Transcript Patterns of Inheritance

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Blending theory Before 1850,
scientists thought that some fluid
substance in the blood of animals
or in the sap of plants was the
hereditary material. The
combination of the parent's
characteristics in the offspring
was thought to occur by a
"blending" of this fluid.
If so, a white dog that mated with
a brown dog should produce only
tan puppies. A tall person who
had a child with a short person
should produce all "medium-size"
children, etc. Clearly not the
case! Even though people
recognized problems with this
theory, it was the top theory of
the day!
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http://ed.ted.com/lessons/how-mendel-s-pea-plants-helpedus-understand-genetics-hortensia-jimenez-diaz
The earliest scientific experiments
about how traits are inherited were
done in the mid 19th century by an
Austrian monk named Gregor
Mendel.
Mendel studied patterns of gene
inheritance in everyday garden peas.
He noticed that some peas were
yellow and some green, some plants
were short and some were tall, and
that there were different colours of
flowers on the plants.
His lifelong studies provided the
basis for the first basic understanding
of heredity. Mendel discovered that
traits are inherited in discrete units
(we now know these to be genes).
This was a long time before there was
any knowledge of cells, DNA or
genes
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When the information is inherited from both
parents is the same, i.e. both chromosomes
carry the same allele for a particular
characteristic, the chromosome is said to be
homozygous.
When the information inherited from both
parents is different, with each chromosome in
the pair carrying different instructions about a
trait (different alleles) the chromosomes are
referred to as heterozygous.
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When two different sets of instructions are present at the
same time, both characteristics can not be expressed.
E.g. In the case of curly hair, the hair cannot be both straight
and curly; a person has one or the other.
One instruction can be “stronger” than another. This is
called dominant and will be the characteristic shown in the
offspring.
The other gene remains hidden or masked and is called
recessive.
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The term PHENOTYPE is used to describe the visible
characteristics in an individual that are produced by
genes (i.e. straight hair).
The genetic combination that produces the
characteristic is the GENOTYPE. It refers to the alleles
that an organism inherited from each of its parents
(i.e. CC or Cc or cc).
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Representing inheritance patterns with letters
C = curly hair c = straight hair
Genotype
of the
person
Phenotype
of the
person
Homozygous Heterozygous
dominant
(Cc)
(CC)
Homozygous
recessive
(cc)
Curly hair
Straight
hair
Curly hair
To solve this type of genetics problem, called a
monohybrid cross, use the following method:
(Punnet Square!!)
Sample Problems:
a)
In a family, a father is homozygous for curly
hair. The mother is homozygous for straight
hair. Determine the likelihood that they will
have a curly haired child.
b)
If one of those children, when grown up,
marries and has children with another
heterozygous curly haired individual, what
possible phenotypes can the offspring have?
Also indicate the probability of each.
1.
2.
A woman has eye cataracts (a clouding that develops in the
lens of the eye obstructing the passage of light). This is a
dominant trait (E) and she is homozygous. Her husband is
homozygous and normal (e). What will be the predicted
genotypes and phenotypes of the children?
a) Being able to roll your tongue is dominant (R), while not
being able to roll your tongue is recessive (r). What will be
the predicted genotypes and phenotypes of the offspring of
2 individuals who can’t roll their tongue?
b) Is there any chance that these 2 individuals could
produce an offspring that is able to roll his/her tongue?
c) One of the offspring (called F1 generation) ends up
having 2 children who can roll their tongues and 2 children
who cannot. What are the genotypes of the parents of these
4 children? Draw a punnett square for this cross.
3.
In guinea pigs, the allele for a black coat is
dominant over the allele for a white coat. A
black guinea pig was crossed with a white
guinea pig. All if the first generation offspring
have black coats.
a.) Describe how you could determine
whether or not the black parent is homozygous
or heterozygous for the gene. Indicate which
letter you would use to represent the alleles.
b.) If 10 offspring were produced, indicate
how many you would expect to have black
coat colour if the black parent was
heterozygous.