Download Genetic crosses: Rules of the game

GENETIC CROSSES: RULES OF THE GAME
MAKING MELANIN PIGMENT
The TYR gene is just one of many genes on the human number-11 chromosome.
The TYR gene encodes a protein that functions as the enzyme, tyrosinase. This
enzyme catalyses a step in the pathway that produces the pigment, melanin.
Melanin pigment is seen in the hair, the skin and the irises of a person’s eyes. Melanin
pigment is present not only in people, but also in other vertebrate species in their skin,
eyes and fur (in the case of mammals), feathers (in the case of birds) and scales (in
the case of reptiles).
Melanin pigmentation is produced in a multi-step pathway in special cells known as
melanocytes.
The enzyme tyrosinase catalyses one step in the pathway that produces melanin.
Without a functioning tyrosinase enzyme, melanin production cannot occur and this
results in a condition known as albinism.
The TYR gene in humans has two common
alleles:
 allele A that produces normal tyrosinase enzyme,
resulting in normal pigmentation
 allele a that produces a faulty protein that cannot
act as the tyrosinase enzyme, resulting in a lack of
pigment (albinism).
Persons with genotypes AA or Aa have
normal pigmentation. In contrast, persons
with the aa genotype lack a functioning
tyrosinase enzyme and so have albinism.
From this, we can conclude that normal
pigmentation phenotype is dominant to
albinism
RULES OF THE GENETIC GAME
A cross involving the alleles of the TYR gene is a monohybrid cross because it involves
the alleles of just one gene at a time.
When the alleles of two genes are involved, the cross is termed a dihybrid cross. A
monogenic cross involves the segregation of alleles of the same gene into
separate gametes. This segregation, or separation, occurs when homologous
chromosomes disjoin at anaphase 1 of meiosis.
Tracey and John are planning their next pregnancy. One of their first-born nonidentical twin children, Fiona, has the condition of albinism and the parents want to
know about the chance of this condition appearing in their next child.
Both parents are carriers of albinism and have the genotype Aa.
MONOHYBRID CROSSES: PIGMENT OR NOT?
For the TYR gene on the number-11 chromosome, which controls
pigment production, the cross can be seen 
During meiosis, the pair of number-11 chromosomes disjoin, carrying
the alleles to different gametes. Tracey’s eggs have either the A
allele or the a allele. This also applies to the sperm cells produced by
John.
This separation of the alleles of one gene into different gametes that
occurs during meiosis is known as the segregation of alleles. For each
parent, the chance of a gamete with A is 1 in 2 and the chance of a
gamete with a is also 1 in 2.
These probabilities can also be incorporated into a Punnett square.
The AA and the Aa genotypes both result in normal pigmentation.
The aa genotype causes albinism.
A Punnett square shows the chance of each possible outcome, not what will happen.
So, Tracey and John asked,
‘What is the chance that our next child will have albinism?’
The answer to their question is 1 in 4, or ¼. The chance that their next child will have normal
pigmentation is ¾.
If the next child has normal pigmentation, what is the chance that this child will be a
heterozygous carrier of albinism?
There are three ways a child with normal pigmentation can result, so the chance that this child
will be a heterozygous Aa carrier of albinism is 2 in 3.
MONOHYBRID CROSS: ABO BLOOD TYPE
The four blood groups in the ABO blood system are A, B, AB and O.
These different blood group phenotypes are controlled by the ABO gene on the number-9
chromosome.
Each phenotype is determined by the presence or absence of specific proteins, known as
antigens, on the plasma membrane of the red blood cells.
Antigens present on
red blood cells
ABO blood group
Frequency in Australian
population*
antigen A
A
38%
antigen B
B
10%
antigens A and B
AB
3%
neither antigen
O
49%
* based on Australian Red Cross data
MULTIPLE ALLELES FOR ONE GENE
Blood type is an example of a multiallelic system where
by three or more alleles of a gene exist among the
members of a population.
Blood groups are therefore said to be a multialleic trait.
The fact that there are more than two alleles responsible
for determining the blood group makes no difference to
their transmission, which takes place in normal Mendelian
fashion.
Tracey and John are both blood
group B and their twins are both
blood group O. This means that
both parents have the heterozygous
genotype IBi, while the twins have
the genotype ii. The cross between
Tracey and John for the ABO gene
is:
The chance that Tracey and John’s next
child will be blood group B is 3 in 4
and the chance that it will be blood
group O is 1 in 4.
Remember that ratios, such as 3 to 1,
identify the chance or the probability
of a particular outcome occurring; they
do not identify a certain outcome.
VARIATIONS OF THESE CROSSES
Co-dominance
Incomplete dominance
http://highered.mheducation.com/sites/0073525286/student_view0/chapter4/gie_
_incomplete_dominance_-_red_white_pink_flower_color.html
Lethal genes
LETHAL GENES
Some genotypes can result in death early in embryonic development. Offspring with
such lethal genotypes do not develop and so do not appear among the offspring in a
cross.
An example of this is fur colour in mice.
Read page 305 – 306
Parental Phenotype
Yellow x Yellow
Parental Genotype
Yy x Yy
Possible Gametes
YY and Yy and yy
Gametes
Y
y
Y
YY
Yy
y
Yy
yy
GENOTYPIC AND PHENOTYPIC RATIOS
When you finish a Punnett square you are often required to provide a summary of the
results e.g. 1 in 4 will be recessive for the trait, 50% will be brown etc.
You may be asked also to represent the results as a ratio.
If we were to look at the growth of daffodils and used the letter G to represent growth. The
ratios would be:
1 GG : 2 Gg : 1 gg
3 Tall : 1 Short
(Remember this is complete dominance!)
MONOGENIC VS. POLYGENIC TRAITS
Monogenic
 Mono = one
Genic = gene
 Traits that are controlled by a single gene.
 Typically show discontinuous variation; meaning a small
number of phenotypes often just two are produced
Polygenic
 Poly = many
Genic = gene
 Traits that are controlled by many genes
 Typically show continuous variation; where a range of
phenotypes are expressed
 Typically quantitative and are often described in numerical
values, such as in height 175cm, 6ft 2in
POLYGENIC INHERITANCE
Polygenic inheritance is the transmission of a characteristic that
is controlled by two or more genes (polygenes), for example
human skin colour and height.
Unlike Mendelian inheritance where plants were either tall or
short, humans have a range of heights.
In addition to involving multiple genes, polygenic inheritance
also looks at the role of environment in someone's development.
When a trait shows a range of phenotypes, this is called
continuous variation.
POLYGENES AND HUMAN HEIGHT
Adult human height shows a large range of
phenotypes.
Adult height can be affected by illnesses in
childhood, diet, genetics
For this polygenic trait in humans it is
reasonable to predict that the distributions of
height in a large sample of adults will show a
bell-shaped distribution.
What could influence skin colour?
TRUE OR FALSE?
Polygenic traits typically show discontinuous variation
The greater the number of polygenes, the greater the number of possible phenotypic
classes
Polygenic traits measured over a large sample of people would be expected to show
a bell-curve type distribution.
TEST CROSSES
https://www.youtube.com/watch?v=Exqy9r5E8Gw