Mendel Lecture for CHS IB BIO

1. Classical Genetics

2. Mendel And The Laws Of
Inheritance
• Gregor Johann Mendel (1822-1884) -
father of genetics
• Austrian monk
• Conducted landmark studies from
1856-1864, - thousands of crosses
• Kept meticulously accurate records that
included quantitative analysis

3. Mendel Chose Pea Plants as His
Experimental Organism
• Hybridization
– The mating or crossing between two
individuals that have different characteristics
• Purple-flowered plant X white-flowered plant
• Hybrids
– The offspring that result from such a mating
– Presumed to be a blending of the parent traits
– Often observed to be different than either
parent (hybrid vigor)
• Mendel observed them to be like one of
the parents with respect to some traits –
no blending

4. Mendel Studied Seven Traits
That Bred True
• The morphological characteristics of an
organism are termed characters or traits
• A variety that produces the same trait
over and over again is termed a true-
breeder

5. Mendel’s Experiments
• Crossed two variants differing in only
one trait
– a monohybrid cross

6. DATA FROM MENDEL’S MONOHYBRID CROSSES
P Cross F1 generation F2 generation Ratio
Tall X All tall 787 tall, 2.84:1
dwarf stem 277 dwarf
Round X All round 5,474 round, 2.96:1
wrinkled seeds 1,850 wrinkled
Yellow X All yellow 6,022 yellow, 3.01:1
Green seeds 2,001 green
Purple X All purple 705 purple, 3.15:1
white flowers 224 white
Axial X All axial 651 axial, 3.14:1
terminal 207 terminal
flowers
Smooth X All smooth 882 smooth, 2.95:1
constricted 229 constricted
pods
Green X All green 428 green, 2.82:1
yellow pods 152 yellow

7. Interpreting the Data
• For all seven traits studied
1. The F1 generation showed only one of the
two parental traits
2. The F2 generation showed an ~ 3:1 ratio
of the two parental traits
• These results refuted a blending
mechanism of heredity

8. Interpreting the Data
• Data suggested a particulate theory of
inheritance
• Mendel postulated the following:

9. Law of Segregation
• A pea plant contains two discrete hereditary
factors, one from each parent
• The two factors may be identical or different
• When the two factors of a single trait are
different
– One is dominant and its effect can be seen
– The other is recessive and is masked
• During gametogenesis (meiosis), the paired
factors segregate randomly so that half of the
gametes received one factor and half of the
gametes received the other

10. Review a few modern terms
– Mendelian factors are now called genes
– Alleles are different versions of the same
gene
– An individual with two identical alleles is
termed homozygous
– An individual with two different alleles, is
termed heterozygous
– Genotype refers to the specific allelic
composition of an individual
– Phenotype refers to the outward appearance
of an individual

11. • Prevalent alleles in a population are termed wild-
type alleles
– These typically encode proteins that
• Function normally
• Are made in the right amounts
• Alleles that have been altered by mutation are
termed mutant alleles
– These tend to be less common in natural populations
– They are likely to cause a reduction in the amount or
function of the encoded protein
– Such mutant alleles are often inherited in a recessive
fashion
• A particular gene variant is not usually considered
an allele of a given gene unless it is present in at
least 1% of the population.
– Rare gene variants (<1%) are termed polymorphisms
rather than allelic variants

12. Modern Genetic Terminology
• Recessive
– Null – no functional protein is produced
– loss-of-function alleles (LOF)
• a protein that doesn’t function as much as or in the same way as
the wildtype protein
– Wildtype alleles are dominant to recessive alleles
• Dominant
– Gain-of-function (GOF)
• protein functions is a new way
• more protein is made than in wildtype
• protein can not be regulated as in wildtype
– Dominant-negative (DN)
• a mutated protein disrupts the function of wildtype proteins
– dominant alleles are dominant to wildtype