PRINCIPLES OF HEREDITY

Sources:Scott/Forseman, Biology, 1980, Knowles Genetics, Society and Decisions, 1985, http://www.cc.ndsu.nodak.edu/instruct/mcclean/plsc431/mendel/, http://www.people.virginia.edu, Kimball's online Biology,

 

5.1 Genetics and Evolution

a. Explain the inheritance of traits which are determined by one or more genes including dominance, recessiveness, sex linkage, phenotypes, genotypes and incomplete dominance
b. Solve problems that illustrate monohybrid and dihybrid crosses

c. Compare sexual and asexual reproduction
d. Explain how the coding of DNA (deoxyribonucleic acid) controls the expression of traits by genes
e. Define mutations and explain their types and causes
f. Explain the process of DNA replication
g. Describe evidence, past and present, that supports the theory of evolution, including diagramming relationships that demonstrate shared characteristics of fossil and living organisms
h. Explain the theory of natural selection including adaptation, speciation, and extinction
i. List major events that affected the evolution of life on Earth (e.g. climate changes, asteroid impacts)

1) Explain the inheritance of traits which are determined by one or more genes including dominance, recessiveness, sex linkage, phenotypes, genotypes and incomplete dominance

Genetics is based on sexual reproduction (meiosis). See below for a comparison between sexual and asexual reproduction.

 

To help you understand the concept, study the animation below. The red flower genes (pair) are referred to as "Red Factors" as visualized by Mendel. The animation involves a Punnet square, a system to determine the potential inherited characteristics of the offspring plants. Experimentally, each square represents a cross between the emasculated plant 1 (top of Punnet square) with the male sex cells of plant 2 (left side of Punnet square).

To begin the animation, click on the green circle at the upper right of the animation. To stop the animation (while it is running) click on stop (the red circle). To rewind the animation by 5 frames, click on the yellow left arrow.

The squares labeled offspring 1, 2, 3 and 4 represent separate plants produced in the crosses performed by Mendel.

 

 

Likewise, offspring who receive the trait from each parent of white flower color will be a pure strain of white flowered plants. The Punnet square animation below proceeds automatically without stopping. Click on "stop" (red circle) at any time to stop the animation. Clicking the green circle will restart the animation.

 

 

 

 

NOTICE: EACH OF THE OFFSPRING HAVE RECEIVED TWO WHITE FACTORS (GENES). THE FLOWER COLOR OF ALL THE OFFSPRING PLANTS OF THESE CROSSES WILL ALSO BE RED (PUREBRED WHITE).

Therefore, offspring plants that inherit a gene from a white-colored flower parent and a red-colored flower parent will be mixed, or hybrid for flower color. Click on the green arrow to begin the animation below. Like the first one, it will automatically stop four times. Click on the green circle to continue the animation.

 

NOTICE: EACH OF THE OFFSPRING HAVE RECEIVED ONE WHITE FACTOR (GENE) FROM THE PUREBRED WHITE-COLORED FLOWER AND ONE RED FACTOR (GENE) FROM THE PUREBRED RED-COLORED FLOWER. THE OFFSPRING ARE HYBRID PLANTS. WHAT WILL THE FLOWER COLOR OF ALL THE HYBRID PLANTS BE?

DID YOU GUESS PINK? Good guess, but incorrect.

ALL OF THE FLOWERS OF THE OFFSPRING in the first generation (from the original cross) (F1 generation) WERE RED!!

 

 

 

NOTICE: Genotype: One homozygous recessive (purebred white), one homozygous dominant (purebred red) and two heterozygous genotypes (hybrid red).

Phenotype: 3 red: 1 white (ratio 3:1)

 

 

 

 

2. Solve problems that illustrate monohybrid crosses- read material below carefully. Click on the link ("Monohybrid cross: Tall and Short Pea Plants") when you get to it for monohybrid cross practice.

During meiosis, the parental genotypes are segregated to the sex cells (parental gametes).

We will review what we have learned in the study performed by Mendel where he tracked the single trait of plant height.

Choose Symbols: Tall plant= D; Dwarf plant=d

The cross between a homozygous dominant tall plant and a homozygous recessive dwarf (small) plant is shown by CLICKING ON "MONOHYBRID CROSS-TALL AND SHORT PEA PLANTS"BELOW. Note how similar it will be to flower color.

Monohybrid Cross-tall and short pea plants

In the data, note how the 3:1 ratio exists with literally all traits (genes or alleles)

 

Codominant- click on link to the left

Codominance (as described in the link above) exists, not only for flower color in snapdragons but also for many human proteins. The proteins produced by BOTH alleles are expressed.

For example, alpha1-antitrypsin is a protein present in the fluids and blood serum of the body. Its purpose is to prohibit the destruction of proteins by other enzyme-"eating" proteins. Each human has two alleles or genes for each trait.

Homozygous normal is MM. In this situation the person has large quantities of this protease inhibitor(alpha1-antitrypsin). Protease is the general name for a protein-degrading protein. Human alpha1-antitrypsin moves through a gel across an electric current at a medium speed. Homozygous abnormal is ZZ. This person has extremely low levels of alpha1-antitrypsin. This person has hardly any protease inhibitor and is at risk of getting emphysema. The protein in this person moves VERY slowly in the same electric field, more slowly than any other genetic form of the inhibitor (thus the use of the last letter of the alphabet).

If an MM male and a ZZ female get married, all their children will have an MZ genotype (Check it out using the Punnet Square). Both proteins (intermediate and slow migrating) will be seen...they both are expressed. In other words, they are both dominant.

 

CLICK ON "PEDIGREE CONSIDERATIONS"BELOW

Pedigree Considerations-Click here: A dominant gene or allele is found in each generation, however, a recessive gene or allele skips one generation. Think of the pea flower examples: RR x RR = RR, (the original parents were white and red. In the F1 generation, there were no white).

F1 cross: RR x RR = RR, RR, RR, RR (F1 parents were only red and white showed up again in the F2 generation

 

2. Solve problems that illustrate dihybrid crosses- read material below carefully. Click on the link ("Mendel's Second Law: Law of Independent Assortment"- deals with color and smoothness of seeds) when you get to it for dihybrid cross practice.

During meiosis, the parental genotypes are segregated to the sex cells (parental gametes). The F1 genotype will also separate when the sex cells are formed as shown in the drawing to the left.

We will review what we have learned in the study performed by Mendel where he tracked two traits at one time, shape and color of the pea seeds.

The genes for yellow smooth or yellow wrinkled seeds

 

and the genes for green smooth or green wrinkled seeds

of the pea plants follow the same principles as described above and the Laws described below. Mammalian genetics also follows similar principles.

Choose Symbol Seed Color:

Yellow = G; Green = g (Yellow is dominant, green is recessive)

Seed Shape: Round = W; Wrinkled = w (Round is dominant and wrinkled is recessive)

The dominance relationship between alleles for each trait was already known to Mendel when he made this cross. The purpose of the dihybrid cross was to determine if any relationship existed between different allelic pairs. Let's now look at the cross using our gene symbols. This will introduce us to the second Law of Mendel.

 

CLICK ON "MENDELS SECOND LAW-LAW OF INDEPENDENT ASSORTMENT"BELOW

 

Chromosome Theory of Heredity

What are chromosomes?

Detailed Discussion: Chromosomes

CHROMOSOME THEORY OF HEREDITY
MENDELIAN THEORY OF HEREDITY
1. Chromosomes occur in pairs 1. Mendelian factors occur in pairs

2. Paired chromosomes separate during meiosis. Each sex cell (gamete) receives only one chromosome of the pair.

2. Mendel's paired factors separate during meiosis. Each sex cell (gamete) receives only 1 factor of the pair.
3. Chromosome pairs separate independently of other of other chromosome pairs during meiosis. 3. Mendel's factor pairs separate independently of other factor pairs.
4. Individual chromosomes return to their original paired numbers in the fertilized egg. 4. Mendel's individual factors become paired factors in the offspring.

 

 

 

CLICK ON "SUMMARY DESCRIPTION AND ANIMATION FOR CROSSING OVER DURING MEIOSIS" below- Click on the "Refresh" option on the top of the Internet browser of the new window to see the animation once again.

CLICK ON "DETAILED DISCUSSION:CROSSING OVER" BELOW TO LEARN ABOUT CROSSING OVER

CLICK ON "AUTOSOMES VS SEX CHROMOSOMES/ABNORMALITIES IN SEX CHROMOSOMES" BELOW

 

The inheritance of abnormal genes found on the 22 pairs of human chromosomes (autosomes) obey the principles of Mendelian genetics. The inheritance of abnormal genes on the X-chromosomes also are based on Mendel's principles.

Gender issues complicate this process only in regard to the Y chromosome. Males have only one X chromosome whereas females have a pair of X-chromosomes. The expression of genetic abnormalities in the offspring of affected people takes on a new dimension when the Y-chromosome is included in the "equation". SEE BELOW

 

SEX-LINKED RECESSIVE INHERITANCE OF A GENETIC DISORDER (F1 GENERATION)

XY = male; XX= Female; The red superscript "a" on the female X chromosome represents an abnormal gene on that particular chromosome. The lower case "a" means it is a recessive gene (allele).

Click "go" until all the possible crosses (genotypes) of the F1 Generation are represented.

FROM THE DATA

There is a 50% chance that the couple will have a girl or a boy.

There is a 50% chance that a daughter will be born who is a carrier for the trait (does not have the effects of the genetic abnormality...just carries the abnormal gene). (REMEMBER: THE FEMALE HAS TWO X-CHROMOSOMES. THE OTHER CHROMOSOME HAS A NORMAL GENE- DOMINANT. IN A HETEROZYGOUS SITUATION, THE RECESSIVE GENE IS SUPPRESSED AND THE PERSON IS NORMAL BUT IS A CARRIER)

There is 50% chance the couple will give birth to a boy with the genetic abnormality (NOTE: A RECESSIVE TRAIT IS EXPRESSED ONLY IN MALES IN SEX-LINKED INHERITANCE BECAUSE OF THE ABSENCE OF A DOMINANT GENE. THE GENE FOR THIS TRAIT IS NOT PRESENT ON THE Y-CHROMOSOME. AS A RESULT, IT ACTS AS IF IT WERE A HOMOZYGOUS RECESSIVE WITH THE MALE EXPRESSING THE GENETIC ABNORMALITY). In the female, the extra X-chromosome produces the normal product and compensates for the chromosome with the abnormal gene.

What would happen if the abnormal recessive gene were on the X chromosome of the male (MEANING THE FATHER HAS THE GENETIC ABNORMALITY), instead of on one of the female X-chromosomes. USE THE PUNNET SQUARE TO FIGURE IT OUT.

Answer: Sons will not possess the abnormal trait, so they will be normal but all daughters born to the couple will be CARRIERS.

 

 

SEX-LINKED RECESSIVE INHERITANCE OF A GENETIC DISORDER(F2 GENERATION)

XY = male; XX= Female; The red superscript "a" on the female and male X-chromosomes represent an abnormal gene on those particular chromosomes.

This represents the cross between a sister who is a carrier and brother who expresses the genetic abnormality (consanguineous marriage- can also be cousins) and illustrates why this kind of marriage is not permitted by law.

BASED ON THE DATA

In the F2 Generation, there is a 50% possibility of having a boy with the genetic abnormality. This person would be like a homozygous recessive.

There is a 50% chance of the daughters to be born with the genetic abnormality and an equal chance for the daughter to be a CARRIER.

A consanguineous marriages increase the incidence of genetic abnormalities.

 

 

SEX-LINKED DOMINANT INHERITANCE OF A GENETIC DISORDER (F1 GENERATION)

XY = male; XX= Female; The red superscript "A" on the female X-chromosome represent an abnormal gene on those particular chromosomes. The normal gene of the allelic pair is on the other X-chromosome. Since the defect is expressed by a dominant gene, the mother would have the genetic abnormality. Like the Mendelian example, the mother would have a heterozygous dominant genotype (Aa).

Dominant genes are expressed in this genotype and thus the mother has the genetic abnormality.

BASED ON THE DATA

Daughters have a 50% chance (1 in 2) of receiving the dominant allele and expressing the genetic abnormality.

Likewise, sons have the same risk as the daughters. The affected son would have the genetic abnormality.

WHAT WOULD HAPPEN IF THE "A" GENE WOULD BE ON THE X-CHROMOSOME OF THE FATHER RATHER THAN ON ONE OF THE X-CHROMOSOMES OF THE MOTHER? TRY IT USING A PUNNET SQUARE BEFORE YOU LOOK AT THE ANSWER.

ANSWER: 100 PERCENT OF THE DAUGHTERS WOULD RECEIVE THE DOMINANT ALLELE AND HAVE THE GENETIC ABNORMALITY. NONE OF THE SONS WOULD RECEIVE THE GENE. THEY WOULD BE NORMAL.

 

 

CLICK ON LINK BELOW TO LEARN ABOUT DETAILS OF:

 

 

3. Compare sexual and asexual reproduction

 

 

4. Explain how the coding of DNA (deoxyribonucleic acid) controls the expression of traits by genes

1) Protein synthesis from DNA and RNA

2) Concept of genes and how their traits are expressed

 

5. Define mutations and explain their types and causes

Patterns of Gene Inheritance-Genetic Disorders

Biochemical basis of genetic disorders: The classic concept of the gene has been that it is 1) a unit of STRUCTURE within which crossing over or recombination cannot occur, 2) it is a unit of FUNCTION, having one primary function and 3) it is a unit of MUTATION.

MUTATION: Any sudden heritable change in DNA (definition is restricted to POINT MUTATIONS and does not include the whole chromosome).

REVIEW: The genetic messages on RNA are read by the ribosomes in groups of three nucleotides or triplets (see left). (Remember, the actual change would occur in the DNA and be passed on to the RNA during RNA synthesis).

POINT MUTATIONS: involves the substitution of one DNA base for another within a triplet.

The Genetic code above refers to RNA sequences. If the middle A in the RNA sequence AAA (coding for the amino acid lysine- Lys) were to change to a C (cytosine) as a result of the change in the DNA through a mutational event, the resulting RNA triplet would be ACA.

From the picture above, ACA codes for the amino acid threonine (Thr). Therefore, lysine changes to isoleucine at that point in the amino acid sequence of the protein. In this example, in the DNA, the nucleotide would have changed from a Thymine to a Guanine (G).

RESULTS OF THE CHANGE IN THE PROTEIN CHAIN DUE TO A SINGLE POINT MUTATION: 1) It may be a SILENT MUTATION and not affect the function of the protein or 2) It may cause a change in the three dimensional folding of the protein causing it to lose partial or complete activity.

POINT MUTATIONS may also be caused in one of the DNA bases IF a nucleotide is either removed or a new one inserted in the DNA chain. These are much rarer than the single change of one base to another.

RESULTS OF THE CHANGE IN THE PROTEIN CHAIN DUE TO INSERTION OR DELETION: Since the RNA is read in triplets at the point of the insertion or deletion, there would be a shift in reading the triplet codes. Most of the amino acids after the mutation would be different resulting in a protein that would most likely be completely nonfunctional.

 

 

VARIATION IN THE EXPRESSION OF GENES

NO MATTER WHAT KIND OF INHERITANCE IS INVOLVED, THERE ARE OTHER FACTORS THAT GOVERN THE EXPRESSION OF GENES OR WHAT IS SEEN WHEN THE GENE IS EXPRESSED.

 

 

Polygenic inheritance

More than one gene controls a single trait

Over a thousand human traits are inherited according to Mendelian rules, however, others are controlled by many genes (3 or more) Examples: height, weight, intelligence, skin color

One gene may influence more than one trait.

Marfan's syndrome: One gene causes abnormally long fingers, legs, toes, lenses of the eyes are in abnormal positions and heart problems may occur. Sickle Cell Anemia: The sickle cell gene affects blood, bones, kidney and the brain

Multiple Alleles- Many forms of the same gene

Examples : In sweet clover there are over 200 alleles preventing flowers from self-fertilization; In cattle there are over 250 forms of a gene controlling a single blood type.

 

 

CRITERIA FOR AUTOSOMAL DOMINANT, AUTOSOMAL RECESSIVE, X-LINKED DOMINANT AND X-LINKED RECESSIVE INHERITANCE- A COMPARISON

Criteria for Autosomal Dominant Inheritance
Criteria for Autosomal Recessive Inheritance
Criteria for X-Linked Dominant Inheritance
Criteria for X-Linked Recessive Inheritance
Trait appears in every generation (no skipping) Trait characteristically appears only in siblings (i.e. skips generations) Transmission by females follows the same pattern as a homozygous dominant. X-LINKED DOMINANT CANNOT BE DISTINGUISHED FROM AUTOSOMAL DOMINANT INHERITANCE. Incidence of the trait is much higher in males than in females because males do not have an extra X chromosome with a normal gene for the same trait like females do.
Trait is transmitted by an affected person to half the children On the average one-fourth of the siblings are affected (risk is one in every four births will be affected) Affected males transmit the trait to all their daughters and to none of their sons Trait is transmitted by an affected man from all HIS daughters to half THEIR sons
Unaffected people do not transmit the trait to unaffected people Parents of the affected child may be consanguineous (related- like cousins) Affected females who are homozygous transmit the trait to all their children. Trait is never transmitted directly from father to son
The occurrence and transmission of the trait are not affected by sex (males and females equal chance) Males and females are equally likely to be affected. In rare X-linked dominant disorders, affected females are twice as common as affected males but are likely to express the condition in a milder form
  Tests for carrier states (has gene but is normal-usually heterozygous)) are sought for.   Trait may be transmitted to a series of carrier females. If so, the affected males in a kindred are related to one another through females.

 

THE INFORMATION ABOUT DIFFERENT INHERITANCE TYPES IN THE TABLE ABOVE, THE PRINCIPLES AND LAWS OF MENDELIAN GENETICS, POINT MUTATIONS, CHROMOSOMES AND GENES, AS WELL AS THE PRINCIPLES OF EXPRESSIVITY AND PENETRANCE WILL GIVE YOU A BASIS TO APPRECIATE THE EXISTENCE OF THE GENETIC ABNORMALITIES LISTED BELOW. The list is incomplete.

THE GENETIC DISORDERS ARE CATEGORIZED ACCORDING THE THE FOUR INHERITANCE TYPES LISTED IN THE TABLE.

OTHER EXAMPLES NOT NECESSARILY RELATED TO GENETIC ABNORMALITIES ARE ALSO INCLUDED.

 

 

CLICK ON INHERITANCE DISORDER LINKS BELOW TO SEE DETAILS ABOUT EACH GENETIC DISORDER

 

 

 

List major events that affected the evolution of life on Earth (e.g. climate changes, asteroid impacts)

Impact Cratering

Asteroids

Climate changes

Plate tectonics and the evolution of climate

 

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