Biology 3201
Unit 3 – Genetic Continuity
Ch. 16 – Genetics and Heredity
16.1 – Genetics of Inheritance
Genetics – the branch of biology dealing with the principles of variation and
inheritance in organisms; how traits are passed on from generation to generation
traits – distinguishable characteristics or phenotypic features of an individual
ex. Eye color, height, hair color
heredity – the passing of genetic traits such as the color of eyes or hair from one
generation to the next resulting in similarities between members of one family or strain
The study of genetics allows us to greater understand why certain traits are
characteristic of a certain family (such as the occurrence of diseases) and it helps
determine the likelihood of inheriting certain characteristics.
For many years, people really had no clue about how babies were conceived and
why children looked similar to their parents. In fact, in the early 1800s it was believed
that complete human embryos were contained in the sperm, while the only thing females
did was provide a place for the embryos to develop.
That all changed with the work of an Austrian monk named Gregor Mendel. He
conducted a series of experiments using pea plants to model his ideas about heredity.
There were four main reasons why Mendel chose to use pea plants:
1. very common in Europe
2. easy to grow and they mature quickly
3. easy to control which plants reproduced
4. different varieties of the plant had different traits which were readily
observable, and each trait only had two possible forms.
Ex.
Height – tall or short
Seed pod color – yellow or green
Seed pod shape – round or wrinkled
Important terms related to Mendelian genetics
Blend theory: an early theory in genetics that states that factors from parents were
blended in their offspring. This theory, however, was unable to explain the appearance or
disappearance of distinct traits from one generation to another.
P generation – the designation for the parent generation
F1generation – the first filial generation, offspring from the cross (mating) of the P
generation
F2 generation – the second filial generation, offspring from the cross of the F1
generation
Unit characters – a term describing Mendel’s “factors” of inheritance (genes),
which are inherited as independent units
Gene – a specific sequence of DNA that guides the expression of a particular trait
and can be passed on to an offspring
Allele – alternate form of a gene
Unit theory – a term describing Mendel’s law of inheritance, from his discovery
that genes (which he called “factors”) are inherited as independent units
Dominant – type of trait in which the characteristic is always expressed, or
appears, in an individual
Recessive – having an allele that is latent (present but inactive) and is therefore
not usually expressed unless there is no dominant allele present
Purebred – having descended from ancestors of a distinct type or breed. Purebred
organisms in a given species or variety all share similar traits
Hybrid – an organism heterozygous for a trait
Homozygous – describes an individual with two alleles at one locus that are
identical (two dominant or recessive alleles for one trait)
Heterozygous – describes an individual with two different alleles at a locus (one
dominant and one recessive allele for a single traits)
Genotype – genetic makeup of an organism; remains constant throughout an
individual’s life. Usually indicated by the combination of letters in a Punnett square
Phenotype – the physical and physiological traits of an organism
Punnett Square – simple grid used to illustrate all possible combinations of
gametes from a given set of parents
Product rule – rule that states that the probability, or chance, that two or more
independent events will occur together is the product of their individual probabilities
occurring alone
Ex. When flipping a coin, the chance of getting heads twice in a row is
½x½=¼
Mendel’s First Experiment – A Monohybrid Cross (see fig 16.5, p. 529)
Monohybrid cross – a cross of two heterozygous individuals for one particular
trait
Mendel crossed a true breeding tall plant with a true breeding short plant (both
purebreds). In the F1 generation, all the plants were tall; no short plants were produced.
This was unexpected, as blend theory would have predicted that some medium-sized
plants were produced. Since all the plants were tall, Mendel concluded that tall was a
dominant trait and the tall “factor” (gene) was dominant over the short one.
Principle of dominance – when individuals of contrasting traits are crossed, the
offspring will express only the dominant trait
In the next experiment, Mendel bred two plants from the F1 generation (both tall).
In this case ¾ of the offspring were tall, but ¼ of them were short. Mendel repeated this
experiment several times, and with different traits and got the same results. From these
results, Mendel came up with the following conclusions:
1) Each parent in the F1 generation started with two hereditary “factors”.
One was dominant while the other was recessive
2) The factors separate in the parent. Only one factor from each parent is
contributed to the offspring.
3) Each offspring inherits one factor from each parent. If the dominant
factor is present it will be expressed even if the recessive factor is also present.
4) The recessive factor will be expressed only if another recessive factor is
present as well.
Law of segregation – Mendel’s first law of inheritance, in which the hereditary
traits are determined by the pair of alleles from each parent. These alleles are separated
during gamete formation, giving each offspring only one allele from each parent.
(see fig. 16.6 and 16.7, p. 530)
16.2 – Complex Inheritance Patterns
Mendel’s Second Experiment – A Dihybrid Cross
Dihybrid cross – cross of two heterozygous individuals for two different traits
Mendel wanted to know if the inheritance of one characteristic influenced another
(for example, does height affect the pea shape). So he crossed two plants that were
purebred for two different traits.
(see fig. 16.12, p. 536)
Mendel crossed plants that had round, yellow seeds with plants with green,
wrinkled seeds (round and yellow are dominant traits). The F1 generation had seeds
which were all yellow and round.
In the F2 generation, something unusual happened. He got the following numbers
of offspring:
- 320 round yellow
- 101 wrinkled yellow
- 104 round green
- 26 wrinkled green
That works out to be a 9:3:3:1 ratio. Mendel realized that this ratio was only
possible if the “factors” for one trait were independently assorted from the “factors” of
the other trait.
Law of independent assortment – Mendel’s second law of inheritance, stating that
the inheritance of alleles for one trait does not affect the inheritance of alleles for another
trait.
Beyond Mendel’s Laws
Mendel’s experiments focused on traits that involved only two alleles where one
factor was clearly dominant over the other. However, this is not always the case.
Incomplete dominance
→ blending of the two traits of two different alleles at one locus that
occurs when neither allele is dominant
→ for example, there are 3 possible colors of snapdragons: red, white, or
pink. Red snapdragons have two red alleles, white snapdragons have two white alleles,
and pink snapdragons result from one red allele and one white allele.
(see fig. 16.15, p. 541)
Note that R stands for red and R’ stands for white since neither allele is dominant
Co-dominance
→ in genetics, describes a situation in which two alleles may be expressed
equally. The situation occurs when two different alleles for a trait are both dominant.
→ for example, feather color in chickens governed by two dominant
alleles, black (B) and white (W). Birds that are heterozygous (BW) have a barred pattern
(see fig. 16.16, p. 541)
Multiple Alleles
→ pattern of inheritance when a gene may have more than two alleles for
any given trait
→ for example, human blood type is controlled by 3 alleles: IA, IB, and i
(A, B, and O respectively). This results in four possible blood phenotypes: A, B, AB and
O. A and B are both dominant to O, but co-dominant to each other.
Test Crosses
If you have an unidentified organism displaying a dominant trait, it could be
either homozygous or heterozygous. To determine which one it is, a test cross can be
performed.
Test cross – cross of an individual with unknown genotype with a homozygous
recessive individual; used as a method to identify an unknown genotype.
Ex. Suppose you had an unknown tall pea plant. You could breed it with a short
plant and see the ratios of the offspring produced.
If the unknown plant was homozygous, the test cross would look like this.
Unknown Gametes
Test plant gametes
T
T
t
Tt
Tt
t
Tt
Tt
Genotypic ratio: all Tt
Phenotypic ratio: all tall
If the unknown plant was heterozygous, the test cross would look like this:
Unknown Gametes
Test plant gametes
T
t
T
Tt
tt
T
Tt
tt
Genotypic ratio: ½ Tt: ½ tt
Phenotypic ratio: ½ tall: ½ short
Pedigrees
Pedigree – diagram that illustrates the genetic relationships among a group of
individuals
(see fig. 16.17, p. 544)
16.3 – Chromosomes and Heredity
The chromosome theory of inheritance
In the early 1900s, two scientists named Walter Sutton and Theodor Boveri
observed that the behavior of chromosomes during meiosis was related to the behavior of
Mendel’s “factors”
(see fig. 16.19, p. 546)
The chromosome theory of inheritance states 2 things:
1. Mendel’s factors, or genes, are carried on chromosomes
2. it is the segregation and independent assortment of chromosomes during
meiosis that accounts for the patterns of inheritance.
Thomas Morgan and Sex-Linked Characteristics
Thomas Morgan was a scientist who did genetic work with fruit flies, particularly
looking at how eye color was inherited. In these flies, having red eyes is a dominant
condition to white eyes. He crossed two red-eyed flies and produced a white eyed male.
This was not unusual; however, when Morgan crossed a red-eyed female (offspring of the
white-eyed male) with a red-eyed male, all the female offspring had red eyes while ½ the
males had red eyes and ½ had white eyes. Not one female had white eyes. This led
Morgan to conclude that the gene for eye color was located on the X chromosome.
As well, Morgan discovered that some genes do not sort independently but are
linked together. However, Morgan also found that some linked genes can separate. This
occurs if genes are located further apart on a chromosome, where crossing over during
meiosis is more likely to occur
(see fig. 16.20, p. 546)
Modern Meaning of the Law of Independent Assortment
→ if crossing over does not take place, genes that are located on different
chromosomes will sort independently while genes located on the same chromosome will
be inherited together.
Sex-linked inheritance – the transfer of genes on the X or Y chromosome from
one generation to the next.
→ genes only located on the X chromosome are X-linked. Genes
located only on the Y chromosome are Y-linked.
→ very few Y-linked traits in humans, mainly because Y
chromosome is very small
(see fig. 16.21 and 16.22, p. 547)
Chromosomes and Gene Expression
Males and females produce the same amount of proteins coded for by the X
chromosome, even though females have 2 copies of the X chromosome. This is due to
the fact that only one X chromosome is functional in each female cell; the other is
inactive.
Barr body – the inactivated X chromosome, one of the two chromosomes
found in every cell in females. Barr bodies are picked at random; in ½ of a females cells
it could be the first X chromosome, in the other half it could be the second X
chromosome.
Polygenic inheritance – pattern of inheritance in which a trait is controlled by
more than one gene
Ex. In corn, length of the ears is controlled by two separate sets of alleles.
Continuous variation – range of variation in one trait resulting from the protein
products produced by many genes.
Ex. Since ear length in corn is controlled by two separate sets of alleles,
there are many different sizes, including medium.
Modifier genes – genes that work along with other genes to control the expression
of a trait.
Ex. Eye color in humans is controlled by a pigment called melanin.
Brown eyes are dominant over blue eyes (no melanin). As well, modifier genes help
produce other eye colors.
Changes in Chromosomes
Changes in Chromosome Structure (see fig. 16.27, p. 550)
1) Deletion
→ change in the physical structure of a chromosome in which a
portion of the chromosome is lost.
→ can be caused by viruses, radiation, various chemicals
2) Inversion
→ change in the physical structure of a chromosome where a
certain gene segment becomes free from its chromosome momentarily before reinserting
in the reverse order.
3) Duplication
→ change in the physical structure of a chromosome where a gene
sequence is repeated one or more times within one or several chromosomes.
4) Translocation
→ change in the physical structure of a chromosome where a part
of one chromosome changes places with part of the same chromosome or with part of
another homologous chromosome.
Change in Chromosome Number
Nondisjunction – a situation where a person has too few or too many
chromosomes. Results when chromosomes or chromatids do not separate during meiosis.
Examples of nondisjunctive disorders:
1) Down Syndrome
→ genetic disorder resulting from nondisjunction during gamete formation
→ during fertilization, an individual receives an extra copy of
chromosome 21 (see fig. 16.29, p. 553)
→ also called trisomy 21
2) Turner Syndrome
→ when a person inherits only a single X chromosome. Results in
improper development (no ovaries, infertile, heart defects, etc.)
3) Klinefelter Syndrome
→ a male with XXY. Results in inactive sex organs, no facial hair, and
maybe some breast development
4) Jacobs Syndrome
→ a male with XYY. Results in speech and reading problems and
persistent acne.
16.4 – Human Genetics
Various genetic disorders:
Autosomal Recessive Disorders
→ are carried on the autosomes and are caused by recessive alleles
→ not specific to any sex
→ people can be carriers if heterozygous (no symptoms)
Examples:
1) Tay-Sachs disease
→ untreatable autosomal recessive condition in children, who
appear normal at birth but whose brains and spinal cords begin to deteriorate at about
eight months
→ a disorder where lipids in the brain cannot be broken down
because there is a lack of a certain enzyme. These lipids build up in brain cells and
destroy them
2) phenylketouria (PKU)
→ autosomal recessive disorder that affects young children, in
which an enzyme that converts phenylalanine to tyrosine is either deficient or defective.
→ This disorder causes brain damage. A special diet free of
phenylalanine can prevent brain damage, but there is no cure.
3) Albinism
→ genetic condition in which the eyes, skin, and hair have no
pigment. Affected individuals either lack one of the enzymes required to produced
melanin or lack the means of getting the enzyme to enter the pigment cells that produce
melanin.
Codominant Inheritance
→ occurs when an individual have both of a defective dominant allele
→ heterozygous advantage – a survival benefit for those individuals who
inherit two different alleles for the same trait
ex. People heterozygous for sickle-cell anemia are less likely to
contract malaria
1) Sickle-cell anemia
→ co-dominant genetic disorder in which a defect in the
hemoglobin in red blood cells leads to sickle-shaped blood cells that can clog arteries and
reduce blood flow to vital organs
Autosomal Dominant Inheritance
→ very rare in human populations
→ appears in both homozygous dominant and heterozygous individuals
1) Progeria
→ rare disorder that causes a child to age rapidly; results from a
random and spontaneous mutation of one gene
→ occurrence is 1 out of 8 million
2) Huntington Disease
→ autosomal dominant condition; a lethal disorder in which the
brain progressively deteriorates over a period of 15 years, and symptoms usually occur
after age 35.
Incomplete Dominance
→ the heterozygote exhibits a phenotype midway between both dominant
and recessive traits.
Ex. Familial hypercholesterolemia (FH)
About 1 in 500 people are heterozygous and have twice the normal
level of blood cholesterol. Homozygous recessive people (1 in 1000000) have 6 times
the blood cholesterol of normal people.
X-linked Recessive Inheritance
→ diseases more common in men than women; women can be carriers
(have the defective gene but not show symptoms of the disease)
1) Color blindness – a condition where people cannot distinguish red and
green
2) Hemophilia – a condition where the blood does not clot properly
3) Muscular Dystrophy – a condition where muscles degenerate over a
period of time.