Mechanisms of Heredity

I.  Overview
    Have you ever wondered why offspring look like their parents?  The answer - genetics.  During the next two classes we will study how this process works.

II.  Sex determination - a review
    Let's start with sex!  

    Sex chromosomes are those that carry important information for determining sex.  There are two sex chromosomes in mammals, X and Y.  The X chromosome, which is much larger than the Y, has a centrally-positioned centromere. All other chromosomes are termed autosomes.  Humans have 46 total chromosomes:  44 autosomes and 2 sex chromosomes.  Or, 22 pairs (homologous chromosomes) of autosomes and 1 pair of sex chromosomes.  The sex chromosomes are homologous to one another. 

    Normal females have two XX chromosomes, normal males have an X and Y chromosome.  Nettie Sloan first discovered this is 1900.  Thus, after meiosis, all female eggs will have one X chromosome, whereas, 50% of sperms will carry the X chromosome and 50% the Y chromosome (see diagram in class).  When sperm and egg recombine, there is a 50% chance of having a female, and 50% chance of having a male.  We can set up a Punnet square to represent this situation.   

    Thus, there is 1:1 ratio of male to female offspring or 1 in 2 (1/2) chance of having a boy or girl or 50% chance of having a girl or boy.  In practice slightly more males are conceived, possibly because the Y carrying sperm swim faster.

    Folk tales state that the male determines the sex of the child.  This is true only in the sense that sperms come in two "flavors" (X and Y), whereas eggs in only one (X).  Thus, depending upon which sperm fertilizes the egg, that will determine the sex of the offspring.

III.  Nondisjunctions
     Non-disjunction at meiosis (or mitosis) is one cause of unusual combinations of chromosomes (a cell/individual with odd numbers of chromosomes is termed aneuploid).  Nondisjunction is the failure of chromosomes to properly segregate during meiosis.  In other words, it is a violation of the Law of Segregation.  Nondisjunction can occur at meiosis I or II.  (see diagrams).

    Common aneuploids include:  XXX - triple X syndrome, female, 47; XXY - Kleinfelters syndrome, male, 47, (frequency about 1 in 1000 live births); XYY - male, 47; XO - Turner's syndrome, female, 45 (about 1 in 5000 live births); Down's syndrome (trisomy 21; frequency increases with maternal age).  

    Cells can tolerate imbalances of sex chromosomes better than imbalances of autosomes.  With the exception of Down's syndrome and a couple of others like Edwards syndrome (trisomy 18) or Patau's (trisomy 13), there are few live births of autosomal aneuploids and individuals are severely afflicted. Why are cells able to tolerate extra sex chromsomes?  Because....

1. Extra X chromosomes are inactivated so they don't "mess things up".  Mary Lyon first observed that in normal females only one X chromosome remains active, any additional ones are inactivated during development (after about 10 embryonic divisions).  The inactivated X chromosome forms a small dark staining spot in nucleus, called a Barr body.  This forms the basis for the sex test.  Thus, only one remains active, the others inactivate as Barr bodies.  Barr body number:  XX - 1 Barr body; XXX - 2 barr bodies; XXY - 1 Barr body; XYY - no Barr body; XY - no Barr body. (Note:  the chromosome that is inactivated is at random.); and 
   

2. Y chromosomes are small and carry comparatively little genetic information that will "mess things up". 

IV.  Karyotypes
    A karyotype is used to determine the chromosome composition of an individual. White blood cells are typically used (red cells lack a nucleus), they are treated with colchicine which prevents the formation of microtubules.  Thus, cell division stops at metaphase, allowing a good view of the spread chromosomes.  The chromosomes are stained, photographed and arranged from large to small, matching homologs (see exercise).

    Some parents want to know if the offspring they are carrying is "normal."  Amniocentesis (removal of amniotic fluid containing fetal cells is obtained) or chorionic villi biopsy (remove some of fringe around embryo) permit sampling of fetal tissue to determine karyotype and other genetic conditions. 

V.  Sex-Linked Genes

A. Coat color in cats: X-linked trait
    Let's talk about cats, specifically calico cats which are orange, black and white.  The coat color gene is carried on the X chromosome.  Thus this gene is called X-linked (or sex-linked for a more general term).  There are two forms of expression (called alleles) for this trait - X^O and X^B.  The X^O allele codes for orange pigmentation and the X^B for black spots.  If both alleles of a pair are the same, we call it homozygous; if they are different, heterozygous.  Thus, in a diploid cell there are three possible combinations of alleles:  X^O X^O, X^O X^B, and X^B X^B.  Cats with these combinations appear:  white with orange spots, white with black and orange spots (calico), and white with black spots, respectively.  The actual genetic composition is called the genotype, and the expression of the genes is the phenotype.  Thus, calico is the phenotype for a cat with the genotype X^O X^B. 

    Note that so far we have only considered female cats.  Male cats can have the genotype X^B Y  or X^OY.  The phenotype of the first would be white with black spots, the second will have orange spots.  Thus, all calico cats are female!  (there is the rare sterile male with the genotype X^O X^B Y). 

    As an aside, recall that the X chromosome that becomes inactive as a Barr body does so randomly.  This explains why calico cats have random splotches of black or orange, depending upon which chromosome carrying which allele was inactivated in the cell line that gave rise to that part of the cat. Also note that these alleles are codominant; that is, both are expressed in the heterozygote.  In many cases, one allele (dominant) dominates over the expression of the other (recessive).  Thus, heterozygotes will express the dominant trait.  

B.  X-linked traits in humans
    These include hemophilia, colorblindness (recessive), muscular dystrophy (recessive), Lesch-Nyhan (recessive), and hypophosphatemia (dominant).

    Let's use hemophilia, a disorder where the blood fails to properly clot, as a model.  There are two alleles for this trait H - normal and h - hemophilia.  Thus, there are five possible genotypes (and phenotypes):  X^H X^H(normal female), X^H X^h (normal female, carrier of hemophilia), X^h X^h (female hemophiliac),  X^H Y(normal male), X^h Y (hemophiliac male).  Note that in the heterozygous condition, that normal condition dominates over the expression of hemophilia.  Thus the normal allele is dominant to the hemophilia allele which is recessive.  For recessive traits to appear in the phenotype, they need to be present in the homozygous condition.  Dominant traits are expressed in the homozygous or heterozygous state.

     Because females must receive two recessive alleles to express the trait, X-linked traits are usually rarer in females than males. 

C.  Y-linked traits in humans
    There aren't too many.  These traits would obviously be passed from father to son.  Hairy ears and testosterone production (SRY gene) are two genes located on the Y chromosome.

VI.  Autosomal Traits in Humans
    So far we have only discussed genes that are carried on the sex chromosomes.  Obviously, the autosomes carry genes, too.  In fact, the majority of the genes are carried on the autosomes.  

    There are a variety of traits in humans that seem to inherit as though they are controlled by a single gene (for a listing click here). As you study this list and complete the handout, remember that each trait is specified by a single gene that is found on one of the 22 pairs of homologous autosomes.  Each homolog is carrying one allele for the gene.  The two alleles may be the same (homozygous) or different (heterozygous).  One allele may dominate (dominant) over the expression of the other (recessive).

    Geneticists use one or three letter abbreviations to represent a gene.  Thus we used the letter "H" to represent the gene for blood clotting above.  In the case where one allele is dominant over another, the dominant allele is expressed with capital letters and the recessive allele with lower case (for blood clotting example:  H = normal clotting; h = hemophilia)..  

    Let's give another example.  If we are interested in the tongue-rolling gene, we could symbolize it with the letter "R".  We would use a capital "R" to represent the dominant allele and a lowercase "r" to represent the recessive allele.  If neither allele is dominant, different letters can be used (as in the calico cat example) or perhaps the prime (') symbol.  For example, we could express the two alleles as R and R'.

    Since these alleles are not found on the sex chromosome and their expression is unrelated to the sex of the individual, we don't need to include the designation X or Y when writing the genotypes of the autosomes.  Thus, tongue rolling is symbolized RR or Rr or rr not X^RX^r or X^rX^r or X^RY or X^rY.

    Another symbol commonly used is "+" to represent the "wild type" or normal condition.  Then, the "mutant" condition would be represented with a letter symbol.

Some take-home-lessons from the Single Gene Handout:  

1. Dominant traits are not necessarily the most common ones; 

2. You may not have enough information to determine your genotype for many traits - those that you show the dominant phenotype.

3. There are two ways to determine your genotype of an individual that show the dominant phenotype:
   
   (a) Pedigree analysis - study the parents.  If one parent shows the recessive trait, then the offspring must be heterozygous; and
    
   (b) Perform a Test Cross - mate an individual with the dominant phenotype to an individual that is homozygous recessive.   If the recessive phenotype occurs in the offspring, then the individual must have been heterozygous.  Obviously this is not possible with humans, but is commonly used for other organisms.

VII.  Solving Genetics Problems
    Now you are almost ready to solve some genetics problems involving a single gene - called monohybrid crosses.  Here are a few additional tips:

1. List the information that you  know, i.e., how many genes are involved, how many alleles are involved, what are the alleles, do the alleles show dominance/recessive or codominance? do you know the genotypes of parents or offspring?; 
    

2. What is the ratio of the offspring.  If the offspring exhibit a 3:1 ratio then the parent's must be heterozygous.  A 1:1 ratio indicates that one parent is homozygous recessive and the other is heterozygous.  A 1:2:1 ratio of phenotypes indicates that the alleles exhibit incomplete dominance as in snapdragon flower color. A 9:3:3:1 ratio of offspring indicates a mating between two parents heterozygous for two genes. A 1:1:1:1 ratio indicates a mating between two parents, one heterozygous and the other homozygous recessive for two traits.
    

3. Rule of Multiplication - to determine the probability of two unrelated events occurring, multiply the probability of the individual events.  Consider a couple who want two children.  

• What is the probability that child #1 is a girl?      Answer = 50% or 0.5  

• What is the probability that child #2 is a girl?      Answer = 50% or 0.5

• What is the probability that both children will be girls?  Answer = since there are independent events, the probability is the product of their individual probabilities:  0.5 x 0.5 = 0.25 = 25% or 1/4 

4. Rule of Addition - to determine the probability of an event occurring in two or more different ways, add the probabilities of the individual events.  What is the probability that one of the children will be a girl and the other a boy?  answer - the probability of the first child being a boy and the second child being a girl is 0.25 (0.5 x 0.5); and the probability of the first child being a girl and the second child a boy is 0.25.  Thus the probability of having a girl and a boy, in either order, is 0.25 + 0.25 = 0.5 = 50%  or 1/2. 
    

VIII.  Genetics Problems
    Check out the problems at the end of the chapter or provided on-line. The more problems you work, the better you will understand it.  We'll work some together in class.

IX.  Lessons From a Monk:  Gregor Mendel 
    Called the "Father" of genetics.  Normally, I would use gender-inclusive language and call such a person the "Founder" of Genetics.  In this case, "Father" is correct because Mendel was monk in the monastery of St Thomas, Brunn (now Brno) near Vienna.  He was born in 1822.  He published the first work on genetics in 1865 called "On the Nature of Inheritance".  He studied the inheritance of traits in peas. 

    Peas were a terrific choice because:  

1. there were lots of hybrids that had previously been studied; 

2. there were true breeding varieties available (i.e., when allowed to self pollinate or mated with another individual of the same type, they produced more of the same variety); 

3. they are small, easy to grow and have a reasonably short life cycle; 

4. the flowers are good for breeding studies (readily self pollinate, easy to remove stamens for cross pollination studies).

    Mendel's work was ignored for 35 years, when it was simultaneously "discovered" by three botanists (Tschermak, Correns, de Vries).  It was ignored because it was:  

1. too sophisticated using brilliant, carefully controlled experiments (he was a man ahead of his times); 

2. too much math/stats for people to understand; 

3. published in an obscure journal; 

4. Mendel was not in the scientific mainstream; and

5. he stated his ideas poorly (a good reason to do well in Symposium!).

    Mendel studied 7 different traits (genes) each with two forms of expression (alleles)  These are 

    These genes are located on 4 of the 7 pairs of homologous chromosomes (2n=14).  Flower color and seed color are on chromosome 1; flower position, pod shape, and plant height are on chromosome 4; pod color on chromosome 5; and seed shape on chromosome 7.

So, what were Mendel's actual contributions? He determined:  

1. Hereditary information is transmitted in particles or units (that we now call genes).  [A gene specifies a particular trait or feature of an individual.  The phenotype is the outward expression of the genes and the genotype refers to the specific genetic makeup that translates into the phenotype]; 

2. There are two factors (allele) for every trait in a diploid individual (on homologous chromosomes) [An allele is the form of expression of a gene]; 

3. The two alleles may be the same (homozygous) or different (heterozygous); 

4. Alleles don't blend their expression, like mixing a can of paint.  This seems obvious now, but the dominant paradigm of the day was that alleles blended.  For example cross a red flower with a white flower to get pink which becomes would become diluted more and more in subsequent crosses.  In contrast, if alleles are discrete, then mix red and white and get pink, but mix pink and pink and you could get back red and white; 

5. One unit from every pair moves into gametes during meiosis (Law of Segregation).  In other words, homologous chromosomes separate during meiosis; 

6. The expression of one allele (dominant) usually predominates over the other (recessive);

7. Genes act independently of one another (i.e., Law of Independent Assortment, homologous chromosomes separate randomly in meiosis).
    

X. Multiple Alleles
    This is the situation where there are more than two alleles at a locus responsible for a particular trait.  (versus two alleles for all the traits that we have discussed so far).  A good example is the ABO blood groups in humans.   

    Three alleles are responsible for ABO blood type.  These are:  

• I^A - this allele codes for A antigens on the cell surface of the red blood cells (an antigen is a substance that stimulates the production of an antibody.  Specifically, this gene causes galactosamines to be attached to lipids on the cell surface);  

• I^B - specifies B antigens on rbc (adds galactose to cell surface lipids); and 

• i (or O) -  no antigens on cell surface.

    Thus, an individual can have the following phenotypes/genotypes:

Note:  These alleles are codominant (one does NOT dominant over the expression of the other).  In other words, in the heterozygote, both are expressed in the phenotype.  As an aside, if you have type A blood, you have antibodies against type B antigens.  Antibodies react with the antigens that stimulated its production to remove it from the body.  Individuals with type B blood have antibodies against type A antigens, type O blood has antigens against both A and B antigens and individuals with AB blood have no antibodies.  Thus, type O blood is the universal donor (since it has no antigens on rbc, it won't be attacked by either antibody) and type AB is the universal recipient (because it has no antibodies, won't react with either A or B antigens).

Genetics of blood types:  Crosses work just like others described above.  For example:    

1. Consider a mother with type AB blood and a father with type AB.  What progeny and in what proportions do you expect?  Answer:  1/4 type A, 1/4 type B, 1/2 type AB.

2. Consider two parents, one type A and the other type B.  Can they have a child with type A blood?  type B?  type AB?  type O?  Answers:  It depends on the actual genotypes of the parents.   

3. Consider a family in which the father has type B blood, the child type A blood and the mother type O blood.  Dad says he's not the father of the child.  What do you conclude?  Answer:  Dad is right.  Since we know the mother of the child has type O blood the child must have at least one "i" allele.  Thus, the other allele is "A".  Since the father is type B, he could not have contributed the A allele, and therefore, is not the father.  Note:  blood genetics can confirm a person is NOT a parent, but cannot prove that a person IS the parent 

Rh Antigens 
    These are another type of antigen found on red blood cells.  They were first recognized in rhesus monkeys, hence the abbreviation.  Either your rbc cells have the antigens (RH+) or not (Rh-).  Rh positive individuals never produce antibodies against Rh antigens.  Rh negative individuals don't normally produce antigens but can be induced to do so if exposed to Rh+ blood (as might happen in a transfusion, which is unlikely in modern medicine, or it could easily happen if an Rh- mom gives birth to an Rh+ child).  In the latter case, the mother is given a shot of rhogam soon after delivery to scavenge up the Rh+ antigens to prevent her immune system from producing Rh antibodies that might affect subsequent children.  

Last updated: July 14, 2009     � Copyright by SG Saupe