MENDELIAN GENETICS
I. For years, scientists have observed similar traits in individuals in the same family.
A. Such patterns have been observed in both plant and animal populations.
B. However, the reasons for these similarities have only recently been discovered.
II. EARLY VIEWS ABOUT GENETICS
A. Early biologists believed that proteins were responsible for producing individual
traits in plants and animals. This was proven incorrect in the early 1900’s.
1. Today, we know that _____________ is the major genetic material in
living organisms.
B. Some early biologists supported an idea known as Blending. According to this view, an
individual’s traits were a perfect blend of the traits of the individual’s parents.
1. By the late 1800’s this was shown to be incorrect.
III. IMPORTANT GENETIC TERMS
A. Cross-breeding-sperm from one organism fertilizes the egg of another organism.
B. Genes-sequences of DNA on chromosomes. These produce traits and enzymes in living
organisms.
1. Alleles-different segments of a gene. ALL GENES CONTAIN 2 ALLELES FOR A
PARTICULAR TRAIT.
a. Types of Alleles
1) Dominant alleles-alleles that are expressed in an individual. These are
abbreviated with a capital letter (A).
2) Recessive alleles-alleles that are usually not expressed in an individual.
These are abbreviated with a lower case letter (a).
b. ALLELES ALWAYS OCCUR IN PAIRS!!!
c. There are 3 distinct genetic combinations that occur for every trait in living
organisms:
1) Homozygous Dominant Individuals-have 2 dominant alleles for a
particular trait. How are these abbreviated?
2) Homozygous Recessive Individuals-have 2 recessive alleles for a
particular trait. How are these abbreviated?
a) This is the only case where an individual will express a
recessive trait.
3) Heterozygous Individuals-have a dominant and a recessive allele for a
trait. How are these abbreviated?
a) Which allele is expressed in these individuals?
b) The recessive allele may not appear in these individuals;
however, it may be passed on to offspring.
2. Genotype-
3. Phenotype-
4. Punnet Squares-squares that are used to represent the possible genetic outcomes of
specific crosses. These are also known as test crosses.
IV. GREGOR MENDEL-The Father of Modern Genetics
A. He lived in a monastery in the current Czech Republic.
B. He did his genetic research in the mid to late 1800’s. Remember that he know absolutely
nothing about chromosomes, genes or DNA. DNA had not even been discovered when he
started his research.
C. Mendel wanted to address the issue of blending. He specifically did not believe in blending;
therefore, he set out to prove it wrong.
D. Mendel used pea plants for his genetic studies. Why did he choose pea plants?
E. 2 points to remember about Mendel:
1. He knew nothing about DNA or genes.
2. He physically crossed, planted, grew and counted the # of traits in his peas.
V. MENDEL’S WORK
A. Initially, Mendel conducted crosses that focused on only one trait. Such crosses are known as
monohybrid crosses.
B. For his monohybrid crosses, Mendel selected flower color as the trait that he wanted to look at.
1. He knew that most pea plants produced purple flowers. He also knew that a few pea
plants were capable of producing white flowers. Mendel did not understand why this
occurred however.
2. Mendel was attempting to disprove blending. He had never seen a pea plant produce
3. Today, we know that purple flower color is dominant to white flower color in pea plants.
C. Mendel’s First Monohybrid Cross
1. In his first cross, Mendel crossed a purple-flowered plant with a white flowered plant.
a. What are the genotypes of the parents in this cross if P is dominant for purple
flowers and p is recessive for white flowers?
b. Punnet Square of this first Monohybrid Cross:
1) What are the genotypic and phenotypic ratios of the offspring in
this cross?
2) What color(s) flowers did Mendel observe in his crosses.
3) Next Mendel decided to cross the offspring from this first cross.
D. Mendel’s Second Monohybrid Cross:
1. In this cross, he crossed two offspring from his previous crosses. What are the
phenotypes and genotypes of the parents in this second cross?
2. Punnet Square of this second cross:
a. What is the genotypic ratio of the offspring in this cross?
b. What is the phenotypic ratio of the offspring in this cross?
E. Mendel’s Conclusions from his Monohybrid Crosses
1. Alleles do not blend in heterozygous individuals.
2. Each sexually-reproducing individual (plant or animal) has 2 “factors”
(alleles) for every individual trait.
3. The above-mentioned factors segregate (separate) during meiosis. This produces
gametes that contain 1 of the factors.
4. All possible combinations of factors can occur in the gametes.
F. Today, these conclusions are referred to as Mendel’s Law of Segregation.
G. Next, Mendel decided to examine two traits. These types of crosses are known as Dihybrid
Crosses. Specifically, he wanted to see if one trait had an influence on the appearance of
another trait.
H. Mendel’s Dihybrid Crosses
1. These were conducted in much the same fashion as the monohybrid crosses.
2. For these crosses, Mendel selected flower color and seed shape as his two traits. Today,
we know the following about these traits:
3. Mendel’s First Dihybrid Cross:
a. In this cross, Mendel crossed a purple flowered, round seeded individual with
a white flowered, wrinkled seeded individual. What were the genotypes of these
individuals?
b. Punnet Square of this Cross:
c. What are the genotypes and phenotypes of the offspring in this cross?
4. Next, Mendel decided to cross the offspring from the previous cross.
5. Mendel’s Second Dihybrid Cross
a. What is the Phenotypic Ratio of the offspring in this cross?
6. Mendel’s Law of Independent Assortment
a. Each pair of genes is distributed independently during meiosis.
7. Mendel’s work was the foundation of modern genetics. Unfortunately, all of this
work was published in a small journal that sold only 13 copies. His work went
unnoticed for over 30 years. When it was discovered, it truly revolutionized the world
of genetics.
8. Last Points on Mendel:
a. In 1900, Hugo Devries, Karl Correns and Erich Tscermak announced the
discovery of Mendel’ research. They actually were able to prove that Mendel
was correct.
b. By 1900, many researchers were beginning to indicate that chromosomes were
the key to genetics in living organisms.
c. Walter Sutton-1900-proposed The Chromosomal Theory of Inheritance. Much of
this theory was based on Mendel’s work. They key points in this theory include:
1) Chromosomes occur in pairs-each living organism receives one from the
mother and one from the father.
2) Synapsis is the pairing of homologous chromosomes. They separate
during meiosis.
3) Chromosomes never breakdown during cell division.
4) Chromosomes carry genes. Genes contain alleles (these were Mendel’s
“factors”).
5) Meiosis creates new variations of a gene-through crossing over. This
explains Mendel’s Law of Independent Assortment.
6) All genes on one chromosome are inherited together. Chromosomes
carry different genes.
VI. SEX CHROMOSOMES
A. Animals contain sex chromosomes which determine the sex of an individual. The discovery of
these chromosomes occurred early in the 1900’s.
B. Females-have 2 X sex chromosomes (XX).
1. All of the eggs produced by a female have an X chromosome.
2. In general, the second X chromosome provides very few traits to females. What can
females do with the second X chromosome?
C. Males-have an X and a Y sex chromosome.
1. The Y chromosome carries very few traits. It does lead to the production of
D. Who determines the sex of an offspring?
VII. THOMAS HUNT MORGAN-did genetic research in the early 1900’s.
A. He did his work on the organism Drosophila melanogaster.
1. What is this organism?
2. Why did he choose this organism?
B. Morgan knew that fruit flies had red eyes. One day, a student in Morgan’s lab found a
white-eyed fruit fly. This white-eyed fly was a male. Morgan decided to use this fly to examine
genetics in animals.
C. Morgan’s Research
1. He crossed the white-eyed male with a normal, red-eyed female fly. All of the offspring
in this cross were red-eyed individuals. What did Morgan conclude about red eye color?
2. Next, Morgan crossed the offspring from the previous cross and he observed a perfect
Mendelian ratio of three red-eyed flies for every one white-eyed fly. However, Morgan
noticed that all of his white-eyed flies were males.
a. How did Morgan explain this?
1) He was able to show that the gene for white eye color was on an X
chromosome. Specifically, it was located on the second X chromosome
of the female flies. Therefore, white eye color would not show up in the
female flies, but they could pass the chromosome on to male offspring
where white eye color would appear.
2) However, with continued crossing, the white eye color would appear in
some female offspring. This occurs since the male can then pass the sex
chromosome (and the white eye color gene) onto his female (and male)
offspring.
3. Morgan referred to this process as linkage.
4. Linkage-a situation in which a specific trait is determined by a gene on a distinct
chromosome (such as white eye color on an X chromosome).
VIII. VARIATIONS ON MENDEL’S WORK
A. Incomplete Dominance-occurs when the phenotype of a heterozygous individual is intermediate
that of the homozygous dominant and homozygous recessive types.
B. Codominance-occurs when there are two dominant alleles for a trait. Both of the dominant
alleles can be expressed in the individual.
1. Example: The Human ABO Blood System
a. Human Blood Types are the result of three alleles: two dominant alleles and
one recessive allele.
b. What are the dominant alleles associated with human blood?
c. What is the recessive allele in human blood?
d. Genotypes in Human Blood:
C. Epistasis-occurs when the alleles of one gene cover or mask the effects of another gene.
D. Pleiotrophy-refers to a gene that influences multiple traits.
IX. MENDELIAN TRAITS IN HUMANS
A. Tongue-curling
B. Earlobes
C. Dimples
D. Hair on finger
E. Hitchhiker’s thumb
F. Widow’s peak
G. Thumbs over