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A. Gregor Mendel
Mendelian Genetics Worksheet
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 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 true time and again, we now have two laws that can be
T t
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
Y y
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.
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What is Mendel’s first law?
Explain the first law in terms of plant height (T).
What is Mendel’s second law?
Explain the second law in terms of both height and pea color (T and Y).
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.
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If red fur is dominant over blue fur, what are the alleles for the different furs?
If yellow peas are dominant over green peas, what are the alleles for the different color peas?
What would the alleles for a heterozygous grey alien be?
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
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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.
16. Using a Punnett square, what are the possible offspring of the parents?
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.
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According to the pedigree on the right, individual II-2 is what sex?
According to the pedigree, individual I-2 is what sex?
If the trait being traced is brown eyes, what phenotype is individual II-3?
What is the phenotype of individual III-1?
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F. Sex-Linked Traits
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Sometimes a particular trait is found on a sex chromosome, usually X. These genes are
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called sex-linked genes only because they are located on the sex chromosome X. The
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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 sex-linked 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 Xh 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 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.
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What is a sex-linked trait?
Why are males more prone to inherit the disease or disorder?
Why are females considered carriers? Why can’t males be carriers?
Cross a male afflicted with colorblindness and a normal woman.
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 most brain tissue to disease. This disease is unique in that the person does not show
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Autosomal Dominant Inheritance
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 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.
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What is the difference between an autosomal disorder and a sex-linked disorder?
What is the difference between a dominant and recessive autosomal disorder?
Are there carriers in autosomal dominant disorders? Why or why not?
Why does Huntington’s still exist if it is deadly and dominantly inherited?
Cross a male with Huntington’s disease with a normal female. What are the chances a child will have Huntington’s?
What do two parents with Achondroplasia have to think about before having children of their own?
Why does Tay-Sachs still exist even though an individual afflicted dies by the age of 5?
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
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
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H’H’
H’h
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 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 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).
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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?
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.
K. Codominance
The final complex pattern of heredity is termed codominance where two dominantly inherited alleles code for the same trait. Since
both alleles are dominant, the phenotype shows BOTH phenotypes, not a blend, but each phenotype equally. In the case of
humans, two particular traits use codominance. The first trait is sickle-cell anemia where the heterozygous individual has both
round red blood cells (R) and sickle-shaped red blood cells (S). The genotype for this may be RS. Another human trait that uses
codominance is blood type where type A and type B are dominant, therefore an individual with the IA and IB allele is said to be
blood type AB. This type of inheritance pattern is also frequently found in flower petals, and certain furry mammals such as roan
cattle, a codominant red and white phenotype (RW).
46. Sickle-cell anemia is a codominant trait. Cross two heterozygous individuals.
47. What percent of offspring may contract sickle-cell?
48. Why is blood type in humans considered both multiple alleles and codominance?
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