Download INHERITANCE AND VARIATION OF TRAITS UNIT FIVE: GENETICS

INHERITANCE AND VARIATION OF TRAITS
UNIT FIVE: GENETICS, COMPLEX INHERITANCE AND HUMAN HEREDITY
MAIN IDEA: MENDEL EXPLAINED HOW A DOMINANT ALLELE CAN MASK THE PRESENCE OF A
RECESSIVE ALLELE
OBJECTIVE 1: RELATE THE TERMS TRAITS AND GENES TO ONE ANOTHER AND DESCRIBE
MENDEL’S MONOHYBRID CROSSES AND APPLY THE TERMS HYBRID, P1, F1, and
F2
A. Heredity is the passing on of characteristics from parents to offspring and the branch of biology that
studies heredity is called genetics.
B. Characteristics that are inherited are called traits.
C. The instructions for each trait are located on chromosomes, which are found in the nucleus of a cell.
1. The DNA on a chromosome is arranged in segments to control the making of proteins.
2. These DNA segments are called genes and each chromosome is made of 100’s to 1000’S of
genes, which determine the characteristics and function of the cell. Each gene can have several
variants, called alleles, which code for different variants of the traits in question.
3. Every cell of any individual organism contains the identical set of chromosomes.
D. Human body cells have 46 chromosomes, getting 23 from each parent.
E. When organisms reproduce, genetic information is transferred to their offspring. In species that
reproduce sexually, each cell contains two variants of each chromosome, one inherited from each
parent. Thus sexual reproduction gives rise to a new combination of chromosome pairs with
variations between parent and offspring.
F. The chromosomes that make up one pair, one chromosome from each parent, are called
homologous chromosomes.
1. Homologous chromosomes have the same length, centromere position, and the same genes for
the same trait in the same order, although the traits might not be identical, i.e. earlobes (see page
270)
G. Gregor Mendel - first person to succeed in predicting how traits would be transferred from one
generation to the next.
H. Mendel performed cross pollination by transferring male gametes from the flower of a true-breeding
green seed plant to the female organ of a flower from a true-breeding yellow-seed plant (see page
278)
1. The offspring produced were called hybrids; they came from parents that had different forms of a
trait. (yellow vs. green peas)
2. His crosses were called monohybrid; the parents differed by a single trait – seed color.
I. To describe the generations of his monohybrid cross, Mendel used the following terms:
1. P1 – the original parents
2. F1 – offspring of the cross pollinated parents (F stands for “filial” or son or daughter).
3. F2 – second filial generation; offspring of F1’s self-pollinating.
4. Mendel studied seven traits and found the F1 generation plants to show a 3:1 ratio
J. Mendel concluded that each organism has two factors that control each of its traits.
1. These factors are called genes and are located on chromosomes. Genes are segments of DNA
that code for a polypeptide or protein.
2. Genes exist in alternative forms called alleles and are passed from generation to generation.
3. An organisms two alleles are located on different copies of a chromosome – one inherited from
the female parent and one inherited from the male parent. During fertilization, half of the
chromosomes (DNA) come from one parent and the other half from the other parent.
OBJECTIVE 2: DESCRIBE MENDEL’S LAWS OF DOMINANCE AND APPLY THE TERMS
DOMINANCE , RECESSIVE, PHENOTYPE, GENOTYPE, HOMOZYGOUS,
HETEROZYGOUS
A. Traits are described as either dominant or recessive.
1. Dominant trait – the trait that is expressed (shows up) in the hybrid ex. Yellow (Y)
2. Recessive trait - the trait hidden in the hybrid (not expressed) ex. green (y)
B. The Law of Dominance - when an organism is a hybrid, (Yy) for a certain trait, only the dominant trait
is expressed. (Y).
C. When recording the results of crosses, use the same letter for different alleles of the same gene. An
upper case letter is used for dominant traits and a lower case letter is used for the recessive traits.
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1. Tall (T) vs short (t), Round (R) vs. wrinkled (r), or Yellow (Y) vs green (y).
D. Genotype – The gene combination and organism contains. It is established at fertilization and will
determine the phenotype.
1. Homozygous - two alleles for the trait are the same: RR, tt, YY
2. Heterozygous – two alleles for a trait differ from each other: Rr, Tt, Yy
E. Phenotype – The way an organism looks and behaves regardless of the genes it contains
F. You can’t always know an organism’s genotype by looking at its phenotype.
OBJECTIVE 3: SUMMARIZE THE LAW OF SEGREGATION
A. Two alleles for each trait must separate during formation of gametes (eggs and sperm), and the
parent passes on at random only one allele for each trait to each offspring.
B. (See page 279) During fertilization, two alleles for that trait unite to produce the genotype for that
specific trait.
OBJECTIVE 4: DIFFERENTIATE BETWEEN A MONOHYBRID AND A DIHYBRID CROSS
A. A cross that involves hybrids for a single trait is called monohybrid cross.
1. During the self fertilization of the F1 generation, the male gametes (Yy) randomly fertilize the
female gametes (Yy)
2. Such a cross results in the following genotypes: YY,Yy, yY, yy.. The genotypic ratio is 1YY: 2Yy:
1yy
3. Such a cross results in phenotypic ratio of 3 yellow seeds : 1 green seed.
B. Dihybrid Cross – considers two different traits at the same time. Traits are inherited independently of
one another. For example Round seeds (R) are dominant to wrinkled seeds (r) and Yellow seeds (Y)
are dominant to green seeds (y).
1. RRYY x rryy = RrYy (dihybrid), round and yellow for all the F1’s
2. Cross the F1’s: RrYy x RrYy (dihybrid cross)
a. first identify all the possible gametes: use the foil method: First, Outside, Inside, Last
b. possible gametes are RY, Ry, rY, and ry
C. Law of Independent Assortment
1. Mendel concluded that a random distribution of alleles occurs during gamete formation. Genes
on separate chromosomes sort independently during meiosis.
2. (see page 280, figure 10.11) The law of independent assortment is demonstrated in the dihybrid
cross by the equal chance that each pair of alleles (Yy and Rr) can randomly combine with each
other. So Rr will separate from Yy.
a.
Alleles in à
in parental cell
gamete à
formation
possible allele combinations
In gametes
Y
YR ¼
y
Yr 1/4
R
yR ¼
R
yr
¼
b. The random assortment results in 4 possible gametes, each of which is equally likely to
occur.
c. He also counted four different phenol types and found a ratio of about 9:3:3:1
d. If the alleles for seed shape and color were inherited together, only two kinds of seeds would
be produced: RY, ry
OBJECTIVE 5: BE ABLE TO USE A PUNNETT SQUARE TO WORK OUT THE POSSIBLE RESULTS
OF VARIOUS TYPES OF GENETIC CROSSES. DISCUSS THE PROBABILITY OF
EXPERIMENTAL RESULTS
A. Punnett Square – A shorthand way of finding the expected proportions of possible genotypes and
phenotypes in the offspring of a cross.
B. Makes it easier to keep track of the possible genotypes involved in a cross.
1. Takes into account that fertilization occurs at random.
2. Steps:
a. Identify all of the possible parent gametes and place the females on the left edge and the
male gametes on the top edge.
b. Put the possible allele combinations of the zygotes (fertilized egg) in the inner boxes:
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Parents: Male = TT, Female = Tt
T
T
T
t
Genotypes = TT, Tt
Phenotypes = All Tall
Cross two heterogyotes:
Genotypes =
Phenotypes =
Cross a homozygous recessive with a heterozygote:
Genotypes =
Phenotypes =
c. All the above crosses were Monohybrid because they only looked at one trait.
C. A dihybrid cross uses a larger Punnett Square: four boxes on each side with a total of 16 boxes (see
page 282)
1. Remember the dihybrid cross has a phenotypic ratio for the F2 generation was 9:3:3:1
D. Punnett squares show the possible combinations and the likelihood that each will occur.
E. Genetics follows rules of chance; in reality you don’t get the exact ratio
F. Calculate the probability of an event occurring: desired outcomes/total # of outcomes
1. probablity of getting tails in a coin toss is one in two chances or 1:2 or ½
2. probability of getting either a dominant or recessive allele for a specific trait is ½
G. Results predicted by probability are more likely to be seen when there are a large number of
offspring.
OBJECTIVE 6: DISCUSS HOW MEIOSIS EXPLAINS MENDEL’S RESULTS
A. The segregation of chromosomes in Anaphase I of meiosis explains Mendel’s observation that each
parent gives one allele for each trait at random to each offspring, regardless of whether the allele is
expressed.
B. The segregation of chromosomes at random in Anaphase I in meiosis explains Mendel’s observation
that factors, or genes, for different traits are inherited independently of one another.
C. Today, Mendel’s laws of heredity form the foundation of modern genetics
MAIN IDEA: THE CROSSING OVER OF LINKED GENES IS A SOURCE OF GENETIC VARIATION
OBJECTIVE 7: EXPLAIN HOW GENE LINKAGE CAN BE USED TO CREATE CHROMOSOME MAPS
A. The new combination of genes produced by crossing over and independent assortment is called
genetic recombination
1. Review possible combinations of genes in Objective 5.
B. Chromosomes contain many genes that code for proteins and genes that are located close to one
another on the same chromosome are said to be linked. Linked genes usually travel together during
gamete formation.
1. The linkage of genes on a chromosome results in an exception to Mendel’s law of independent
assortment because the linked genes usually do not segregate independently. (see page 283,
Fig. 10.14)
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C. From studies it was found that linked genes can separate during crossing over.
D. Crossing over occurs more often between genes that are far apart than those that are close together.
A drawing called a chromosome map shows the sequence of genes on a chromosome and can be
created using cross over data.
1. Chromosome map percentages are not actual chromosome distances, but represent relative
positions of the genes.
2. The higher the crossover frequency, the farther apart the two genes are.
OBJECTIVE 8: DEFINE THE TERM NONDISJUNCTION AND DISCUSS ITS VARIOUS EFFECTS IN
REGARDS TO POLYPLOIDY
A. The failure of homologous chromosomes to separate properly during meiosis is called
nondisjunction. Occasionally, both chromosomes of a homologous pair move to the same pole of
the cell.
B. The result of nondisjunction is a change from the usual number of chromosomes in the diploid cell.
C. Nondisjunction can have many forms:
1. Trisomy – results when a gamete with an extra chromosome is fertilized by a normal gamete.
The zygote will have an extra chromosome.
a. Extra chromosome 21 in humans results in Down Syndrome
2. Monosomy - results when a gamete missing a chromosome fuses with a normal gamete. The
zygote lacks a chromosome. In humans, most zygotes with monosomy do not survive.
a. An example of monosomy that is not lethal is Turner’s Syndrome. Females only have a
single X chromosome instead of two.
3. Polyploidy - involves a total lack of separation of homologous chromosomes results in zygotes
that are 3n or 4n.
a. Rare in animals and always lethal to the human zygote.
b. Polyploidy occurs roughly in one in three species of plants and the fruits from these plants
tend to be larger and healthier. Therefore is can be beneficial and of commercial value. Plant
breeders have learned to artificially produce polyploid plants using chemicals that cause
nondisjunction.
c. Bread wheat (6n), oats (6n), sugar cane (8n)
MAIN IDEA: THE INHERITANCE OF A TRAIT OVER SEVERAL GENERATIONS CAN BE SHOWN AS
A PEDIGREE
OBJECTIVE 9: DESCRIBE HUMAN GENETIC DISORDERS THAT ARE CAUSED BY INHERITANCE
OF RECESSIVE ALLELES
A. Most genetic disorders are caused by recessive alleles and in order to be expressed, the person
would need to be homozygous recessive.
B. An individual who is heterozygous for a recessive disorder is called a carrier.
C. Many of these alleles are rare, but a few are common in certain ethnic groups. (see page 297, Table
11.2)
D. Punnett squares can be used to calculate the chance that offspring will be born with some of these
genetic disorders.
E. Cystic fibrosis
1. The most common genetic disorder among white Americans.
2. About 1 in 20 are carriers and 1 in 3500 children born to white Americans inherits the disorder.
3. Defective protein in the plasma membrane results in the formation and accumulation of thick
mucus in the lungs and digestive tract.
4. Life expectancy is mid-twenties.
F. Albinism
1. Genes do not produce the normal amount of pigment called melanin.
2. Results in no color to skin, hair, and eyes (eyes are blue)
3. Associated with vision problems and skin is highly susceptible to UV damage.
4. There is no cure, but life span not affected.
G. Tay-Sachs
1. Recessive disorder of the central nervous system in which there is an absence of an enzyme that
normally breaks down a lipid produced and stored in tissues of the CNS. Causes accumulation
of lipids in the cells.
2. Life expectancy is approximately five years.
3. Common in the U.S. among the Amish and those with Eastern European Jewish ancestors.
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H. Galctosemia
1. Recessive alleles cause the absence of an enzyme that breaks down galactose.
2. Accumulation of galactose causes enlarged liver, kidney damage and mental disabilities.
3. There is no cure but those that are homozygous recessive should limit their intact of lactose and
galactose in the diet by avoiding milk products.
OBJECTIVE 10: DESCRIBE HUMAN GENETIC DISORDERS THAT ARE CAUSED BY THE
INHERITANCE OF SINGLE DOMINANT ALLELE
A. Many traits are inherited just as the rule of dominance predicts
1. Tongue rolling, hanging earlobes, almond shaped eyes, and thick lips
B. Some disorders are caused by a single dominant allele.
C. Huntington’s Disease is a lethal genetic disorder caused by a rare dominant allele.
1. Occurs in 1 in 10,000 people in the U.S.
2. Results in the breakdown of certain areas of the brain causing loss of brain function,
uncontrollable movements, and emotional disturbances.
3. Occurs in people between ages of 30-50, after they may have already had children and passed
along the dominant allele.
4. There is a test to determine if you are a carrier….if positive you can decide if you want to risk
having children… you will also know that you will most likely develop the disease.
D. Another trait inherited by a single dominant allele is a form of dwarfism called achondroplasia.
1. Occurs in 1, 25000 people in the U.S. and the gene that affects normal bone growth is dominant
instead of recessive!
2. People with achondroplasia have a small body size and limbs that are comparatively short. Most
will have an adult height of four feet and an average life span.
3. 75% of the people with achondroplasia are born to parents of average size. SO….
a. In a dominant genetic condition, the genotype is seen as the phenotype
b. When children with achondroplasia are born to parents of average height, the conclusion
is that the condition occurred because of a new mutation or genetic change.
OBJECTIVE 11: DESCRIBE AND INTERPRET A PEDIGREE
A. A pedigree maps the inheritance of genetic traits from generation to generation.
B. A pedigree uses a set of symbols to identify gender, individuals affected by the trait being studied,
and family relationships through several generations.
1. circle- female
2. square – male
3. shaded symbols – individuals showing the trait being studied.
4. Unshaded symbols – individuals not showing the specific trait
5. Half-shaded symbol – carriers, heterozygous individuals
6. Horizontal line – connects parents
7. Vertical line – connects parents to their offspring
C. Each horizontal row designates a generation, with the most recent row at the bottom.
D. Review analysis of pedigree charts pages 299 and 300 in text and Lab 35: Human Inheritance.
E. Pedigrees are used to infer genotypes from the observations of phenotypes.
1. By knowing physical traits, genealogists can determine what genes an individual is most likely to
have.
2. Pedigrees can help genetic counselors determine whether inheritance patterns are dominant or
recessive.
3. When recessive traits are expressed, the ancestry of the person expressing the trait is followed
for several generations to determine which parents or grandparents were carriers of the recessive
allele.
F. If good records have been kept within families, disorders in future offspring can be predicted.
However, more accuracy can be expected if several individuals within the family can be evaluated.
MAIN IDEA: COMPLEX INHERITANCE OF TRAITS DOES NOT FOLLOW INHERITANCE
PATTERNS FOLLOWED BY MENDEL
OBJECTIVE 12: DISTINGUISH BETWEEN INCOMPLETELY DOMINANT AND CODOMINANT
ALLELES AND GIVE EXAMPLES OF CODOMINANCE IN HUMANS
A. Many inheritance patterns are more complex than those studied by Mendel.
1. Most traits are not simply dominant or recessive.
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B.
Incomplete Dominance
1. Neither trait is dominant over the other, the traits appear to blend
2. A red flower is crossed with a white flower makes a pink flower.
3. The heterozygous genotype has a phenotype different than either homozygous dominant or
homozygous recessive.
4. Cross two heterozygotes:
R
R’
R
R’
Genotypic ratio =
Phenotypic ratio =
C. Codominance
1. Both alleles of the pair are expressed, but in different places. Some dogs have brown and white
hairs.
2. Causes the phenotypes of both alleles to be produced in heterozygous individuals; both alleles
are expressed equally.
3. An example of codominance in humans is sickle-cell anemia. It is a blood disorder with
abnormal hemoglobin (the oxygen carrying protein) that causes red blood cells to deform and
block small blood vessels. This blockage causes pain and tissue damage. Sickle-shaped cells
do not survive as long as normal red blood cells so the person is anemic.
a. Individuals that are heterozygous for the allele produce both normal and sickle-celled
hemoglobin. Symptoms are not as severe as those that are homozygous.
b. The sickle cell allele increases resistance to malaria
OBJECTIVE 13: COMPARE MULTPLE ALLELIC AND EPISTASIS INHERITANCE AND BE ABLE TO
GIVE EXAMPLES OF EACH
A. It is common for more than two alleles to control a trait in a population.
B. Traits controlled by more than two alleles have multiple alleles
1. Example: Four alleles of a single gene govern fur color of rabbits resulting in ten possible
genotypes and four phenotypes. (see page 304, figure 11.7)
2. Human blood types are determined by the absence or presence of certain proteins on the
surface of red blood cells, and there are three alleles.
GENOTYPES
A A
A
I I or I i
B B
B
I I or I i
A B
I I
ii
SURFACE PROTEIN
A
B
A and B
none
PHENOTYPE
A
B
AB
O
3. Determining blood type is necessary before receiving transfusions: Different surface proteins will
cause the RBC’s to clot
4. Determining blood type may be useful in determining parentage but it cannot prove with certain
man is definitely the father; only DNA testing can determine actual parentage.
a. Who could be the father of baby Jane? Jane is type O, her mother is type A and her two
possible fathers are Jim (type B) or John (type AB). Prove your answer in a punnett square.
Genotypic ratio =
Phenotypic ratio =
C. Epistasis is an interaction in which one allele hides the affects of another allele as in coat color of
Labrador retrievers. (see page 305, figure 11.8)
1. Coat color is controlled by two sets of alleles:
a. The dominant allele E determines whether the fur will have dark pigment (E_bb, E_B_)
b. Alleles ee cause no pigmentation.
c. Allele B determines how dark the pigment will be
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OBJECTIVE 14: DETERMINE THE SEX OF AN ORGANISM BY ITS CHROMOSOMES
A. Humans have 46 chromosomes or 23 homologous pairs
B. 22 of the matching pairs are called autosomes.
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C. The 23 pair differ in males and females and are called the sex chromosomes.
1. XX = female
2. XY = male
D. Because males produce two kinds of gametes during meiosis, either X or Y, males determine the sex
of the offspring.
1. Do a punnet square to determine the ratio of expected males to females after fertilization.
X
Y
X
X
Genotypic ratio =
Phenotypic ratio =
2. Because fertilization is governed by probability, the ratio usually is not exactly 1:1 in a small
population.
OBJECTIVE 15: EXPLAIN DOSAGE COMPENSATION AND ITS RELATIONSHIP TO CHROMOSOME
ACTIVATION
A. X chromosomes are larger than Y chromosomes and the X chromosome carries genes necessary for
the development of both females and males. The Y chromosome has genes mainly for male
development.
B. Because females have two X chromosomes and males have just one, to balance the difference in the
dose of X-related genes, one of the X chromosomes stops working in the females body cells.
1. This is called dosage compensation or X-inactivation
2. Inactivation is a completely random event and occurs in all mammals.
C. Chromosome inactivation results in different coat colors of a calico cat.
D. The inactivated X chromosomes can be observed in cells. Darkly stained, inactivated X
chromosomes are called Barr bodies and are seen as a condensed, darkly stained structure in the
nucleus. Only females will have Barr bodies in their nucleus.
OBJECTIVE 16: BE ABLE TO EXPLAIN WHAT A SEX-LINKED TRAIT IS AND DISCUSS ITS IMPACT
ON THE MALE POPULATION
A. Sex-linked traits are those traits controlled by genes located on a sex chromosome
1. In humans it usually is on the X chromosome and is recessive
2. Because the X and the Y are not homologous, the Y chromosome has no corresponding allele to
one on the X chromosome.
B. Males have only one X chromosome so even if their defective allele is recessive they will have the
trait. Females could have a normal second X and only be a “carrier” for the trait; they have to have
two defective alleles in order for the sex-linked trait to be expressed.
C. Sex-linked traits in humans are certain forms of color blindness (red/green) see page 307, hemophilia
(see page 308), one form of muscular dystrophy, and fragile X syndrome.
OBJECTIVE 17: DESCRIBE POLYGENIC INHERITANCE AND GIVE SOME EXAMPLES
A. Polygenic inheritance is the inheritance pattern of a trait that is controlled by two or more genes.
(see page 309)
1. Genes may be on the same or on different chromosomes. (RRTtSsQq)
2. Examples are eye color, skin color, height, or size/color of fruit
3. The phenotypes of these traits will show a continuous range of variability from the minimum value
of the trait to the maximum. Results in a bell-shaped curve. (see page 326)
a. All heterozygotes are intermediary and all alleles represented by an upper case letter
contributes a small, but equal, portion to the trait being expressed.
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OBJECTIVE 18: SUMMARIZE HOW INTERNAL AND EXTERNAL ENVIRONMENTS AFFECT GENE
EXPRESSION
A. The genetic makeup of an organism at fertilization determines only the organism’s potential to
develop and function.
B. As the organism develops, many factors can influence how the gene is expressed, or even whether if
it is expressed at all.
C. Genes interact with each other and the environment to form a complete picture of inheritance.
1. individuals known to have a particular gene fail to express the phenotype specified by that gene.
D. External environmental influences include temperature, nutrition, light, water, chemicals, and
infectious agents.
1. Example: leaves may have different color and shape depending on how much light they receive
E. Internal environments of males and females are different because of hormones and structural
differences.
1. Example: feather color in peacocks are expressed differently in males and females because of
hormones, which are determined by different sets of genes.
2. Age also affects gene function as the internal environment changes as an organism ages.
F.
By studying twins, scientists can better determine if inheritance patterns are influenced by genetic or
environmental contributions.
1. Traits that appear frequently in identical twins are at least partially controlled genetically.
2. Presume traits that are expressed differently in identical twins are strongly influenced by
environment.
3. A percentage of twins that both express a given trait is called a concordance rate (see page 310).
A large difference between fraternal twins and identical twins show a strong genetic influence.
MAIN IDEA: CHROMOSOMES CAN BE STUDIED USING KARYOTYPES
OBJECTIVE 19: DISTINGUISH NORMAL KARYOTPES FROM THOSE WITH ABNORMAL NUMBERS
OF CHROMOSOMES AND DEFINE AND DESCRIBE THE ROLE OF TELOMERES
A. Scientists study whole chromosomes by using images of chromosomes stained during metaphase.
B. The staining bands identify or mark identical places on homologous chromosomes.
C. The pairs of homologous chromosomes are arranged in decreasing size to produce a micrograph
called a karyotpe, (see page 311, figure 11.17) which is useful in pinpointing unusual chromosomes
numbers and sizes.
D. Scientists have found that chromosomes end in protective caps called telomeres.
1. Telomere caps consist of DNA associated with proteins.
2. Telomeres might be involved with both aging and cancer.
OBJECTIVE20: DESCRIBE NONDISJUNCTION AND ITS EFFECTS ON AUTOSOMAL AND SEX
CHROMOSOMES IN HUMANS
A. Mistakes in meiosis, usually due to nondisjunction, may result in abnormal number of
chromosomes. Autosomal or sex chromosomes can be affected. (see page 312, fig. 11.18)
1. Nondisjunction can occur in Meiosis I, Meiosis II, or both.
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2. Trisomy of 21 pair – Down’s Syndrome (0nly autosomal trisomy in which affected individuals
survive to adulthood.)
3. X, XXX, XXY, XYY – any individual with at least one Y chromosome is male, and any individual
without a Y is a female. Most lead normal lives, but are sterile and may suffer from varying
degrees of mental retardation. (see page 314, Table 11.4)
SUMMARY: VARIATION OF TRAITS
A. Variation among individuals of the same species can be explained by both genetic and environmental
factors.
B. Individuals within a species have similar, but not identical, genes.
C. In sexual reproduction, variations in traits between parent and offspring arise from the particular set of
chromosomes (and their respective multiple genes) inherited, with each parent contributing half of a
chromosome pair.
D. Genes along with environmental factors can modify an individual’s specific development, appearance,
behavior and likelihood of producing offspring.
E. The set of variations of genes present, together with the interactions of genes with their environment,
determines the distribution of variation of traits in a population.
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