Genetics Worksheet

HANDS ON ACTIVITYGenetics
Name________________________
Objectives
• Define the terms allele, genotype, phenotype, homozygous, heterozygous, dominant allele,
recessive allele, monohybrid, dihybrid
• State an individual’s genotype and phenotype
• Complete a monohybrid Punnett square
• Complete a dihybrid punnett square
Things you should be able to explain to someone else after this lab:
Genotype
Phenotype
Homozygous dominant
Homozygous recessive
Heterozygous
Allele
Monohybrid
Dihybrid
Punnett Square
Gregor Mendel
Law of Segregation
Law of Independent Assortment
In this activity we will look at several traits that appear to be controlled by single genes in a
classic Mendelian pattern in order to better understand the expression of dominant and recessive
traits. Mendelian inheritance (Mendelian genetics) is a scientific description of how hereditary
characteristics are passed from parent organisms to their offspring and it underlies much of
genetics. This theoretical framework was initially derived from the work of Gregor Mendel. It
was re-discovered in 1900 at which time it was very controversial. In 1915 Mendel’s theories
were integrated with the chromosome theory of inheritance by Thomas Hunt Morgan and they
became the core of classical genetics.
Mendel discovered that when crossing white flower and purple flower plants, the resulting
flower is not a blend of purple and white. Instead, the offspring was purple flowered. In
response, he conceived the idea of heredity units, which he called “factors”. He believed one of
the factors was a recessive characteristic and the other dominant. Mendel said that factors (now
called genes) normally occur in pairs in ordinary body cells. He also noted that these factors
must segregate during the formation of sex cells. Each member of the pair becomes part of the
separate sex cell. The dominant gene (i.e. the purple flower in Mendel’s plants) will hide the
recessive gene (aka the white flower in Mendel’s plants).
After Mendel self-fertilized the F1 generation and obtained the 3:1 ratio, he correctly theorized
that genes can be paired in three different ways for each trait: AA, aa, and Aa. The capital “A”
represents the dominant factor and lowercase “a” represents the recessive. (The last combination
listed above, Aa, will occur roughly twice as often as each of the other two, as it can be made in
two different ways, Aa or aA.)
Mendel stated that each individual has two factors for each trait, one from each parent. The two
factors may or may not contain the same information. If the two factors are identical, the
1
individual is called homozygous for the trait. If the two factors have different information, the
individual is called heterozygous. The alternative forms of a factor are called alleles.
The genotype of an individual is made up of the many alleles it possesses. An individual’s
physical appearance, or phenotype, is determined by its alleles as well as by its environment. An
individual possesses two alleles for each trait; one allele is given by the female parent and the
other by the male parent. They are passed on when an individual matures and produces gametes:
egg and sperm. When gametes form, the paired alleles separate randomly so that each gamete
receives a copy of one of the two alleles. The presence of an allele doesn’t promise that the trait
will be expressed in the individual that possesses it. In heterozygous individuals the only allele
that is expressed is the dominant. The recessive allele is present but its expression is hidden.
Mendel summarized his findings in two laws; the Law of Segregation and the Law of
Independent Assortment.
•
•
The Law of Segregation essentially states that when any individual produces gametes, the
copies of a gene separate so that each gamete receives only one copy aka one allele. A
gamete will receive one allele or the other.
The Law of Independent Assortment, also known as “Inheritance Law” states that
separate genes for separate traits are passed independently of one another from parents to
offspring. More precisely the law states that alleles of different genes assort
independently of one another during gamete formation.
The above information was obtained from Wikipedia.org: http://en.wikipedia.org/wiki/Mendelian_inheritance
Write your answers directly on this worksheet or type your answers directly into the worksheet.
ACTIVITY 1: GENETICS DEFINITIONS and QUESTIONS
1. Differentiate between the human genotype and human phenotype.
2. Define an allele.
3. What is the minimum number of human alleles associated with each human gene? (Hint:
what process produces human gametes, how many gametes are required to form a
complete human).
4. What does it mean if an allele is dominant? How many dominant alleles does a person
need for the dominant trait to be expressed?
2
5. What does it mean if an allele is recessive? How many alleles does a person need for a
recessive trait to be expressed? Explain your answer.
6. Julie is heterozygous for a widow’s peak. The widow’s peak (W) is dominant to a
straight line (w). Use this information to identify the following:
a. Julie’s Genotype:
Julie’s Phenotype:
7. Julie’s brother Sam is homozygous recessive for the same trait of widow’s peak. Use this
information to identify the following:
a. Sam’s Genotype:
Sam’s Phenotype:
8. Julie marries Jason, who is also homozygous recessive for widow’s peak. Draw a Punnett
square below illustrating this cross.
9. What is the chance Julie and Jason have a child with a widow’s peak?
10. For another trait, Jason is homozygous dominant for a dimpled chin (D) which is
dominant over no dimple (d). Jason’s brother, Josh is heterozygous for a dimpled chin.
Use the information in the lab to determine the following.
a. Jason’s Genotype:
Josh’s Phenotype:
b. Which of the brothers has a dimpled chin?
3
11. So far we know Jason is homozygous recessive for widow’s peak and homozygous
dominant for dimpled chin. Julie is heterozygous for both widow’s peak and dimpled
chin. Draw a dihybrid Punnett square on the next page showing the mating between Jason
and Julie for both of these traits (dihybrid).
12. What is the chance Julie and Jason will have a child who has dimples but no widow’s
peak?
In addition to looking at physical traits like widows peak, freckles, and tongue rolling, Punnett
squares can be used to predict the probability of genetic diseases. One recessive genetic disease
is cystic fibrosis which causes a buildup of thick, sticky mucus that damages organs like the
lungs and pancreas. There is no cure for cystic fibrosis but there are treatments to help manage
the symptoms.
As a recessive disease, someone would have to be homozygous recessive to have cystic fibrosis.
An individual who is heterozygous with one normal allele and one recessive allele would not
have cystic fibrosis but could pass the recessive trait to an offspring.
1. Sallie does not have cystic fibrosis but her Dad does suffer from cystic fibrosis. Use this
information to identify the following:
a. Sallie’s Genotype:
Sallie’s Phenotype:
2. Sallie is married to Greg who is normal and does not have a history of cystic fibrosis in
his family. Use this information to identify the following:
a. Greg’s Genotype:
Greg’s Phenotype:
3. What is the probability that Sallie and Greg have a child with cystic fibrosis? Use a
Punnett Square to determine the answer for this question. Your answer should include
the Punnett square and probabilities.
4
5