At the time Mendel started his work, the blending concept of inheritance was prevalent. Mendel disproved this concept
through well-designed experiments that offered statistical evidence; by analyzing the 3:1 results among the F2
generation of a monohybrid cross. Mendel arrived at the law of segregation: factors (alleles) for a trait occur in pairs in
an organism: they separate into different sex cells during gamete formation. This explains why the recessive
phenotype—absent in the F1 generation—reappeared in the F2 generation.
Solving genetics problems requires distinguishing between the genotype (genetic makeup) and the phenotype
(appearance) of an individual. For any pair of alleles, the dominant allele is represented by an uppercase letter, and the
recessive allele is represented by a lowercase letter. A cross is done by using the laws of probability, most often by
employing a Punnett square, which offers a mechanism whereby all possible types of sperm fertilize all possible types of
eggs. The results can be expressed as the phenotypic ratio or can be used to state the chances of an individual having a
Mendel used a monohybrid cross to verify his law of segregation. A cross represented by AA x aa offers the best chance
of producing a recessive offspring. Today the testcross is used to determine whether an individual is heterozygous or
The F2 results of a dihybrid cross allowed Mendel to formulate his law of independent assortment: during gamete
formation, the factors of one pair separate independently from the factors of other pairs. This law explains why
Mendel’s F2 generation contained four types of genotypes—that is, all possible combinations of dominant and recessive
characteristics. The Punnett square can also be used to solve two-trait problems including the two—trait testcross.
Studies of human genetics have shown that many autosomal genetic disorders can be explained on the basis of simple
Mendelian inheritance. Genetic counselors often construct pedigrees to show the pattern of inheritance of a
characteristic within a family. The particular pattern indicates the manner in which a characteristic is inherited.
Tay-Sachs disease, cystic fibrosis, PKU and sickle cell disease are autosomal recessive disorders that have been studied in
detail. Neurofibromatosis and Huntington disease are autosomal dominant disorders that have been well studied.
There are many exceptions to Mendel’s laws. The phenotypes of individuals arise from many kinds of inheritance
patterns. Multifactorial inheritance controls some traits. Dominance, for example, can be complete or incomplete.
Codominance can also exist among the offspring of a genetic cross. Genes for some traits have multiple alleles. Many
traits are polygenic and are influenced by the environment.
11.1 – Gregor Mendel (p182-183)
• Mendel discovered certain laws of heredity after doing experiments with garden peas during the mid-1800s.
1. When Mendel began breeding experiments, other breeders had different ideas about heredity. Place a check next to
the statements that represent the ideas at that time:
a._____ A cross between a red flower and a white flower results in all offspring having pink flowers.
b._____ A cross between a red flower and a white flower results in some offspring having white flowers.
c._____ In a genetic cross, both parents contribute equally to the offspring.
d._____ Parents of contrasting appearance will produce offspring of intermediate appearance.
e _____ The instability of genetic material explains unexpected cross results.
2. Mendel’s work reflected several methods and advantages that contributed to his success. Place a check next to the
statements that represent those methods and advantages:
a .______Each trait studied (e.g., seed shape, flower color) displayed many different phenotypes.
b. ______Breeding experiments have a statistical basis.
c_______The garden pea plants used had a long generation time.
d. ______The plants used were easy to cultivate.
e. ______The plants used could not self-pollinate.
11.2 – Mendel’s Law of Segregation (p184-188)
• When Mendel performed one-trait crosses, he found that each organism contains two factors for each trait, and
the factors segregate during formation of gametes.
• Today it is known that alleles located on chromosomes control the traits of individuals.
3. Mendel arrived at the law of segregation by interpreting the results of his monohybrid crosses. Place a check next to
the interpretations he used.
a._______ F1 organisms contain one copy for each hereditary factor.
b._______ Factors segregate when gametes form.
c._______ Gametes fuse randomly during fertilization.
d._______Allelic pairs assort in a dependent manner.
4. The length of stem in the plants that Mendel studied had two alleles: T (tall) and t (short). Using these letters, write
the alleles for the heterozygous genotype a. _____________, the homozygous dominant genotype b. __________,
and the homozygous recessive genotype c. _____________
5. When Mendel crossed true-breeding tall plants with true-breeding short plants, the F1 plants were a. __________,
when he crossed F1 x F1, the offspring included b. __________, plants for every c. __________, plant. Because some
of the F2 plants were short, he concluded that the F1 generation was Tt; therefore, each original parent plant had
passed on only one factor. Mendel’s law of segregation states: d. __________,
6. a. Complete the following table to show the difference between genotype and phenotype:
c. The genotypic ratio among the offspring from this cross is: ________________________
d. The phenotypic ratio among the offspring from this cross is: _______________________
7. Among humans, the allele for dark hair (D) is dominant to the allele for blonde hair (d). Consider the cross Dd x Dd. To
answer these questions, use fractions except when asked for a percentage.
a. What is the chance that either parent will produce a gamete with a dominant allele? ____________________
b. Using the multiplicative law of probability, calculate the chance of a homozygous dominant offspring (dark hair).
Show your work. ____________________
c. What is the chance this couple will have a homozygous dominant offspring? ____________________
d. What is the chance either parent will produce a gamete with a recessive allele? ____________________
e. Using the multiplicative law of probability, calculate the chance of a homozygous recessive offspring (blonde
f. What is the chance this couple will have a homozygous recessive offspring? ____________________
g. Using the multiplicative law and the additive law, calculate the chance of a heterozygous offspring (dark hair).
h. What is the chance this couple will have a heterozygous offspring? ____________________
i. Using the additive law, calculate the chance of an offspring with the dominant phenotype (dark hair).
j. Your calculations indicate that the phenotypic ratio for this cross is 1/4 dark hair to Y4 blonde hair, or a
phenotypic ratio of ____________________
8. a. In peas, yellow seed color is dominant to green. The key is: Y= __________, y = __________
b. Fill in this Punnett square for the cross Yy x Yy.
c. The genotypic ratio among the offspring from this cross is: ____________________________________________
d. The phenotypic ratio among the offspring from this cross is: _______________________________(yellow: green).
9. The gametes combine at a. _____________and usually, a b. _____________number must he counted before a 3:1
ratio can be verified.
One- Trait Testcross (pp. 187-188)
10. Researchers do a a. _____________ (i.e., the dominant phenotype is mated to the recessive phenotype) to
determine whether the dominant phenotype is homozygous or heterozygous. If the individual is homozygous
dominant, the F1 generation is expected to be b. _____________ . If the individual is heterozygous, a phenotypic
ratio of c. _____________is expected.
11. In humans, widow’s peak (W) is dominant to straight hairline (w). Consider the cross Ww x Ww. The chance of a
child with widow’s peak is a a. _____________%, and the chance of a child with straight hairline is b.
12. Consider the cross Ww x ww. The chance of a child with widow’s peak is a. _____________% and the chance of a
child with straight hairline is b. _____________.
13. Among humans, dark eyes (B) is dominant to blue eyes (b). In a family, one parent has dark eyes and the other has
blue eyes. Among their offspring, two develop dark eyes and two develop blue eyes. Most likely, the genotypes of
the parents are a. _____________ (dark-eyed parent) and b. _____________ (blue-eyed parent).
14. In fruit flies, a cross between long-winged (L) flies and short-winged (I) flies produces only long-winged Flies. Most
likely, the genotypes of the parental flies are a. _____________ (long-winged parents) and b. _____________
11.3 Mendel’s Law of Independent Assortment (188-191)
When Mendel and others performed two-trait crosses, they found that every possible combination of factors is
present in the gametes.
Today it is known that homologous pairs of chromosomes separate independently during meiosis I, and this
produces all possible combinations of alleles in the gametes.
15. In pea plants. T= tall and t = short, and G = green pods and g = yellow pods. When Mendel crossed homozygous tall
plants having green pods with pure, short plants having yellow pods, the F1 plants all had the genotype
a. _____________ and the phenotype b. _____________. If T always stayed with G and t always stayed with g in
the gametes, then how many different phenotypes would be among the F2 plants? c. _____________ Mendel
observed four different phenotypes and formulated his law of independent assortment, which states:
16. The process of meiosis explains the law of segregation and the law of independent assortment. Considering the
movement of chromosomes, why is only one allele for each trait present in the gametes? a. _____________. Why
are all combinations of alleles present in the gametes? b. _____________
Two-Trait Genetic Problems (p189-191)
A testcross can also be used to determine the genotype of an individual that is dominant in two traits.
17. In horses, black (B) is dominant to brown (b) and a trotter (T) is dominant to a pacer (t). Use fractions in your
Consider the cross Bb x bb. Among the offspring, the chance of black coat is a. _____________and the chance of
brown coat is b. _____________.
Consider the cross Tt x Tt. Among the offspring, the chance of a trotter is c. _____________. a pacer is
Consider the cross RbTt) x bbTti, and use the multiplicative law of probability to determine the chances of the
Black trotter (e) ______________
Black pacer (f) ______________
Brown trotter (g) ______________
Brown Pacer (h) ______________
What is the phenotypic ratio expected for the preceding cross? i. _____________________________________________
Check your answer by doing a Punnett Squire
18. Given the cross BbTt x BbTt, determine the ratio and phenotypes expected. ________________ _____________ :
_________________ _________________ : ______________ _________________ : _____________ _____________
19. a. Do a Punnett square for the cross BbTt X Bbtt.
b. determine the ratio and phenotypes expected.
_________________ _________________ : ______________ _______________ : ____________ ____________
20. In rabbits, black (B) is dominant to brown (b), and spotted coat (S) is dominant to solid coat (s). A black, spotted
rabbit is mated to a brown, solid one, and all ten of their offspring are black and spotted. The genotypes of the
parents are a. _____________, and b. _____________.
21. In humans, widow’s peak (W) is dominant to straight hairline (w), and short fingers (S) are dominant to long fingers
(s). If the two parents are heterozygous in both regards, what is the chance of any offspring having widow’s peak
and short fingers? a. _____________If one parent is heterozygous in both regards and the other is homozygous
recessive, what is the chance of an offspring with widow’s peak and short fingers? b. _____________ or
11.4 Human Genetic Disorder (p192-195)
• Many human genetic disorders are inherited according to Mendel's laws.
• The pattern of inheritance indicates whether the disorder is recessive or dominant.
• Recessive disorders require the inheritance of two recessive alleles; dominant disorders appear if a single
dominant allele is inherited.
22. Answer the questions for the following pedigree:
What is the mode of inheritance shown in this pedigree?
What is the genotype of person 1?
What are the chances of person 1 having normal children?
What are the chances of person 3 having normal children?
23. Answer the questions for the following pedigree
a. What is the mode of inheritance in this pedigree?
b. For person 2, the genotype is
c. For person 2, the phenotype is
d. For person I, the genotype is
e. How did you determine this?
f. What are the chances that person 3’s children will be normal?
24. An unaffected man carrying the allele for PKU reproduces with an unaffected woman carrying the same allele. The
chances (percent) of the offspring having PKU are a. and of the offspring being normal are a. _______________.
25. A man who is heterozygous for neurofibromatosis reproduces with a woman who is normal. The chances (percent)
of the offspring having neurofibromatosis are a. _______________, and of the offspring being normal are
26. Match the descriptions to these types of disorders.
1. Cystic fibrosis
2. Huntington disease
4. Phenylketonuria (PKU)
5. Tay-Sachs disease
a. _________ lysosomal storage disease
b. _________benign tumors in skin or deeper
c. _________ progressive nervous system degeneration
d. _________ disorder affecting function of mucous and sweat glands
e. _________ essential liver enzyme deficiency
11.5 – Beyond Mendelian Genetics (p196-199)
• There are forms of inheritance that involve degrees of dominance, multiple alleles, and polygenes.
• Environmental conditions can influence gene expression.
27. When a curly-haired person reproduces with a straight-haired person and their children have wavy hair, this is an
example of _______________________________________
28. Both a man and a woman have sickle cell trait. List all phenotypes among the offspring, as well as the chance
(percent) of each occurring. a. ______________________, b. ______________________,
29. A man with blood type A reproduces with a woman who has blood type B. Their child has blood type 0. Give the
genotype of all persons involved: man a. ______________________, woman b. ______________________, and a
child c. ______________________.
30. If a child has type AB blood and the father has type B blood, what could the genotype of the mother be?
31. If a child has type BO blood and the father has type 0 blood, what could the genotype of the mother be?
32. Roan cattle have red hairs and white hairs because of codominance of two alleles. Using the key in the diagram
that follows, cross a roan cow and a roan bull, and give the phenotypic ratio. a. ______________________What
ratio would have resulted if red hair were dominant over white hair? b. ______________________.
Key: RR = red
RW = roan
WW = white
33. An investigator notes that a population contains a range of phenotypes that fits a bell-shaped curve. What type of
inheritance pattern is this? a. _____________The investigator decides that three pairs of alleles are involved. List
all the possible genotypes for an intermediate phenotype. b. __________________________Explain your answer.
34. Consider a model in which there are three gene pairs of alleles; a dominant allele in any pair adds pigment to the
skin. Use the letters A, B, C to indicate pigment formation and a, b, c to indicate lack of pigment formation.
a. What is the genotype for the darkest individual? __________________________
b. What is the genotype for the lightest individual? __________________________
c. What is the genotype for the offspring from a cross of the individuals from a and b? _______________________
d. How does the skin color of this person compare to either of the parents? _______________________________
35. In humans, A = a normal amount of the pigment melanin and a = albinism (absence of pigment), and B = brown
eyes and b = blue eyes.
a. What is the eye color for the genotype BBAa? __________________________
b. What is the eye color for the genotype bbaa? __________________________
c. If a man with the genotype BBAa reproduces with a woman having the genotype bbaa, what is the chance
(percent) of producing a child with normal amounts of melanin? __________________________
d. What is the chance of producing a child with albinism? __________________________
36. In rabbits, there are four alleles for coat color, but each rabbit has only two of these. What type of inheritance
pattern is this? _________________________________________________________________________________