Mendelian Genetics 101
Inheritance• Parents and offspring often
share observable traits.
• Grandparents and
grandchildren may share
traits not seen in parents.
• Why do traits disappear in
one generation and reappear
in another?
Definitions
• Genetics = the branch of biology that studies heredity
• Heredity = the passing on of characteristics from parents to offspring
– ***from the Latin word hered-, meaning “heir.”
Background• Organisms usually
resemble their parents because they inheritcertain traits from them.
• These characteristics, variants called traits, are determined by genetic information on chromosomes.
• Genetic information = segments of DNA = genes
Gregor Mendel
Father of genetics
• Gregor Mendel, an Austrian monk, carried out the first important studies on heredity (1800s).
History
• Mendel was the first person to succeed in predicting how traits would be transferred from one generation to the next.
• Earlier observers looked at many traits at once-- Mendel focused on one at a time
Mendel Combined:
• Plant breeding
• Statistics
• Careful record keeping
Mendel’s findings of transmission of traits are
now considered the Laws of Inheritance.
Mendel’s Experiments
Mendel Artificially Cross-Pollinated Pea Plants
Mendel studied the pea plant Pisum sativum.
- easy to cultivate and a short life cycle
- easy to control pollination
- keep unwanted pollen out
- cross-fertilize artificially
-had discontinuous characteristics
-Ex: flower color, seed texture
-knew of at least 34 such traits
-He focused on 7
Mendel studied pea traits, each
with two distinct phenotypesCharacteristic
Mendel’s Experimental Design
Mendel’s Experimental Design
Mendel’s Experimental Design
TERMINOLOGY
PHENOTYPE - appearance ("pheno-
"=visible, as in "phenomenon")
GENOTYPE - genetic make-up;
combination of alleles
- not always visible, but
detectable by performing
crosses
ALLELES - variants of a gene.
Three important written conventions for writing genotypes:
• The same letter is used for different alleles of the same gene.
• Uppercase letters are used for dominant alleles and lowercase letters are used for recessive alleles.
• The letter for the dominant allele is always written first.
Gene Seed shape Seed color Flower
color
Flower
position
Pod shape Pod
color
Plant
height
Dominant
allele
Round (R) Yellow (Y) Purple
(P)
Axial (A) Inflated
(I)
Green
(G)
Tall
(T)
Recessive
allele
Wrinkled
(r)
Green (y) White
(p)
Terminal
(a)
Constricted
(i)
Yellow
(g)
Short
(t)
Mendel‘s Experiments
Mendel’s Experimental Design: What we know now
Mendel’s Experimental Design: What we know now
Mendel’s Experimental Design: What we know now
TERMINOLOGY
HOMOZYGOUS - having two alleles
that are alike
HETEROZYGOUS - having two
unlike alleles
DOMINANT - showing a phenotypic
effect in heterozygous form
RECESSIVE - showing a phenotypic
effect only when homozygous
Production of gametes: Meiosis
• The number of chromosomes is reduced (by half).
• Alleles for genes are separated into the gametes.
• Gametes then combine during sexual reproduction to form zygotes with complete sets of chromosomes.
• Mendel’s flowers examples
• Dog examples handout
Gametes for Single Characteristic
Two characteristicsHeterozygous for both characteristics
_____
_____
__________
Mendel’s Laws of Inheritance
• “LAW OF DOMINANCE” – One factor in a pair may prevent the other from having an effect; that factor dominates or masks the other.
in modern terms:
• “LAW OF DOMINANCE” – recessive alleles will always be masked by dominant alleles
Mendel’s Laws of Inheritance
• “LAW OF SEGREGATION” – paired factors separate during the formation of reproductive cells
in modern terms:
• "LAW OF SEGREGATION" - alleles separate from one another during meiosis in the formation of gametes.
Mendel’s Laws of Inheritance
• “LAW OF INDEPENDENT ASSORTMENT” – factors for individual characteristics are not paired together.
in modern terms:
• "LAW OF INDEPENDENT ASSORTMENT“ - genes at different locations are separated independently of one another during gamete formation.– Far away from each other on the same chromosome
– On separate chromosomes.