Genetics Unit
Solving Genetics Problems
Upon the completion of this unit the learner will be able to:
1. Apply genetic terminology while solving five types of genetic problems.
2. Predict the genotypes of organisms within a pedigree.
Vocabulary
Genetics
Chromosome
Gene
Dominant
Recessive
Homozygous
Heterozygous
Hybrid
Purebred
Homozygous Dominant
Homozygous Recessive
Pedigree
Genotype
Phenotype
Punnett Square
Monohybrid
Dihybrid
Parental Generation
F1 Generation
F2 Generation
1
Genetics Vocabulary Lecture
Heredity.
Allele –
Gene –
Dominant trait –
Recessive trait –
Homozygous – (Purebred)
Heterozygous – (Hybrid)
Genotype – the DNA for a trait, i.e. R or r.
Phenotype – the appearance of a trait determined by a genotype, i.e. round or wrinkled peas.
Seven Steps:
Cross a hybrid white flower with a homozygous purple flower.
2
Sex Linked Traits Lecture
The study of inheritance of genes located on sex chromosomes was pioneered by T. H. Morgan and his students at the beginning of
the 20th century. Although Morgan studied fruit flies, the same genetic principles apply to humans. Since males and females differ in
their sex chromosomes, inheritance patterns for X-chromosome linked genes vary between the sexes
What are sex chromosomes?
What are sex-linked traits?
Which sex chromosome is home of the most sex-linked traits
What are examples of traits that are sex-linked?
1) Color blindness – (red-green)
2) Hemophilia –
3) Muscular Dystrophy –
What letters should we use to designate these normal and mutant traits?
B = _________
b = _________
H = _________
h = _________
M = _________
m = _________
How should we write these genotypes to help us remember they are sex-linked?
Color Vision
Genotypes
XBXb
Color Vision
Phenotypes
Genotypes
Phenotypes
Complete this chart:
XHXh
Man with
hemophilia
Normal Vision
Girl
XMY
Woman carrier of
muscular
dystrophy
3
Try This:
Cross a colorblind male with a carrier female with healthy vision
1.
2.
3.
4.
5. & 6.
7. Chance of Normal _________
Chance of Carrier _________
Chance of colorblind __________
Incomplete Dominance Lecture
Sometimes genes that are present to not completely dominate their other allele. In these cases the
end result is a blend of the two phenotypes. All genes in the problems today are Incompletely
Dominant.
Example: Some species of flowers when you cross a red petal and a white petal produce a pink
petal. We change the nomenclature to show the user what is happening in these problems.
1. Dominant = Red, Recessive = White, Incomplete = Pink
2. Red = R, White =W, Pink = RW
3. Crossing a red and a white = RR x WW
4. Sperm = R or R
Egg = W or W
5. & 6.
7. What are the chances for: Red ___________ Pink ___________ White __________
4
Blood Type Lecture
Multiple Alleles
So far we have been working with traits that are usually either/or situations. Tall or short, green or
yellow. Sometimes there are more then two choices. Human eye color is one example of this.
Human blood type is another example.
There are many different ways that we type blood. Today, we’ll look at two systems, ABO and the
Rh- factor.
ABO Typing:
Each red blood cell has proteins on its surface that let the body identify it as belonging to yourself.
Blood Type
Antigen
A
Antigen
Antibody
B
anti-A
Antibody
Anti-B
A
yes
no
no
yes
B
no
yes
yes
no
O
no
no
yes
yes
SB
yes
yes
no
no

Genes “A” and “B” are __________________ genes.

Gene “o” is the _______________ gene.
Sample Problem:
Mom is hybrid type B blood, Dad is hybrid type A blood.
1.
2.
3.
4.
5. & 6.
7. Chance of Each Blood Type:
5
Rh Factor Blood Type
Rh+ means that
Rh- means that
The dominant gene is ________
The recessive gene is ________
Let’s assign letters for each genotype:
Phenotype
Rh+
Rh-
Genotype
Try This:
Dad is heterozygous Rh+ factor and mom is homozygous for Rh- factor blood.
1.
2.
3.
4.
5. & 6.
7. Chance of each blood type _________
Does this type of problem remind you of a type you’ve already done?
Note: There is a danger if a mom with Rh- blood has a Rh+ fetus in her body her antibodies
may attack the fetus and cause her to be sick or die.
6
Pedigree Lecture
Pedigree –
A sample pedigree
Symbol
Description
Unaffected male
Unaffected female
Affected male
Label the following on your pedigree: P generation, F1 generation,
F2 generation.
Affected female
Deceased male
Example #2:
Deceased female
Possibly affected male
Possibly affected female
Carrier male
Carrier female
7
Dihybrid Lecture
1. Definition –
Try This:
Given B = brown hair, b = blond hair AND T = Tall while t = short.
BbTt = ______________ phenotype
Bbtt = ______________ phenotype
bbTT = ______________ phenotype
bbtt = ______________ phenotype
2. When drawing your sperm and egg for the 7 steps in this type of problem each:
a. Sperm contains __________________________ genes?
b. Egg contains _____________________ genes?
3. Predict the sperm or eggs possible for a critter of BbTt.
4. Arrow Rule:
5. What sex cells could be obtained from the genotypes below?
a. Bbtt: ___________, ___________, ___________, ___________
b. bbtt: ___________, ___________, ___________, ___________
c. BBTT: ___________, ___________, ___________, ___________
d. BbTt: ___________, ___________, ___________, ___________
e. Hint repeated sperm or eggs can be crossed out.
6. Cross a totally heterozygous brown eyed tall girl with a heterozygous brown eyed
homozygous tall guy.
8
Multiple Genes Lecture
For many traits there are two or more genes the produce variations of one trait. For example, 6
pairs of genes, determine one’s skin color. This results in 4,096 different genotypes of skin color.
Example: The redness of some wheat kernels seems to be determined by two pairs of genes acting
in an additive way.
1. Four dominant genes produce very, dark red kernels.
2. Three dominant genes produce dark red kernels.
3. Two dominant genes produce a medium red kernel.
4. One dominant gene produces a light red kernel.
5. A lack of any dominant genes produces a white kernel.
If R and S are genes related to the redness of wheat kernels (r and s represent the lack of redness)
what will the phenotypes of each of the following genotypes be?
RrSs
RRss
RrSS
RrSs
Rrss
RRSS
Give the different genotype combinations that could be present in each of the above genotypes:
(Hint: The arrow rule)
Cross medium red wheat (RrSs) with dark red wheat (RrSS).
1.
2.
3.
4.
5./6.
7.
9