Introduction to Genetics
Understanding Punnett Squares Notes (Chapter 11 and 14)
Probability
Probability: the ____________________ that a particular event will occur, it can be used to predict
the outcomes of _______________________.
Paperclip Demonstration
1) What is the probability of picking a red paperclip?
2) What is the probability of picking a red paperclip two times in a row?
3) What do you think would make your results close to the calculated probability?
Punnett Squares
Punnett Squares: a diagram that shows the ______________ of each possible outcome of a cross.
Monohybrid Crosses
1. Brown eyes are dominant over blue eyes. If two heterozygous brown-eyed people marry, what
are the chances that they will have a blue-eyed child?
Key:
Cross:
Genotype:
Phenotype:
Dihybrid Crosses
Dihybrid Cross: Used to determine the probability of specific genotypic and phenotypic ratios when
looking at __________ traits at the same time.
** FOIL each parent’s genotype independently sorts to set up the cross ***
2. In guinea pigs, black coloring is dominant over brown coloring and rough fur is dominant over
smooth. Cross two heterozygous black guinea pigs with rough fur.
Key:
Cross:
Genotype:
Phenotype:
Incomplete Dominance Crosses
Incomplete Dominance: A cross between organisms with two different phenotypes can produce
heterozygous offspring with a _________ phenotype that is a _____________of the parental traits.
IMPORTANT: Different letters are used to represent the different alleles and combinations are used
for the heterozygous pair.
Snapdragon Flowers
3. What results may be expected in a cross between two pink flowered snapdragons?
Key:
Red = ___________
White = ___________
Pink = _________
Cross:
Genotype:
Phenotype:
Codominance Crosses
Codominance: This happens with a cross between organisms with two different phenotypes
produce offspring with a ________ phenotype in which both of the parental traits appear together.
Tortoise Shell Cats
4. A cross between a black cat and an orange cat produces a striped condition know as tortoise
shell. Cross a cat that has tortoise shell with an orange cat.
Key:
Black = ________
Orange = ________
Cross:
Genotype:
Phenotype:
Tortoise -Shell = ________
Blood Groups (Codominance)
Pages 344 to 345
There are three possible alleles for blood types ____________________
There are four different blood types ____________________________
A and B are codominant
____________
O is recessive
____________
1. A man with blood type AB marries a woman with blood type O. Work out the possible blood
types of their offspring.
Key:
Cross:
Genotype:
Phenotype:
Table 1: Human Blood Groups
Phenotype
(Blood Types)
Genotype
Safe Transfusions
To
From
Sex Linked Genes
pg 350 to 353
One pair of chromosomes (X and Y) determines the sex of an individual; genes located on
them are said to be _______________________ genes.
Females: Females have _________ copies of the X - chromosome.
Male: Males have just ________ X - chromosome. Thus, all X-linked alleles are
_________________ in males, even if they are recessive. The Y - chromosome is much
smaller and appears to contain only a few ___________.
Eye color is a sex-linked trait in fruit flies carried on the X chromosome. Determine the sex
and eye color for a father who has white eyes and mother who is homozygous dominant for
red eyes.
Key:
Genotype:
Cross:
Phenotype:
X- Chromosome Inactivation (page 352)
If just one X chromosome is enough for cells in males, how does the cell “adjust” to the extra
X chromosome?
Chromosomal Disorders (page 353)
Explain what happens during nondisjunction.
Genetic Practice Problem Sets
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Assign each trait a LETTER—Make a Key
Dominant trait = Capital letter (D)
Recessive trait = small letter (d)
Show the cross by determining the Genotypes of the Parents
True Breeding — the same
(DD) Homozygous Dominant
(dd) Homozygous Recessive
Hybrid — mixed
(Dd) Heterozygous
Set up and solve the Punnett Square.
Then list the probable genotype and phenotype for the offspring.
Answer the question(s).
Monohybrid Crosses
1. Black is dominant to brown coat color in mice. Cross a heterozygous black mouse with a
brown mouse. What are the chances of having a brown mouse?
2. Hitchhiker’s thumb is dominant to straight thumb in humans. Cross a heterozygous
hitchhiker with an individual who is homozygous for the trait. Could they have a child with
a straight thumb?
Dihybrid Crosses
3. In horses, pacers are dominant over trotters, and chestnut is dominant over black
coloring. What are the chances of getting a black pacer from a cross between two
heterozygous chestnut pacers?
Incomplete Dominance Cross
4. When red petunias are crossed with white ones, the resulting offspring are pink! Show a
cross between one with pink flowers one with a red flowers. What are the chances of
getting red flowers from such a cross?
Codominance Crosses
5. Red coat color in cattle is codominant to white. The heterozygous is called “roan” (a red
coat with white hairs). Cross cattle with a white coat with one that has a red coat.
Blood Group Crosses
6. A woman sues a man for the support of her child. She has type A blood, her child has
type O, and the man is type B. Show how this man could be the father of her child.
9 A woman sues a man for the support of her child. She has type B blood, her child has
type A, and the man is type AB. Show how this man could be the father of her child.
Sex Linked Crosses
10 In humans, colorblindness is a recessive trait that is carried on the X chromosome. A
colorblind man marries a woman who is homozygous normal.
11 Pattern baldness is a sex-linked trait that is recessive. A man with a full head of hair
marries a woman whose is heterozygous for baldness. Show this cross.