Genetics: Patterns of Inheritence

How Does Sexual Reproduction Generate Variation Among Offspring?

So how do we explain the tremendous amount of variation in offspring produced by sexual reproduction?  First, it is important to know that the original source of variation occurs due to gene mutations.  This occurs when the genetic code itself is altered by changes in one or more nitrogenous bases within the DNA of a gene, as shown in this diagram.  If a mutation occurs within the DNA of a cell that undergoes meiosis, it will eventually be passed to the gametes and thus to the next generation.  Mutations are the major way in which variation is brought about in organisms that reproduce asexually.  Mutations are also important in sexually reproducing organisms, but represent only a minor source of the genetic variation.  In sexual reproduction, there are three additional sources of variation that result from gene shuffling and recombination within chromosomes.  We will now examine these three processes.

The third factor that produces variation among offspring is the process of fertilization.  When haploid gametes unite, the resulting diploid cell has received a chromosome set from each parent.  The processes of independent chromosome assortment and crossing over during meiosis have already produced a huge amount of variation among individual gametes.  Now, two of these gametes are randomly selected to create a new individual.  The combination of the two chromosome sets can interact in different ways to increase variation among offspring.

In animals, the gametes that fuse at fertilization are an egg and a sperm.  A human egg and sperm are shown here during an in vitro fertilization process

The process of evolution requires genetic variation within a population so that natural selection can act.  And of course, it is also necessary that the “selected” individuals pass their genetic traits to the next generation.  Darwin understood that heritable variation is what makes evolution possible, but he could not explain the source of the variation.  Another biologist, Gregor Mendel worked at the same time as Darwin and published a theory of inheritance that gave insight into this question.  Unfortunately, Mendel’s work was not appreciated until 1900, after both men were dead.  In the next topic, you will learn how Mendel discovered the basic rules for inheritance of genetic traits.

How Are Genes Inherited?

Gregor Mendel
Before the work of Mendel (1822-1884), heredity was poorly understood.  For example, one explanation could be “blending” in which characteristics of each parent would be mixed together in the offspring like red and yellow paint making orange.  We now know that this is not true.  If it were, all members of a population would look the same after many generations.  Instead, characteristics are inherited as discrete units, which we now call genes.  Genes, while not always expressed, are passed along unchanged from generation to generation.  Gregor Mendel was the first to use the experimental method to elucidate the principles of heredity—a branch of biology that we now call genetics.  Mendel was an Austrian monk with prior training in the experimental method and an interest in plant heredity.  He performed experiments with pea plants in the abbey garden and used mathematics to analyze the results.  As we describe some of Mendel’s experiments, you will see how they led to our modern concepts of homologous chromosomes, independent assortment during meiosis, and dominant vs. recessive genes.

Genetic Terminology
From Mendel’s experiments, you know the meaning of dominant and recessive genes.  This illustration shows some additional terms.  The physical traits of an organism are its phenotype.  Thus the phenotype of pea plant flowers can be either purple or white.  The genes that an organism possesses are its genotype.  In the case of the pea plants, the genotype for flower color can be PP, Pp or pp depending on which alleles for flower color are present.   When the alleles for a characteristic are the same, the genotype is homozygous for that characteristic (PP or pp in the examples here).  If the alleles are different, the genotype is heterozygous.  Thus a hybrid organism is always heterozygous for the hybrid trait.

Remember that one allele came from each parent.  This is because one chromosome of each homologous pair is contributed by each parent during fertilization.  An allele represents a single gene on a chromosome.  Here we see a pair of homologous chromosomes that bear the gene for flower color.  Note that the alleles for flower color occupy the same position on each chromosome.  This spot is called the gene locus.

Mendel’s Laws
Here are the two major statements that arise from Mendel’s work.  Both the law of segregation and the law of independent assortment should be familiar to you, since they are consequences of meiosis.

The law of segregation simply means that when a diploid cell undergoes meiosis, each gamete receives one chromosome of each homologous pair and thus only one allele for a given gene.  The diagram illustrates a cross between two pea plants:  one is homozygous for yellow seeds, the dominant gene, and the other is homozygous for green seeds, the recessive gene. When meiosis occurs all gametes from one parent contain a yellow allele and all from the other parent a green allele.  Thus all F1 offspring are hybrids with yellow seeds.

Segregation is best demonstrated when 2 hybrid plants are crossed.  Now each gamete will contain either an allele for yellow or an allele for green.  In other words, alleles from the Yy cells are segregated during meiosis so that each gamete contains only one allele for seed color.  As you know by now, this type of cross produces a 3:1 phenotypic ratio in offspring as random gametes participate in fertilization.  You already know that homologous chromosomes assort independently during meiosis.  So if the pea plant shown here was also a hybrid for flower color, the offspring would have both purple and white flowers, but if flower color and seed color are on different chromosomes, each plant could have either seed color associated with either flower color.  This is what the law of independent assortment means.

Punnett Squares
You have seen several examples of how Punnett squares can be used to visualize genetic crosses. If you are not sure how to set up a Punnett square or how to interpret the results that it produces, play this video.

We are going to cross two tall plants that are both heterozygous for height.  Big T is the allele for tall, which is dominant, and little t is dwarf, which is recessive.

Let’s set up the genotypes for the cross.  Each parent has big T and a little t in its genotype, although both plants have the tall phenotype. Since we are looking at just one trait and are crossing hybrid plants, this is called a monohybrid cross.

Now we are going to produce the gametes for each parent.  We will put the gametes from parent 1 along the side of the Punnett square and the gametes from parent 2 along the top. 

Now we will perform all the possible combinations of gametes that could occur during fertilization.  To do this, we just fill in each box with the gamete in its row and column.

Now the Punnett square is finished.  There are 4 zygotes that can be produced from this cross.  Three of them have at least one big T and so will have the tall phenotype.  The fourth zygote has only small t’s and will be a dwarf plant.

Genetics and Probability
The results of a genetic cross are determined by the rules of probability.  In the monohybrid cross shown here, probability tells us that half of the sperm and half of the eggs will contain the dominant allele (R) and half will contain the recessive allele (r).  Each time a zygote is formed, it has a 50% chance of receiving big R and a 50% chance of receiving little r from each gamete--just like flipping a coin.  Thus the chance of any of the 4 genotypes shown in the Punnett square being formed during fertilization is 1/2 x 1/2 = 1/4.  Since there are 3 ways that the genotype can give a dominant phenotype, but only one way that a recessive phenotype can result, the ratio of dominant to recessive phenotypes is 3:1.

What Are Some Complexities of Genetics?

When Mendel performed his breeding experiments on pea plants, he happened to pick characteristics in which the alleles were completely dominant or completely recessive, relative to one another.  There are also cases in other organisms where this is true.  For example, the dark hair color of mammals is due to a dominant gene, whereas white color is recessive.  We now must ask how a genotype is expressed as a phenotype.  Since genes code for proteins, it is the specific protein that determines phenotype.  Most cellular proteins are enzymes; hair color requires the action of a specific enzyme to produce brown-black pigment.  Most recessive genes have been altered in such a way that their protein product is not made or is defective.  In the case of white hair color, the recessive allele has been mutated and does not produce the enzyme required to make pigment.  The bear cub shown here is white and thus homozygous for the recessive gene.  His black mother must be heterozygous since she passed on a recessive gene to her cub.  One normal allele in the mother bear is sufficient for expression of the full black color, because enough enzyme is made to produce the required pigment.  

Here we see an example of complete dominance from Mendel’s experiments in which the allele for round seeds is dominant and the allele for wrinkled seeds is recessive.  What causes the homozygous recessive seeds to wrinkle?  The dominant allele codes for an enzyme that converts sugar within the seed to starch, but the recessive allele makes a defective form of the enzyme.  As a result, sugar accumulates in the seed and water is drawn in by osmosis causing the seed to swell.  When the seed matures and dries out it wrinkles.  Again, one dominant gene produces sufficient enzyme, so the heterozygous seed is round and smooth

Degrees of Dominance
In many genes, one allele is not completely dominant over another.  Instead, there is a spectrum of dominance with complete dominance at one end and co-dominance at the other.  In terms of phenotype, an example of complete dominance is seen in Mendel’s peas where plants heterozygous for purple and white flower color are all dark purple.  At the other extreme, both alleles in a heterozygous organism are expressed and affect the phenotype in separate, distinguishable ways.  If heterozygous pea plants were like this, their flowers might be purple with white spots.  Between the extremes of complete dominance and co-dominance are various degrees of incomplete dominance in which the phenotype lies somewhere between the two parental varieties.  If pea plants showed incomplete dominance for flower color, their heterozygous flowers would be pale purple.  We will next examine the underlying basis for incomplete dominance and co-dominance.

There are only a few good examples of co-dominance, where both alleles of a gene are expressed.  A classic example is the roan coat color found in cattle and horses.  In the example shown here, horses homozygous for red color have reddish brown hair.  Horses homozygous for the white allele have a white coat.  In horses heterozygous for these alleles, the coat is a mixture of red and white hairs giving the red-roan color.  Note that the individual hairs are not pink as they would be in incomplete dominance.  Instead, the coat contains both completely red and completely white hairs. 

If two roans from the F1 generation are crossed, the F2 generation contains all three phenotypes.  Co-dominance is usually the result of previous mutations in both alleles that caused them to differ slightly from one another while retaining full function.

Levels of Organization
Phenotype can be defined at several levels of organization and the spectrum of dominance of alleles may differ at each level.  An example is Tay Sachs disease.  This is a genetic disease found in children homozygous for a recessive gene that influences lipid metabolism.  The normal allele codes for an enzyme required to metabolize certain lipids.  The other allele produces a defective form of the enzyme.  The child shown here has Tay Sachs disease and has severe neurological symptoms due to accumulation of lipid within brain cells.  Children with this disease die within a few years of birth.

If the phenotype of the child is examined at the organismic level, complete dominance is observed.  Heterozygous parents are completely normal, but carry a defective allele.  Their progeny have a 1 in 4 chance of receiving two defective alleles, in which case they will have Tay Sachs disease.

But what if we look at the cellular level and measure activity of the lipid-metabolizing enzyme?  In this case the heterozygous individuals are not completely normal since they have enzyme activity intermediate between the two homozygous conditions.  This is the definition of incomplete dominance.  Fortunately for these individuals the enzyme activity is sufficient to prevent the symptoms of Tay Sachs.

We can also examine gene expression at the molecular level by measuring the number of proteins produced by each gene.  Now we find that the heterozygous individuals have an equal number of normal vs. defective enzyme molecules.  This is the definition of co-dominance, since the products of both alleles are present within the cell.  So whether alleles appear to be completely dominant, incompletely dominant, or co-dominant relative to each other can depend on which phenotypic trait is considered.

Multiple Alleles
Another complexity of genetics is the presence of multiple alleles for a given characteristic.  A good example of this is the human blood type.  Blood type is determined by antigens, specific glycoproteins, which extend from the surface of red blood cells. There are 3 alleles for this gene.  Allele A produces antigen A, allele B produces antigen B, and the third allele does not produce any antigens.  This gives 4 possible blood types, as shown here.  Note that if one allele is A and the other is B, the blood type is AB.  Since both antigens are present, this is also a good example of co-dominance.  If neither antigen is present, the blood type is called O. 

Here we see the possible genotypes for each of the 4 phenotypes.  Superscripts indicate if the allele is for antigen A or antigen B, and small “i” is the allele that does not produce an antigen.  Multiple allelic forms of a gene, rather than just two alleles, is quite common.  How many allelic variations can a gene have? To give an extreme example, a gene that is involved in eye development (called Pax6 ) has 302 known alleles.

Environmental Factors
Finally, it is important to remember that environment also plays a role in determining the phenotype of some traits.  In the previous examples of polygenic inheritance, exposure to the sun would also influence skin tone, and poor nutrition during childhood would reduce growth and thus alter individual height.  In the example shown here, the color of hydrangea flowers is determined by soil conditions.  These two flowers have similar genotypes, but the blue flower was grown on soil more acidic than the pink flower.