Transcript GeneticsandHeredity - Winston Knoll Collegiate

Genetics and
Heredity
History
• Genetics is the scientific study of genes,
heredity and variation in living organisms.
• Heredity is the passing of traits to offspring
from its parents or ancestor.
• Inheritance is how traits, or
characteristics, are passed on from
generation to generation.
Gregor Mendel
• Austrian Monk.
• Experimented with “pea plants”.
• Used pea plants because:
• They were available and were pure breeding
(offspring identical to themselves).
• They reproduced quickly
• They showed obvious differences in the traits
Understood that there was something that
carried traits from one generation to the
next- “FACTOR”.
Mendel cont……
In the mid-1800s, the rules underlying
patterns of inheritance were uncovered in
a series of experiments performed by an
Austrian monk named Gregor Mendel.
Mendel's Plant Breeding Experiments
Gregor Mendel was one of the first to apply
an experimental approach to the question
of inheritance.
For seven years, Mendel bred pea plants
and recorded inheritance patterns in the
offspring.
Particulate Hypothesis of Inheritance
Parents pass on to their offspring separate
and distinct factors (today called genes)
that are responsible for inherited traits.
To test the particulate hypothesis, Mendel crossed truebreeding plants that had two distinct and contrasting traits (a
specific characteristic that varies from one plant to another)—for
example, purple or white flowers.
What is meant by “true breeding?”
For each monohybrid cross, Mendel cross-fertilized true-breeding plants that
were different in just one character—in this case, flower color. He then allowed
the hybrids (the F1 generation) to self-fertilize.
Typical breeding experiment
P generation (parental
generation)
F1 generation (first filial
generation, the word filial
from the Latin word for
"son") are the hybrid
offspring.
Allowing these F1
hybrids to self-pollinate
produces:
F2 generation (second
filial generation).
It is the analysis of this
that lead to the
formulation of Principle
of Dominance states
that some alleles are
Mendel studies seven characteristics in the garden pea
:
Statistics indicated
a pattern.
Chromosomes
Homologous chromosome: one of a matching pair of
chromosomes, one inherited from each parent.
Sister chromatids are identical
Alleles: alternative versions of a gene.
The gene for a particular inherited character resides at a specific locus
(position) on homologous chromosome.
For each character, an organism
inherits two alleles, one from each
parent
How do alleles differ?
Dominant allele
Recessive
allele
Recessive allele
Recessive allele
Dominant - a term applied to the trait (allele) that is expressed irregardless of
the second allele.
Recessive - a term applied to a trait that is only expressed when the second
allele is the same (e.g. short plants are homozygous for the recessive allele).
Segragation:
Mendel wanted the answer to another question: Had the
recessive alleles disappeared or they were still present in the
F1 plants:
To answer this question he allowed all seven kinds of F1 hybrid
plants to produce an F2 generation by self pollination.
The results were that the traits controlled by recessive allele
reappeared in F2 generation.
Mendel formulated his second law, The law of Segregation:
When each F1 plant flowers and produces gametes, the two
alleles segregate from each other so that each gamete carries
only a single copy of the gene. Therefore, each F1 plant
produces two types of gametes- those with the allele for
tallness and those with the allele for shortness.
Probability and Punnett Squares
Punnett square: diagram showing the probabilities of the
possible outcomes of a genetic cross
Genotype versus phenotype.
How does a
genotype ratio differ
from the phenotype
ratio?
Punnett squares - probability diagram illustrating the possible
offspring of a mating.
Ss X Ss
gametes
Independent Assortment :
• After showing that the alleles segregate during formation of gametes,
Mendel wondered if they did so independently. Or, does the
segregation of one pair of alleles affect the segregation of another
pair of alleles.
• To test this he did a Two Factor cross or a Dihybrid Cross: He crossed
true breeding plants that produced round yellow peas with wrinkled
green
Principle of Independent
Assortment:
This principle states that the genes for
different traits can segregate independently
during the formation of gametes.
Independent assortment helps to account for
the many genetic variations observed in
plants, animals, and other organisms.
Some alleles are neither
dominant nor recessive, and
many traits are controlled by
multiple alleles or multiple
genes.
Incomplete Dominance: One
allele is not completely
dominant over the other.
The heterozygous phenotype is somewhere in between the two
homozygous phenotypes.
Codominance
Both alleles contribute to
the phenotype .
Multiple alleles:
Many genes have more than two
alleles. But an individual can have
only two allels
Polygenic Traits
• Traits produced by interaction of several genes.
• The wide range of skin colour in humans comes about partly because
more than four different genes control this trait