Genetic Crosses Dominant and
Recessive Genes Incomplete Dominance
Pedigree Studies Sex Determination Gregor Mendel
Mendel’s 1st Law Mendel’s 2nd
Law Summary
of Mendel’s Laws
Genetics
is the science of heredity and how new life changes and varies in
their characteristics. Sexual reproduction in humans involves 2 gametes; one male gamete, the sperm,
and one female gamete, the egg.
As was discussed in the cell
division webpage the gametes are haploid
(n). When fertilisation occurs the resulting fertilised egg is diploid (2n) Genetic variations occur
as a result of this union.
As was discussed in the heredity
webpage the physical characteristics of organisms are developed as the
protein builds up their bodies. These proteins are formed as a result of genes
carried on chromosomes.
In genetics genes are
represented by letters. There are usually 2 different types of the same
gene; one is dominant and one is recessive. An example of this
is: T is a gene for tall and t is a gene for short. The two versions
of the same gene are called alleles.
The two alleles are formed at the same position, or locus, on the chromosome.
REMEMBER: THE 2 ALLELES ARE GOTTEN FROM
THE 2 GAMETES OF SEXUAL REPRODUCTION. THAT IS WHY THEY CAN DIFFER.
IN ASSEXUAL REPRODUCTION THE NEW LIFE
IS ALWAYS IDENTICAL TO THE PARENT CELL BECAUSE THERE IS ONLY 1 PARENT.
Dominant
genes will always prevent the recessive gene from working. If a person
has 2 dominant alleles for tallness: T T then he will be tall. If a person has 1 dominant and
I recessive allele for tallness: T t then he will be tall. The only way the recessive gene
will be expressed is if he has 2 recessive alleles for short: t
t then he will be short.
If the pair of genes controlling the characteristic has
identical alleles, TT or tt, we call it a homozygous pair of genes. If the pair
of genes controlling the characteristic are different alleles, Tt or Cr, or Cw we call it a heterozygous pair of genes.
When we express the genotype
of a characteristic we state the pair of alleles. Genotype examples are TT, Tt, tt, BB, Bb, bb, etc.
When we state the physical characteristic expressed by the
genotype we are stating is phenotype.
The phenotype for TT is tall while
the phenotype for tt is short. These phenotypes could vary
because of environment effects. This
is especially true in terms of genotypes and phenotypes for intelligence. The
upbringing, environment, and education experiences greatly affect the
phenotype.
REMEMBER: THE 2 ALLELES ARE GOTTEN FROM
THE 2 GAMETES OF SEXUAL REPRODUCTION. THAT IS WHY THEY CAN DIFFER.
When working on genetic crosses you must state the capital letter which represents the
dominant allele and the small letter
that represents the recessive allele.
The following is an example of how to
work out genetic crosses:
|
A is a dominant characteristic. |
a is a recessive characteristic. |
|
This bird has two genes for red feathers. Its genotype is AA. Its phenotype is RED |
This bird has two genes for blue feathers. Its genotype is aa. Its phenotype is BLUE |
This Punnett Square shows how we can diagram
the genes.
The orange bird has two
dominant A genes. We put two A s along the top of the square.
The blue bird has two
recessive a genes. We put two a s down along the left side of the
square.
All the offspring have
the genes Aa.
They will all have orange feathers (phenotype), but will carry a recessive gene for blue
feathers (genotype).The progeny are the offspring produced.
This is called
the F1 generation progeny.
Now suppose that two individuals from the F1
generation become parents. Here they are!
The baby birds are called the F2 generation. You can see how their genes work
out. The offspring are coded in the squares. One bird will be orange with two AA genes.
Two birds will be orange with genes coded Aa.
One bird will be blue and will have two recessive aa genes.
Individual nests of birds may not turn out exactly like this, but if there are
many baby birds, they will work out genetically with the ratios 1:2:1.
Genotype: 1 AA, 2 Aa,
1 aa
Phenotype: 3 orange feathers, 1 blue feathers
Punnett
Square Animated Example
Incomplete
dominance is the situation where two different alleles are equally
dominant. When this occurs the heterozygous genotype
that is produced is an intermediate phenotype (blend) between the two respective homozygous genotypes. This is
also called codominance.
In this
example AA genotypes have red, Aa genotypes pink and aa genotypes whitish flowers. Note that the heterozygous genotype Aa is a blend of red and white.
Note: This
is the F1 generation.
In the F2 generation two pink flowers will
produce 2 pink phenotypes, 1 red phenotype, and 1 white phenotype.
A pedigree is a diagram showing the genetic history of a group of related organisms.
A pedigree showing the occurrence of a recessive
trait in three generations of a family
.
Circles in a pedigree represent females and squares
represent males. A horizontal line between a circle and a
square indicates a marriage or
partnership. Vertical lines indicate
the children from the marriage
or partnership. In this activity, a
filled-in circle or square shows that the individual has both alleles for the
trait. A half-filled-in circle or square indicates that the individual has one
recessive allele for the trait.
Our cells have 46 chromosomes. There
are 23 pairs. Remember, we get 23 from our mother and 23 from our father. We have
22 pairs of chromosomes called autosomes.
These are the chromosomes that control our body growth, enzymes, etc. We have 1 pair of sex chromosomes.
If the person is a male that pair is composed of an X and a Y chromosome. The male’s genotype is XY.
If the person is a female she will have a sex
chromosome composed of 2 Y chromosomes. The
female’s genotype is XX.
The
If
the child has a genotype of XX then it
becomes a girl. If the child has a genotype of XY then it becomes a boy.
As you can see the ratio of males to females is
1:1.
Gregor Mendel studied 7 characteristics of pea plants. He studied:
As a result of Mendel’s work the study of genetics
began. He discovered that, although an organism may have genotypes for 2
different physical traits (phenotypes)
the organism will only exhibit one of those traits.
The work he did for the height of the plants is a
follows:
He
discovered although the parents of the first generation had TT and tt genotypes all the progeny of the F1 generation were
tall.
He
then discovered that in the F2 generation there was a 3:1 ratio between tall
and short.
The
The
Note
that in the F1 generation the parents were homozygous. One was TT and one was tt. All the progeny were tall because all the progeny had Tt genotypes with T being the dominant characteristic.
In
the F2 generation a Tt plant was used to self-pollinate itself. All of the
phenotypes in this combination were Tt. As a result
the F2 progeny were 3 Tt and1 tt.
Mendel’s Laws
First Law- The Law of Segregation
1.
This
law states:
a.
In
diploid organisms, chromosomes occur in matching pairs. These pairs are called homologous chromosomes. These are
chromosomes that pair at meiosis, have the same length and banding pattern plus
carrying the same genes at the same locus. They have the same sequence of
genes.
Notice that these two chromosomes are homologous
because they have alleles at the same position on the chromosome but one allele
is for purple flowers and the other for white flowers.
b.
For each characteristic or trait organisms inherit
two alternative forms of that gene, one from each parent. These alternative
forms of a gene are called alleles.
c.
When
gametes (sex cells) are produced, allele pairs separate or segregate leaving
them with a single allele for each trait.
d.
When the two alleles of a pair are different, one is
dominant and the other is recessive.
Example:
If your eyes are blue, green or grey
you have two alleles for blue eyes (bb),
then your gametes must have a blue allele (b); if your eyes are brown you might have two brown
allele (BB), then
your gametes have one allele for brown (B)
or you might have one
allele of each kind (Bb),
in which case you make two kinds of gametes some contain the brown allele (B) and some contain the blue allele
(b).
Second Law- Law of Independent Assortment
Mendel’s Second
Law involves dihybrid crosses.
Dihybrid crossing involves the study of 2 characteristics at the same time.The Law of
Independent Assortment states that alleles
for different traits are
distributed to sex cells (& offspring) independently of one another.
Mendel noticed during all his work that the height of the plant and the
shape of the seeds and the color of the pods had no impact on one
another. In other words, being tall didn't automatically mean the plants
had to have green pods, nor did green pods have to be filled only with wrinkled
seeds, the different traits
seem to be inherited independently.
The genotypes of our parent pea plants will be:
RrGg x RrGg where
"R" = dominant allele for
round seeds
"r" = recessive allele for
wrinkled seeds
"G" = dominant allele for
green pods
"g" = recessive allele for
yellow pods
Notice that we are dealing with two different traits: (1) seed texture (round or
wrinkled) & (2) pod color (green or yellow). Notice also that
each parent is hybrid for each trait (one dominant & one recessive allele
for each trait).
|
|
RG |
Rg |
rG |
rg |
|
|
RG |
RRGG |
RRGg |
RrGG |
RrGg |
|
|
Rg |
RRGg |
RRgg |
RrGg |
Rrgg |
|
|
rG |
RrGG |
RrGg |
rrGG |
rrGr |
|
|
Rg |
RrGg |
Rrgg |
rrGg |
rrgg |
|
We need to
"split" the genotype letters & come up with the possible gametes
for each parent. Keep in mind that a gamete (sex cell) should get half as
many total letters (alleles) as the parent and only one of each letter.
So each gamete should have one "R
or r" and one "G or g"
for a total of two letters. There are four possible letter combinations:
RG, Rg, rG, and rg. So, when the two parents’ gametes form a new organism
the punnett square will look like this:
The results from a dihybrid cross
are always the same:
9/16 boxes (offspring) show dominant phenotype
for both traits (round & green),
3/16 show dominant phenotype for first
trait & recessive for second (round & yellow),
3/16 show recessive phenotype for first
trait & dominant form for second (wrinkled & green), &
1/16 show recessive form of both traits
(wrinkled & yellow).
So, as you can
see from the results, a green pod can have round or wrinkled seeds, and the
same is true of a yellow pod. The different
traits do not influence the inheritance of each other. They are inherited
INDEPENDENTLY.
Interesting to
note is that if you consider one trait at a time, we get "the usual"
3:1 ratio of a single hybrid cross (like we did for the Law of Segregation).
For example, just compare the color trait in the offspring; 12 green & 4
yellow (3:1 dominant: recessive). The same deal with the seed texture; 12
round & 4 wrinkled (3:1 ratio). The traits are inherited
INDEPENDENTLY of each other.
Animation
Explaining Independent Assortment of Genes
|
LAW |
PARENT CROSS |
OFFSPRING |
|
DOMINANCE |
TT x tt |
100% Tt |
|
SEGREGATION |
Tt x Tt |
75% tall
|
|
INDEPENDENT ASSORTMENT |
RrGg x RrGg |
9/16 round seeds & green pods |
Click
here to go to practice questions using Punnett squares
Click
here to go to practice questions about genetics and Mendel’s Laws
Interactive
animation with Mendel’s peas
TOP
Linkage (Gene Linkage)
Gene
linkage occurs when traits for 2 separate characteristics occur on the same
chromosome.
The characters Mendel examined happened to be on separate chromosomes. That is
why he observed independent assortment. If,
however, the genes are on the same chromosomes, they will be inherited together.
For example, consider the following parental nuclei. Both father and mother
have a pair of chromosomes with alleles for two different genes:
If we look at this with a punnet square what is going
to happen in the next generation:
The phenotype ratios are still 3:1,
but there are fewer genotype combinations than in the usual cross involving two
alleles.
Remember: With independent assortment
the phenotypes resulted in a 9:3:3:1 ratio.
View
this animation of crossing over
Sex-linked Genes or
Sex linkage
Genes
or traits whose controlling genes are on the X sex chromosome but not on the Y
sex chromosome. As a result recessive
phenotype occurs more often in males than in females.
There is yet another, unrelated, special
case that means something totally different, yet has a similar-sounding name.
This is sex-linked genes, genes located on one of the sex chromosomes (X or Y) but not the other. Since,
typically the X chromosome is longer, it bears a lot of genes not found on the
Y chromosome, and thus most sex-linked genes are X-linked genes. One
example of a sex-linked gene is fruit fly eye colour. An X chromosome carrying
a normal, dominant, red-eyed allele would be symbolized by a plain X,
while the recessive, mutant, white-eyed allele would be symbolized by X'
or Xw. A fly with genotype XX'
would normally be a female with red eyes, yet be a carrier for the
white-eyed allele. Because a male typically only has one X chromosome, he would
normally be either XY and have normal, red eyes or X'Y and have white eyes. The
only way a female with two X chromosomes could have white eyes is if she would
get an X' allele from both parents making her X'X' genotype. The cross between
a female carrier and a red-eyed male would look like this:
|
|
X |
Y |
||||
|
X |
|
|||||
|
X' |
||||||
Notice that while there is a “typical”
ratio of ¾ red-eyed to ¼ white-eyed, all of the white-eyed flies are males.
Sex-linked traits act just like recessive
ones except they also bow to the will of the sex of the child. Genes are
carried on things called chromosomes. Most people already know that in
humans the man has an X and Y chromosome and the female has two X chromosomes.
This is the reason that only the man can determine the sex of the child. Women
can only provide X chromosomes while a man can provide either. Now, the X
chromosome is bigger and can carry more genetic information on it than the Y
can.
This is where sex-linked traits come in.
Because the X is bigger it means that some genes carried on it are not carried
on the Y chromosome. These genes can be expressed even without a corresponding
partner on an X chromosome. They also cannot be blocked out unless there is
another X chromosome carrying a dominant partner.
In humans, two well-known X-linked traits
are haemophilia and red-green colour-blindness. Haemophilia is the
failure (lack of genetic code) to produce certain substance needed for proper
blood-clotting, so a haemophiliac’s blood doesn’t clot, and (s)he could bleed
to death from an injury that a normal person might not even notice.
One human sex-linked trait is
Haemophilia. Haemophilia is a disease that keeps a person's blood from clotting
when he is cut. Haemophiliacs can easily bleed to death and must be very
careful not to injure themselves. Many also take daily injections to help the
problem. Because haemophilia is a sex-linked disease most of the people who
have it are men. Women can carry the gene for haemophilia but will not be
affected by it because their second X chromosome will block it out with a
healthy gene. They must have two copies of the defective gene to display the
disease. Inheriting two copies is highly unlikely. Men carrying haemophilia do
not have another X chromosome so they will have haemophilia with only one gene
for it. Mothers carrying one gene for haemophilia take a great risk with having
children because there is a 50% chance their sons will end up with the disease.
Here's how it works:
As you can see, at least half of her
children (boxes 1 and 2) will inherit the defective gene. One, a daughter, will
only carry the gene. The other, a son, will have the disease haemophilia. The
last two children (boxes 3 and 4) will carry healthy genes. Of course these are
only the possibilities of what her children could end up with. She could very
well end up giving it to all her children or none at all. It's just a matter of
chance. Now let's take a look at what will happen if this woman's haemophiliac
son has children with a healthy woman:
In this case half the children will
still inherit the gene. All the sons will be safe but all the daughters will
end up carrying haemophilia and could end up passing it on to their children.
It is in this way that sex-linked genes can disappear and reappear from
generation to generation.
1.
Which of the following is a possible abbreviation for a genotype?
A. BC
B. Pp
C. Ty
D. fg
2. What is the best way to determine the phenotype of the feathers on a
bird?
A. analyze the bird's DNA
(genes)
B. look at the bird's feathers
C. look at the bird's beak
d. examine the bird's droppings
3. Which of the following pairs