Heredity &
Genetics
Basic Vocabulary
Heredity: The transmission of
traits from one generation to the next
Genetics: The study of heredity
Character: A heritable feature that varies among individuals
Trait: A variant for a character
Gregor Mendel
Austrian monk
Worked with pea plants
Conducted scientifically rigorous
experiments
Mendel’s Experiments
Created “true breeding” lines of pea plants
True breeding: varieties for which self fertilization produces offspring that are all identical to parents
Hybrids: offspring of two varieties
P generation: parental generation
F generation: filial generation
Mendel’s Experiments
Monohybrid cross
Mendel’s Experiments Monohybrid Cross
What he deduced:
1) Alternate versions of genes account for variations in inherited characteristics (we now call these “alleles”).
2) For each character, an organism inherits two alleles (one from each parent). An organism may be
homozygous or heterozygous for a given character.
3) If two alleles at a given locus differ, one allele
determines the organism’s appearance (it is said to be the dominant allele).
4) The two alleles for a heritable character separate during gamete formation and end up in different gametes (law of segregation).
Mendel’s Experiments
Monohybrid cross
Results of Monohybrid crosses
Mendel concluded that parents pass on
“heritable factors”—what we now call genes.
He believed that these factors retain their identity generation after generation .
He based his assumptions about an
organism’s genotype (its genetic make-up)
on the observed phenotype (what he could
see).
Homologous
Chromosomes
The test cross
To determine if an
organism who exhibits the dominant trait in their phenotype is a pure bred organism or a carrier, one can
perform a test cross.
Test crosses always use a homozygous
recessive organism as
the probe.
Law of Independent Assortment
Mendel also conducted experiments to see what would happen if he crossed plants for two
characters. He wanted to know if the alleles for one character move as a package with those for another character or if the alleles move
independently of each other.
This is known as a dihybrid cross.
An example would be to cross true-breeding plants for both seed color (Yellow = Y vs green = y) and seed shape (Round = R vs wrinkled =r) which would have only one possible outcome:
All offspring would be hybrids (RrYy)
Law of Independent Assortment
Law of Independent Assortment
What would happen with a cross of the F
1generation (RrYy vs RrYy)?
There are two possibilities:
The traits are passed to offspring as a package.
The traits are passed to offspring separately.
Dependent Assortment
Independent Assortment
Mendel’s Law of
Independent Assortment
Mendel found that when he carried out dihybrid crosses, he always obtained data very close to the 9:3:3:1 ratio.
These results supported the idea that alleles for different characters segregate
independently of each other during meiosis.
His law of Independent Assortment states that inheritance of one character has no effect on the inheritance of another
character.
Extensions to Mendelian Genetics
Degrees of Dominance:
1) Incomplete Dominance: Hybrids have a phenotype that is between two parental varieties. This is not blending, but rather an issue of dosage.
2) Codominance: Two alleles affect phenotype in separate and distinguishable manner.
3) Multiple Alleles: There are more than two possible alleles for a given gene locus.
4) Pleiotropy: One gene impacts many phenotypic qualities.
Incomplete Dominance
Codominance
Variations on Mendel’s Laws
Many genes have more than two alleles. Genes that have more than two versions are known as multiple alleles. However, any given organism can only carry two alleles for each character.
In humans, the ABO blood group involves three alleles of a single gene.
IA: carbohydrate A is added to red blood cells
IB: carbohydrate B is added to red blood cells
i: no carbohydrate is added to red blood cells
IA and IB are codominant; both alleles are expressed in heterozygous individuals.
ABO Blood Group
Pleiotropy
Pleiotropy is a property that occurs when one gene controls many characters.
Ex: Sickle cell anemia
In sickle cell anemia, an individual who is homozygous for the sickle cell gene
produces abnormal hemoglobin. This then leads to damaged organs and other
associated problems.
Polygenic Inheritance
A single character may be controlled by more than one gene.
These characters are known as quantitative characters.
Polygenic inheritance is when several genes contribute to a single phenotypic character in a summative manner.
Human skin color and height are examples of polygenic traits which are inherited
separately.
Polygenic Inheritance
Variations on Mendelian Genetics
Epistasis: This occurs when a
gene at one locus alters the
phenotypic
expression of a
gene at a second
locus.
More on Epistasis
Mendelian Genetics
Norm of Reaction: How much a genotype responds to environmental conditions.
- ABO blood type has no breadth to its norm of reaction. This genotype group has one possible phenotype.
Gene Expressivity: the degree to which one expresses a gene
Gene Penetrance: the degree to which a
particular gene generates any phenotype at all.
Human Traits and Mendelian Genetics
Many human traits follow Mendelian patterns of inheritance.
Rather than carrying out selective breeding, geneticists look at pedigrees, a table which describe inheritance of a trait (or traits)
across generations.
As a general rule, a square represents a
male, a circle represents a female.
Pedigrees
Recessively Inherited Disorders
An allele that causes a genetic disorder is one that codes for a malfunctioning protein or no protein at all.
A carrier for a disorder is an individual that presents with a normal phenotype but is
heterozygous for that character.
Recessively Inherited Disorders
Cystic Fibrosis:
1/2500 people of European descent suffer from this disorder (with 1/25 individuals acting as carriers).
N = has gene that codes for a membrane protein that helps transport Cl- between cell and extracellular fluid.
n = missing protein; [Cl-] is high outside of cell causing build-up of mucus
This has a pleiotropic impact; pancreas, lungs, digestive tract all impacted.
Recessively Inherited Disorders
Sickle-Cell Anemia
1/400 people of African descent impacted.
N = normal hemoglobin
n = abnormal hemoglobin
In times of low O2, sickle-cell hemoglobin molecules can form aggregates, clumping up and clotting blood vessels.
This is a codominant trait—an individual can produce both normal and sickle-cell hemoglobin molecules.
Recessively Inherited
Disorders
Recessively Inherited
Disorders
Dominantly Inherited Disorders
Not all disorders are recessive—some are dominant.
All lethal alleles arise in mutations in cells that produce sperm and egg cells. In the case of dominant lethal alleles, if the
mutation causes death prior to reaching reproductive age, the organism will not pass the trait on.
Achondroplasia, a form of dwarfism, is a dominant disorder. People who are
heterozygous for this trait have the dwarf phenotype. Offspring born with two
dominant genes rarely live past childhood.
Dominantly Inherited
Disorders
Sex Chromosomes and Sex-linked Genes
In humans, there are 44 autosomal chromosomes and two sex chromosomes in each cell.
The two sex chromosomes are X and Y.
Females have two X chromosomes, males have one X and one Y chromosome.
All eggs contain an X chromosome, but sperm can either end up with an X or a Y chromosome.
The SRY gene, located on the Y chromosome, triggers testis development. This gene is
activated about two months into development.
Sex Chromosomes
Sex Chromosomes and Sex-linked Genes
Both the X and Y chromosomes carry mostly genes that have nothing to do with femaleness or
maleness.
The Y chromosome has about 78 genes which code for approximately 25 proteins. About half of these genes are only expressed in the testis.
A sex-linked gene is a gene that is located on either chromosome. A male who inherits a recessive X-
linked gene is said to be hemizygous. Some examples of this are colorblindness, Duchenne muscular dystrophy, and hemophilia.
Sex Chromosomes and Sex- linked Genes
X-Inactivation:
For genes that are located on the X chromosome, scientists have found that females do not make twice as much of a protein that is coded for by these genes.
One X chromosome becomes inactive (by methylation), thus males and females have same “dosage” of these proteins. The inactivated chromosome is called a Barr Body.
Selection of chromosome that becomes inactivated occurs randomly.
Once a chromosome is inactivated, all further mitotic descendants will have the same X chromosome inactivated.
Barr bodies also have an active XIST gene. It produces multiple RNA molecules which attach (and eventually cover) the inactivated chromosome.
Barr Body
Linked Genes
Further experimentation with dihybrid crosses found that some dihybrid crosses did not produce the expected 9:3:3:1 ratio.
Some dihybrid crosses produced a 3:1 ratio.
Scientists now know that genes that are located close to each other on a given
chromosome tend to travel together. These
genes are called linked genes.
Linked Genes
“Parental type” refers to offspring who have phenotype similar to their parent.
“Recombinant type” refers to offspring who have non-parental phenotypes.
If 50% of offspring are recombinants, a geneticist would say there is a 50% frequency of recombination. This indicates that genes are on different chromosomes (unlinked).
Crossing over breaks physical connection between specific alleles on the same chromosome. The farther apart two genes are on a
chromosome, the higher the probability that a crossover event will occur between them (higher recombination frequency). This
information can be used to develop a genetic map.
1 map unit = 1% recombinant frequency
Linked Genes
Morgan did experiments to see if genes were located on the same chromosome.
He carried out dihybrid crosses using true-
breeding wild-type flies (gray with normal wings) with true-breeding double mutants (black with vestigial wings). All offspring were
heterozygotes.
He then carried out a testcross with the F1
generation (heterozygous) and a double mutant.
What would ratio be if genes were linked? If they were on different chromosomes?
Linked Genes
Linked Genes
So what’s really happening?
Crossing over is occurring.
Linkage Map
We can use recombination frequencies to calculate relative distances between genes.
The farther apart two genes are, the higher the probability that a crossover will occur thus the higher the recombination
frequency.
Recombination frequency is calculated by dividing the number of recombinant
offspring found in a dihybrid cross and
dividing this number by the total offspring.
Linkage Map
Abnormal Chromosome
Number
Abnormal Chromosome Number
Nondisjunction occurs when members of a homologous pair do not move apart correctly in meiosis I or the sister
chromatids do not separate in meiosis II.
When a gamete that has an abnormal chromosome number fuses with a normal gamete, the resulting zygote will have an abnormal chromosome number. This is known as
aneuploidy.
May be monosomic for a chromosome (or 2n-1) or trisomic (2n + 1).
Polyploidy refers to organisms with more than two complete chromosome sets. This is common in plants and can lead to generation of new species.
Alteration of Chromosome Structure
Errors in meiosis or damage due to
environmental factors (such as radiation) can cause breakage within a chromosome.
There are four main types of changes to chromosome structure:
1) Deletion 2) Duplication 3) Inversion
4) Translocation
