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Name: ______________________________
Date: _______________
Unit 5C: Genetic Inheritance
NOTES
The Work of Gregor Mendel & Applying Mendel’s Principles
Heredity- the delivery of characteristics from ________________ to _______________
Genetics- the scientific study of heredity
 The Modern study of genetics was founded by a monk named Gregor Mendel in the
mid- 1800’s
 Mendel studied genetics in ______________________
Traits- Specific characteristics of an individual
Example: in pea plants- tall or short, round or wrinkled
 In humans, brown eyes, blue eyes
Crossing Organisms
To study genetic inheritance, Mendel crossed pea plants and looked at how certain traits were
passed from generation to generation
• To cross means to cause one organism to ___________________ with another
(like breeding)
Mendel Arrived at Two Conclusions
1. An individual’s characteristics are determined by factors that are passed from one
parental generation to the next.
- Today, these “factors” are called ________________
- Each gene has (at least) ____________ contrasting varieties
- The different forms of the gene are called __________________
- Example: the gene for height in pea plants comes in alleles for tall and short.
Additional Notes/Summary:
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2. The Principle of Dominance- some alleles are dominant and some are recessive
- Organisms carry ___________ alleles for each trait
- Only one dominant allele needs to be present for that trait to be expressed
(it “dominates” over the recessive allele)
- _________________________ of the recessive allele must be present for that trait to be
expressed.
TT
Tt
tt
Why do organisms have two alleles for each gene?
_____________________________________________________________________________
_____________________________________________________________________________
If an allele is dominant does that mean it codes for the more common trait in a population?
_____________________________________________________________________________
_____________________________________________________________________________
 If more people in an area have a recessive trait, their offspring will have it too, continuing
the trend.
 Example: Almost ________________ of Northern Europeans have
__________________ even though light eyes are ________________________to brown
eyes.
Additional Notes/Summary:
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Allele Combinations
Homozygous- organisms with two _____________________ alleles for a particular gene (Ex:
TT is homozygous __________________, and tt is homozygous ______________________)
Heterozygous- organisms that have two _______________________ alleles for the same gene
(Tt)
Genotype- the _____________________ makeup of an organism (Ex: TT)
Phenotype- the observable characteristic or _____________________ (Ex: Tall)
A plant with the genotype _________ has a phenotype of short.
A plant with the genotype of Tt has a phenotype of ____________.
Using Punnett Squares
_____________________ can be used to predict the outcome of genetic crosses.
 Example: refer to the “Using Punnett Squares” page
When organisms are crossed to determine the inheritance of one gene it is called a
_________________ cross.
The parents used are called the “_____” generation and the offspring are called the “________”
(first filial) generation.
 Ex: what percent of F1’s will be tall (T) if both parents are heterozygous tall? _________
 What is the phenotype ratio of tall (dominant) to short (recessive)? _____________
Additional Notes/Summary:
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Test Cross
(Refer to the page titled “Test Cross”)
Two Factor Crosses
Experiments testing how _________ genes are passed down are called two-factor or
______________ crosses.
R= round
r= wrinkled
Y=yellow
y=green
 If one parent is homozygous recessive for both genes and the
other is homozygous dominant for both genes, then all
offspring are __________________________ for both traits
(refer to the page titled “Heterozygous Dihybrid”)
Predictable Phenotypic Ratios for Offspring of Heterozygous Parents
Monohybrid Cross:
 Tt X Tt (Two heterozygous parents)
 Offspring ratio of dominant phenotype to recessive phenotype will be _____________
Dihybrid Cross:
 TtGg X TtGg (Two parents heterozygous for both genes)
 Offspring phenotypic ratio of both dominant to one dominant, one recessive to one
recessive, one dominant to both recessive will be ____________________
Additional Notes/Summary:
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Beyond Dominant and Recessive
Not all genes follow the principle of dominance
There are 4 exceptions to the principle of dominance:
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Incomplete dominance
Codominance
Multiple alleles
Polygenic traits
Incomplete Dominance
Occur when one allele is ______________________________
dominant over the other
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Example: four o’clock flowers
Red (RR) x White (WW) = Pink (RW)
Neither red nor white is dominant
Heterozygous phenotypes are a __________________ of the two homozygous
phenotypes
Codominance
The phenotypes of _________ alleles are ___________________
 Example: In some chickens, black feathers are codominant with
white feathers
 Heterozygous chickens will have both black and white feathers
 Black and white are _______ blended, they appear
___________________
Additional Notes/Summary:
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Multiple Alleles
A gene with more than two alleles has “multiple alleles”
 Individuals only have two copies of each gene
 But, ____________ alleles for the gene can exist in a ______________________
 Example:
o There are multiple alleles (3) for human blood type: IA, IB, and i.
o Alleles IA and IB are codominant
o Each person inherits 2 of these alleles, one from mom and one from dad.
(Refer to the page “Two different Patterns of Inheritance”)
Polygenic Traits
Traits that are produced by interactions between _________________ genes
 Examples: At least three genes work together to make the reddish-brown pigment in the
eyes of fruit flies
 The variety of skin color in humans is because multiple genes interact to produce skin
color.
Sex-Linked Inheritance
Sex-linked genes- genes located on the sex chromosomes (X and Y)
 Genes on the Y are only found in ____________ and are passed from father to son
 Genes on the X are found in both sexes, but remember… males have just one X
The Consequence of only having one X Chromosome
Recessive disorders related to genes on the X chromosome are
___________________________ in males since they only have one X.




Example: color blindness
3 genes work together to produce color vision and ALL are located on the X chromosome
If any of these alleles is defective in males, they will experience problems seeing colors
In females, if one of these genes is defective, they still may have a
______________________ of the allele on their other X chromosome!
(Refer to the page “Two different Patterns of Inheritance”)
Additional Notes/Summary:
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Human Pedigrees
A pedigree is a chart that shows ____________________ of inheritance in a family.
How to read a pedigree
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Circles represent females
Squares represent males
Shaded shapes mean the individual expresses the trait
Not shaded shapes mean the individual DOES NOT express the trait
Vertical lines connect parents to offspring
Horizontal lines represent a marriage
This pedigree is for the dominant “white forelock” trait.
The grandfather expresses the trait.
What is the genotype of the circled individual? ____________________
What do you think the genotype of the grandfather must be? ______________
Additional Notes/Summary: