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Chapter 6
6-3 What is genetics? The study of heredity (passing traits from
parents to offspring)
The chemical factors that determine traits are called genes.
Gregor Mendel is considered the “father of genetics”.
Mendel used pea plants in his studies because he could easily
manipulate their reproductive parts. Male parts produce pollen
(male sex cells) and female parts produce eggs (female sex
cells). A new plant is formed when the pollen fertilizes the egg.
Trait – a distinguishing characteristic that is inherited, ex.
seed color or plant height
Plants can self-pollinate and inherit traits from a single parent, or
cross-pollinate when two plants are involved.
Pure-bred or true-bred – traits are from a single parent, or
the same from 2 parents
Hybrid – traits from 2 parents are different
From Mendel’s monohybrid cross results, Mendel concluded that:
1) traits are inherited as discrete units and
The Law of Segregation (Mendels’ First Law)
2) Organisms inherit 2 copies of each gene, one from each
parent
3) Organisms donate only 1 copy of each gene in their
gametes. So, the 2 genes separate,
or segregate, during gamete formation.
6-4 Genes and Dominance
Gene – piece of DNA that provides a set of instructions to a
cell to make a certain protein
Locus – location of a gene on a chromosome
Homozygous – organisms that have 2 identical alleles for a
trait ex. TT or tt
Heterozygous – organisms that have 2 different alleles for
the same trait ex. Tt
Genotype – genetic makeup; the letters ex. TT, Tt, tt
Phenotype – physical characteristics or appearance; the
words ex. Tall
P – parental generation; original pair of plants
F1 – first filial generation; filius is Latin for “son”
F2 – second filial generation
1) Traits passed from P to F1 through chemical factors (genes).
Traits are found in contrasting forms. The alternative (different)
forms of a gene are called alleles.
2) The principle of dominance states that some alleles are
dominant and others are recessive.
Dominant genes (alleles) can mask a recessive trait. It is
represented by a capital letter.
When Mendel crossed a tall plant with a short plant, the F1 plants
inherited an allele for tallness and an allele for shortness.
TT – tall
Tt – would still be tall because T is dominant
Recessive genes (alleles) will only be shown if a dominant allele
for that trait is NOT present. It is represented by a lower case
letter.
gg – green peas
Gg – yellow peas
All of an organism’s genetic material is called its genome.
6-5 Probability and Punnett Squares
Probability? The likelihood that a particular event will occur. What
is the probability that a coin will land heads up? ½ or 50%
Punnett square – grid system for predicting all possible
genotypes resulting from a
-The axes represent the possible gametes of each parent.
- The boxes show the possible genotypesof the offspring.
Monohybrid crosses examine the inheritance of only 1 specific
trait.
Ex. Tt X Tt
Phenotypic ratio? 3 tall : 1 short
1 tt
Genotypic ratio? 1 TT : 2 Tt :
Probabilities predict averages
The larger the number of individuals, the closer the resulting
offspring numbers will get to the expected values.
 If you want to determine if a tall pea plant is homozygous
or heterozygous, what cross would you perform?
 Cross it with a short purebred plant (tt). If all offspring are
tall, then it is homozygous. If some are short, then it is
heterozygous.
Dihybrid Crosses – crosses that examine the inheritance of 2
different traits
Are alleles for different traits related to each other? Ex. color and
texture of seeds
The Principle of Independent Assortment states that genes for
different traits can segregate independently (without influencing
each other’s inheritance) during the formation of gametes.
Ex. YyRr X YyRr
Summary of Mendel’s Principles
 Offspring inheritance is determined by genes passed from
parents
 A gene for a trait may be dominant or recessive
 The two copies of a gene segregate from each other
during gamete formation.
 The alleles for different genes usually segregate
independently of one another.
6-6 Independent Assortment & Gene Linkage
•Sexual reproduction creates unique combination of genes. Any
human couple can produce a child with one of about 70 trillion
different combinations
–independent assortment of chromosomes in meiosis
–random fertilization of gametes
–Crossing-over (exchange of chromosome segments
between homologous chromosomes during prophase I of
meiosis I) helps create even greater variation
•Unique phenotypes may give a reproductive advantage to some
organisms.
Locus – a gene’s place on a chromosome
The further apart 2 genes are located, the more likely they are to
be separated during crossing-over. So, genes that are located
together tend to be inherited together.
This is called gene linkage. Genes that are far apart are more
likely to assort independently.