Download Gregor Mendel

Gregor Mendel
The basic laws of heredity
were first formed during
the mid-1800’s by an
Austrian botanist monk
named Gregor Mendel.
• Mendel’s paper published in 1866, but
was not recognized by Science until the
early 1900’s.
• Because his work laid the foundation to
the study of heredity, Mendel is referred
to as “The Father of Genetics”
Mendel’s Pea Plants
• Mendel based his laws on his studies of
garden pea plants.
• Mendel was able to observe differences
in multiple traits over many generations
because pea plants reproduce rapidly,
and have many variable traits such as:
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Mendel’s Experiments
• First: Mendel noticed that some plants always
produced offspring that had a form of a trait
exactly like the parent plant. He called these
plants purebred plants.
• For instance, purebred purple flowered plants
always produced purple flowered offspring
and purebred white flowered plants always
produced white flowered offspring.
Parent:
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Mendel’s Second Experiment
• Mendel Crossed purebred plants with opposite forms
of a trait. He called these plants the parental genera,
or P generation. For instance, purebred purple
flowered plants were crossed with purebred white
flowered plants.
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Parent (P) Generation
Offspring (F1 Generation)
•Mendel observed that all of the offspring grew to have purple
flowers. None resembled the white flower. He called this
generation of offspring the first filial, or F1 generation.
Dominant and Recessive
Genes
• Mendel went on to reason that one factor
(gene) in a pair may mask, or hide, the other
factor. For instance, in his first experiment,
when he crossed a purebred purple flowered
plant with a purebred white flowered plant, all
offspring were purple flowered. Although the
F1 offspring all had both purple and white
factors, they only displayed the purple factor.
He concluded that the purple factor masked
the white factor.
• Dominant Trait: mask or hide other
alleles, such as the “purple flower”.
• Recessive Trait: is masked, or covered
up, whenever the dominant allele is
present, such as the white flower allele.
Mendel’s Third Experiment
• Mendel then crossed two of the offspring (F1) purple plants
produced from his first experiment.
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Offspring (F2) 3/4
Purple
• Parent (F1)
1/4 White
• Mendel called this second generation of plants (F2). To his
surprise, Mendel observed that this generation has a mix of
purple and white flowered plants. This occurred even though
none of the F1 parents had white flowers.
Mendel’s Law of Segregation
• Mendel’s first law, the Law of Segregation,
has three parts. From his experiment,
Mendel concluded that:
• 1. Plant traits are handed down through
“hereditary factors” in the sex cells.
• 2. Because offspring obtain hereditary factors
from both parents, each plant must contain
two factors for every trait.
• 3. The factors in a pair segregate (separate)
during the formation of the sex cells, and
each sperm or egg receives only one
member of the pair.
• Today, scientists refer to the “factors” that
controls traits as genes. The different forms
of a gene are celled alleles.
• Alleles that mask or hide other alleles, such
as the “purple flower” allele, are said to be
dominant.
• A recessive allele, such as the white flower
allele, is masked, or covered up, whenever
the dominant allele is present.
Dominant/Recessive Alleles
• Mendel observed a variety of dominant and recessive alleles in
pea plants other that the purple/white flower allele.
Dominant
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Recessive
Homozygous Genes
• What Mendel referred to as a “purebred” plant we now know
this to mean that the plant has two of the same alleles for a
particular trait.
•
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P generation (Both white flowers)
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Offspring (F1)
ALL white flowers
• According to Mendel’s Law of Segregation, each parent
donates one flower color gene to the offspring. Since each
parent had only white flower genes to donate, all offspring
will have white flower genes (homozygous) and will
therefore have white flowers.
Heterozygous
• When both alleles for a trait are present or
both alleles are different.
•
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• P generation one of each color
flowers
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F1 generation all purple
• Although the offspring have both a purple
flower and a white flower allele, only the
purple flower allele is expressed and is
therefore dominant over the white flower
allele.
Law of Independent Assortment
• Mendel’s second law, the Law of Independent
Assortment, states that each pair of genes
separate independently of each other in the
production of sex cells.
• Genotype: the genetic makeup of an
organism with reference to a singe trait, set
of traits, or all of the organisms traits.
• Phenotype: The appearance of an
organism resulting from the interaction of
the genotype and the environment.