Download Genetics Supplement

Genetics Supplement
(These supplementary modules, a Genetics Student Handout, and Teacher Preparation Notes with
background information are available at http://serendip.brynmawr.edu/sci_edu/waldron/#genetics.
By Drs. Scott Poethig, Ingrid Waldron, and Jennifer Doherty, Dept. Biology, Univ. Pennsylvania, © 2014.)
We all know that children tend to resemble their parents in appearance. Parents and their children tend
to have similar appearance because children inherit genes from their parents and these genes influence
characteristics such as skin and hair color.
How do genes influence our characteristics?
1. A gene is a segment of a ________ molecule that gives the instructions for making a protein. Different
versions of the same gene are called alleles, and different alleles give the instructions for making
different versions of a __________________. The different versions of a protein can result in different
characteristics, as illustrated in this table.
Genotype 
Protein

Phenotype (characteristics)
AA or Aa

Enough normal enzyme to make
melanin in skin and hair

Normal skin and hair
color
aa

Defective enzyme for melanin
production

Very pale skin and hair
color (albino)
If both copies of a gene have the same allele, the person is homozygous for that gene. If the two copies
of a gene have different alleles, the person is heterozygous.
Often, in a heterozygous individual a dominant allele determines the observable characteristic and the
other recessive allele does not affect the phenotype. Thus, a heterozygous person has the same
phenotype as a person who is homozygous for the dominant allele. In our example, the A allele is
dominant because it codes for normal, functional enzyme and, even in a heterozygous individual, there
is enough of this normal, functional enzyme to produce enough melanin to result in normal skin and hair
color. The a allele is recessive because it codes for a non-functional enzyme which does not affect skin or
hair color in a heterozygous individual.
2. Circle the genotypes in the table that are homozygous. Explain how the two different homozygous
genotypes result in different phenotypes.
3. What are the two different genotypes for the albinism gene that result in the same phenotype?
- Explain how two people with different genotypes can have the same phenotype.
How does a baby inherit genes from his or her mother and father?
As you know, each gene is a part of a DNA
Mother
molecule. Each DNA molecule is contained in a
Meiosis ↓
chromosome. You will see that we can understand
egg
how a baby inherits genes from his or her mother
and father by understanding how the genecarrying chromosomes move during meiosis to
form gametes and fertilization to form the zygote
that develops into a baby.
Father
Meiosis ↓
sperm
Fertilization
zygote
Mitosis ↓
baby
Inheritance of Albinism
To learn more about how genetic traits are inherited, we will start with a specific question:
If each parent has one A allele and one a allele (i.e. both parents are Aa), what different
combinations of A and/or a alleles could be observed in the children of these parents?
To answer this question your group will use model chromosomes to demonstrate meiosis and
fertilization. The pair of homologous chromosomes for each parent will include one model chromosome
with an A allele and another with an a allele.
4. One of you should be the mother and
use your model chromosomes to
demonstrate how meiosis produces
different types of eggs, and another
should be the father and demonstrate
how meiosis produces different types of
sperm. In this chart, write in the
genetic makeup of the two types of
eggs and the two types of sperm
produced by meiosis.
Next, model fertilization, using the
model chromosome for each type of
sperm to fertilize each type of egg.
Write the genetic makeup of the
resulting zygotes in the chart.
Biologists use a similar chart to analyze inheritance However, biologists omit much
of the detail shown above and use a simplified version called a Punnett Square.
5. In this Punnett square, circle each symbol which represents the genetic makeup of
a gamete produced by the heterozygous mother or father.
- Use an * to indicate the genotype of each zygote.
A
a
A
AA Aa
a
Aa
aa
6. The genotypes of the zygotes in the Punnett square represent the possible genotypes of the children
of this couple. Explain why each child will have the same genotype as the zygote that he or she
developed from.
7. For the Aa mother in this Punnett square, what fraction of her eggs have an
a allele? _____
- What fraction of the Aa father's sperm have an a allele? _____
A
a
A AA Aa
a
Aa
aa
- What fraction of this couple's children would you expect to be aa? _____ Explain your reasoning.
8. Complete this Punnett
square for two parents who
are homozygous AA.
Complete this Punnett
square for two parents who
are homozygous aa.
Complete this Punnett square for a
mother who is heterozygous Aa and a
father who is homozygous aa.
9. For each of the four Punnett squares on this page, indicate the phenotypes of each parent and each
child (N = normal skin and hair color or O = albino).
Notice that most of the children have the same phenotypic characteristic as one or both parents.
- Circle the only example of a child who has a phenotypic characteristic that is not observed in either
parent.
10. Explain why two albino parents won't have any children with normal skin and hair color, but two
parents with normal skin and hair color could have an albino child.
11. Albino children are rare in the general population. Based on this observation, what is the most
common genotype for parents? Explain your reasoning.
Genetics of Sex Determination
As you probably know, human males have an X and a Y chromosome (XY), whereas females have two X
chromosomes (XX). The gene that results in the development of male anatomy is located on the Y
chromosome. This gene is called SRY, which stands for Sex-determining Region of the Y chromosome. If
a zygote has a Y chromosome with the SRY gene, the embryo will develop testes and male anatomy. If a
zygote does not have a Y chromosome with the SRY gene, the embryo will develop ovaries and female
anatomy.
1. In the figure below, use X and Y to show the separation of the sex chromosomes in meiosis (mother
on the left and father on the right). Then show the formation of a zygote that will develop into a female
(on the left) or a male (on the right).
Meiosis
Fertilization
2. Draw a Punnett Square which shows the inheritance of the sex chromosomes. Use X to indicate an
egg or sperm with an X chromosome and Y to indicate a sperm with a Y chromosome.
3. Based on this Punnett Square, what percent of children would you expect to be male?
4. To test this prediction, begin by writing down the initials of all the children your mother has had.
Arrange these initials in order from the youngest to the oldest, indicating whether each was male or
female.
- Use this information and the information from the other students in your group to complete the upper
rows of the table on the next page. Add your group's data on the total number of children and total
number of males to the class data your teacher is collecting.
5. Complete the following table.
Sex of each child
1st 2nd 3rd 4th 5th
+
Total
number
of children
Total
number
of males
%
males
Your mother's children
Children of the mother of another
student in your group
Children of the mother of another
student in your group
Children of the mother of another
student in your group
Children of the mothers of all the
students in your class
Predicted percent from Punnett square
(see question 2)
6. Use your group's data and data from nearby student groups to answer the following questions.
- If a mother's first child is a son, is the next child necessarily a daughter?
- If a mother's first child is a daughter, is the next child necessarily a son?
- If a mother's first two children are the same sex, is the next child necessarily the opposite sex?
These observations illustrate that you cannot predict the sex of the next child based on the sex of a
previous child or children. Each time a sperm fertilizes an egg, this fertilization event is independent of
any previous fertilizations that resulted in older brothers or sisters.
7. Compare the predicted percent male from the Punnett square with the observed percent male for
your mother’s children and for the children of the mothers of each of the other students in your group.
How similar to the prediction are the observed results for each of the families?
- How can you explain any differences between the Punnett square prediction and the observed results?
Random variation in which sperm fertilizes which egg has a bigger effect on the percent male in small
samples such as an individual family, but random variation can also influence the percent male for larger
samples such as the children of the mothers of all of your classmates.
- How similar is the class result to the prediction from the Punnett square?
Pedigree Analysis
Geneticists illustrate the inheritance of a gene within a family by using a pedigree chart. In a pedigree
chart, males are symbolized by a square (□) and females are symbolized by a circle (○). People who are
affected by a condition or disease are symbolized by a dark square or circle.
This pedigree chart shows the inheritance of albinism in three generations of a family. 1 and 2 represent
a couple who had five children, including a son who is labeled 3 and a daughter who is labeled 5. Only
one of their children, 5, was an albino. One of their sons (3) and his wife (4) had four children, including
a son (6) who was an albino.
1
3
2
4
5
7
6
1. Write the genotypes of each individual who is labeled with a number in the pedigree. Use A to
represent the dominant allele and a to represent the recessive allele. Begin by writing in the genotypes
of 5 and 6. How do you know their genotypes?
- Explain how you can determine the genotypes of 1 and 2. Show the Punnett Square for these parents,
and write their genotypes in the pedigree.
- Write the genotypes of 3 and 4 in the pedigree.
- Explain how you can figure out the genotype of 7 and write her genotype in the pedigree.
Many other conditions are the result of homozygous recessive alleles, so these conditions are inherited
in the same manner as albinism. These include:
 sickle cell anemia
 cystic fibrosis (a genetic disease that results in difficulty in breathing and serious illness)
 phenylketonuria (a genetic disease that results in mental retardation unless phenylketonuria is
detected at birth and treated with a special diet).
This pedigree shows the inheritance of a different condition called achondroplasia (ay-kon-druh-playzhuh), a form of dwarfism. Dark circles or squares indicate individuals with achondroplasia.
1
3
2
4
5
6
7
2. Think about 5 and 6 and their children. Based on this family, is the allele that causes achondroplasia
recessive or dominant? How do you know? Include a Punnett square for 5 and 6 in your answer, using D
to represent the dominant allele and d to represent the recessive allele.
- In the pedigree, write the genotypes of 5 and 6.
- Write the genotypes of 2, 3 and 7. How do you know their genotypes?
3. Determine the genotypes of 1 and 4 and write them in the pedigree. Explain your reasoning.
4. Based on the frequency of dwarfs among the people you have seen in your lifetime, do you think that
the allele for achondroplasia is common or rare in the population? Explain your reasoning.
Challenge Questions
5. Most people who have the achondroplasia allele did not inherit this allele from their parents. For
people with the achondroplasia allele who did not inherit it from their parents, what biological process
is the most likely explanation for their achondroplasia allele?
We have analyzed two types of models of inheritance: a Punnett Square and a pedigree. A model is a
simplified representation of a biological process that demonstrates important aspects of the process.
Models can make it easier to understand important features of a complex biological process.
6. What are some advantages of a Punnett Square as a model of inheritance?
- What is one limitation of a Punnett Square as a model of inheritance?
- What is one advantage of a pedigree as a model of inheritance?
- What is one limitation of a pedigree as a model of inheritance?