Download Dominant Phenotype Recessive Phenotype Genotypes

Genotypes & Phenotypes
Name _____________________________
Period _____ Date ___________
Many genes that determine the traits present in a person’s appearance have two types
of alleles. Generally, one allele is dominant over the other allele. If two dominant alleles
are present (XX) the dominant trait will be expressed. If only one dominant allele is
present (Xx) the dominant trait will still be expressed. The recessive trait will be
expressed only when two recessive alleles are present without a dominant allele (xx).
The expressed trait is the phenotype and the alleles responsible are the genotype.
Simple Traits: Look at the chart below and determine if the traits you have are dominant
or recessive. Circle your phenotype for each trait. Then decide which genotype(s) you
could have and circle the one or two possible genotypes that you may have.
Dominant Phenotype
Recessive Phenotype
Genotypes
Detached earlobe
Attached earlobe
EE
Ee
ee
Can roll tongue
Can NOT roll tongue
TT
Tt
tt
Dimples
No dimples
DD
Dd
dd
Right handed
Left handed
RR
Rr
rr
Freckles
No freckles
FF
Ff
ff
Widow’s peak
Straight hairline
WW
Ww
ww
Cleft chin
Smooth chin
CC
Cc
cc
Curly hair **
Straight hair
HH
Hh
hh
(Wavy)
**Note: Curly hair is an example of a trait that shows incomplete dominance. A person
who is heterozygous (Hh) will have wavy hair.
Sex-linked traits: Some genes are carried on the X chromosome. For a woman to
express the recessive phenotype, she must have two copies of the recessive allele (ss).
For a man to express the recessive phenotype, he only needs one copy of the recessive
allele (sY). Both genders still have the possibility of having the dominant allele and
expressing the dominant trait. For example, seeing normal colors is a dominant sexlinked trait. Being colorblind is recessive sex-linked trait. A woman has three possible
genotypes for this trait, NN, Nn, nn and a man could have two possible genotypes for
this trait, NY or nY. Explain why it is more common for a man to be colorblind than a
woman.
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Read pages 168-169 before completing this side of your worksheet.
Punnett Squares: A Punnett square is used to predict possible genotypes and
phenotypes of offspring from a given cross. For example, if you cannot roll your tongue
you are homozygous recessive for that trait (tt). You will only be able to pass on
recessive alleles. If you marry an individual that is also homozygous recessive (tt), none
of your children will be able to roll their tongues. If you marry an individual that is
homozygous dominant (TT), all of your children will be able to roll their tongues. If you
marry an individual who is heterozygous dominant (Tt) for that trait, 50% of your children
will be able to roll their tongues. Different combinations create different possibilities in
the offspring. Fill in the Punnett squares below to show all the possible offspring for
each possible cross and circle all of the offspring that will be able to roll their tongues:
What percentage of the offspring in all of these crosses combined would be able to roll
their tongues? (tongue rollers divided by total offspring x100 = %) _______________
Conclusion Questions:
1) Based on all possible combinations, what percentage of offspring would you expect
to not have a Widow’s peak? _________
2) What percentage of offspring would you expect to have wavy hair? __________
3) If a woman has freckles, but one of her parents has no freckles, what is her
genotype? __________
4) If that same woman married a man with no freckles, how many of their children would
have freckles? __________________
5) If a colorblind man has a son, will that son also be colorblind? EXPLAIN.
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