Name ___________________________ Period ______
Unit 6: Genetics
Essential Skills
6-1.
Be able to show the expected phenotype and genotype ratios for any single trait Punnett Square cross.
(CCS: 2g, 3a)
6-2. Recognize the terms genotype, phenotype, dominant, recessive, heterozygous and homozygous.(CCS:
2g, 3a-b)
6-3. Use a pedigree to determine the probable mode of inheritance of a trait. (CCS: 3c-d)
Vocabulary – You should be able to define and use the words in bold. (see website)
Study Guide
1. Using the height of plant (Tall (T) & Short (t)). Explain how Mendel obtained his P generation, the
results of the F1 generation and the results of the F2 generation.
Mendel developed purebred lines of plants (homozygous tall and homozygous short) by continuously
self-fertilizing a group of plants. He then crossed the two purebred lines (P generation). The offspring
of this generation, called the F1 generation, where all tall. This suggested that the tall trait could coverup the short trait (were dominant). He then took two plants from the F1 generation and crossed them
to create the F2 generation. In the F2 generation he observed that ¾ of the offspring were tall and ¼
were short. Because the recessive trait is present in the F2 generation, Mendel predicted that the two
alleles from the F1 generation separate during gamete formation. If two recessive alleles are united
during fertilization, the offspring will display the recessive trait.
2. How did Mendel’s results with pea plants show that alleles are either dominant or recessive? Use
examples to explain.
The offspring in the F1 generation were all tall, even though one of their parents was short. This
suggests that the tall allele masks the presence of the short allele. Mendel observed a similar
phenomena when he crossed purebred purple plants with purebred white plants. All of the offspring
were purple and the white trait was covered up.
3. Explain how the following terms relate to each other:
a.
DNA – Chromosome Chromosomes are made of DNA
b.
Gene – Chromosome Genes are located on chromosomes
c.
Gene – Trait A gene codes for a specific trait
d.
Trait – Allele An allele is an option for a trait
4. What is the Law of Segregation? How is this related to the Law of Independent Assortment?
The Law of Segregation says that alleles during gamete formation. The Law of Independent Assortment
says that the separation of alleles (on homologous chromosomes) is random.
5. Explain how the process of Meiosis supports Mendel’s Laws. How does crossing-over relate to
Mendel’s Laws?
Both segregation and independent assortment occur during Meiosis I. Crossing over also contributes to
the random separation of alleles and allows alleles located on the same chromosome to become
separated.
6. Compare and contrast the following sets of terms:
a. Dominant vs. Recessive – Dominant alleles mask recessive alleles
b. Genotype vs. Phenotype – Genotype is the genetic make-up of an individual, the phenotype is
the physical appearance of the individual
c. Homozygous vs. Heterozygous – Homozygous individuals have 2 of the same alleles for a
gene where heterozygous individuals have 2 different alleles for a gene.
d. Purebred vs. Hybrid – Purebred is homozygous, hybrid is heterozygous
7. How does genotype affect phenotype? Use the terms dominant, recessive, homozygous and
heterozygous.
The genetic make-up (genotype) of an individual determines what physical trait(s) (phenotype) they
will have.
Honors Biology 2008-2009
8. Complete the following table that summarizes the 4 basic genetic crosses: (B=brown; b =blue)
Word Cross
Homozygous dominant
x Homozygous recessive
Homozygous Dominant
x Heterozygous
Heterozygous x
Heterozygous
Homozygous recessive x
heterozygous
Genotype
Cross
BB x bb
Offspring Phenotype Ratio
100% brown
BB x Bb
2 BB: 2 Bb
100% brown
Bb x Bb
1 BB: 2 Bb: 1 bb
3 brown: 1 blue
bb x Bb
2 Bb x 2 bb
2 Brown : 2 Blue
9. Short tails are dominant to long tails in dogs.
a.
Offspring
Genotype Ratio
100% Bb
What are the phenotypic and genotypic ratios of a cross between a dog with a long tail
and a dog with a short tail (who had a parent with a long tail)?
Alleles: S = short tail, s = long tail
Cross: ss x Ss
s
s
S
Ss
Ss
Genotypic ratio: 2 Ss: 2 ss
s
ss
ss
Phenotypic ratio: 2 short tail: 2 long tail
b.
If you had a short tailed dog and did not know the phenotypes of its parents, how could you
conclusively determine the genotype of your dog? Explain using examples of a Test Cross.
Cross the short tailed dog with a long tailed dog. If any of the offspring have long tails, the
dog was heterozygous. If none of the offspring have long tails, the dog was likely to have
been homozygous dominant.
S
?
S
s
S
S
s
Ss
?s
s Ss
ss
s Ss
Ss
s
Ss
?s
s Ss
ss
s Ss
Ss
10. About 70% of Americans perceive a bitter taste from the chemical phenylthiocarbamide (PTC). The
ability to taste this chemical results from a dominant allele (T) and not being able to taste PTC is the
result of having two recessive alleles (t). Albinism is also a single locus trait with normal pigment being
dominant (A) and the lack of pigment being recessive (a). A normally pigmented woman who cannot
taste PTC has a father who is an albino taster. She marries a homozygous, normally pigmented man
who is a taster but who has a mother that does not taste PTC. What are the genotypes of the possible
children in this dihybrid cross?
Alleles: T = taster, t = non-taster & A = normal pigment, a = albino
Cross: Aatt x AATt
Possible female gametes: At & at
Possible male gametes: AT & At
At
at
AT AATt AaTt
Genotypic ratio: 1 AATt: 1 AaTt: 1 AAtt: 1 Aatt
At AAtt Aatt
Phenotypic ratio: 2 normal pigmented tasters : 2 normal pigmented non-tasters
11. What is the key to recognizing a trait that is inherited by incomplete dominance? What is the
notation you would use to show the genotypes of a trait with incomplete dominance?
In incomplete dominance, the heterozygote will have a phenotype that is an intermediate or blend of
the two homozygous phenotypes. You would use a capital letter to show the “more dominant” allele and
a lower case letter to show the “more recessive” allele.
12. In northern California there is a creature know as a wildcat. It comes in three colors, black, red, and
maroon. This trait is controlled by a single locus gene with incomplete dominance. A homozygous (BB)
individual is black, a homozygous (bb) individual is red, and a heterozygous (Bb) individual is maroon.
What would be the genotypes and phenotypes of the offspring if a black wildcat were crossed with a red
one?
Alleles: BB = black, Bb = maroon, bb = red
Cross: BB x bb
B
B
B Bb Bb
Genotypic ratio: 100% Bb
B Bb Bb
Phenotypic ratio: 100% maroon wildcats! (Go cats Go!)
Name ___________________________ Period ______
13. What is the key to recognizing a trait that is inherited by Co-dominance? What is the notation you
14.
15.
16.
17.
18.
19.
20.
would use to show the genotypes of a trait with Co-dominance?
In co-dominance, the heterozygote will have a phenotype that displays both of the alleles. You would
use two different capital letters, one for each dominant allele, as the notation.
In corn plants, there are two alleles for kernel color, red (R), and yellow (Y). The red and yellow alleles
are co-dominant. What would be the genotype and phenotype ratios in the offspring produced by a
cross between a plant with red kernels and a plant with both red and yellow kernels?
Alleles: RR = red, RY = red & yellow, YY = yellow
Cross: RR x RY
R
R
R RR RR
Genotypic ratio: ½ RR: ½ RY
Y RY RY
Phenotypic ratio: ½ red kernels: ½ red & yellow kernels
Differentiate between a multiple allele trait and a polygenic trait.
A multiple allele trait has more than two options for the trait, but is affected by a single gene. A
polygenic trait is affected by two or more genes that interact.
Show the expected genotype and phenotype ratios of a cross between a man with type A blood and a
woman with type B blood (both the man and woman had at least one parent with type O blood).
Alleles: A = type A, B = type B, AB = type AB, OO = type O
Cross: AO x BO
A
O
B AB BO
Genotypic ratio: 1 AB: 1 BO: 1 AO: 1 OO
O AO OO
Phenotypic ratio: 1 type AB: 1 type B: 1 type A: 1 type O
What is the difference between Autosomal recessive trait, an Autosomal dominant trait?
Autosomal recessive traits are caused by a recessive allele on an autosome. Autosomal dominant traits
are caused by a dominant allele on an autosome.
What is the difference between an autosomal trait and a Sex-Linked trait? What sex chromosomes
determine male? Female? Why do sex-linked traits affect males more than females?
A sex-linked trait is controlled by a gene located on a sex-chromosome (the X or Y chromosome). A
male will have an X & a Y chromosome. A female will have 2 X chromosomes. Sex-linked traits affect
males more commonly than females because males only have 1 X chromosome and most sex-linked
traits are on the X chromosome.
Red-green color blindness is a recessive sex-linked trait. Show the expected genotype and phenotype
ratios of a cross between a man who is colorblind and a woman who has normal vision (but her father
was colorblind).
Alleles: XN = normal vision, Xn = red-green color blind
Cross: XnY x XNXn
Xn
Y
N
X XN Xn
XNY
Genotypic ratio: 1 XN Xn: 1 Xn Xn: 1 XNY: 1 XnY
n
n
n
n
X X X
XY
Phenotypic ratio: 1 normal female: 1 color-blind female: 1 normal
male: 1 colorblind male
The following pedigree shows the individuals who have the genetic disorder thunderfanemia.
Based on the pedigree, is this disorder caused by a dominant or recessive allele? Fill in the genotype for
each individual.
The disorder is recessive.
Honors Biology 2008-2009