Simple Genetics
Exploring Unlinked Genes to SexLinked Genes
Overview
● Autosomal Genes
o
Punnett squares
● Dihybrid Crosses
o
Bigger Punnett squares...
● Sex-Linked Genes
o
Thank your mom
The Allele
Any of the alternative versions of a gene that may
produce distinguishable phenotypic effects.
● Phenotype: The observable physical and physiological traits of an
organism, which are determined by it’s genetic makeup.
● Genotype: The genetic makeup, or set of alleles, of an organism.
Alleles are found on chromosomes
● One per chromosome
● Separated in meiosis
● Maternal vs. Paternal chromosomes
● Dominant vs. Recessive
● Codominance vs. incomplete dominance
H-words
● Homozygous: Having two identical alleles for a given
trait
o Dominant vs. Recessive
● Heterozygous: Having two different alleles for a given
trait
Mendell (He’s the pea dude)
● Independent Assortment: Each pair of alleles assort
independently of each other during gamete formation
● Law of Segregation: Two allele pairs segregate into
different gametes
Biology’s favorite shape
The Punnett Square: A diagram used in the study of
inheritance to show the predicted genotypic results of
random fertilization in genetic crosses between
individuals of a known genotype
● Shows genotypes of possible
offspring of two parents
Monohybrid Crosses
A cross where only one trait is being explored
● Based on when a single set of chromosomes is separated
Diagramed by a 2 x 2 Punnett Square
1. Define alleles
2. Set up square
3. Cross
A
a
a
Aa
aa
a
Aa
aa
Sample: A heterozygous green pea plant is crossed
with a yellow pea plant. Show the cross.
A = green
a = yellow
Sample Problems
1. Two pea plants heterozygous for pod color are crossed. Define allele
notation and diagram a Punnett Square for this cross.
2. A homozygous dominant red flower is crossed with a homozygous
recessive white flower. They produce all pink flowers, what is the reason
for this? What would be produced if a pink offspring was crossed with
another pink offspring? Diagram a Punnett Square.
3. A blue eyed man and brown eyed women have four children, none of
which have blue eyes. Does blue eyes appear to be dominant or
recessive?
Dihybrid Crosses
A cross where two genes are explored simultaneously
●
Physically based on alleles located on two sets of chromosomes, which
then segregate into gametes.
● Genes must be unlinked (not on the same chromosome)
● 9:3:3:1
Requires a 4 x 4 Punnett Square
1. Determine possible gamete formations
2. Set up square
3. Cross!
Remember this? It’s a dihybrid cross!
9:3:3:1, the magic number
Refers to the phenotypic ratio expected from a normal
dihybrid cross of two heterozygous parents. This was
devised by Mendell.
9: the expected number of offspring with all dominant traits
3: the expected number of offspring with one dominant and one recessive
trait
3: the reciprocal of the first ‘3’
1: expected number of offspring with all recessive traits
Sample Problem
Cross: AaBb x AaBb
Possible gametes (Parent 1): AB, Ab, aB, ab
Possible gametes (Parent 2): AB, Ab, aB, ab
AB
AB
Ab
aB
ab
Ab
aB
ab
Practice Problems
1. Two dogs heterozygous for brown fur and blue eyes are crossed. Diagram
this cross. What is the probability of the dogs having a white fur-green
eyed dog?
2. A dog true-breeding for brown fur and blue eyes is crossed with a dog
true-breeding for white fur and green eyes. Diagram the cross. What is
the probability that these parents will have a white fur-green eyed
offspring?
Sex-Linkage
● The miracle of the Y chromosome
● Does not include alleles found on it’s homologous X
chromosome
● Males have increased susceptibility to these types of
alleles
● Two X chromosomes makes a female
one X and one Y makes a male!
Crosses for traits that are sex-linked
Also known as x-linked traits
● Notation: XA Xa and Y for males
● X and Y represent the sex chromosome
● When crossing, X X will always represent female and
the X Y will always represent male.
Carrier: An individual who is heterozygous for a specific
trait, thusly it affects their offspring, but not their
phenotype.
Sample Problem!
Hemophilia, a sex linked disorder, causes difficulty in blood clotting, it is
a recessive disorder. A hemophiliac man marries a woman whose father
was also a hemophiliac, but she is healthy. What is the probability that
this couple will have a hemophiliac daughter?
1. Determine genotypes of parents
2. Remember, this is a monohybrid cross, set up a square
3. Preform the cross (do not forget correct notation)
XH
Father: XhY
Mother: XHXh
XH= healthy
Xh= hemophiliac
1/2 Chance of
hemophilic daughter D:
Xh
Xh
XHXh
XhXh
Y
XHY
XhY
A Last Problem...
1. A healthy man and a healthy women have a hemophilic
child. The man believes this is evidence that his wife
has had an affair. What is the genotype of the man
and his wife? Is this evidence of an affair?