Download Alleles - mykingbiology

Genetics
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Heredity – the
passing of traits from
parent to offspring
Genetics- the study of
heredity
Part I: Mendel and
the Gene Idea
I. Gregor Mendel
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He was a monk, gardener, mathematician
1st to apply statistics to selective breeding
Published work on pea plant inheritance
patterns in the 1860’s.
(nothing known about the
cell for inheritance…)
Why was the pea a good choice?
1) Cheap and available
2) Produce offspring quickly/short generation
time
3) Characteristics are determined on 1 gene
with only 2 alleles.
Ex. Flower color: purple or white
4) No trait blending
5) Can control breeding. (Easy access to male
and female sex parts)
Mendel’s Questions
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Mendel crossed (bred) two different plants
to discover what traits the offspring would
show.
Ex: Will a purple flower plant crossed with
a purple flower plant produce all purple
offspring?
II. Basic genetic
concepts
A. Mendel studied 7
different characters in
peas:
Ex)
Height: tall vs. dwarf
Seed shape: smooth vs.
wrinkled
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Characters – inherited features of an organism.
Traits – variations of a character.
Ex) character:
fur color
possible traits:
brown, black, red
Review of chromosomes
A. Each = one DNA molecule
B. Gene – DNA sequence found on a particular
chromosome, that codes for a particular trait.
C. Each may have thousands of genes for a
particular set of genes
•
Ex. Human chromosome 11 has the genes for
making the enzymes hemoglobin, catalase, and
insulin (and thousands of other genes)
Genes express alleles
Alleles are possible forms of the same
gene.
Eye color
brown
blue
More about alleles:
Dominant allele – the “stronger” allele. In a
heterozygous combination the dominant allele will
be expressed.
Symbol is a capital letter
Ex) trait: pea plant height
tall is dominant, tall = T
Recessive allele – “weaker” allele. In the
heterozygous individual these are hidden, and the
dominant form of the trait will be expressed.
Symbol is a lowercase letter
Ex) trait: pea plant height
dwarf is recessive, dwarf = t
Allele combinations for genes
HOMOZYGOUS – both gene are the same.
 Ex) Seed coat trait:
wrinkled seed allele
and wrinkled seed allele
 HETEROZYGOUS – genes are different
 Ex) seed coat trait
wrinkled seed allele and round seed allele f
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Genotype: the alleles an individual has
Phenotype: the way those alleles are
expressed
Ex) pea plant height
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Possible genotypes: T T, T t, or t t
Possible phenotypes: tall or dwarf
Possible combinations:
Ex) pea plant height
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Homozygous dominant = TT, tall
Homozygous recessive = tt, dwarf
Heterozygous = Tt, tall
Example of an individual’s allele
combinations on three gene loci
Mendel’s Experiments
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True-breeder – an individual that always
produces offspring with the same expression for
a given trait.
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P1 – the parent generation. Two true breeders
with different traits for the same character are
crossed
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Ex) purple flower plant always produces purple flower
offspring
Ex) true breeding purple flower pea X true breeding
white flower pea
F1 – first filial generation. The offspring
(progeny) of the P1
F2 – second filial generation. The offspring of
two individuals from the F1 generation.
Experiment
Results
C. Mendel’s Conclusions
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Mendel’s Rules of Inheritance –
generalizations made by Mendel
Law of Segregation
Law of Independent Assortment
Mendel’s Rules of Inheritance
1)
Different versions of genes account for
variety in organisms.
2) For each character, an organism inherits
two alleles, one from each parent.
3) If two alleles differ, then one (dominant)
is fully expressed, and the other
(recessive) has no effect.
Mendel’s Laws of Inheritance
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Law of Segregation - The two alleles a parent has
for each character segregate during gamete
production.
Ex. Heterozygote pea for flower color
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Pp : half of gametes get P, other half get p
Law of Independent Assortment – for each gene the
alleles separate independently of alleles for other
genes
Ex) heterozygote for flower color (Pp) and seed
color (Gg)
some gametes will get PG, some will get Pg, pG, or
pg, in a ¼ ratio
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Refers to two or more genes/characters
A Test Cross
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WHY? To determine whether an organism that
has the dominant trait is homozygous dominant
or heterozygous.
HOW? Cross with a homozygous recessive
organism. Does the recessive phenotype show
up?
How to read the results?
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100% dominant offspring = homozygous dominant
parent
ANY recessive offspring = heterozygous parent.
Patterns of Inheritance NOT
revealed by Mendel’s studies
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Incomplete dominance
Codominance
Multiple alleles
Polygenic traits
Incomplete dominance
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In heterozygotes both alleles are
expressed so the trait blends.
Example: flower color in snapdragons
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allele F R = red
allele F W = white
In heterozygotes (F R F W) = pink
Codominance
In heterozygotes both alleles are
expressed in separate distinguishable ways
 Example: Roan horse
 RR – there are red hairs
 rr – there are white hairs
 Rr – some hairs are
white and some are red
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Multiple alleles
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There are more than two possible alleles for
a character.
Example: human ABO blood group
3 alleles A, B, and O (allele = O)
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O is recessive, A and B are codominant
4 possible phenotypes
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Type
Type
Type
Type
A blood (genotypes: AA or AO)
B blood (BB or BO)
O blood (OO)
AB blood (AB)
Human Blood Types
Polygenic inheritance
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-many genes affect one phenotype.
Ex) human skin pigmentation
Sex-linked genes
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– genes that are on the X chromosome
show unique inheritance patterns
The Y chromosome (ONLY male gender).
The X chromosome has other genes on it.
Sex-linked traits
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can pass from mom to sons OR daughters
can pass from the dad to ONLY daughters
Mendelian Inheritance in humans:
- from dominanat/recessive
alleles on one gene.
Examples:
• Widow’s peak
• Attached or free earlobes
• Recessive disorders*: Ex.s) cystic fibrosis,
Tay-Sachs disease, sickle-cell disease
• Dominant disorders*: Ex.) Huntington’s
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*Determined with pedigree analysis, genetic
testing and counseling