Download The Science of Genetics

The Science of Genetics
Original Power Point Created by
Andy Harrison
Modified by the GA Agriculture Education Curriculum Office
July 2002
Introduction
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Genetics began as people noticed the
similarity of offspring to parents
Animals or plants were selected based on
certain desirable traits
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Later became known as selective breeding
Gregor Mendel
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Austrian monk: 1822 –1884
Noticed the texture of seeds differed
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For example, some were smooth and round,
others were not smooth and not round
Also noticed the difference in colors of
flowers, length of stems, etc.
Mendel’s Experiments
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Used purebred plants
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Chose plants with traits that would be carried to
next generation each time seeds produced.
Crossed purebred plants with other purebred
plants.
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Removed petals
Removed pollen from stamens and placed on
stigma of pistil of different type of plant
Mendel’s Experiment
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Some produced red flowers, and some only
white
He then crossed all red with all white
Yielded some plants with all red flowers
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Reasoned that some factor must cause the red to
be dominant over white
Mendel’s Experiment
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Law of Dominance – overriding or dominant
factors make certain recessive traits disappear.
Allele – matched pair of genes that control a trait
Law of Segregation – alleles responsible for trait
separate and then combine with other parent at
fertilization – each parent provides one of two
genes for the trait
Inheritance
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Mendel let some plants self-pollinate
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Offspring called an F1 generation
Some all red and some all white
Let F1 reproduce
Plants from all red produced some white
flowers
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Called the F2 generation
Inheritance
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Mendel concluded that traits or factors
associated with red or white flowers
separated before pollen merged with egg
Law of segregation - alleles responsible for
traits from each parent are separated and
then combined with factors from other
parents at fertilization.
Inheritance
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Law of Independent Assortment – factors or
genes for certain characteristics are passed
on to the next generation separate from the
factors or genes that transmit other traits.
Punnett Square
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Developed by mathematician R.C. Punnett
Illustrates the possible combinations of a
particular trait
Punnett Square
R
r
75%
R
RR Red
Rr
Homozygous
Heterozygous
Red
25% White
3:1
r
Rr Red
rr
Heterozygous
Homozygous
White
Red
Red to
White
Genetics
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The study of how traits are passed on to
future generations
Genotype – the genetic composition of an
individual
Phenotype – how the allele expresses itself
Genetics
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Homozygous – Genes that contain 2
dominant alleles (RR) or 2 recessive alleles
(rr)
Heterozygous – Genes containing 1
dominant allele and 1 recessive allele (Rr)
Gene Transfer
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Traits are located on chromosomes
Chromosomes are made of strands of DNA
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Contains proteins and amino acids
Each segment of the chromosome that controls a
trait is a GENE
Some traits are controlled by one gene, others
under multi-genetic control
In Mendel's law of dominance, one allele is
expressed and one is hidden
Meiosis
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Similar to mitosis
Has an additional step
Chromosomes are
divided so each sperm
or egg contains a
chromosome
Results in one egg or 4
sperm
Gene Transfer
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At fertilization the egg and sperm unite
Newly formed cell contain two chromosomes
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Haploid
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For example, sex cells have only one chromosome
Diploid
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There is a pair of alleles for each trait
Chromosomes are in pairs
Some cells have more than two pair
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Also known as polyploid
Sex Determination
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Each parent contributes one chromosome
which pairs with a chromosome from the
other parent
Sex Chromosomes
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X: comes from the female
Y: comes from the male
Sex Determination
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Female has XX
Female
XX
X
X
XX
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Male has XY
Male XY
X
Y
XY
Gene Transfer
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DNA shaped in spiral a coil
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Also known as a double-helix
If flattened out it would resemble a ladder
The two “sides” of the ladder are connected by
nitrogen containing bases
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Adenine - A
Thymine - T
Cytosine - C
Guanine - G
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Adenine can only pair with Thymine
Cytosine can only pair with Guanine
Gene Transfer
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During mitosis and meiosis, the
chromosome separates
Prior to cell division, DNA copies itself by
replication
Messages in DNA are transferred by RNA
(a messenger substance)
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This “message” determines how the molecules
of new DNA are to be arranged
Plant Breeding
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Systematic process of matching genetic
factors from parent plants to produce
offspring that are superior to parents
Early man used a primitive form of plant
breeding
Plant Breeding
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All plants came from wild plants that were
domesticated
The domesticated species are called
cultivars
What Plant Breeders Look For
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Increased yield
Disease resistance
More efficient water use
Temperature tolerance
Ease of harvest
Uniform maturity
Quality of fruit or usable portion
Plant Breeding
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Breeders have increased adaptation of plants by a
systematic system of plant breeding
Hybrid – crossing two purebred lines to result in a
superior plant
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Corn most common hybrid
Many vegetables and flowers now hybrids
Heterosis – the result of the crossing of unrelated
parents
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Results in increased performance
Hybrid Corn
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First developed in 1909
Information on use taught to growers by
Extension Service and Vocational Ag
Classes in schools
Breeder develops parent lines by inbreeding
selections from populations
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Then crosses parent lines to create hybrids
Hybrid Corn
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Crosses are made by collecting pollen from
one plant and depositing on silk of recipient
plant.
A
B
AB
Animal Breeding
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Less animals than in 1950, but production
has increased
Most slaughter animals are crossbred due to
hybrid vigor (heterosis benefit)
Selected for weight gain, size,
conformation, litter size, etc.
Animal Breeding
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Some new breeds have been developed by
crossbreeding
Santa Gertrudis - cross of two different
species: Bos taurus X Bos indicus
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Most cattle in America or Europe – Bos taurus
Cattle from India - Bos indicus (Brahman)
Santa Gertrudis
Developed
All
on the King Ranch in Texas
Santa Gertrudis are descendants of
the bull “Monkey”
Animal Breeding
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Texas cattlemen wanted to combine gentle
nature and carcass quality of the Shorthorn
breed with vigor, heat resistance, and insect
resistance of the Brahman cattle
Other breeds developed by crossing Bos
indicus – Brangus, Simbrah, Braford, and
Charbray
Animal Breeding
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Computers have aided in sorting data
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Results in expected progeny differences
(EPD’s)
Use data of offspring to make selections of
which animals to breed to others or to keep
Animals selected with leaner carcasses and
better quality muscle result in increased
flavor