Patterns of Inheritance
Heredity is the transmission of traits from one generation to the next
Genetics is the scientific study of heredity
Gregor Mendel
Born in 1822
Was a priest at St. Thomas Augustinian Monastery in the Czech republic
Studied mathematics and plant breeding
Before Mendel’s work, people had only vague ideas about heredity, many of which
were wrong
Essentially, the underlying genetic basis for what they observed was a mysterious
black box
A character is a heritable feature that varies among individuals
A trait is a variant of a character
Each of the characters Mendel studied occurred in two distinct traits
Worked with the garden pea, Pisum sativum
He understood the basics of plant anatomy
Pea plants are normally self-fertilizing
Mendel figured out how to force cross-fertilization
He used varieties that were true-breeding for certain traits
He carried on crosses between true-breeding individuals displaying distinct forms of
the same character
Genetic terminology
Parental plants are the P(Parental) generation
Their offspring are the F (Filial) generation
1
The offspring of two F1 parents are the F2 generation
The distinctive traits make up an organisms’ phenotype
The underlying genetics make up its genotype
Mendel analyzed seven traits
He determined which form of each trait was dominant and which was recessive
But the real important part of Mendel’s work involved the F generations
2
When F plants either self-fertilize or fertilize each other, their offspring are the F
1
generation
If the F1 plants were hybrids, this would be a monohybrid cross
2
Monohybrid crosses
The F plants of any particular cross all looked the same but the subsequent F plants
1
2
showed a mixture of phenotypes
Mendel concluded that:
1. the heritable factor (gene) for white flowers did not disappear in the F1
plants
2. only the purple flower factor was affecting the F1 plants
3. the F1 plants must have carried two factors for the flower color trait, one for
purple and one for white
Mendel developed the following hypotheses:
1. There are alternative forms of genes (alleles).
2. There are two genes for each inherited characteristic, one coming from each
parent.
3. A sperm or egg (haploid) only carries one allele for each inherited
characteristic because the allele pairs present in the parent segregate
(separate) during the production of gametes. Fertilization restores the paired
condition (diploid).
4. When two different alleles are present, one can be fully expressed
(dominant) while the other has no noticeable effect (recessive).
A Punnett square is used to predict or explain the results of a genetic cross
Mendel’s principle of segregation
Pairs of alleles segregate (separate) during gamete formation; the fusion of gametes
at fertilization restores pairs of alleles again.
What about when there are two traits?
Mendel also crossed true-breeding plants having round yellow seeds (RRYY) with
plants having wrinkled green seeds (rryy)
Were the two characteristics transmitted as a package deal or were they inherited
independently?
What did Mendel find?
A mixture of phenotypes including combinations not seen in the parents
A phenotypic ratio of 9:3:3:1
Why?
The traits were inherited independently!
With all the dihybrid crosses that Mendel conducted, he always found the same
9:3:3:1 phenotypic ratio in the F generation
2
Mendel’s principle of independent assortment
Each pair of alleles segregates independently of the other pairs during gamete
formation.
Mendel published his work but it wasn’t until 16 years after his death that the
significance of his findings were recognized
A useful tool - the testcross
A testcross is a mating between an individual showing the dominant phenotype (but
with an unknown genotype) and a homozygous recessive individual
Inheritance is based on probabilities
Mendel’s principles apply to the inheritance of many human traits
Pedigrees
To determine how particular traits are inherited, we need to track these traits
Geneticists can develop a family pedigree
It’s like a “genetic family tree”
Particular pedigree patterns give us an idea of the type of inheritance involved
The idea of dominance and recessiveness only comes into play in heterozygous individuals
It does not extend to populations
Dominance ≠ more common
Recessive ≠ less common
Human disorders controlled by a single gene
Our ~21,000 genes are distributed among the 24 different types of chromosomes (22
autosomes and 2 different sex chromosomes)
Inheritance is pretty straight-forward for the autosomal gene disorders
Recessive disorders
Most humans genetic disorders are recessive
Cystic fibrosis
Affects one in:
17,000 African Americans
90,000 Asian Americans
1800 Caucasian (European ancestry) births
Allele is carried by 1 in every 29 Caucasians
Dominant disorders
Achondroplasia
Affects about 1 in 25,000 people
Head and torso develop normally but arms and legs are short
Genetic testing
Today many tests can detect the presence of disease-causing alleles
Most genetic tests are performed during pregnancy if the prospective parents are
aware that they have an increased risk of having a baby with a genetic disease
In amniocentesis, a physician uses a needle to extract about 2 teaspoons of the fluid
that bathes the developing fetus
In chorionic villus sampling (CVS), a physician inserts a narrow, flexible tube through
the woman’s vagina and into her uterus, removing some placental tissue
Newer genetic screening procedures involve isolating tiny amounts of fetal cells or
DNA released into the mother’s bloodstream
These newer technologies are less invasive, more accurate, and can be
performed earlier
Variations on Mendel’s laws
Incomplete dominance
The hybrids display an intermediate phenotype
The genotypic and phenotypic ratios are the same
Incomplete dominance in humans
Hypercholesterolemia is characterized by dangerously high levels of cholesterol in
the blood
In hypercholesterolemia,
heterozygotes have blood cholesterol levels about twice normal, while
homozygotes have about five times the normal amount of blood cholesterol and
may have heart attacks as early as age 2
Codominance
Occurs when different alleles each have an independent effect on the phenotype
Both alleles are clearly seen in the phenotype
Multiple alleles
ABO blood types
Governed by three alleles IA, IB, i
This is also a case of codominance
Pleiotropy
Pleiotropy is when one gene influences several characters
Sickle-cell disease results in abnormal hemoglobin proteins, and causes disk-shaped
red blood cells to deform into a sickle shape with jagged edges
In most cases, only people who are homozygous for the sickle-cell allele have sicklecell disease
Polygenic inheritance
Many traits are governed by more than one gene
Often, each gene has several alleles as well
Usually in polygenic inheritance, the phenotype is determined by the interaction of
multiple genes, each having an additive effect on the characteristic
Polygenic traits frequently exhibit continuous phenotypic variation, rather than
discontinuous (incremental) variation
There are no discrete phenotypic classes
Thus, these display a bell-shaped distribution curve
Epigenetics and environmental effects
Often, the environment can affect the phenotype displayed
Temperature, exposure to chemicals, nutrition, acidity of soil, etc.
Sometimes the role of the environment is pretty simple
This is also seen in Himalayan rabbits
Rabbits raised at 20°C or less show black at the extremities
Those raised at 30°C or more show none
But sometimes it is much more complex
In general, only genetic influences are inherited and the effects of the environment are not
passed to the next generation
In recent years, however, biologists have begun to recognize the importance of epigenetic
inheritance, the transmission of traits by mechanisms not directly involving DNA
sequence
Linked genes
Linked genes
are located near each other on the same chromosome and
tend to travel together during meiosis and fertilization
Such genes are often inherited as a set and therefore often do not follow Mendel’s
law of independent assortment
Sex determination in humans
Many animals, including all mammals, have a pair of sex chromosomes (X and Y), that
determine an individual’s sex
Sex chromosomes
Autosomal inheritance is pretty straightforward; the sex chromosomes are a whole
other story
Sex-linked genes
Any gene located on a sex chromosome is a sex-linked gene
There are many more X-linked genes
Because of this, inheritance patterns do not follow Mendel’s laws
The sex chromosomes contain genes for other traits besides sex determination
These are called sex-linked genes
Most of the genes on the X chromosome do not have a corresponding copy on the Y
Therefore, females have two copies of all the genes located on the X chromosome;
males have only one copy
What happens if that one copy is defective?
Sex-linked disorders are much more likely to affect males than females
Red-green color blindness is a sex-linked recessive disorder
With sex-linked disorders, how much more likely are they to affect males than
females?
Hemophilia A is another sex-linked recessive disorder
Hemophilia affects ~1 in 7,000 males