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Content Standard
11
Identify Mendel’s laws
of genetics
ELIGIBLE CONTENT
• Know and apply the Law of Dominance
• Know and apply the Law of Segregation
• Know and apply the Law of Independent
Assortment
• Define and recognize homozygous,
heterozygous, genotype, phenotype, and
alleles
• Know how to work a monohybrid cross
Overview
• Mendel discovered that when crossing white flower and
purple flower plants, the result is not a blend. Rather
than being a mix of the two, the offspring was purple
flowered. He then conceived the idea of heredity units,
which he called "factors", one of which is a recessive
characteristic and the other dominant.
• Mendel said that factors, later called genes, normally
occur in pairs in ordinary body cells, yet segregate
during the formation of sex cells. Each member of the
pair becomes part of the separate sex cell.
Define
Homozygous
• Homozygous - both alleles are alike
(same) (AA or aa)
• Both alleles are dominant (TT)
or
• Both alleles are recessive (tt)
• Describes an organism with both alleles
the same for a trait.
Define
HETEROZYGOUS
•
•
•
•
Hybrid
One dominant and one recessive allele
Tt
Describes an organism with two
DIFFERENT alleles
Define
GENOTYPE
•
•
•
•
•
•
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The genetic make up of an organism
Always represented by letters
T = tall
t = short
Genotype for tall = (TT) or (Tt)
Genotype for short = (tt)
You can NOT tell the genotype by looking at an
organism.
Define
PHENOTYPE
• Way an organism LOOKS and or BEHAVES.
• You tell the phenotype by LOOKING at the organisms or
its parents etc.
• Examples of phenotypes
• The person is tall
• The person is short
• The organism has a tail
• The organisms has red hair
Define
ALLELES
• The different forms of a trait that a gene may have
• Alleles determine traits
• All cells in your body except sex cells have two alleles
for each trait
• Sex cells have only one allele
• Dominant allele is represented by a CAPITAL letter (T)
• Recessive allele is represented by a lower case letter (t)
• Traits will carry two alleles (pair of alles = trait)
• ex. (tall (Tt) or (TT) or short (tt))
Law of Segregation (The "First Law")
• The Law of Segregation states that when any
individual produces gametes, the copies of a gene
separate so that each gamete receives only one
copy. A gamete will receive one allele
• Alleles of genes separate when gametes are formed.
• Parent = tall (Tt) The offspring will receive only ONE
of the alleles
• Father = Tt
Mother = Tt
• Parents cells = two alleles
• Father could pass on only ONE allele either a T, or t
• Mother could pass on only ONE allele either a T, or t
• Gametes are sex cells and sex cells have only ONE
allele for a trait
• One from the father and one from the mother
Law of Independent Assortment
(The "Second Law")
• states that alleles of different genes assort independently
of one another during gamete formation
• the law of independent assortment; during gamete
formation the segregation of the alleles of one allelic pair
is independent of the segregation of the alleles of
another allelic pair
• In independent assortment the chromosomes that end
up in a newly-formed gamete are randomly sorted from
all possible combinations of maternal and paternal
chromosomes
Law of Independent Assortment
(The "Second Law")
• In other words when Tt sorts during meiosis (gamete
formation) no one allele has an affect or influence on the
other allele. So T does not affect the t when forming sex
cells
•
These allele pairs are then randomly united at
fertilization.
• This means that traits are transmitted to offspring
independently of one another.
Law of DOMINANCE
• If the organism has the dominant allele then that allele
will be expressed.
• The dominant allele will mask or hide the recessive allele
when heterozygous
• Tt Dominant allele is T = Tall
• TT Dominant allele is T = tall
• The recessive allele will be expressed ONLY when it is
homozygous
• tt both recessive = t = short
Monohybrid crosses
• Perform the following crosses
• TT * TT
• TT * tt
• TT * Tt
• Tt * tt
• Tt * Tt
TT * TT
T
T
T
TT
TT
T
TT
TT
All will be tall
TT=100 % tall
0% short
TT * tt
t
t
T
Tt
Tt
T
Tt
Tt
All will be tall
Tt =100 % tall
0% short
Recessive is masked
Tt * tt
t
t
T
Tt
Tt
t
tt
tt
Tt =50% tall
Tt =50% short
TT * Tt
T
t
T
TT
Tt
T
TT
Tt
All will be tall 100% tall
TT= 50 % tall
Tt= 50% tall
0% = short
Tt * Tt
T
t
T TT
Tt
t Tt
tt
75% = tall
25% - short