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Genetic variations in meiosis are due to:
Independent assortment of chromosomes during meiosis
(random lining up of homologous chromosomes in
metaphase I. It can produce about 8 million different
combinations of chromosomes.
Random fertilization of gametes (depends of which one of
the 8 million possible varieties of sperm meats one of the 8
million possible varieties of egg.
Crossing over is exchange of chromosome segments
between homologous chromosomes during prophase I of
meiosis I. (also known as recombination)
Genetic linkage means that genes located closer together
tend to get inherited together.
Traits, genes, alleles
Gene: is a piece of DNA that provides a set of
instructions to a cell to make a certain protein.
(Protein relates to the organism’s characteristic) Each
gene has a locus (= address, specific position on a pair
of homologous chromosomes).
Allele: any of the alternative forms of a gene that may
occur at homologous chromosomes on which the
locus for that gene is found. Each allele comes from
one parent. The two alleles may be same or different.
(So gene refers to general characteristic like eye color,
allele is specific and carries the information whether
you will have green or brown eyes). We use one letter
abbreviation for alleles – example “G” for green, “B”
for brown
Homozygous: describes two of the same alleles at the
specific locus (example- you got 2 alleles, one from
mom and one from dad, and they both code for
brown eyes, you are “BB”)
Heterozygous: describes two different alleles at a
specific locus (example- you got 2 alleles, one from
mom and one from dad, and one codes for brown
eyes and the other codes for green eyes, you are
“BG”)
Genome: is all of the organism’s genetic material.
Unless you have identical twin, you have unique
genome that determines all of your traits. (It includes
all the information whether used or masked)
Genotype: typically refers to the genetic makeup of a
specific set of genes (think of it as the map giving our
body directions, in our example “BB” and “BG” are
examples of genotype for eye color)
Phenotype: is the physical characteristics, or traits, of
an individual organism (think of it as something we
can observe, like brown or green eye color)
Dominant allele: is the allele that is expressed when
two different alleles or two dominant alleles are
present. The dominant allele is marked with capital
letter (and recessive with lower case letter).Take our
eye color example – suppose that brown “B” eye
color is dominant over green “g” eye color. Than both
genotypes “BB” and”Bg” result in brown eye color.
Recessive allele: is the allele that is only expressed
when two copies are present. Taking the example of
our eye color – only genotype “gg” results in green
eye color.
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Mendel and heredity:
Traits: are distinguished characteristics that are inherited (like eye color, leave shape,
length of tail)
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Genetics: is the study of biological inheritance patterns and variation in organism
Purebred: is genetically uniform line. The offspring of purebred parents inherit all of the
parent organism’s characteristics.
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Cross: is the mating of two organisms in study of genetics. Example is crossing whitecolored pea with purple-colored pea (this is the parental or P generation). The resulting
offspring is the first final or F1 generation. Crossing F1 generation would give F2
generation…
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Mendel’s conclusions:
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Traits are inherited as discrete units (traits are not blended or diluted over
successive generations)
Law of segregation:
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Organisms inherit two copies of each gene, one from each parent
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Organisms donate only one copy of each gene in their gametes (the 2
copies of each gene segregate, or separate, during meiosis = gamete
formation)
Law of independent assortment: allele pairs separate independently from each
other during gamete formation (meiosis). This means that different traits
appear to be inherited separately.
Monohybrid cross involves one trait (like flower color), dihybrid cross involves two traits (like
flower color and plant height). Testcross – using homozygous recessive parent to find out if
the other unknown parent is homozygous dominant or heterozygous (it is determined based
on the cross results)
Homozygous x homozygous cross
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Purple flower colors “F” are dominant over white flower
colors “f”
Parents: FF (homozygous dominant),ff (homozygous
recessive)
All offspring’ genotypes are Ff (heterozygous) and all
phenotypes are purple.
Heterozygous x heterozygous cross
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Both parents are heterozygous – genotype is Ff (have allele
for white and purple flowers), phenotype is purple
(because purple color is dominant)
Offspring genotypes are: one FF (homozygous dominant),
two Ff (heterozygous), and one ff (homozygous recessive).
The genotypic ratio is 1:2:1 (FF:Ff:ff)
Offspring phenotypes are 3 purple (because 3 carry
dominant purple allele) and 1 white (no purple allele). The
ratio is 3:1 (purple: white). There is 75% chance the
offspring is going to have purple color and 25 % chance it is
going to be white.
Heterozygous with homozygous
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One parent is homozygous recessive (genotype –ff,
phenotype-white) and one is heterozygous (genotype- Ff,
phenotype-purple)
Offspring genotype: two Ff, two ff. The ratio is 1:1 (Ff:ff)
Offspring phenotype: two purple and two white. The ratio
is 1:1 (purple: white). There is 50% chance the offspring is
going to be either purple or white.
Homozygous recessive parent is used in testcross where
we do not know the genotype of the other parent