead
of red or green
Why is this trait so rare in
females?
O Because males have only one X chromosome, they
are affected by recessive X-linked traits more often
than are females.
O Females are less likely to express a recessive X-linked
trait because the other X chromosome may mask the
effect of the trait.
O In genetics, a carrier is an individual who has the gene
but not the disease. For X-linked traits, the carrier will
be a female.
O The possible outcomes of a sex linked characteristic
are as follows:
Females: Normal, Normal Carriers, Inflicted (have the
condition)
Males: Normal or inflicted
Ex. Cross a colorblind male
with a female who is normal,
but is a carrier.
Ex. Cross a colorblind male
with a female who is normal,
but is a carrier.
Hemophilia
O Hemophilia, another recessive sex-linked
disorder, is characterized by delayed clotting of
the blood.
O Hemophilia is sometimes called “free
bleeding.”
O more common in males than in females
Ex. Cross a normal female
with a male who is a
hemophiliac.
Ex. Cross a normal female
with a male who is a
hemophiliac.
Multiple Alleles
Not all traits are determined by two alleles.
Some forms of inheritance are determined by
more than two alleles referred to as multiple
alleles. An example of such a trait is human
blood group.
Blood groups in humans
The ABO blood group has three forms of alleles
O IA is blood type A; IB is blood type B; and i is blood
type O. Type O is the absence of AB
O Each individual can only have two of the alleles
O Note that allele i is recessive to IA and IB.
O However, IA and IB are codominant; blood type AB results
from both IA and IB alleles.
O the ABO blood group is an example of both multiple
alleles and codominance.
Blood Allele Combinations
X A
A
B
O
B
O
Blood Allele Combinations
X A
A
B
O
B
O
Blood
Types
A
B
AB
O
Possible
Genotypes
AA or AO
BB or BO
AB
OO
O Blood typing is not connected in any way with the
sex chromosomes
O Blood type is genetically determined by the
presence or absence of a specific complex
carbohydrate found on the surface of red blood
cells
Rh Factors
O The Rh blood group includes Rh factors, inherited
O
O
O
O
O
from each parent.
Rh is another important antigen found on the
surface of red blood cells
Rh factors are either positive or negative (Rh+ or Rh-);
People who have this protein are said to be Rh+
85% of humans have the Rh+
Rh+ is dominant.
The Rh factor is a blood protein named after the
rhesus monkey because studies of the rhesus
monkey led to discovery of the blood protein.
Rh- mothers must be careful!
O If both parents are positive or if both are negative
O
O
O
O
everything is ok.
If the father is Rh- and the mother is Rh+ everything is
ok
BUT if the mother is Rh- and the father is Rh+ problems
can arise – a condition known as Erthroblastosis
When an Rh- mother gives birth to an Rh+ infant, the Rhmother begins to make anti-Rh antibodies which can be
fatal for future pregnancies.
The treatment is typically “RhoGAM” a blood product
that can suppress the ability to respond to Rh+ red blood
cells
More Multiple Alleles
O Multiple alleles can
demonstrate a hierarchy of
dominance.
O In rabbits, four alleles code
for coat color: C, cch, ch, and c.
O the hierarchy of dominance
can be written as C > cch >
ch> c.
O Full color is dominant over
chinchilla, which is dominant over
Himalayan, which is dominant over
albino.
In humans….
O Eye color and skin tone are both
examples of traits that have multiple
alleles
Importance of Multiple Alleles
O The presence of multiple alleles increases the
possible number of genotypes and phenotypes.
GENETIC
DIVERSITY!
Visible traits can be influenced
by more than the genotype.
Hormones and Chemicals
O Genes are present in both male and females, but
show up only in one sex due to the presence of sex
hormones. The examples of this are a man’s beard,
development of breasts and the plumage in birds.
These occur because of the hormone testosterone (in
males) and estrogen (in women).
Genes – Protein - Hormones
O Hormones are like messengers in your body that give
orders to your cells to do particular things.
O Chemicals that are manufactured by glands and
released into the bloodstream, sort of holding a sign up
with a type of cell's name on it (like a limo driver at the
airport waiting to pick up a client.)
O Most cells ignore the hormone, but when it comes
across the right one its chemical "message" is read and
the cell starts to do what it has been told.
O The X and Y chromosomes code for different hormones.
Environmental Influences
The environment also has an effect on phenotype.
O For example, the tendency to develop heart disease
can be inherited. However, environmental factors
such as diet and exercise also can contribute to the
occurrence and seriousness of the disease.
O Sunlight, water, and temperature are environmental
influences that commonly affect an organism's
phenotype.
O The cat's tail, feet, ears, and nose are dark. These areas
of the cat's body are cooler than the rest. The gene that
codes for production of the color pigment in the
Siamese cat's body functions only under cooler
conditions. Therefore, the cooler regions are darker; and
the warmer regions, where pigment production is
inhibited by temperature, are lighter.
Polygenic Traits
O You have examined traits determined by a pair of
genes.
O Many phenotypic traits, however, arise from the
interaction of multiple pairs of genes. Such traits
are called polygenic traits. Traits such as skin color,
height, eye color, and fingerprint pattern are
polygenic traits.
O Human skin color is thought to be controlled by 12
genes!
END DAY 1
More Genetics
Karyotype Studies
O The study of genetic material does not involve the
study of genes alone.
O Scientists also study whole chromosomes by using
images of chromosomes stained during
metaphase.
O The staining bands identify or mark identical
places on homologous chromosomes.
Karyotype
O During metaphase of mitosis, each
chromosome has condensed greatly and
consists of two sister chromatids.
O The pairs of homologous chromosomes are
arranged in decreasing size to produce a
micrograph called a karyotype
Karyotype
O Karyotypes of a human male and a human
female, each with 23 pairs of chromosomes
Notice that the 22 autosomes are matched
together with one pair of nonmatching sex
chromosomes.
Karyotype
Nondisjunction
During cell division, the chromosomes separate, with
one of each of the sister chromatids going to opposite
poles of the cell. Therefore, each new cell has the
correct number of chromosomes.
If sister chromatids fail to separate properly in cell
division nondisjunction occurs
Nondisjunction
O If nondisjunction occurs during meiosis I or meiosis
II, the resulting gametes will not have the correct
number of chromosomes.
O When one of these gametes fertilizes another
gamete, the resulting offspring will not have the
correct number of chromosomes.
Nondisjunction
O Notice that nondisjunction can result in extra
copies of a certain chromosome or only one copy
of a particular chromosome in the offspring.
O Trisomy set of three chromosomes of one kind
O Monosomy only one of a particular type of
chromosome
Nondisjunction
O Nondisjunction can occur in any organism in which
gametes are produced through meiosis.
O In humans, alterations of chromosome numbers
are associated with serious human disorders,
which can be fatal.
Down Syndrome
O One of the earliest known human chromosomal
disorders is Down syndrome. It is the result of an
extra chromosome 21.
O one out of 800
O The frequency of Down syndrome increases with the
age of the mother. Studies have shown that the risk
of having a child with Down syndrome is about six
percent in mothers who are 45 and older.
Sex Chromosomes
O Nondisjunction occurs in both autosomes and sex
chromosomes. Some of the results of
nondisjunction in human sex chromosomes are:
O Turner’s syndrome and Klienfelter’s syndrome
O Note that an individual with Turner's syndrome
has only one sex chromosome. This condition
results from fertilization with a gamete that had
no sex chromosome.
Pedigree
O a scientist studies a family history using a pedigree,
a diagram that traces the inheritance of a particular
trait through several generations.
O A pedigree uses symbols to illustrate inheritance of
the trait. Males are represented by squares, and
females are represented by circles
Pedigree
O Pedigrees help genetic counselors determine whether
inheritance patterns are dominant or recessive. Once
the inheritance pattern is determined, the genotypes
of the individuals can largely be resolved through
pedigree analysis.
Analyzing a Pedigree
O One who expresses the trait being studied is
represented by a dark, or filled, square or circle
O One who does not express the trait is represented
by an unfilled square or circle
O A horizontal line between two symbols shows that
these individuals are the parents of the offspring
listed below them.
O Offspring are listed in descending birth order from
left to right and are connected to each other and
their parents.
O A pedigree uses a numbering system in which
Roman numerals represent generations, and
individuals are numbered by birth order using
Arabic numbers.
O This pedigree shows the inheritance of the dominant
genetic disorder polydactyly. People with this disorder have
extra fingers and toes.
O Recall that with dominant inheritance the trait is expressed
when at least one dominant allele is present. An individual
with an unaffected parent and a parent with polydactyly
could be either heterozygous or homozygous recessive for
the trait. Each unaffected person would be homozygous
recessive for the trait.
O individual I2 has polydactyly, indicated by the dark
circle. Because she shows the trait, she is either
homozygous dominant or heterozygous. It can be
inferred that she is heterozygous—having one
dominant gene and one recessive gene—because
offspring II3 and II4 do not have the disorder.
O Notice that II6 and II7, two unaffected parents, have
an unaffected offspring—III2. What can be inferred
about II2, based on the phenotype of her parents
and her offspring?