Notes on Genetics:
I. Terms in Genetics:
A. Chromosomes have hundreds of genes and these genes come in Pairs.
They code for the same trait but maybe in different versions. A pair of genes
are called Alleles.
B. These Alleles are either Recessive or Dominant. If they are Recessive it
takes two to express the trait. If Dominant only takes one to express the trait.
Example: Tallness is Dominant (T) and Shortness is Recessive (t).
TT or Tt is a tall person. Only tt is short person.
C. Homozygous (Purebreed) means that a person has the same genes on
both chromosomes. (TT or tt)
Heterozygous (Hybrid) means that a person carries both types of genes (Tt)
So a person can be Homozygous Tall (TT) or Homozygous Short (tt).
A person can be Heterozygous for Tall (Tt).
D. The Phenotype is what you see (Bill is Tall) and the Genotype is both genes
inside the cell ( Bill has Tt – so he is a carrier of the short trait).
E. Law of Segregation : Two alleles for every trait separate during Meiosis but
reunite after fertilization. {one from mom & one from dad}
F. Law of Independent Assortment: Alleles separate randomly during
formation of gametes (meiosis). It’s by chance each sex cell gets certain genes.
G. Gene Linkage : Genes that are close together on a chromosome usually
don’t get broken up during crossing over and are linked together. (but can still
be separated by crossing over)
H. Karyotype : An individual’s chromosomes.
I. Polygenetic Traits : Some traits are determined by more than two alleles.
(height, eye color, skin color)
II. Predicting percentages of having offspring with certain traits : {punnett
squares, monohybrid crosses, & dihybrid crosses}
A. Parent Generation = P1
First Generation = F1
Second Generation = F2
B. Basic Monohybrid Genetic Problems:
In Bengal Tigers, the allele (gene) for an orange coat is dominant to the
allele for a white coat. If a Homozygous recessive tiger is crossed with a tiger
Heterozygous for coat color, what is the percent probability that one of the
offspring will be orange?
Orange Coat = C
White Coat = c
P1 = Parent Generation Homozygous recessive is cc X Heterozygous is Cc
Cc
Cc
Orange Coat
Orange Coat
cc
cc
White Coat
White Coat
Results would be 2 orange coat (50%) and 2 white coat (50%)
Divide the boxes into 25% each for a total of 100%.
Phenotype is 2 orange coat : 2 white coat
Genotype is 2 Cc : 2 cc (Blueprint- what each child can pass on to
next generation)
C. How to read a Dihybrid Cross: (crossing two traits at one time)
S = straight tail s = curved tail
B = brown coat b = white coat
16 total boxes so 16 possible combinations (count the results below)
S? B? total of 9 so 9 cats have Straight tails and Brown Coats
ss B? total of 3 so 3 cats have Curved tails and Brown Coats
S? bb total of 3 so 3 cats have Straight tails and White Coats
ss bb total of 1
so 1 cat has Curved tail and White Coat
Sidenote about Crosses: If you cross hybrids in a monohybrid cross you will
always get a 3:1 ratio : cross two traits with hybrid parents you will get a
9:3:3:1 ratio.
III. Different Types of Crosses:
A. Recessive Diseases & Traits : Must inherit a recessive gene from each parent.
If both parents are carriers then odds are 25%. If one is a carrier and one is not
you will not get the disease but may be a carrier.
B. Dominant Diseases & Traits: Only requires one gene from one parent to be
seen in a Phenotype.
C. Incomplete Dominance : When two different alleles are both expressed in
the phenotype. The genes blend their traits.
D. Co Dominance and Multiple Alleles : This is when a trait is determined by
more than 2 genes. Blood type has 3 alleles (A, B, and O) that determine a
person’s blood type. A and B are both Dominant to O and share dominance
when together (AB).
E. Sex – Linked Traits : Some traits are carried only on the X chromosome.
The mother is the only parent who can give her sons these traits. (color
blindness, hemophilia) Daughters can be carriers or have the disease if they
inherit both alleles from their parents.
XhXh = Girl with Hemophili
XHXh = Girl who is Carrier
XhY = Boy with Hemophilia
III. Pedigrees : A chart that allows you to read the history of a genetic disease
or trait in a family’s history.
Represents a Male (male have rough edges)
Represents a Female (Females are soft and round)
If Square or Circle is fully colored that person has the trait.
If Square or Circle is half colored that person is a carrier of the trait.
If Square or Circle is clear then that person does not have the trait.
If a pedigree has several circles that are half colored and the squares are
either clear or fully colored than this represents a trait that is Sex Linked.
Example:
Other Examples:
A. Dominant Disease Pedigree
B. Recessive Disease Pedigree