Chapter 9 Patterns of Inheritance Part 2

Inheritance of Two Genes  How two genes get sorted into gametes depends, at least in part, on whether the two genes are on the same or different chromosomes.  Homologous chromosomes get separated during meiosis I when the pairs line up at the center of the cell during Metaphase I.  Each homologue of each pair becomes attached to opposite poles and then the pair is separated when the spindle fibers shorten.  Independent assortment refers to the fact which homologue lines and becomes attached to to which pole is entirely random.  Thus, when the homologous chromosomes separate, either homologue can end up in a particular nucleus.  This random sorting of genes on DIFFERENT chromosomes during meiosis means that genes on one chromosome are sorted into gametes independently of genes on the other chromosome.

Inheritance of Two Genes  But what about two genes on the SAME chromosome?  If two genes are very far apart on the same chromosome, crossing over occurs between them often enough that they are still assorted independently during meiosis, as if they were on entirely different chromosomes.  However, two genes that are very close together on the same chromosome do not assort independently because crossing over does not happen very frequently between them.  Thus gametes usually receive parental combinations of alleles of such genes.  Genes that are almost always inherited together due to the fact that they are very close together on the same chromosome so that crossing rarely happens to separate the two are called linked genes.

Inheritance of Two Genes  An example of this would be red hair and lighter skin color in humans.  When a child inherits the gene for red hair, the child also usually inherits the gene for lighter skin color, as well.  This combination of hair shade and skin color are usually inherited together due to the closeness of these two genes for these two traits on the same chromosome.

Inheritance of Two Genes  Consider the case of the family in Figure 9.8 on page 162.  Would you consider that this family’s case lends support to the idea of independent assortment or to the idea of linked genes? Give reasons for your answer.

Beyond Simple Dominance: More Complex Patterns of Inheritance  The examples that we have considered up to this point are examples of simple dominance, in which the dominant gene completely masks the expression of the recessive one.  However, as we will see, this is not always the case.  In some cases, both genes are expressed at the same time or, in other cases, several genes influence the same trait.  In still other cases, the expression of one gene can affect many different traits.

Beyond Simple Dominance: Codominance  Two non-identical alleles of a gene may be codominant, meaning that both alleles are fully expressed when inherited together and both are dominant over the recessive allele of that same gene.  An example would the alleles of the gene that codes for the carbohydrates on human red blood cells that give then their identity.  The enzyme encoded by the ABO gene modifies the carbohydrates on the red blood cell membrane.  Both the A and B alleles encode slightly different versions of the enzyme, resulting in a difference in the modification of the carbohydrates of the red blood cell.  The O allele has a mutation that prevents the enzyme from being active at all, resulting in no red blood cell membrane carbohydrates and no “identity” on your red blood cells.

Beyond Simple Dominance: Codominance  Every person carries two of the three ABO gene alleles.  The two alleles that you carry (your genotype) determine the form of carbohydrates on your red blood cells, which, in turn, determines your blood type (phenotype).  Since the A and B alleles are codominant, if you have both the A and B alleles, you have both versions of the enzyme and therefore, both versions of the modified carbohydrates on your red blood cells, making your blood type AB.  Since the O allele is recessive to both the A and B alleles, if you have an o with either or an A or a B, you will have only the A or B form of modified carbohydrates n your red blood cells, making you blood type A or B.  Only when neither the A or B allele is present will you have type O blood.  People who have type O blood are considered to be “universal donors” since their red blood cells have no “identity” so, therefore, the immune system (which would normally attack non-self cells) can’t recognize and attack these cells.

Beyond Simple Dominance: Codominance

 Why is Type AB blood considered to be the “universal recipient”?  A student who has type AB blood is learning about genetics in her Biology class. She learns from her parents that her mother has Type O blood and her father has Type AB blood. Upon revealing this information to their daughter, they also feel it is time that they told her that she was adopted as a baby. Why did the parents feel the need to finally reveal this information to their daughter?

Beyond Simple Dominance: Incomplete Dominance  In the case of incomplete dominance, one allele of a gene pair is not completely dominant over the other allele, resulting in the individual with a heterozygous genotype having a phenotype that appears to be a “blend” of the tow homozygous phenotypes.  An example of this would be the gene that influences the flower color of snapdragons.  The dominant allele codes for an enzyme that produces red pigment.  The recessive allele carries a mutation so that the enzyme produced is nonfunctional and so no pigment is produced at all.  Plants that are homozygous dominant produce lots of red pigment.  Plants that are homozygous recessive produce no pigment at all.  Plants that are heterozygous produce only a small amount of red pigment so that their flowers appear pink.

Beyond Simple Dominance: Incomplete Dominance

 Consider that the dominant form of the gene for fur color in rabbits causes black pigment to be produced. The recessive form carries a mutation, resulting in no pigment being produced at all, causing the rabbits fur to be white. If we consider that the fur color in these rabbits is an example of incomplete dominance, what results would you expect if a rabbit with black fur were bred with a rabbit with white fur?  How do these results compare with the results if the gene for black fur were completely dominant over the mutated recessive gene?

Beyond Simple Dominance: Epistasis  Some trait are affected by the products of many genes.  This phenomenon is responsible for what is called polygenic inheritance or epistasis.  Human skin color is an example of an epistatic trait.  The products of several different genes interact to produce the color of your skin.

Beyond Simple Dominance: Epistasis  Another example is the coat color of labrador retrievers.  Coat color of these dogs can be black, yellow, or brown.  One gene involved in coat color is responsible for the production of the pigment melanin by cells in the skin called melanocytes.  The dominant allele (B) of this gene specifies black fur while the recessive allele (b) specifies brown fur.  Another gene for coat color determines how much melanin will be deposited.  The dominant allele (E) for this gene causes the melanin to be deposited in the fur, while the recessive allele (e) reduces deposition of the melanin.  Combinations of these two genes result in either black, brown, or yellow fur in the puppies.

Beyond Simple Dominance: Epistasis Explain how/why the genotypes above cause the coat colors that they cause.

Beyond Simple Dominance: Pleiotropy  When one gene affects multiple traits, this is called pleiotropy.  Mutations in a gene like this result in complex genetic disorders such as sickle cell anemia, cystic fibrosis, and Marfan’s syndrome.  People with Marfan syndrome are tall, thin, and loose-jointed, but so are plenty of people who don’t have Marfan syndrome.  Therefore, Marfan syndrome is difficult to diagnose.

Beyond Simple Dominance: Pleiotropy  The pleiotropic gene involved in Marfan syndrome encodes the protein fibrillin. This protein fiber is responsible for the elasticity of many tissues in the body including the heart, skin, blood vessels, tendons, etc.  Mutations in the fibrillin gene result in the production of defective fibrillin or no filbrillin at all, resulting in reduced elasticity of tissues.  The largest blood vessel in the body, the aorta, is especially affected by this lack of elasticity.  This results in muscle cells in the thick wall of the aorta not working well as well as affecting the elasticity of the wall of the aorta itself.  Since the aorta should be elastic and expand under pressure, this lack of elasticity eventually results in the aorta becoming thin and leaky.  Calcium deposits can also accumulate in the aortic wall.  These things result in inflammation, thinning, and weakening of the aortic wall, which can then suddenly rupture (burst) during exercise.  This is exactly what happened to 21 year old basketball star Harris Charalambous during warm-up exercises in  Charalambous had undiagnosed Marfan syndrome.

Complex Variations In Traits  Most organic molecules are made in metabolic pathways that involve a number of enzymes.  Genes encoding these enzymes can mutate in any number of different ways, resulting in a “spectrum” of enzyme activity from excessive to none at all.  Add to this the fact that environmental factors can add even more variation.  In the end, you see a lot of variation in many traits from one extreme to the other due to complex interactions between gene products and the environment.

Complex Variations In Traits: Continuous Variation  When the individuals of a species have traits that vary within a small range, this is called continuous variation.  The more genes and environmental factors that influence the trait of the species, the more continuous the variation will be.  When a bar graph of the variations of the trait is created, if a line drawn across the top of the bars forms a bell curve, then we know that that trait varies continuously.  Examples of traits that vary continuously in humans include height, skin color, and eye color.

Complex Variations In Traits: Continuous Variation Eye Color

Complex Variations In Traits: Environmental Effects on Phenotypes  Variations in traits are not always caused by differences (mutations) in alleles, but can also be caused by environmental factors such as temperature, day length, or pH.  For example, seasonal changes in temperature and day length affect the production of melanin and other pigments that color the skin and fur of many animals.  These animals have different color phases in different seasons or different patches of color on areas of the body that have different temperatures. (Ex. Rabbits)  In another example, the pH of the soil can affect the phenotype of a plant.  The presence of predators can also affect the phenotype (expression of the genes) in many animals.  In these instances, the predators give off chemicals that trigger the change in phenotype of the individual. (Ex. Daphnia)

Complex Variations In Traits: Environmental Effects on Phenotypes Daphnia

Complex Variations In Traits: Environmental Effects on Phenotypes  Other environmental factors that can affect phenotype include altitude and depression (in humans).  Altitude has been found to affect the height of plants, such as Yarrow.  In one experiment, cuttings from the same plant (genetically identical) grew to different heights at different altitudes. Since all of the plants were genetically identical, genotypic differences were not the cause for the height differences, so it must have been the differences in altitude that were responsible for the height differences.  In humans, there is a genes that encodes for a protein that allows seratonin to cross the membrane of brain cells. When the seratonin crosses the brain cell membrane, it results in a lowering of anxiety and depression during traumatic times.  Mutations in the seratonin transporter gene reduces our ability to cope with stress by inhibiting the transport of seratonin across brain cell membranes. Only when we experience stress and/or emotional hardship in our environment do we experience the phenotypic effect of this mutation, depression.

Complex Variations In Traits: Environmental Effects on Phenotypes