Study Guide "Chapter 11 & 15"
11.1 Objectives
Summarize Mendel’s studies of inheritance in pea plants
Explain Mendel’s conclusion about inheritance
What is the Principle of dominance
What happens during segregation
Genetics is the scientific study of heredity. Gregor Mendel is known as the father of genetics and studied
pea plants at the monastery garden where he worked. He knew that plants carried out sexual
reproduction in order to reproduce. Part of each plant produces both the male and female reproductive
cells. (pollen the male gamete and the ovum which is the female gamete).
Concept of unit characters inherited traits are controlled by genes which occur in pairs with multiple
alleles
True-breeding
Tall plants would only produce tall plants, short plants
only short offspring, green peas will produce only green ones etc…..Pea plant normally self pollinate,
thus the seeds (zygotes) that are produced inherit all their traits from the single plant thus no variation.
He found that if pea plants were allowed to self pollinate they would result in identical pea plants with
no variation. This is known as true-breeding. True-breeding pea plants were the basis of Mendel’s
experiments.
Cross-breeding
Tall plants and short plants would produce tall plants, green pea plants and yellow pea plants would
produce yellow peas, etc…….Mendel wanted to produce seeds by joining gametes from two
different types of pea plants…..this is known as cross-pollination. He did this by removing the male
pollen producing portion from the flower of one type of plant and dusting that flower with pollen from
another plant. (Figure 11-2 pg. 264) From this experiment Mendel would conclude that some alleles are
dominant while others are recessive.
Mendel studied seven different pea plan traits. Each trait had a contrasting trait. Example: green
peas or yellow peas, tall plants or short plants etc….The seven types of pea plant traits were crossed and
the offspring were studied. (Figure 11-3 pg. 264)
Principle of Dominance
The P generation was the original parent plants. The initial offspring generation was referred to as the
F1 generation or first filial.(filiusor filiaare the Latin words for “son” or “daughter”) He found the
F1 generation to his surprise showed only one of the traits of the two parents, it seemed that one of the
two characteristics for each plant had disappeared. (Figure 11-4 pg. 265)
Example: Tall plant X Short plant = All tall plants.
Mendel drew two conclusions from this:
1.) Biological inheritance is determined by factors (genes) that are passed down from one generation to
the next, resulting in one gene from the mother and one gene from the father. Different versions of
genes for the same trait are called alleles.
Example: Tall (T) and (t) short…….both genes for height of the plant.
2.) Dominance/recessiveness ….that some traits were dominant and would mask or cover the recessive
one. A recessive trait would only show if the genes of the allele for the trait were both recessive.
Examples: Homozygous Tall Plant (T T) dominate trait shows
Heterozygous Tall Plant (T t) dominate trait shows
Homozygous short plant (t t) recessive trait shows
Principle of dominance states that some alleles are dominant and others are recessive
Segregation
Mendel wanted to know it the recessive alleles disappeared or were they still present in the
F1generation. To answer this he cross-bred the F1 generation for each of the seven traits. The offspring
of this cross is referred to as the F2 generation. (Figure 11-4 pg. 265) Mendel found that the resulting
F2 generation produced the recessive trait about ¼ of the time. This re-emergance of the recessive trait
indicated that at some point the allele for shortness had been separated from the one for tallness. This
suggested that the alleles for tallness and shortness were segregated from each other during the
formation of gametes. (Figure 11-5 pg. 266)
Example: T t X T t = ¾ Tall plants and ¼ short pants
Law of segregation states that a pair of genes which each allele for the trait resides on each homologous
chromosome pair and will be separated during the formation of gametes. (Meiosis)
11.2 Objectives
Principles of probability
Use of the Punnett square
Mendel carefully categorized and counted the many offspring from his experiments and realized the
principles of probability could be used to explain the results. An example of this principle is the flip of a
coin. Each flip of the coin is an independent and singular event so one flip and the chances of it coming
up heads with one (flip) would result in one chance for heads out of the two possibilities heads or tails
(sides of the coin), thus ½ or 50% every flip of the coin would have the probability of coming up
heads. But if you flip the coin 3 times and wanted to determine the odds or probability of all three flips
turning up heads it would be ½ x ½ x ½ =1/8or 12.5% (Figure 11-6 pg. 267)