Download Autosomal Non-Mendelian Inheritance

Non-Mendelian Inheritance
Slide 2
In organisms, the majority of genes are located on autosomes. Simply put,
autosomes are chromosomes that are not sex chromosomes, or do not determine the sex
of an organism. The genes that coded for the phenotypic characters Mendel studied in pea
plants, for example, were located on autosomes. While Mendel was fortunate to have
studied genes with fairly simple inheritance, many other genes do not follow such clear
patterns, even when they are located on autosomes. In this lesson, we will discuss
examples of inheritance of autosomal genes that differ from typical Mendelian
inheritance. In the following lesson, we will discuss inheritance patterns of genes located
specifically on sex-chromosomes.
Slide 3
Many genes are present in populations in more than two versions, or
alleles, and code for slightly different protein products. Even when genes have only two
alleles, however, as in Mendel’s studies, the alleles may not be clearly dominant and
recessive. One instance of this is called incomplete dominance. Incomplete dominance is
expressed in heterozygous offspring that are intermediate in appearance between their
homozygous dominant parent and their homozygous recessive parent. In this illustration,
the gene for flower color in snapdragons has two alleles. When an individual is
homozygous for the dominant allele of this gene, it produces red flowers. Homozygous
recessive individuals produce white flowers. Heterozygous individuals, however, produce
pink flowers, rather than the red flowers that would be expected if the dominant allele
were completely dominant over the recessive allele. In this case, the gene of interest
codes for a protein that makes pigment in the flowers. Individuals with two dominant
alleles of this gene are able to make enough of this enzyme to produce red flowers.
Heterozygous individuals, on the other hand, are able to produce only half as much
enzyme, because they have only one dominant copy of the gene. Because of this, their
flowers have less pigment and appear pink.
Slide 4
In some instances, both alleles of a gene are clearly expressed in the
phenotype of a heterozygous individual. This is called codominance. An example of
codominance is found in human blood types. In humans, blood type is determined by
three alleles of the same gene: IA, I B, and IO. Individuals with type AB blood produce two
types of antigens, or receptor proteins on the surface of their red blood cells – antigens A
and B. Individuals of type A or B blood, on the other hand, produce only A or B antigens,
respectively. The synthesis of these receptors is linked to two alleles of the same gene –
alleles A and B. Since these two alleles are both expressed in the heterozygote phenotype,
they are codominant.
Slide 5
Mendel’s pea plants had two versions of each character that he studied,
such as green vs. yellow seeds, or white vs. purple flowers. As he demonstrated, each
version was due to a different allele of a particular gene. In other words, each gene had
only two alleles. Many genes, however, have numerous alleles, each one coding for a
slightly different phenotype. The different alleles of a gene arise from mutations
occurring in the DNA. An example is the gene responsible for the coat color of domestic
rabbits. In rabbits, there is one gene that determines coat color, but it has four different
alleles. When an offspring inherits two of the alleles from its parents, its coat color will
range from dark gray to white, depending on exactly which of the two alleles it inherited.
In this case, the multiple alleles of a single gene can result in multiple phenotypic traits.
Slide 6
In many cases, an organism’s phenotype is determined by more than one
gene, rather than by one gene with two or more alleles. Multiple genes that determine a
phenotype often act in such a way that the activity of one gene is affected by the activity
of another. When the expression of one gene affects the action of another gene or genes,
we call this epistasis.
The diagram on this slide displays a well-studied example of epistasis that occurs in
mice. In mice, coat color is determined by two genes. For simplicity, we’ll call these two
genes gene A and gene B. Gene A controls the formation of pigment that is deposited in
the mouse’s coat. Gene B controls whether the pigment has a continuous distribution on
each hair, or whether it appears in alternating bands. In both gene A and gene B, the
recessive allele codes for a non-functional enzyme. As a result, mice homozygous
recessive for gene A (genotype aa) are unable to produce pigment; mice homozygous
recessive for gene B (genotype bb) are unable to produce a banding pattern, and instead
deposit pigment continuously throughout each hair.
Phenotypically, mice homozygous recessive for gene A are albino, since the recessive
version of gene A will block all pigment production. Note that in mice homozygous
recessive for gene A, the genotype for gene B is inconsequential, because there is no
pigment to distribute in the coat. In mice heterozygous or homozygous dominant for gene
A, the genotype for gene B determines whether mice will be dark grey or sandy brown.
Individuals that are homozygous recessive for gene B will develop a dark grey coat,
because the enzyme produced by gene B is non-functional, hence, pigment will be
deposited continuously in each hair. Individuals homozygous dominant or heterozygous
for gene B, on the other hand, will develop a sandy brown coat, as the dominant version
of gene B codes for a functional enzyme, hence pigment will be deposited in alternating
bands. In any of these cases, note how the activity of one gene can affect the activity of
another – this causal relationship between genes is the type of activity that defines
epistasis.
Slide 7
While multiple genes may combine to affect one phenotypic characteristic,
the opposite may also be true – one gene may have multiple phenotypic effects. This
phenomenon is called pleiotropy. One of the most well-known examples occurs in
Siamese cats. In this particular breed of cat, the gene that codes for coat color also results
in the crossed eyes common to this breed.
Slide 8
Some organisms, such as the snowshoe hare, or the hydrangeas on this
slide, present another common theme in the link between genotype and phenotype. In
many organisms, the physical expression of genes, or the phenotype, is affected by the
environment. In hydrangeas, flower color depends largely on the pH of the soil. In
relatively acidic soils, hydrangeas will produce blue flowers. In more basic soils, pink
flowers will be produced. On a molecular level, this change in color is caused by the
availability of free aluminum ions in the soil. In acidic soils, free aluminum ions are
typically in higher concentration than in basic soils. Aluminum ions free in the soil are
taken up by the hydrangea, and ultimately bind to a pigment found in the vacuoles of the
flower cells. This binding causes a conformational change in the pigment, changing the
wavelengths of light it reflects and making the flowers appear a different color.
In the case of the snowshoe hair, the phenotype of coat color is affected by seasonal
changes in the environment. In summer, coat color is a mottled brown; in winter, coat
color is white. As you can guess, these colors help the snowshoe hair blend in to its
environment and avoid detection by predators. The snowshoe hair is an excellent
example of both environment affecting phenotype, and of a species adapting to their
environment. Remember that species have adapted over time to their environments, so it
should be no surprise that the expression of specific genes may be affected by changes in
the surroundings of an organism.
Slide 9
In many cases, the alleles of two or more genes appear together in
offspring more often than would be expected by typical Mendelian inheritance. Such
results provide an exception to Mendel’s second law, the law of independent assortment,
which states that the alleles of different genes assort independently of one another during
gamete formation. When two or more genes do not assort independently of one another,
the genes in question are said to be linked. For example, in the illustration here, a fruit fly
heterozygous for two genes is crossed with a fly homozygous recessive for both genes
and the offspring phenotypes are observed. Note that the heterozygous parent fly has a
brown body and normal wings, while the homozygous recessive parent has a black body
and reduced, or vestigial, wings. In the offspring, one would expect a random assortment
of the alleles of the two genes that determine body color and wing size, hence equal
proportions of all of the possible combinations of the phenotypic traits. However, the
observed phenotypic ratios differed from the expected phenotypic ratios. The majority of
the offspring produced exhibited the parental phenotypes. The genes that determine these
two characters are said to be linked, because they are often inherited together.
Slide 10
Gene linkage occurs when two or more genes are located on the same
chromosome. Recall that during crossing over of meiosis I, chromosomes physically
exchange portions of their chromatids. If two genes are located close together on the
same chromosome, there is a greater chance that they will be transferred together during
crossing over. The alleles of these genes will then have a greater chance of appearing in
the gametes in the same combination as is found in the parents. Hence, offspring also
have a greater chance of inheriting the same combination of alleles that their parents had
if the genes under consideration are linked.
Slide 11
Geneticists often use the concept of gene linkage to identify the
approximate location of genes on chromosomes by creating a genetic map. On a genetic
map, two or more genes are mapped along the length of a chromosome in relation to one
another. The units of distance separating different genes are called centimorgans, or cM.
Each centimorgan corresponds to a one percent chance of recombination of the alleles of
two genes during meiosis. For example, when investigating the linkage between the
genes for eye color and body color, tests resulted in recombination frequencies of thirtyone percent. In other words, thirty-one percent of the offspring had phenotypic
combinations of eye color and body color that were not present in either parent. These
genes were then able to be mapped in relation to one another at a distance of 31 cM.
Slide 12
On a final note, remember that some of a cell’s DNA is non-nuclear. Both
mitochondria and chloroplasts carry their own DNA. In sexually reproducing organisms,
these organelles are passed down to the next generation only through the maternal
gamete, or egg cell. For this reason, inheritance of mitochondrial or plastid DNA is often
referred to as maternal or cytoplasmic inheritance. In addition, mitochondria and plastid
inheritance patterns are further complicated by the fact that the DNA of these organelles
typically has a higher mutation rate than that of nuclear DNA. This mutation rate,
coupled with the high number of these organelles in some cell types, can lead to a great
amount of genetic variation even within single cells.