Chapter 25 – Mendelian Genetics
25.1 – Mendel’s Principles of Heredity
The Study of Heredity
When two gametes fuse, a zygote is formed. The haploid number of each gamete combine to form a diploid
number zygote. The offspring may have some similarities with one or both of the parents. Genetics is the study
of hereditary information as it is passed on from parents to offspring. Gregor Mendel was the first to study
genetics in the 1800’s.
Gregor Mendel was a monk who used pea plants to study genetics. The advantages to using pea plants as his
model organism were; ease of growing them, matured relatively quickly, and traits or characteristics were easily
visible to the naked eye. He kept records of the following characteristics, which came in two forms;
1. seed shape – round or wrinkled
2. seed color – yellow or green
3. seed coat color – grayish brown or white
4. pod color – green or yellow
5. pod shape – inflated or wrinkled
6. stem length – long or short
7. flower position – lateral or terminal
He used mathematics to explain his results and he published his work in a journal. His ideas were not widely
accepted at the time and he died without recognition.
The Law of Dominance
Mendel discovered that he could create purebred plants. For example, when he allowed short plants to self-
pollinate, eventually 100% of the offspring would be short. The same was true for the tall plants. Then he
wondered what size offspring would result from crossing a short plant with a tall plant. The short and tall plants
are called the parent generation or the P generation. The offspring are called the first filial generation or the F1
generation. When Mendel crossed a purebred tall and a purebred short P generation, the F1 generation was
100% tall. From this cross Mendel concluded that the tall trait was the dominant one. More accurately, he
concluded that when an organism is hybrid for a pair of contrasting traits, only the dominant trait can be seen in
the hybrid. This is the law of dominance. Then, to satisfy his curiosity, Mendel crossed two of the tall
offspring (no longer purebred) from the F1 generation and found something interesting. The F2 generation of
the second filial generation was 75% tall and 25% short.
Law of Segregation
Taking the F1 and F2 data into account, Mendel had to explain why the short or recessive trait was not seen in
the F1 generation and was seen in the F2 generation. Rather than using the word gene, which did not exist to
Mendel, he used the word factor to explain his results. He said there were two factors for a trait and each factor
could be one of two forms. In this case the factors were tall or short and each plant could have a tall or short
factor, two tall factors, or two short factors.
Mendel theorized that in the hybrid F1 cross each plant had one tall and one short factor. He went on to theorize
that the factors separate from each other during the formation of gametes or meiosis and then recombine
together during fertilization. This is Mendel’s Law of Segregation.
The Gene-Chromosome Theory
Long after Mendel’s death, a graduate student, W.S. Sutton, observed homologous chromosomes in diploid
cells separate during spermatogenesis. Sutton realized that the separated chromosomes would be recombining
in the fertilization process. After looking at Mendel’s work, Sutton concluded that Mendel’s factors are carried
on the chromosomes, since the chromosomes were doing the same thing Mendel’s factors were theorized to do,
that is separate and recombine. In Sutton’s day, the word gene was used instead of factors and the Gene-
Chromosome Theory was developed (we will discuss this theory in detail in chapter 26).
25.2 – Fundamentals of Genetics and Genotypes and Phenotypes
1. Alleles – different forms for one gene (for example, regarding the gene that determines plant height; the
alleles are short (t) or tall (T))
2. Homozygous – the alleles are the same (for example, in a purebred tall plant, both alleles are tall (TT))
3. Heterozygous – the alleles are different (for example, in a hybrid tall plant, one allele is tall the other is short
4. Genotype – the genetic makeup of an organism (for example, (Tt))
5. Phenotype – the physical trait of an organism (for example, tall)
Probability in Genetics
The law of probability, or chance, says if there are several possible events that might happen, and no one of
them is more likely to happen than any other, then they will all happen in equal numbers over a large number of
The Punnett Square
The Punnett square diagram is an easy way to show the results of any cross. Refer to the bottom of page 504
for directions on how to use this helpful tool.
The Punnett square results for a purebred cross is always 100% hybrid genotype showing the dominant
phenotype. The Punnett square results for a hybrid cross is 25% pure dominant and 25% pure recessive
genotypes, 50% hybrid genotypes, 75% tall phenotypes, and 25% short phenotypes. Refer to the top of page
The Test Cross
When you are trying to find out the genotype of an organism, you simply cross it with a known pure recessive
mate. If 100% of the F1 generation show the dominant phenotype, the unknown organism was purebred for the
dominant trait. If 50% of the F1 generation was dominant and 50% was recessive, then the unknown organism
was a hybrid. Refer to the Punnett squared on page 507 for an illustration of this concept.
25.3 – Other Concepts in Genetics
The Law of Independent Assortment – (refer to the bottom of page 508) states that during meiosis, genes for
different traits are separated and distributed to gametes independently of one another (note: there are exceptions
to this rule). Using this law in a dihybrid cross (TtRr x TtRr, for example) always produces a 9:3:3:1
phenotype ratio. (See page 509)
Incomplete Dominance – When organisms have this type of inheritance, the hybrid produces a trait that is a
blend of each trait. Examples of this are the four o’clock flower (RW is pink), and Andalusian chickens (BW
appear blue, which is actually a steel-gray)
Codominance – When organisms have this type of inheritance, the hybrid exhibits both traits equally.
Examples of this include AB blood type and roan coat in cattle and horses. (See page 510 for picture of this)
Multiple Alleles – For some traits there are more than two alleles for that trait. In humans, examples of traits
that have multiple alleles are blood type ( A,B,O alleles), Rh blood factors, and eye color.