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Genetics Summary
11-1 The Work of Gregor Mendelhapter 11 Introduction to Genetics
Every living thing inherits traits, or characteristics, from its
parents. People have long wondered how these traits are passed
from one generation to the next. Genetics is the scientific study
of heredity. Gregor Mendel did experiments with pea plants to study
inheritance. Pea plants are usually self-pollinating, meaning that
sperm cells fertilize egg cells in the same flower. The pea plants
he studied were true-breeding. True-breeding plants produce
offspring identical to themselves.
Mendel wanted seeds that inherited traits from two different
parent plants. He crossed two plants with different forms of the
same trait. A trait is a specific characteristic, such as height or seed
color. Mendel then grew plants from the seeds formed by each
cross. These plants were hybrids. Hybrids are the offspring of
crosses between parents with different traits. The first generation
of a cross is called the F1 generation. The second generation is
called F2, and so on. Each group of Mendel’s hybrid plants looked like
only one of its parents. In one case, all of the offspring were tall. In another,
all of the offspring had yellow seeds. From these results, Mendel
drew two conclusions:
• Biological inheritance is determined by factors that are
passed from one generation to the next. These factors are
called genes. One gene with two different forms controlled
each trait. Each form of the gene is called an allele.
• Mendel also formed the principle of dominance, which
states that some alleles are dominant and others are
recessive. A living thing with a dominant allele, for a trait
always shows the trait. Recessive alleles are not seen if the
dominant allele is present.
Mendel wondered what happened to the recessive allele. To find
out, he let the F1 plants self-pollinate. Some of F2 plants showed
the recessive trait. The recessive alleles had not disappeared.
Instead, the dominant allele had masked them. From this, Mendel
concluded that when each F1 plant flowers and produces
gametes, or sex cells, the two alleles segregate, or separate, from
each other. As a result, each gamete carries only a single copy
of each gene. Therefore, each F1 plant produces two types of
gametes—those with the allele for tallness and those with the
allele for shortness.
11-2 Probability and Punnett Squares
Probability is the likelihood that a specific event will occur.
The principles of probability can be used to predict the
outcomes of genetic crosses. This is because the ways in which
alleles segregate is completely random. There are two important
points to remember with probabilities:
• Past outcomes do not affect future events.
• Probabilities predict the average outcome of many events.
They do not predict what will happen in a single event.
Therefore, the more trials there are, the closer the numbers
will get to the predicted values. Punnett squares are diagrams that model
genetic crosses. Punnett squares can be used to predict and compare
the genetic variations that will result from a cross. They help predict the
chances an offspring will be homozygous or heterozygous for
a trait.
• Organisms that have two identical alleles for a particular
trait are called homozygous.
• Organisms that have two different alleles for the same trait
are called heterozygous.
Punnett squares allow for the prediction of genotypes and phenotypes of
possible offspring. The genotype is the genetic make of the individual.
Phenotype is the physical appearance of the individual. In the punnett
square above there are three different genotypes: RR, Rr, rr. There are two
different phenotypes: White and grey.
11-3 Exploring Mendelian Genetics
Mendel wondered if genes that determine different traits affect
one another. He did an experiment to find out. Mendel found that
the gene for seed shape did not affect how the gene for seed color
sorted. He summarized his conclusions as the principle of independent
assortment.
The principle of independent assortment states that genes for different
traits can segregate independently during the formation of gametes.
Independent assortment helps account for the many genetic variations
observed in plants, animals, and other organisms.
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What is genetics?
What is a trait?
What did Mendel call the F1 and F2?
What two conclusions did Mendel make from his experiments?
What is a gene?
What is an allele?
What is different about dominant and recessive alleles?
What are gametes?
What happens to the two alleles for the same gene when the
gametes are produced?
What is probability?
What are the two points to remember when using probability for
genetics?
How are punnette squares used?
What does homozygous mean?
What does heterozygous mean?
What is a genotype?
What is a phenotype?
Draw a punnett square. White (R) is dominant to grey (r). Cross
two plants that are heterozygous.
18. From the punnett square you created, what would the genotypes of
the offspring be?
19. From the punnett square you created, what would the phenotypes
of the offspring be?
20. What is independent assortment?
21. In the diagram, what was the genotype of “MOM”
22. What gametes did “MOM” produce?
23. What was the genotype of “DAD”?
24. What gametes did “DAD”produce?
Purple is dominant to white. If a heterozygous purple flower is crossed
with a white flower, what would be the
a. Genotypic ratio of the offspring
b. Phenotypic ratio of the offspring?
If long ears in dominant to short ears in rabbits, what percent of the
offspring of two heterozygous would be
a. Homozygous dominant
b. Long ears
c. Heterozygous
d. Short ears
If smooth leaves is dominant to wrinkled, then what must be the genotypes
of the two plants if one parent is smooth, the other is wrinkled, and all their
offspring have smooth leaves?
Beyond Mendelian Genetics:
Not all genes show simple patterns of dominant and recessive
alleles. Some alleles are neither dominant nor recessive, and
many traits are controlled by multiple alleles or multiple genes.
Some of these patterns are described below.
• In incomplete dominance, one allele is not completely
dominant over another. A red flower breeds with a white flower, all the
offspring have pink flowers.
• In codominance, both alleles appear as part of the phenotype
of the heterozygous offspring. A black horse breeds with a white horse, the
offspring come out with area of both black and white.
• Genes that have more than two alleles are said to have
multiple alleles. Human blood types are examples of multiple alleles.
Humans have three alleles for their blood types, A, B and O .
• A single trait can be controlled by more than one gene.
These are called polygenic traits. There are up to eight different genes that
causes the variety in human skin colors.
Genes do not control all characteristics. Some are due to interactions
between genes and the environment.
14–1 Human Heredity
In order to learn more about humans, biologists often use a
karyotype to analyze human chromosomes. A karyotype is a
picture of a cell’s chromosomes grouped in homologous pairs.
Humans have 46 chromosomes. Two of these, X and Y, are sex
chromosomes. Females have two X chromosomes (XX). Males
have one X and one Y chromosome (XY). The other 44 chromosomes
are known as autosomes. All human egg cells carry a
single X chromosome. Sperm cells carry either X or Y chromosomes.
Half of all sperm cells carry an X chromosome and half
carry a Y chromosome. This ensures that just about half of the
zygotes will be female and half will be male.
To study human inheritance, biologists use pedigree charts.
A pedigree chart shows relationships within a family. The
inheritance of a trait can be traced through the family. From this,
biologists may determine the genotypes of family members.
It is difficult to link an observed human trait with a specific
gene. Many human traits are polygenic, or controlled by many
genes. The environment also affects some traits.
The genes controlling blood type were among the first human
genes to be identified. A number of genes are responsible for blood
groups, but the two best known are the ABO blood groups and the
Rh blood group.
• Red blood cells can carry two antigens—A and B. Antigens
are molecules the immune system can recognize. A person
who has only antigen A has type A blood. A person who has
only antigen B has type B blood. A person who has both antigens
has type AB blood. A person who does not have either
antigen has type O blood. A single gene with three alleles
determines the ABO blood types.
• Red blood cells can also have the Rh antigen. People
with the Rh antigen are Rh positive. Those without it are
Rh negative. A single gene with two alleles determines
the Rh blood group.
There are many human genetic disorders. Some, including
PKU and Tay-Sachs disease are caused by recessive alleles.
Individuals must inherit a recessive allele from each parent. For a recessive
disorder to occur, an individual must receive two recessive alleles, one
from each parent. Other disorders, such as Huntington’s disease, are
caused by a dominant allele. Huntington’s disease is expressed in any
person who has the allele. So for a dominant disorder to show up in an
individual, only a single dominant allele needs to be present. Still other
disorders, such as sickle cell disease, are caused by a codominant allele.
Scientists are starting to learn which changes in the DNA
sequence cause certain genetic disorders. Sometimes, a small
change in the DNA of a single gene affects the structure of a
protein and causes a serious genetic disorder. This is the case
with cystic fibrosis and sickle cell disease.
14–2 Human Chromosomes
Genes on the X and Y chromosomes are said to be sex-linked.
They are inherited in a different pattern than are genes on
autosomes. Most sex-linked disorders are carried on the X chromosome.
Males have just one X chromosome. Thus all X-linked alleles are
expressed in males, even if they are recessive. Some examples of sexlinked disorders are colorblindness and hemophilia.
Most of the time, the mechanisms that separate chromosomes
in meiosis work well, but sometimes errors happen. The most
common error during meiosis is nondisjunction. Nondisjunction
is the failure of chromosomes to separate properly during meiosis.
If nondisjunction occurs, abnormal numbers of chromosomes
may find their way into gametes, and a disorder of chromosome
numbers may result.
• Down syndrome is an example of autosomal nondisjunction.
In this disorder, there is an extra copy of chromosome 21.
• Nondisjunction can also occur in sex chromosomes. Turner’s
syndrome and Klinefelter’s syndrome are two examples.
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Describe incomplete dominance?
Give an example.
Describe codominance?
Give an example
What is multiple alleles?
Give an example
What are polygenic traits?
Give an example.
What is a karyotype?
How many chromosomes do humans have?
What are the sex chromosomes?
What sex chromosomes do females have?
What sex chromosomes do males have?
What are the autosomes?
What do ALL human eggs carry?
What is different about human sperm?
What are antigens?
What makes a person have type A blood?
What makes a person have type B blood?
What must happen for a recessive human disorder to be shown?
What must happen for a dominant human disorder to be shown?
What is a sex-linked trait?
What chromosome do most sex-linked occur on?
Why do males show sex-linked traits more often than females?
What is nondisjunction?
What causes Down sydrome?
A rooster with gray feathers is mated with a hen of the same phenotype. Among
their offspring, 15 chicks are grey, 6 are black, and 8 are white.
a. What is the simplest explanation for the inheritance of color in the chicks?
b. What offspring would you expect from the mating of a grey rooster and a
black hen?
Hemophilia is a sex-linked recessive gene.
a. If a carrier female marries a normal male, what are the chances of their
sons being normal?
b. What are the chances of their sons being a carrier? Why?
Color-blind is a sex-linked recessive trait. A color-blind woman and a man with
normal vision have a son.
a. What is the probability that their first son will be color blind?
b. What are the chances their daughter will be color blind?
In horses, Black (B) is dominant, White (b) is recessive. How is it possible that a
black horse and white horse has colts that have both black and white patches?
In the Karyotype above, Circle the sex chromosomes, and circle the disorder if
there is one?
Circle the correct statememt below that describes the karyotype.
Normal Female
Female with disorder
Normal Male
Male with disorder
Which chromosome numbers are the autosomes?
11-4 Meiosis
Living things inherit a single copy of each gene from each parent.
These copies are separated when gametes form. The process in
which this happens is called meiosis. Meiosis is a process in
which the number of chromosomes per cell is divided in half
through the separation of homologous chromosomes in a
diploid cell.
• Acell that has both sets of homologous chromosomes is said
to be diploid. Diploid means “two sets.”
• Gametes have half the number of chromosomes as their
parent cells. Cells that have only one set of chromosomes are
said to be haploid. Gametes are genetically different from
the parent cell and from one another.
Before meiosis begins, cells undergo DNA replication forming
duplicate chromosomes. Meiosis then occurs in two stages.
Meiosis I:
• Two cells form.
• Each cell has sets of chromosomes and alleles that are
different from each other and from the original cell.
Meiosis II:
• The cells divide again, but this time the DNA is not
copied first.
• Four daughter cells form.
• Each daughter cell contains half the number of chromosomes
as the original cell.
One of the most significant events of meiosis happens during Prophase I.
Homologous chromosomes, the two chromosomes that have the same
genes at the same physical locations on the chromosomes, pair up next to
each other. The chromosomes cross over each other, are cut and reconnect.
This allows for an exchange of the alleles and produces new combinations
of the genes on the chromosomes.
Each chromosome is now genetically different from the others.
Although they sound the same, meiosis and mitosis are different.
Mitosis makes two identical cells. These cells are exactly like the
parent cell. Meiosis, however, forms four cells. Each cell has
only half the number of chromosomes as the parent cell. The
cells are also genetically different from one another.
Questions:
27. What happens to genes during meiosis?
28. Define meiosis?
29. What does diploid mean?
30. What does haploid mean?
31. What events happen during Meiosis I?
32. What events happen during Meiosis II?
33. What makes two chromosomes homologous?
34. What is a crossover?
35. How does crossing over change the chromosomes?
36. Describe the cells after MITOSIS.
37. Describe the cells after MEIOSIS
Meiosis
Mitosis
On the diagram above, label the following terms where they apply:
Some terms may be found in the same locations.
Haploid, Meiosis I, Meiosis II, diploid, DNA Replication, Gametes,
n, 2n, Chromosomes