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Genetics (Chapter 11)
11.1 Objectives
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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.(filius or filia are 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
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Genetics (Chapter 11)
Segregation
Mendel wanted to know it the recessive alleles disappeared or were they still present in the F1 generation. 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 reemergance 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
Male Gametes (F2 Generation)
Female Gametes
(F2 Generation)
T
t
T
T T
T T
t
T t
t t
Homologous Chromosomes
T
t
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 = 1/8 or 12.5%. Figure 11-6 pg. 267
Punnett squares
Punnet squares can be used by geneticist to predict and compare genetic variations that can result from a
cross. Using trait genotypes (symbols given to genes to show genetic make-up) and phenotypes (what
characteristic or trait shows) a Punnett square can give a visual representation or diagram of the results of
various crosses. Terms such a homozygous and heterozygous are used to give genotype.
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Genetics (Chapter 11)
Punnett square- special chart for working genetic problems. Punnett squares are used to determine the
probability, or ratio, of expected offspring. Capital letters are used to denote dominant trait. Lower case
letters are used to denote recessive traits. (always put capital letter first in the box)
T
T
T TT TT
t Tt Tt
Each box stands for a zygote and its possible genotype.
Genotype of zygotes: *the ratio of all possible gene combinations in the problem
Example using above cross: TT = 2 = 50% = ½
Phenotype of zygotes: the ratio of physical appearance of the trait
Example using above cross: Tall = 4 = 100% = 2/2
Example problem:
Cross two hybrid pea plants. Tall is dominant over short.
A.) T = tall t = short
B.) T t X T t
C.)
T
t
T
TT
Tt
t
Tt
tt
D.) Genotype: T T = 1 = ¼ = 25%
T t = 2 = ½ = 50%
t t = 1 = ¼ = 25%
E.) Phenotype: Tall = 3 = ¾ = 75%
Short = 1 = ¼ = 25%
A monohybrid cross shows the information for one trait.
A dihybrid cross shows the information for two traits.
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Genetics (Chapter 11)
11.3 Objectives
 Principles of probability
 Use of the Punnett square
Dihybrid cross
ry
R- Round
r - Wrinkled
Y- Yellow
y - Green
ry
ry
ry
RY
RrYy RrYy RrYy RrYy
RY
RrYy RrYy RrYy RrYy
RY
RrYy RrYy RrYy RrYy
RY
RrYy RrYy RrYy RrYy
Principle of independent assortment states that a genes for different traits can segregate independently
during the formation of gametes (Meiosis), Independent assortment accounts for the many genetic variations
observed in plants, animals and other organisms
Summary of Mendel’s Principles
Concept of Unit Characters
Principle of Dominance
Law of Segregation
Law of independent Assortment
The inheritance of biological characteristics is
determined by individual units known as genes. Genes
are passed from parents to their offspring.
In cases is which two or more forms (alleles) of the
gene for a single trait exist, some forms of the gene
may be dominant and others may be recessive.
In most sexually reproducing organisms, each adult
has two copies of each gene- one from each parent.
These genes are segregated from each other when
gametes are formed.
The alleles for different genes usually segregate
independently of one another.
Incomplete dominance- In some cases, genes are neither dominant nor recessive. When one trait does not
completely mask another, this is called incomplete dominance. The hybrid organisms will show up as a blend
of the two traits.
Example: Black + White = Grey
Codominance- is worked the same way but both genes contribute to the genotype
Black + White = Spotted
Example: Human blood type A, B, O
Multiple alleles- more than one pair of genes controls a trait
Example: Human blood type A, B, O
Polygenic traitsExample: Human eye color
Sex linked traitsSex influencedSex limited4
Genetics (Chapter 11)
Sexual reproduction – Male and female (haploid) reproductive cells (gametes) join (fusion), this is called
fertilization
Gametes- Male and female reproductive cells
Sperm (male reproductive cell)
Ovum (female reproductive cell)
Fertilization – the joining of male and female reproductive cells (gametes) to form a diploid cell known as
the zygote
Zygote – a single cell with full number of chromosomes (diploid), resulting from successful fertilization
Haploid- contains only one set of the homologous chromosomes in other words: half the full number of
chromosomes that an organisms somatic cells contain.
Example: Human (sperm cell) 23 chromosomes
Diploid- contains both sets of homologous chromosomes in other words: the full number of chromosomes
that an organisms somatic cells contain.
Example: Human (skin cell) 46 chromosomes
Traits- A specific characteristic that an organism has that varies from one individual to another.
Example: Green peas vs yellow peas, brown hair vs red hair.
Homologous chromosomes- like chromosomes, matching or corresponding set.
Homozygous- Pure traits- the pair of genes for a trait are the same
Examples: Homozygous Tall plants have this genotype T T (dominate)
Homozygous Short plants have this genotype t t (recessive)
Hetreozygous- Hybrid traits – mixed gene pairs
Example: Heterozygous Tall plants have the genotype T t (dominate)
Gene- chemical factors that determine traits, genes are the basic unit of inheritance. In sexual reproduction
one gene is inherited from the male parent and one from the female parent.
Alleles- different forms of a gene Example: T = tall t = short (different characteristics but same type of trait)
Probability- the likelihood that a particular event will occur
Phenotype- what trait shows up in the individual (tall, short, tall)
Genotype- the letters that show what genes an individual has (T T, t t, T t)
Independent assortment- independent segregation of genes during the formation of gametes
Incomplete dominance- situation in which one allele is not completely dominant over another
Codominance-situation in which both alleles of a gene contribute to the phenotype of the organism
Multiple alleles- three or more alleles of the same gene
Polygenic traits- trait controlled by two or more genes
Homologous- term used to refer to chromosomes that each have a corresponding chromosome from the
opposite-sex parent
Diploid- term used to refer to a cell that contains both sets of homologous chromosomes
Haploid- term used to refer to a cell that contains only a single set of chromosomes and therefore only a
single set of genes
Meiosis- process by which the number of chromosomes per cell is cut in half through the separation of
homologous chromosomes in a diploid cell
Tetrad- structure containing 4 chromatids that forms during meiosis
Crossing–over- process in which homologous chromosomes exchange portions of their chromatids during
meiosis
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Genetics (Chapter 11)
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