Download Genetic Inheritance Type Review

Genetic Inheritance Patterns
Mendelian Genetics
A normal cell contains two copies of each gene. These genes are found on the
chromosomes and can come in multiple varieties. Each variety of gene we call an allele. For
example a chromosome can have an allele for brown eyes or an allele for blue eyes. In simple
genetics, one allele is dominant to the others. The dominant allele (shown as a capital letter) is
expressed as long as one copy is present. We only see the recessive trait (shown as a lower case
letter) when both copies of the gene are the recessive allele. Gregor Mendel discovered this
type of inheritance using pea plants. He stated that genes separate from their pair during
meiosis and then reunite with another copy of the gene from the other parent during
fertilization. This gives combination of traits different from either parent.
Practice the simple Mendelian genetics problems below.
1.Oompahs generally have blue faces which is caused by a dominant gene (B). The recessive
condition results in an orange face (b). Develop a "key" to show the genotypes and phenotypes
possible for Oompa Loompas. Use the letter B.
2. Two heterozygous Oompahs are crossed. What proportion of the offspring will have orange
faces.
3. A blue faced Oompah (homozygous) is married to an orange faced Oompah. They have 8
children. How many children will have blue faces?
Incomplete Dominance
In incomplete dominance, one allele is not completely dominant over the other. A
heterozygous individual will show a mix between the two traits. For example in snapdragon
flowers we can have a red color (RR) or a white color (rr). A heterozygous individual would
appear pink (Rr).
Complete the following practice problems.
1.Complete the Punnet square for a cross between a homozygous red-flowered snapdragon
(RR) and a homozygous white-flowered snapdragon (rr). Give the ratio for the phenotype and
the genotype.
Key
RR – red
rr – white
Rr – pink
genotype _________________
phenotype ________________
What happens to the phenotype of the heterozygotes when traits are inherited in an incomplete
dominance pattern?
2. Complete the Punnet Square for a cross between two heterozygous individuals. Give the
phenotype ratio for all possible offspring.
Phenotype ratio ___________________
Co-Dominance
Co-dominance happens when two alleles are both dominant and rather than getting an
intermediate mix, we see both alleles expressed. For example, in chickens feathers can either
be black (BB) or white (WW). A heterozygous (BW) individual would show black and white
spots. A heterozygous individual shows both traits.
Complete the following practice problems.
In Smileys, eye shape can be starred, circular, or a circle with a star. Write the genotypes for
the pictured phenotypes
1. Show the cross between a star-eyed and a circle eyed.
What are the phenotypes of the offspring? ____________
What are the genotypes? __________
2. Show the cross between a circle-star eyed, and a circle eyed.
How many of the offspring are circle-eyed? ____________
How many of the offspring are circle-star eyed? ____________
Multiple Allele
Some traits have more than two alleles and thus have unique inheritance patterns. For
example, human blood type is controlled by three alleles (A, B, O). O is recessive and A and B
are co-dominant. A person must receive two O alleles for them to have type O blood. If a
person receives an A allele and a B allele, their blood type is type AB.
Try these practice problems.
1. a) What are the two genotypes possible for a person who as A blood?
b) What genotype does a person with AB blood have?
c ) What genotype does a person with O blood have?
d) What are the two genotypes possible for a person who as B blood?
2. A man with type AB blood is married to a woman also with type AB blood. What blood
types will their children have and in what proportion?
3. A man has type B blood (genotype BB) is married to a woman with type O blood. What
blood type will all their children have? ________ What is the genotype of the children?
______
Sex-linked
Sex-linked traits are found on the X chromosome and thus gender can affect inheritance
patterns. Males only have one X chromosome and thus express whatever allele is carried by
that one copy. Females have two X chromosomes and thus have the normal dominant and
recessive genotype/phenotype relationship. A woman can be a carrier for a recessive trait if it
is heterozygous. We show sex linked traits as a superscript above the X chromosome. This
means a male is XRY and a heterozygous female is XRXr.
In humans, hemophilia is a sex linked trait. Females can be normal, carriers, or have the
disease. Males will either have the disease or not (but they won’t ever be carriers)
= female, normal
= female, carrier
= female, hemophiliac
= male, normal
= male, hemophiliac
1. Show the cross of a man who has hemophilia with a woman who is a carrier.
What is the probability that their children will have the disease? __________
2. A woman who is a carrier marries a normal man. Show the cross. What is the probability
that their children will have hemophilia? What sex will a child in the family with hemophilia
be?
Polygenic traits
Polygenic traits are traits that are controlled by more than one gene, ie. height, weight,
hair color, skin color (basically anything dealing with color). This allows for a wide range of
physical traits. For example, if height was controlled by one gene A and if AA = 6' and Aa = 5'7"
and as = 5', then people would either be 6', 5'7", or 5'. Since height is controlled by more than
one gene, a wide range of heights is possible.
Pleiotropy
A single gene may be responsible for a variety of traits. This is called pleiotropy. The
complex of symptoms that are collectively referred to as sickle-cell anemia, is an example. A
single gene results in irregularly shaped red blood cells that painfully block blood vessels, cause
poor overall physical development, as well as related heart, lung, kidney, and eye problems.
Another pleiotropic trait is albinism. The gene for this trait not only results in a deficiency of
skin, hair, and eye pigmentation but also causes defects in vision.