BIO212
GENETICS
• LECTURE 2
• INTRODUCTION TO MENDELIAN
GENETICS
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Introduction to Mendelian (Classical) Genetics
• Mendel believed that factors pass from parents to
their offspring, but he did not know of the existence
of DNA.
• We now know that genes are composed of
segments of DNA molecules that control discrete
hereditary characteristics.
• Most complex organisms have cells that are Diploid
(cells have a double set of chromosomes, one from
each parent). E.g. human cells have a double set of
chromosomes consisting of 23 pairs, or a total of 46
chromosomes
• In a diploid cell, there are two genes for each
characteristic.
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Introduction to Mendelian Genetics
• In preparation for sexual reproduction, the
diploid number of chromosomes is reduced to a
haploid number (cells are reduced to cells that
have a single set of chromosomes).
• These haploid cells are gametes, or sex cells, and
they are formed through meiosis
• (In humans: 1pair of sex chromosomes and
22 pairs of autosomes)
• When gametes come together in sexual
reproduction, the diploid condition is reestablished.
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Introduction to Mendelian Genetics
• The offspring of sexual reproduction obtain one gene of each type from each parent.
• The different forms of a gene are called alleles (e.g type of earlobe a person has is determined by the
alleles inherited from the parents).
• The set of all genes that specify an organism’s traits is known as the organism’s genome (≈20,500 in
humans)
• The gene composition of a living organism is its genotype.
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Introduction to Mendelian Genetics
• The expression of the genes is referred to as the
phenotype of a living thing. If a person has attached
earlobes, the phenotype is “attached earlobes”.
• Genotype: three possible genotypes; FF, Ff, and
ff.
• Phenotype: two possible phenotypes: Free &
Attached
• The two paired alleles in an organism’s genotype
may be identical, or they may be different.
• homozygous when two identical alleles are
present for a particular characteristic.
• heterozygous when two different alleles are
present for a particular characteristic
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Introduction to Mendelian Genetics
• When one allele expresses itself and the other does
not, the one expressing itself is the dominant allele
and the “overshadowed” allele is the recessive allele
• Dominant alleles always express themselves, while
recessive alleles express themselves only when two
recessive alleles exist together in an individual.
• FF & Ff = Free earlobe phenotype
• ff = attached earlobe phenotype
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Monohybrid Crosses: Principles of Dominance &
Segregation
• Mendel developed a method of predicting the
outcome of inheritance patterns.
• The garden pea was good choice of experimental
organism because: many varieties were available that
bred true for clear-cut, qualitative traits like
• seed texture (round vs wrinkled)
• seed color (green vs yellow)
• flower color (white vs purple)
• tall vs dwarf growth habit
• pod shape
• pod color,
• and flower location
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Monohybrid Crosses: Principles of Dominance &
Segregation
• Pea plants self-pollinate, resulting in individuals that
are homozygous for particular characteristics over
many generations (these populations are known as
pure lines/true breeding).
• Mendel took pure-line pea plants and crosspollinated them with other pure-line pea plants;
parent generation (P).
• Mendel crossed pure-line tall plants with pure-line
short plants, he observed that all the plants resulting
from this cross were tall. F1 generation (first filial
generation).
• The allele for tall (T) was Dominant over the allele for
short (t)
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Monohybrid Crosses: Principles of Dominance & Segregation
• During Meiosis, F1 plants produce two kinds
of gametes, T & t, in equal proportions; thus
the Alleles SEGREGATE/separate unchanged
• He then crossed the offspring of the F1
generation tall plants among themselves to
produce the F2 generation (second filial
generation).
• Observation: F2 generation; ¾ of the plants
were tall and 1/4 of the plants were short.
• Because of DOMINANCE, 3 of the Genotypes
(TT, Tt, tT) have the same Phenotype (Tall)
• (picture representation next slide)
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Monohybrid Crosses: Principles of Dominance & Segregation
• A Punnett square is a boxed figure
used to determine the probability of
genotypes and phenotypes in the
offspring of a genetic cross.
• The possible gametes produced by
the female are indicated at the top
of the square, while the possible
gametes produced by the male are
indicated at the left side of the
square
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Monohybrid Crosses: Principles of Dominance &
Segregation
• Further crosses: F2 plants were self-fertilized to produce F3.
• All the short plants produced short offspring, demonstrating that they were homozygous for the t
allele (i.e were tt).
• The tall plants produced two categories; 1/3 produced tall offspring, 2/3 produced a mixture of Tall
and short offspring.
• Conclusion: the 1/3 that produced only Tall offspring (true breeding) were TT homozygotes and the
2/3 that were segregating were Tt heterozygotes
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F2
• Mendel continued with crosses of
the other six traits and documented
his observations.
Phenotypes
Genotypes
Genotypic
Ratio
Phenotypic
Ratio
Tall
TT
Tt
1
2
3
Short
tt
1
1
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Monohybrid crosses: test crosses
• Mendel predicted the outcome of a breeding
experiment that he had not yet carried out.
• He crossed heterozygous round peas (Rr) with
wrinkled (homozygous, rr) ones.
• He predicted that in this case one-half of the
seeds produced would be round (Rr) and onehalf wrinkled (rr); 50:50 ratio
•
F1 Gametes
R
r
P
Gametes
r
Rr
r
Rr
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rr
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Monohybrid crosses- test crosses
• He performed the cross and harvested 106 round
peas and 101 wrinkled peas (i.e ≈ 50:50 predicted
ratio)
• This kind of mating is called a testcross. It "tests"
the genotype in those cases where two different
genotypes (like RR and Rr) produce the same
phenotype.
• i.e. To test the genotype of an F1 organism for
instance you must use a homozygous recessive
Parent (P).
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Monohybrid crosses: Reciprocal crosses
• Mendel also showed that reciprocal crosses, which are
two crosses that differ only in the sex of the parents,
give identical results.
• E.g, in a cross between a round seed plant and a
wrinkled seed plant, it makes no difference whether
the pollen comes from the round seed plant or the
wrinkled seed plant.
• (not true for sex-linked traits as seen in the picture)
• Since the sex of the parent doesn’t matter for these
crosses, the term “gamete” is often used to refer to a
generic reproductive cell (pollen/sperm or ovule/egg).
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Reciprocal Crosses
Cross-1
Cross-2
• P: round seed pollen x
wrinkled seed ovule
• P: round seed ovule x
wrinkled seed pollen
• F1: round seed progeny
• F1: round seed progeny
Conclusion: reciprocal crosses give identical results
(NB: in above e.g, ROUND is dominant over wrinkled)
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Mendel’s laws
• Mendel’s analysis and other monohybrid crosses led to the Principles of:
• 1. dominance: When an organism has two different alleles for a trait, one allele
dominates (controls the phenotype).
• 2. segregation: During gamete formation by a diploid organism, the pair of alleles
for a particular trait separate, or segregate, during the formation of gametes (as in
meiosis).
• An Allele is transmitted faithfully/unchanged from one generation to the next
even if it was paired with a different allele in the heterozygote.
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Mendel’s laws
• Mendel continued: He crossed peas that differed in two traits.
• He found that the inheritance of one trait was independent of that of the other and
so framed his third rule/principle:
• 3. the principle of Independent Assortment
• He was lucky as not all genes abide by this rule (exceptions due to recombination &
Crossing over)
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Mendel’s laws
• 1. Mendel’s law of dominance: When an organism has two different alleles for a
trait, one allele dominates.
• 2. Mendel’s law of segregation: During gamete formation by a diploid organism,
the pair of alleles for a particular trait separate, or segregate, during the formation
of gametes (as in meiosis).
• 3. Mendel’s law of independent assortment: The members of a gene pair separate
from one another independent of the members of other gene pairs. (These
separations occur in the formation of gametes during meiosis.)
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