Transcript AG-BAS-02.471-05.4p i

The Science of Genetics
Original Power Point Created by
Andy Harrison
Modified by the GA Agriculture Education Curriculum Office
July 2002
August 2008
Introduction
• Genetics began as people noticed the
similarity of offspring to parents
• Animals or plants were selected based on
certain desirable traits
– Later became known as selective breeding
August 2008
Gregor Mendel
• Austrian monk: 1822 –1884
• Noticed the texture of seeds differed
– 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.
August 2008
Mendel’s Experiments
• Used purebred plants
– Chose plants with traits that would be carried
to next generation each time seeds produced.
• Crossed purebred plants with other
purebred plants.
– Removed petals
– Removed pollen from stamens and placed on
stigma of pistil of different type of plant
August 2008
Mendel’s Experiment
• Some produced red flowers, and some
only white
• He then crossed all red with all white
• Yielded some plants with all red flowers
– Reasoned that some factor must cause the
red to be dominant over white
August 2008
Mendel’s Experiment
• 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
August 2008
Inheritance
• Mendel let some plants self-pollinate
– Offspring called an F1 generation
– Some all red and some all white
• Let F1 reproduce
• Plants from all red produced some white
flowers
– Called the F2 generation
August 2008
Inheritance
• 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.
August 2008
Inheritance
• 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.
August 2008
Punnett Square
• Developed by mathematician R.C. Punnett
• Illustrates the possible combinations of a
particular trait
August 2008
Punnett Square
R
r
75%
R
RR Red
Rr
Red
Homozygous Heterozygou
s
r
August 2008
Rr Red
rr
Heterozygou
s
Homozygous
White
Red
25% White
3:1
Red to
White
Genetics
• The study of how traits are passed on to
future generations
• Genotype – the genetic composition of an
individual
• Phenotype – how the allele expresses
itself
August 2008
Genetics
• Homozygous – Genes that contain 2
dominant alleles (RR) or 2 recessive
alleles (rr)
• Heterozygous – Genes containing 1
dominant allele and 1 recessive allele (Rr)
August 2008
Gene Transfer
• Traits are located on chromosomes
• Chromosomes are made of strands of DNA
– 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
August 2008
Meiosis
• 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
August 2008
Gene Transfer
• At fertilization the egg and sperm unite
• Newly formed cell contain two chromosomes
– There is a pair of alleles for each trait
• Haploid
– For example, sex cells have only one chromosome
• Diploid
– Chromosomes are in pairs
• Some cells have more than two pair
– Also known as polyploid
August 2008
Sex Determination
• Each parent contributes one chromosome
which pairs with a chromosome from the
other parent
• Sex Chromosomes
– X: comes from the female
– Y: comes from the male
August 2008
Sex Determination
• Female has XX

Female
XX
X
X
XX
August 2008
Male has XY
Male XY
X
Y
XY
Gene Transfer
• DNA shaped in spiral a coil
– 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
–
–
–
–
Adenine - A
Thymine - T
Cytosine - C
Guanine - G
• Adenine can only pair with Thymine
• Cytosine can only pair with Guanine
August 2008
Gene Transfer
• 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)
– This “message” determines how the
molecules of new DNA are to be arranged
August 2008
Plant Breeding
• 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
August 2008
Plant Breeding
• All plants came from wild plants that were
domesticated
• The domesticated species are called
cultivars
August 2008
What Plant Breeders Look For
•
•
•
•
•
•
•
Increased yield
Disease resistance
More efficient water use
Temperature tolerance
Ease of harvest
Uniform maturity
Quality of fruit or usable portion
August 2008
Plant Breeding
• Breeders have increased adaptation of plants by
a systematic system of plant breeding
• Hybrid – crossing two purebred lines to result in
a superior plant
– Corn most common hybrid
– Many vegetables and flowers now hybrids
• Heterosis – the result of the crossing of
unrelated parents
– Results in increased performance
August 2008
Hybrid Corn
• 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
– Then crosses parent lines to create hybrids
August 2008
Hybrid Corn
• Crosses are made by collecting pollen
from one plant and depositing on silk of
recipient plant.
A
B
AB
August 2008
Animal Breeding
• 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.
August 2008
Animal Breeding
• Some new breeds have been developed
by crossbreeding
• Santa Gertrudis - cross of two different
species: Bos taurus X Bos indicus
– Most cattle in America or Europe – Bos taurus
– Cattle from India - Bos indicus (Brahman)
August 2008
Santa Gertrudis
Developed
All
on the King Ranch in Texas
Santa Gertrudis are descendants of
the bull “Monkey”
August 2008
Animal Breeding
• 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
August 2008
Animal Breeding
• Computers have aided in sorting data
– 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
August 2008