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Expert answer:Interpret the genotype of the heterozygote if the inheritance pattern for a genetic disorder is shown to be completely dominant.Examine the genotype of the child, the mother, and the possible genotypes of the father for a child with type O blood who is born to a mother with type A blood.Describe the possible genotypes if a homozygous red-eyed Drosophila female is crossed with a red-eyed male.Explain the chances that a color blind woman who is married to a man with normal vision will produceColor-blind sonsColor-blind daughtersCarrier daughtersIdentify the genotype of all individuals involved in a cross between a brown-haired woman and a black-haired man that produce all brown-haired male offspring and all black-haired female offspring assuming X-linkage.Draw a diagram showing the inheritance pattern of an X-linked trait using Fig. 11.19 on p. 202.
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GENETICS WORKSHEET
A. Gregor Mendel
The essence of genetics or the study of heredity comes from the Austrian monk Gregor Mendel. In the
1800s this Catholic priest worked his monastery garden and raised all the fruit and vegetables needed. As
he had a science background, he started studying how traits were passed down from parent (P generation)
to the offspring (F1 generation) and so on. Using hundreds of pea plants he discovered the basics of
genetics, which is why we today call it Mendelian genetics.
1. What are the offspring of the F1 generation referred to as?
2. Why did Gregor Mendel choose pea plants to study as opposed to potatoes or tomatoes?
3. Briefly describe the essence of Mendel’s first experiments with two purebred pea plants. Use the
trait pea color for your description.
B. Mendelian Laws
Through Mendel’s many years and hundreds of pieces data, he threw out many old ideas about heredity
and came up with four hypotheses that turned into two laws that still hold true today. The first hypothesis
stated that individuals have two copies of their genes, one from each parent. The second hypothesis says
that there exist two different versions of the same gene represented by letters. We now call those versions
alleles. The third hypothesis states that if two different alleles occur together, one may be
expressed while the other is not. We say one is dominant and the other is recessive. His
T t
fourth and final hypothesis states that when gametes are formed, alleles for each trait
separate independently during meiosis. From these hypotheses which have been proven
Y y
true time and again, we now have two laws that can be attributed back to Mendel’s
research. The first law is called the law of segregation and it says that because each
individual has two different alleles, it can produce two different types of gametes. If the
gene is represented by the letter R, it can produce R allele or r allele, which represents different forms of
the same trait. The second law is called the law of independent assortment and it states that genes for
different traits are inherited independently of each other. For example, if a person has gene A and gene B
on the same chromosome, they are both inherited without being tied to the other.
4. What is Mendel’s first law?
5. Explain the first law in terms of plant height (T).
6. What is Mendel’s second law?
7. Explain the second law in terms of both height and pea color (T and Y).
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C. General Rules
Alleles are said to be dominant or recessive. A dominant allele expresses (shows) itself even if there is only
one. For example, if the trait is eye color, brown is dominant. Therefore, one B allele will make eye color
brown. A recessive allele is only expressed when no dominant allele is present. Blue eyes are recessive to
brown eyes so the only time blue eyes are expressed is when there are two as in bb. Dominant traits
determine the allele used. For example, if grey is the dominant skin color gene in aliens, white is recessive.
The allele used to represent both colors is g for grey. To differentiate grey and white, G is grey since it is
dominant and g is white since it is recessive. If the combination of the two alleles is such that both alleles
are dominant, it is said to be homozygous dominant. If both alleles are recessive, it is said to be
homozygous recessive. If the two alleles are different, it is said to be heterozygous.
8. If red fur is dominant over blue fur, what are the alleles for the different furs?
9. If yellow peas are dominant over green peas, what are the alleles for the different color peas?
10. What would the alleles for a heterozygous grey alien be?
11. What would the alleles for a homozygous recessive skinned alien be? What color is it?
D. Punnett Squares and Probability
Punnett squares are a method in which all the possible offspring types are
determined based on the parents’ genes. The genes of individuals (represented by
alleles) are called genotypes. The physical appearance or phenotype of an individual
is a result of what the genotype determines. For example, if freckles are dominant
over no freckles, the genotype Ff would have the phenotype of having freckles. The
parents’ genotypes determine what possible alleles are given to the offspring. The
allele type varies and according to the laws of segregation and independent
assortment, two different genes with different alleles separate completely and recombine in four possible
gamete combinations. An easy way for students to remember how to find the possible gametes, the
acronym FOIL (first, outside, inside, last) is often used. For example, if the parent genotype is AaBb, the four
possible gametes are AB, Ab, aB, and ab. Using a Punnett square, the gametes are combined in such a way
as to determine all the possible genotypes. A ratio of the number of genotypes is gathered by adding up all
the same genotypes and comparing them to the others using a colon between the numbers. A ratio of the
phenotypes of the offspring are gathered in a similar manner.
12. What is a genotype?
13. What is a phenotype?
14. If freckles are dominant over plain cheeks, and cleft chin is dominant over a smooth chin, what
would the genotype of a parent be who is heterozygous freckled and heterozygous cleft?
15. What are the possible gametes of the father? Use the FOIL method to determine.
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16. Using a Punnett square, what are the possible offspring of the parents if they both are heterozygous
for freckles and cleft chin?
17. What are the genotypic ratios and phenotypic ratios of the offspring of those two parents?
E. Pedigrees
A pedigree is a type of family tree that traces a particular trait that runs in an entire family. Circles
represent females while squares represent males. Lines connecting two individuals horizontally represent
marriage or a coupling in which offspring were produced. Vertical lines emanating from the horizontal
connector line represent the offspring of the coupling. An individual family member with the trait is shaded
a dark or different color. An individual who carries the trait (heterozygous and phenotype of the dominant
trait) is half-shaded. Each generation has a Roman numeral and each individual of that generation has
Arabic numbers.
18. According to the pedigree on the right, individual II-2 is what sex?
19. According to the pedigree, individual I-2 is what sex?
I
1
2
2
3
4
1
2
II
20. If the trait being traced is brown eyes, what phenotype is individual
II-3?
1
III
21. What is the phenotype of individual III-1?
F. Sex-Linked Traits
Sometimes a particular trait is found on a sex chromosome, usually X.
These genes are called sex-linked genes only because they are located on
the sex chromosome X. The characteristic has nothing to do with the sex of
the individual. Since females have two X (XX) and males only have one (XY),
males have a higher chance of expressing a defective recessive gene since
they don’t have another X to act as the dominant X. Females with only one
defective allele are said to be carriers. Their phenotype is normal and they
do not express the disorder. A Punnett square to determine sex-linked inheritance must include the sex
chromosomes X and Y using a lowercase superscript to denote the defective recessive gene located on the
X chromosome. A few sex-linked disorders are commonly found worldwide. The first is colorblindness
(noted as Xc) in which an afflicted individual inherits a defective gene coding for the color-detecting cones
of the eye’s retina. This individual may have a hard time distinguishing two colors. A second type of sexlinked disorder is the blood clotting defect called hemophilia. An individual with hemophilia cannot
produce adequate blood clots and may bleed to death as a result. This disease is noted as X h where the h is
the defective blood-clotting protein. A third type of sex-linked disease is Fragile X syndrome. A person with
Fragile X inherits an addition of 600+ nucleotides on the X chromosome which results in abnormal facial
features and intellectual disabilities. This is denoted as Xf. The fourth and final common sex-linked disorder
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is Duchenne’s muscular dystrophy (Xd) in which the individual inherits a defective muscle protein causing
progressively weakened muscles. The average life span for someone with Duchenne MD is 25 years.
22. What is a sex-linked trait?
23. Why are males more prone to inherit the disease or disorder?
24. Why are females considered carriers? Why can’t males be carriers?
25. Cross a male afflicted with colorblindness and a normal woman.
26. If a female carrier of Fragile X syndrome has children with a normal male, what are the chances that
a boy will be born with Fragile X syndrome?
27. Cross a male hemophilia with a female carrier of hemophilia. What are the chances they will have a
girl with hemophilia?
28. Cross a female carrier of Duchenne’s muscular dystrophy with a healthy male. What are the chances
the will have a girl with Duchenne?
G. Autosomal Disorders
Most characteristics are found on chromosomes 1-22 or the autosomes.
Since they are not linked to the individual’s sex, they are equally passed
down to males and females. If a dominant allele codes for the defect, that
trait is considered to be dominantly inherited and either the homozygous
dominant or heterozygous genotype will express the defect. One such
autosomal dominant disease is Huntington’s disease where the afflicted
individual inherits the H allele. It is lethal and ends with the individual losing
Autosomal Dominant Inheritance
most brain tissue to disease. This disease is unique in that the person does
not show any symptoms until later in life, usually after having children. As a result, the disease stays in the
human gene pool. The other type of autosomal dominant disorder is dwarfism, in particular, a form called
Achondroplasia. Individuals with dwarfism have a defect in bone growth of the long bones, the arms and
legs. As a result, the average height for Achondroplasia dwarves is about 4’ tall. Dwarfism is caused by one
dominant allele, D. However, two dominant D alleles causes death, termed double dominant lethality. Most
autosomal disorders are caused by recessive alleles, thereby requiring two defective alleles to produce the
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disorder. Any person who is heterozygous is disease free (a healthy phenotype) but is considered a carrier.
There exist five common autosomal recessive disorders. The first one is albinism characterized by a defect
in the pigment melanin. Individuals homozygous for aa are termed albino and may have vision and skin
problems. Another common autosomal recessive disease is cystic fibrosis in which the mucus producing
protein is defective resulting in excessively thick and sticky mucus which can cause death. A person with
cystic fibrosis may live to age 30. Another autosomal recessively inherited disease is the lethal Tay-Sachs in
which the lethal t causes a defect an enzyme in neural cells. If the cells cannot break down lipid or fat, it
accumulates in the nervous tissue and will cause death by the age of 5. Phenylketonuria or PKU is a
recessively inherited autosomal disorder in which the enzyme that breaks down the amino acid
phenylalanine is defective. An accumulation of this amino acid can result in brain damage causing
intellectual disabilities. The final recessively inherited disorder is called neurofibromatosis or NF. NF results
in mostly physical deformities of the skin and/or bone caused by tumors in nervous tissue, which can occur
anywhere on the body.
29. What is the difference between an autosomal disorder and a sex-linked disorder?
30. What is the difference between a dominant and recessive autosomal disorder?
31. Are there carriers in autosomal dominant disorders? Why or why not?
32. Why does Huntington’s still exist if it is deadly and dominantly inherited?
33. Cross a male with Huntington’s disease with a normal female. What are the chances a child will have
Huntington’s?
34. What do two parents with Achondroplasia have to think about before
having children of their own?
35. Why does Tay-Sachs still exist even though an individual afflicted dies by
the age of 5?
36. Cross two PKU carriers. What is the chance a child will be born with PKU?
H. Polygenic Inheritance
In simple inheritance, one gene such as A codes for one trait (albinism). There exist special circumstances in
which many genes code for one particular trait. One clue that many genes are present is the use of more
than one letter, for example A and B or more. In humans, four particular circumstances use polygenic
inheritance: eye color, hair color, skin color, and height. Since many genes code for one trait, there may be
many intermediate phenotypes. Each dominant allele adds to the final tally of the trait, whether it is
pigment or inches in height. In the case of eye color, brown eyes have many pigments, which accounts for
the fact that at least four genes (eight alleles total) code for a person’s eye color. Hair color uses two
different traits, brown melanin and red melanin, each with many genes, to give a person his or her final hair
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color. Skin color is very complex and not completely understood. At least four genes are known to
determine an individual’s skin color. This results in many phenotypes all falling somewhere in between the
two extremes of very dark and very light. Finally, height also has many genes coding for it. It is special in
that a person’s environment or upbringing can determine whether or not he or she reaches his or her
maximum potential for height. If the genes are known and the extremes of the trait are known, it is
possible to calculate how much each dominant allele contributes to the final phenotype.
37. Why is the term “polygenic” a good name for this type of inheritance?
38. If a tree is homozygous for SHORT alleles giving it a genotype of aabbccdd, the tree is only 5 ft tall
(60 inches). If a tree is homozygous for TALL alleles giving it a genotype of AABBCCDD, the tree is a
whopping 25 ft tall (300 inches). How tall would a tree with the genotype AaBbCcDD be?
39. An individual is homozygous dominant for black hair given the genotype AABBCCDD with a melanin
score of 100. Another individual is homozygous recessive with the genotype aabbccdd. This
individual has blonde hair with a melanin score of 20. What is the melanin score of an individual
with the genotype AabbCCdd?
I. Incomplete Dominance
In simple heredity, an uppercase allele means it is a dominant allele and its
phenotype is always expressed. This is not always the case as some “dominant”
alleles aren’t truly dominant. These are called incompletely dominant where the
hh
H’H’
heterozygote shows a blend of the incomplete dominant and the recessive. These
uppercase alleles may be designated with a ‘ or “prime” to signal that they do not
act dominantly. In the case of flowers, snapdragons’ red pigment behaves
incompletely dominant where R’r is pink! In order to see the “dominant”
phenotype, the genotype must be homozygous dominant. The same holds true for
the recessive phenotype, as is typical. In humans, nose size and hair texture act
H’h
similarly. A large nose (L) is incompletely dominant (L’) over a small nose (l)
making the heterozygote (L’l) a medium-sized nose. Curly hair is incompletely dominant (C’) over straight
hair (c) making the heterozygote individual wavy-haired (C’c).
40. Why is using the character ‘ a good practice when noting incomplete dominance?
41. Blue hair in aliens is incompletely dominant over red hair. Cross two heterozygous aliens.
a. What are the genotypes?
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b. What phenotype is the heterozygote?
42. Wide-set eyes are incompletely dominant over close-set eyes.
a. What are the gentoypes?
b. What phenotype is the heterozygote?
J. Multiple Alleles
So far, all genes have been coded for by two alleles, one from the father and one from the mother. This will
always be the case as each parent can only donate one allele. Certain genes
are coded for by more than two alleles, so the phenotype depends on which
alleles are passed down and the order of dominance. This inheritance
pattern is termed multiple alleles and although it uses more than two
alleles for one gene, each allele uses one letter, often times with
superscripts to differentiate variances. A prime example of this is in human
blood types where the letter I denotes the protein immunoglobulin. A
genotype of IA denotes the phenotype type A blood, IB denotes type B
blood, and the recessive i denotes the absence of A or B which is termed
type O blood.
43. How many different letters of genes are used to denote multiple alleles? Why?
44. In corn kernel color, yellow is dominant over white, white is dominant over blue, and blue is
dominant over red. What should the alleles be to represent each phenotype?
45. Cross a pure bred blue corn plant with a heterozygous yellow-white corn plant.
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K. Codominance
The final complex pattern of heredity is termed codominance where two dominantly inherited alleles code
for the same trait. Since both alleles a …
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