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GENETIC OUTCOMES
reflect
In a monastery garden, a curious monk discovered some
of the basic principles of genetics. The work of the monk,
Gregor Mendel, laid the groundwork for the study of
genetics, which has advanced our understanding of
many related areas of science including the genetics of
certain diseases and the process of selective breeding.
Scientists have built upon the discoveries of Mendel,
answering some important questions including, why do
organisms look and act the way they do?
genetics: the study of
how traits are passed from
parents to offspring
Why do offspring resemble their parents? What role can
technology play in genetics? Let’s explore the answers
to these questions.
Genotypes, Phenotypes, and Punnett Squares
An organism’s traits can be predicted based on its parents’ traits. Mendel conducted
breeding experiments with pea plants and concluded that some characters are determined
by two factors. For example, the peas he worked with could have either a smooth texture
or a wrinkled texture. These different values for the texture character are called traits. Traits
are determined by alleles, which are different versions of a gene. Offspring inherit one
allele from each parent in sexual reproduction. The combination of the two alleles is the
offspring’s genotype and determines what trait the organism will have for a character.
In Mendelian genetics two letters, such as Ss (one letter for each allele) represent
genotypes. A capital letter means the allele is dominant. A lowercase letter means the
allele is recessive. One possible genotype for the texture of the peas in Mendel’s breeding
experiments was Ss, meaning that the offspring could have inherited one dominant allele
and one recessive allele. Smooth texture (S) is dominant and wrinkled texture (s) is
recessive. A pea plant with the genotype Ss has smooth peas because the dominant allele
masks the recessive one. The smooth texture is the phenotype. The phenotype is the
physical expression of the alleles. It is the outward appearance of the genotype. So, a pea
with a genotype SS will also have smooth peas, but a pea with the genotype ss will express
the recessive trait, wrinkled peas.
When the genotypes of two parents are known, it is possible to predict the genotypes and
resulting phenotypes of the offspring. A Punnett square is used to find and analyze the
possible gene combinations of the offspring.
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GENETIC OUTCOMES
The Punnett square on the right shows a monohybrid
cross for a single trait represented by a blue A for the
dominant allele and a purple a for the recessive allele.
In this monohybrid cross, the parents have one of each
allele for the trait, allowing us to examine all possible
combinations of the alleles. Therefore, the parents are
both heterozygous, meaning that each parent has two
different forms of an allele for a particular trait. Their
genotypes are written across the top and down the
side of the Punnett square. The genotypes inside the
squares represent the possible allele combinations for
the offspring. Notice that two possible genotypes in the
offspring are homozygous, meaning that they have two
of the same forms of an allele for a particular trait: AA
and aa.
In this Punnett Square, the
offspring have a 75% chance
(3 out of 4) of expressing
the dominant trait and a
25% chance (1 out of 4) of
expressing the recessive trait.
Mendelian genetic crosses include dihybrid crosses as well. A dihybrid cross examines the
possible inheritance of two specific sets of alleles. The Punnett square below shows the
possible genotypes and their frequencies for a trait represented by the letter A (a for the
recessive form) and a trait represented by the letter B (b for the recessive form).
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GENETIC OUTCOMES
what do you think?
For the dihybrid cross on the previous page, calculate the frequency of each phenotype.
For example, combinations containing at least one A and one B appear 9 out of 16 times,
giving offspring a 56% probability of expressing both dominant traits. What is the probability
of the offspring expressing both recessive traits?
Non-Mendelian Genetic Crosses
Geneticists soon discovered that some traits do not follow
Mendel’s patterns of inheritance. These non-Mendelian traits
include phenotypes that are coded by multiple alleles or
do not follow the normal rules of dominance. Some alleles
display co-dominance in which a heterozygous genotype
results in both traits being expressed in the phenotype.
Erminette chicken feathers display co-dominance. A chicken
that has one allele for black feathers and another allele for
white feathers displays both black and white feathers.
Another type of non-Mendelian genetics is incomplete dominance. In this case, a heterozygous
genotype results in a blend of the two traits. For example, a certain breed of snapdragon plant
produces white flowers and red flowers. When they are crossed, a heterozygous offspring is
produced with a third phenotype: pink flowers. Incomplete dominance is popular when breeding
plants and animals in order to obtain a blend of desired traits.
what do you think?
Some flowers exhibit co-dominance. Predict what the offspring of a red flower and a white
flower would look like if they followed a co-dominant pattern of inheritance.
Meiosis and Genetic Variation
Meiosis increases genetic variation in organisms that
mitosis: a form of cell
undergo sexual reproduction. Meiosis is similar to mitosis in
division in which the
that chromosomes replicate and divide into daughter cells.
material from the cell
However, during meiosis, cells undergo two divisions
nucleus divides
(meiosis I and II) resulting in daughter cells with half the
number of chromosomes as the parent cell, which is called
haploid. These cells are the gametes, or sex cells, referred to as sperm and egg.
When eukaryotes reproduce sexually, the gametes from each
parent join together. The two haploid cells combine, giving
the offspring a complete set of chromosomes, which is called
diploid. Sexual reproduction creates greater genetic variety
in two ways. First, an offspring inherits DNA from both of
eukaryote: organism
with a membranebound nucleus that
contains DNA
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GENETIC OUTCOMES
its parents. This causes new random combinations of alleles, resulting in a variety of traits
that differ from the mother and the father. Since genes are randomly assorted when they
are passed to offspring, even two siblings have different combinations of genes and traits
from the same set of parents. Only identical twins have exactly the same DNA.
Meiosis also contributes to genetic variation
through crossing over. During one phase of
meiosis, homologous chromosomes—pairs of
chromosomes containing the same genes, but
possibly different alleles—line up at the center
of the cell. When this happens, sections of the
homologous chromosomes can cross over and
switch position from one chromosome to the
other. This results in a reshuffling of genes on the
individual chromosomes, which provides an even
greater variety of genetic combinations that can be
passed on to offspring.
Crossing over during meiosis I in
the cells shown here resulted in
sections of the two homologous
chromosomes switching places.
look out!
Sexual reproduction is beneficial to organisms because it increases genetic variation, but
asexual reproduction has advantages as well. In asexual reproduction, a single parent
produces offspring that are genetically identical to the
prokaryote: simple
parent and to one another. This type of reproduction is
organism that does not have
mostly associated with prokaryotes. Other simple life
a membrane-bound nucleus
forms such as the hydra and sponge may reproduce
or other organelles; most
sexually or asexually at various stages of their lives.
are unicellular
Although genetic variation is compromised in asexual
reproduction, there are benefits to the parent organism.
Animals that are immobile, such as sponges, would
have great difficulty finding a mate. Asexual reproduction
allows them to produce offspring without having to travel.
Another advantage is that in asexual reproduction the
parent expends much less energy compared to sexual
reproduction. This allows organisms to produce many
offspring without greatly taxing their energy or time. Finally,
in a stable environment, asexual reproduction produces
offspring with the necessary genetic traits to survive and
A bud, or offspring, grows
thrive in their environment.
out of the left side of this
hydra. What do you know
about the bud’s genetic
make-up?
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GENETIC OUTCOMES
Discover Science:
Mapping the Human Genome and Other Genetic Technology
Identifying 25,000 genes and sequencing 3 billion base pairs
genome: all of the
of the human genome is no simple task. But in 2003, after
hereditary information
13 years of research by scientists in more than 18 countries,
within an organism
the Human Genome Project was completed. In addition to
identifying and sequencing all of the genetic material, the
Project’s goals included creating a database to store the information, improving the tools
used for analyzing the data, sharing the technology with companies who could further
the research, and addressing the ethical issues involved with advancements in genetics.
Although the Project has been completed, analyzing the data and applying it will continue
for many years to come.
There are a variety of techniques that are used to study the genomes or smaller sections of
genetic information in organisms.
• Gel electrophoresis: One of the most popular DNA
technologies is gel electrophoresis, in which sections
of DNA are sliced into differently sized fragments and
placed in a gel-covered plate. An electric current is passed
through the gel, causing the fragments to separate
according to size. Gel electrophoresis is commonly used
in DNA fingerprinting to determine the degree to which
two samples of DNA are related based on the number of
matching fragments in their patterns.
• Recombinant DNA technology: DNA can be altered to produce desirable proteins
through recombinant DNA. Small sections of DNA called plasmids containing desired
genes are inserted into host bacterial cells. As the bacteria undergoes protein synthesis,
the inserted genes are translated right along with the host’s own DNA. This technology
is used to study genetic disorders and to explore the possibility of using recombinant
DNA to replace mutated genes.
• DNA and RNA probes: Chosen segments of genetic material are used to label specific
nucleic-acid sequences. These labels can then be followed and examined in more
complex genetic processes and sequences. Nucleic acid probes are often used during
experiments in order to conduct a chromosomal analysis.
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GENETIC OUTCOMES
• Bioinformatics: This field of science combines
genetics and computer technology to store, analyze,
and model the wealth of information held within the
nucleotide sequences of genetic material. Since
DNA is so small, yet so complex, bioinformatics
allows scientists to gain insight into the functions and
changes of genetic material through computations.
• Karyotyping: This technology provides a picture
of an organism’s chromosomes allowing for visual
analysis. The karyotype, or chromosomal image,
can show genetic abnormalities and can be used to
diagnose genetic disorders.
What Do You Know?
Genetic outcomes are the result of combining
chromosomes from two different parents who reproduce
sexually. The processes that take place during meiosis
help contribute to genetic variation among offspring.
Use what you have learned about genetic outcomes
and Punnett squares to solve the following mystery.
The human karyotype above
appears to be normal,
illustrating 23 pairs of
chromosomes. Abnormal
karyotypes might show
a missing, an extra, or a
fragmented chromosome.
A newborn child was brought to the hospital nursery before he was marked with his
parents’ names. The hospital workers must use what they know about genetic outcomes
to determine whom the child belongs to. They use his traits to help them. The baby has
freckles (F dominant), and attached earlobes (l recessive). They believe that the baby most
likely belongs to a couple in which the man has attached earlobes but no freckles. It is
known that the woman is heterozygous for freckles and earlobe type.
Complete the Punnett square on the next page to determine the probability that the child
belongs to this couple. List the possible combinations of the father’s alleles across the top
and the possible combinations of the woman’s alleles down the left side. Then write your
answer in the space below. Include a brief statement explaining how you reached your
answer.
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GENETIC OUTCOMES
Write your answer here:
fl
fl
fl
fl
FL
Fl
fL
fl
To be more certain, the hospital workers performed a gel electrophoresis comparing the
DNA from four fathers against the baby’s DNA shown in column A. The results are shown
below with the four fathers’ DNA labeled B, C, D, and E. According to the data in the gel,
which sample most likely belongs to the baby’s father? Write your answer in the space
provided and include an explanation.
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GENETIC OUTCOMES
connecting with your child
Studying a Genetic Disease
To help students learn more about genetic
outcomes, have your child create an
informational brochure about a genetic
disease. Begin by conducting online
research to find a list of the more commonly
known genetic diseases and have your
child choose one of them to research
further. Instruct your child to gather specific
information about the disease such as its
symptoms, management, potential cures,
and the type of chromosomal abnormality
that causes the disease. Have your child
organize the information in a colorful trifold
brochure that might be distributed in a
doctor’s office or by a health organization.
Make sure two Punnett squares are included
in the brochure to illustrate the chances that
a child will inherit the disease if one parent
carries the disease and if both parents carry
the disease.
Here are some questions to discuss with
students:
• What causes the disease in your
brochure?
• How can doctors use what they know
about the disease to counsel couples
that want to have children, knowing that
one or both of them carry an allele for
the disease?
• What technologies are under
development regarding the treatments
and/or cures for the disease?
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