Download Patterns of Gene Inheritance Mendel`s Laws Gregor Mendel What

Mendel’s Laws
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Patterns of Gene Inheritance
Gregor Mendel
What Mendel Said:
1. Characteristics controlled by unit factors
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exist in pairs in organisms
2. When two unlike unit factors responsible for
single character present in single individual,
one dominant and other recessive.
3. Each individual has two unit factors for each
trait formed in gametes
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Must separate (segregate) factors during gamete
formation so that each gamete receives only one
version of two factors (=law of segregation)
Gene locus
Gregor Mendel –
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Austrian monk
Developed laws of heredity
Worked with pea plants
Investigated genetics at organism level
What We Know Now
• Alternative forms of a gene having the same
position on pair of homologous chromosomes
affect the same trait = alleles
– Alleles occur at same loci (position) on chromosome
• Factors segregate during formation of gametes
– Each gamete has one factor from each pair
– Happens in anaphase of meiosis I
• Fertilization gives each new individual two
factors again
The Inheritance of a Single Trait
- COMPLETE DOMINANCE INHERITENCE • A capital letter indicates a dominant
allele, which is expressed when present.
– An example is W for widow’s peak
• A lowercase letter indicates a recessive
allele, which is expressed in the absence
of a dominant allele.
– An example is w for continuous hairline.
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Widow’s peak
Genotype
- Genotype - genes of individual represented
by two letters or descriptive phrase
– Homozygous - both alleles same; for example,
WW, homozygous dominant and ww,
homozygous recessive.
– Heterozygous -both alleles pair present and
different, Ww.
Phenotype
• Phenotype - physical or observable
characteristic of individual
– WW and Ww result in widow’s peak, two
genotypes with the same phenotype
• NOTE: So far we have only discussed the
concept of complete dominance
– When dominant allele of characteristic is
expressed over recessive allele
– Other types of dominance in allele
expression discussed later
Gamete Formation
• Because homologous pairs separate
during meiosis, a gamete has only one
allele from each pair of alleles.
– If the allelic pair is Ww, a gamete would
contain either a W or a w, but not both.
• Ww represents the genotype of an
individual.
• Gametes are represented by W or w.
One-Trait Crosses
• In one-trait crosses, only one trait such as
type of hairline is being considered.
• When performing crosses, the original
parents are called the parental
generation, or the P generation.
• All of their children are the filial
generation, or F generation.
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Punnett Squares
Monohybrid Cross
• If know genotype of parents, it’s possible
to determine gametes and use Punnett
square to determine phenotypic ratio in
offspring
• When a monohybrid reproduces with a
monohybrid, results are 3 : 1
– Ratio used to state chances of particular
phenotype
– 3 : 1 ratio means there is a 75% chance of the
dominant phenotype and a 25% chance of the
recessive phenotype appearing in F1 progeny
The One-Trait Testcross
One-trait testcross
• It is not always possible to determine a
homozygous dominant from a
heterozygous individual by the phenotype
• A testcross “crosses” the dominant
phenotype with the recessive phenotype
• If homozygous recessive phenotype is
found in offspring, parent must be
heterozygous
The Inheritance of Many Traits
Independent Assortment
• Law of independent assortment states
that each pair of alleles segregates
independently of the other pairs and all
possible combinations of alleles can
occur in the gametes.
• Law dependent on random arrangement
of homologous pairs at metaphase
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Segregation and Independent
Assortment
One-Trait Crosses and Probability
• Laws of probability used to determine
results of a cross
The laws are:
(1) Probability of two or more independent
events occurring together is product of
chance occurring separately
(2) Chance an event can occur in two or
more independent ways is sum of
individual chances
Sponge Question
Probability Exemplar
• In the cross of Ww x Ww, what is the chance of
obtaining either a W or a w from a parent?
• Chance of W = .5 , or chance of w = .5
• The probability of these genotypes is:
The chance of WW = .5 x .5 = .25
The chance of Ww = .5 x .5 = .25
The chance of wW = .5 x .5 = .25
The chance of ww = .5 x .5 = .25
• The chance of widow’s peak (WW, Ww, wW) is .25 + .25 +
.25 = .75 or 75%.
– We know this because the the possible outcomes of 2
or more events are independent of one another, but
can be accomplished in summation to each other =
Sum Law
• Consider the two genes: Y (yellow) and y
(green) and R (round) and r (wrinkled).
• If the P1 generation looks like the following
and thus has the genotype listed:
1. What will be the genotypic and phenotypic
rations of the F1 progeny?
2. If you Cross two F1 progeny, what will be the
genotypic and phenotypic rations of the F2
generation?
Sponge Answer
• Possible Gametes of P1: yR and Yr Cross:
female
yR
male
Yr
YyRr
F1 = 1/1, 100%, YyRr,
yellow round
seeds
• Given the F1 YyRr:
1.) All possible gametesof RrYy: RY, Ry, rY, ry
2.) Where 1 gamete = 1/4 (.25) of 4 gametes
3.) RY Ry rY ry X RY Ry rY ry
Sponge Answer
4.) Complete the punnett
square of: RY Ry rY ry
X RY Ry rY ry
5.) Phenotypic and
Genotypic Ratios:
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Yellow round = 3/4 X
3/4 = 9/16 9
Yellow wrinkled = 3/4
X 1/4 = 3/16 3
Green round = 1/4 X
3/4 = 3/16 3
Green wrinkled = 1/4 X
1/4 = 1/16 1
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Two-Trait Crosses
Dihybrid Cross
• Two-trait crosses - genotypes of parents require
four letters because allelic pairs for two traits
– Any two phenotypes (law of segregation), segregates
the traits together for up to 16 possibilities
– Gametes contain one letter of each in all combinations
• Dihybrid reproduces with a dihybrid the results are
9:3:3:1
• Consider a mating between the following:
– Male parent homozygous dominant for widows peak
and short finger length with a female parent
homozygous recessive for strait hair line and long
finger length
– Further consider a mating between a male F1
progeny to a female just like his mother
Sponge Question
Two-trait Cross
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Sponge Answer Part I
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BB=Black Hair, Bb=Brown Hair bb=Blond Hair
EE=Brown Eyes, Ee=Green Eyes, ee=blue eyes
SS=Dark Skin, Ss=Somewhat Dark Skin ss=light skin
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Male Parent has black hair, brown eyes and dark skin (BBEESS)
Female Parent has brown hair, green eyes and somewhat
dark skin - (BbEeSs)
2.
In humans, black hair is dominant to blond hair AND brown
eyes is dominant to blue eyes AND dark skin pigment is
dominant to light skin pigmentation
You are a family genetics counselor. A couple comes to you
and asks you some questions. They want to know the
following:
1. What are their odds of having a child with dark skin, brown
eyes and brown hair?
2. What are their odds of having a BOY with dark skin, brown
eyes and brown hair?
The male parent has black hair, brown eyes and dark skin and
the female parent has brown hair, green eyes and somewhat
dark skin.
1. Please advise the couple by giving them phenotypic and
genotypic ratios of their F1 progeny?
2. Show them your data by supplying either a punnett square
OR a forked-line diagram
Sponge Answer Part II
• There is a
12.5% chance of
having a child
that has brown
hair, brown eyes
and dark skin.
• If they had a
boy there would
be an 1/16 or 6%
chance of
having just a
boy with these
characteristics
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Genetic Disorders
Pedigrees
Patterns of Inheritance
• When studying human disorders,
biologists often construct pedigree charts
to show the pattern of inheritance of a
characteristic within a family.
• The particular pattern indicates the
manner in which a characteristic is
inherited.
• Pedigree charts represent males as
squares and females as circles.
• Recessive and dominant alleles have
different patterns of inheritance.
• Genetic counselors construct pedigree
charts to determine the mode of
inheritance of a condition.
Autosomal Recessive Pedigree
Complete Dominant Pedigree
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