Transcript Mendelian Genetics

Mendelian Genetics
• Gregor Mendel (1822 – 1884)
performed one of the classic scientific
experiments on inheritance during the
mid-19th century.
• Mendel performed careful experiments
and interpreted quantitative data
applying mathematical concepts to
biology
Mendel’s Experiments
• Mendel examined specific
characteristics of pea plant that were
expressed in one of two ways:
• Seed shape, seed colour, pod shape,
pod colour, flower colour, flower position
on stem and stem length
• Mendel’ experiments came before
knowledge of chromosomes or genes
and he referred to what would be later
known as genes as factors
Summary of Mendel’s
Experiments
1. Inherited characteristics are controlled
by factors (called genes) that occur in
pairs. Each member of a pair of
genes is called an allele. During
cross-fertilization each parent
contributes one of its alleles.
2. One factor, or allele, masks the effect
or expression of another. An
uppercase letter indicates the
dominant allele; the recessive allele is
indicated by a lowercase letter.
3.A pair of factors, or alleles, separate or
segregate during the formation of sex
cells. This is the Law of Segregation.
Sex cells must contain only one
member of the pair of factors, or alleles.
Mendel was very fortunate in his selection
of a species and traits to examine
because these traits displayed a
dominance that is not always found in
most organisms.
Mendel’s Laws of Heredity
1. Each parent contributes one allele
during cross-fertilization. If a purebreeding plant self-fertilizes, each
offspring receives two copies of the
same allele.
2. The dominant allele is always
expressed when the recessive allele is
present.
3. Each pair of alleles segregates during
the formation of sex cells.
Offspring of Cross Pollination
F1 and F2 Generations
• Mendel began his experiments by
crossing two pure breeding plants for
each form of the trait (homozygous for a
particular trait) to produce a generation
of hybrid plants all of which expressed
the dominant form of an allele
(heterozygous for a trait)
• This generation is called the F1 or filial
generation one
• He then crossed two of the hybrid plants
from the F1 generation to produce an F2
generation
• Unlike the initial cross between the two
pure breed plants to produce the F1
generation the results of the F2
generation did not express only the
dominant form of the allele
•
Mendel found that in the F2 generation
of an initial dominant/recessive pure
breed cross for a single trait that:
1. 75% of the offspring expressed the
dominant trait
2. 25% of the offspring expressed the
recessive trait
Punnett Square
• A special chart is used to organize the
result of a cross between the sex cells
of two individuals is called a Punnett
Square
• This chart can be used to predict the
genotypes and phenotypes of the
offspring
• Punnett squares are named for an
English geneticist, Reginald Punnett.
To use a Punnett square you must:
1. Identify the genotype of the parents in a
cross.
2. List the alleles for gametes produced
by each parent based upon the parental
genotypes.
3. Place the possible gametes of each
parent along the top and sides of the
Punnett square.
4. Note all possible allele combinations for
the next generation within the squares.