Mendelian Genetics: Principle of Segregation
VBS Home page,VBS Course Navigator, Transmission genetics, Principle of segregation, Previous Page, Next Page.
VBS Home page,VBS Course Navigator, Transmission genetics, Monohybrid Crosses, Previous Page, Next Page.
The Principle of Segregation. The principle of segregation says that in diploid organisms genes come in pairs and that when gametes get produced each gamete gets one gene at random from each gene pair but not both. When developing this idea Gregor Mendel conducted a series on monohybrid crosses using pea plants.
Monohybrid cross: A monohybrid cross is a cross in which the breeder starts out with parents that are true breeding for alternate forms of a characteristic, for example flower color.
Step 1. Produce parents true breeding for alternate forms of the trait or characteristic being studied. For instance were we studying the inheritance of flower color in peas as Mendel did we would selectively breed for plants that only produced purple flower offspring when bred with themselves. That would be the source of one of the parents. The other parent might be pure breeding for white flowers. These original parents are called the parental generation.
Step 2. Breed these parents together to produce offspring called the F1 generation.
Step 3 Breed the F1 generation offspring with each other to produce the next generation or F2 generation. Examining the appearance offspring resulting from these crosses give us information about the about the pattern of inheritance of the trait being studied.
Phenotype: The phenotype refers to the appearance of the organism. This could refer to some obvious trait such as purple flower color, or to a biochemical trait such as the particular form of an enzyme. In our example the purple or white rectangles represent the phenotype.
Genotype: The genotype refers to the particular combinations of genes that give rise to the phenotype. In our example the letters represent the genes involved in the genotype. AA individuals and Aa individuals can have the same phenotype, both purple for instance but the genotype is different.
For the true breeding organisms in the parental generation notice that both genes are the same for each individual.
The AA individuals only produce gametes carrying
the A allele. The AA individuals produce only gametes gametes carrying the a allele.
Allele: An allele is an alternate form of a gene. For example while a diploid individual might carry two copies of a gene the copies may not be identical. Perhaps each gene codes for a slightly different form of an enzyme.
Heterozygous: Heterozygous refers to an individual having different (non- identical) alleles for each gene in the gene pair. For instance the Aa individuals produced from the cross AA x aa are heterozygous.
In complete dominance, the phenotype of the heterozygote AA cannot be distinguished from one of the homozygotes. So it is said that the allele carried by that homozygote is the 'dominant' allele. This allele is typically written in capital letters. The allele that cannot be detected in the heterozygote is typically written in lower case letters (e.g. Aa).
Tip! Notice when writing the genotype for the heterozygote the dominant allele is always written first. This is simply to avoid visual confusion. The genotype aA is the same thing as Aa, but it makes for less clutter if the genotypes are written in a consistent way.
Punnett Squares. Many times the predicted results of a particular cross are examined using a so called Punnett square, after their inventor. The gametes from one parent are put along the top of the square, and the gametes of the other parent are put along the side. Remember! each gamete gets on gene from each gene pair but not both. Hence the gametes for the male parent (genotype Aa) carry either the A allele or the a allele but not both, This is the essence of Mendel's principle of segregation. The boxes in the square and the pairs of alleles (e.g. AA, Aa aa) represent the genotypes of the offspring from this cross.
The phenotypes of the offspring for complete dominance are shown here by the color of the squares. So we expect that
1/4 of the offspring will be AA and have the purple phenotype
2/4 of the offspring will be Aa and have the purple phenotype and
1/4 of the offspring will be aa and have the white phenotype.
Caution: Be careful to note that dominance is relative to how the organism is being studied. A heterozygous flower may be purple like the dominant flower but when examined biochemically may produce less of a critical enzyme than the homozygous plant, this difference not being obvious visually.
The situations that Mendel examined all involved 'complete dominance following this pattern. Some other common patterns or modes of inheritance you will see in genetics for single gene traits are:
Tip! When doing genetics problems always make sure you:
Tree or branch diagrams are often used in genetics as an aid to solving problems. This particular diagram shows how the 3 : 1 ratio for complete dominance situations analyzed by Gregor Mendel comes about. If the female gametes indeed have one allele from each gene pair but not both then, half of her gametes carry the A allele, he other half the a allele.
Suppose the female gamete has an A allele. The male that she mates with can contribute either an A allele of an a allele leading to two possible offspring AA or Aa Following the other set of branches,, if the female's gamete has an a allele, the male can contribute either an A allele or an a allele but not both, leading to two possible offspring AA or Aa
Adding up all the possibilities gives the genotypic ratio:
1 AA : 2 Aa : 1 aa
and the phenotypic ratio 3 Purple : 1 White flowered offspring.
More on using these branch diagrams and probability in genetics is here.
In so called incomplete dominance the heterozygote is intermediate in phenotype compared to the two homozygotes. In this hypothetical example the heterozygote is lavender as opposed to either purple or white. This comes about when one of the alleles is no longer leading to the production of some sort of substance. For instance if the A2 allele is coding for a non functioning enzyme then one might see significantly less of a pigment produced by whatever biochemical pathway is involved in pigment production.
Incomplete dominance situations are examples of loss of function mutations. In this case the A2 allele is coding for an enzyme that no longer works and so there is no pigment produced in the A2A2 individuals.
Tip! Use subscripts to label the alleles as I have done here for situations when they is no clear dominant allele. Also when doing genetics problems, you may find different notation systems so make sure you understand them.
Codominance is like incomplete dominance in that the heterozygote is different in phenotype from either homozygote. The big difference between the two types of pattern is that in codominance both alleles still code for a functioning product, its just that the product may have different properties or slightly different functions. For example perhaps the A1A1 individuals produce a purple pigment, the A2A2 individuals produce a yellow pigment. The A1A2 heterozygotes since they have both alleles which are quite functional produce both pigments! Sometimes it is difficult to distinguish just by looking whether or not a particular mode of inheritance is codominance or incomplete dominance.
Gain of function mutation. Gain of function mutations are mutations that code for a substance which is functional, but perhaps leads to a slightly different outcome, perhaps coding for an enzyme that functions best at a higher or lower temperature. Gain of function mutations are quite common.
X linkage. X linkage refers to the mode of inheritance common for those organisms that have the XY chromosome system of sex(gender) determination. For example in humans, females are usually XX and males are typically XY. The X chromosome has many genes not found on the y chromosome and this means that many genes associated with the X and y chromosomes in the human male do not come in pairs, contrary to what Mendel concluded about his pea plants. I show to common X linked modes of inheritance:
Tip! Notice when representing the A and a allele I write the A allele as XA to indicate that this allele is linked to or part of the X chromosome.
Notice the male has only one copy of the gene since the y chromosome does not contain a copy of the gene. This is what leads to the odd patterns seen in x linked modes of inheritance.
X linked recessive: Study the diagrams carefully. Notice that the X linked recessive trait leads to a pattern that looks on the surface like a typical complete dominance situation where two heterozygotes are crossed. You do get a 3:1 phenotypic ratio but notice that all the individuals with the recessive phenotype are male!
The various types of color "blindness" are familiar examples of x linked recessive traits.
X linked dominance. Notice the pattern for an x linked dominant trait. In this example a homozygous recessive genotype is mated with a male who carries the dominant allele. He would have the 'purple' phenotype in this hypothetical example and the female would have the 'white' phenotype. Observe that in this situation the male offspring all have the recessive phenotype since they all have the a allele but no A allele to mask it. The female offspring all have the dominant phenotype because they are all heterozygous.
The genetic disorder in humans, soft enamel is a commonly cited X linked dominant trait.
Y linked traits. The y chromosome does have genes on it. These genes would be paired up with corresponding genes of of the x chromosome. For humans this means that every son of a male will inherit the gene determining the train since the y chromosome is only passed from father to son. A trait called hairy ears is just such a trait.
Caution! the xy mechanism of sex(gender) determination is not universal among all animals. Even when it does occur, there are common organisms such as birds in which the females are XX and the males XX
pgd created: 08/08/02 revised 02/21/05