Download Transmission Genetics

Transmission Genetics
How traits are passed down from
generation to generation.
 Transmission of genes and the
phenotypes which come from those genes
 The phenotype determines how the
individual interacts with the world, and
it is the phenotype that is subject to
natural selection.
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Gregor Mendel
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1856 -1863
monk, Czech Republic
Studied 7 traits in pea plants, Pisum sativum
 Established basic rules of transmission genetics
 Good science, but ignored for >30 years
Why peas?
 Many varieties with contrasting traits
 Self-pollinating, with true-breeding varieties
 easy to snip parts to cross pollinate
 Need little space, produce lots of offspring
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His experiments would not have worked out
except:
1) He chose traits that were all dominant
or recessive (“contrasting traits”)
2) He chose traits that were all located on
different chromosomes (pea plants
have 7 chromosomes)
Pretty amazing since he had no idea how
these traits were passed on – he called
them “unit factors”
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Mendel’s unit factors we now call genes.
 The two versions of each gene are called
alleles.
 If an organism has two copies of the same
allele is it said to be homozygous.
True breeding
 If an organism has different alleles of the
same gene it is said to be heterozygous.
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 An
allele is dominant, if it is expressed
whenever that allele is present.
 A recessive trait is one that is hidden by
another.
 It
is only expressed when the allele is
homozygous, that is 2 copies of it and none of
another allele.
 A dominant allele masks, or hides a recessive
allele.
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The Genotype is all the genetic
information the individual has for a
particular trait.
 In
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a diploid organism, that means two copies.
Those traits that are expressed: can be
seen (physical traits) or measured
(chemical traits) are the individual’s
phenotype.
 An
individual heterozygous for a trait may only
show the effect of one allele (a dominant one)
and not the other recessive one.
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Breeding two different, genetically distinct
organisms is called cross breeding or
crossing.
 The offspring of such crosses are called
hybrids.
 The parents are called the parental or
P generation
 The offspring of these parents are called
the F1 generation (first filial)
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Dominant traits are written with capital letters.
Recessive traits are written with small letters.
P = purple pigment (Purple flowers)
p = no pigment (white flowers)
PP = ?
Pp = ?
pp = ?
PP and Pp = purple flowers
pp = white flowers
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Homozgous individuals are true breeding
(produce same trait when self-pollinated)
 What would we get if we crossed a
homozygous purple flower (PP) with a
homozygous white flower (pp)?
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To find out, we can use a Punnett square –
named after Reginald Punnett
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Parent 1
P
a
r
e
n
t
2
pp
P
P
p
Pp
Pp
p
Pp
Pp
(PP)
Gametes made; each has a
50% chance of getting an
allele from either homolog.
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Results: the offspring are all
heterozygous
Genotype of all is Pp
 P is dominant to p, so all have purple
flowers; the white flower phenotype is
masked (p, white, is recessive)
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What would happen if we crossed
members of the F1 generation?
P
p
P
p
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Genotypes: 1: 2 : 1
PP : Pp : pp
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If you come upon a plant with purple flowers,
it could be because the plant has a
genotype of PP, homozygous for purple;
OR it could be heterozygous, Pp, with the
white allele masked.
How do we find out if it is homozygous or
heterozygous ?
We could do a back cross or test cross –
breed the individual with a homozygous
recessive individual.
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
Why breed it with a homozygous
individual?
 Because
it is the only parent that we can be
sure of its genotype from its phenotype!
 PP is purple, but so is Pp (that’s our question)
 Because the only way a recessive trait can be
seen is if there is no dominant trait hiding it,
then a white flower MUST have the genotype
pp which means you can tell what the
genotype is from the phenotype.
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Genotypic ratio 1:1 Pp : pp
If the purple plant was homozygous, the F1
generation would all be purple - Pp
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But, not all traits show simple dominantrecessive relationships. There is also partial
dominance where both traits are expressed.
Some traits show incomplete dominance.
Snap dragons have genes for red flowers (R1)
and white flowers (R2).
A heterozygous flower (R1R2) would be
Pink!
This type of trait gave early scientists the idea
that traits blended in offspring of different
individuals.
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Notice that both
traits are given
capital letters, and
the F2 generation
shows a 1:2:1 ratio
of phenotypes as
well as genotypes.
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Other traits show Codominance where both
alleles are equally expressed.
Blood types: A B O blood groups
Glycolipids on the cell membrane of RBCs.
IA, IB and i (A and B are codominant; O is
recessive)
IAIA = Type A IBIB = Type B
IAi = Type A
IBi = Type B
IAIB = Type AB
ii = Type O
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Mendel’s Crosses Showed:
Principle of segregation: each sexually
reproducing organism has two genes for
each characteristic, and these two genes
segregate or separate during the
production of gametes.
 Principle of independent assortment:
traits which lie on different chromosomes
are passed on independently of each
other.
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Mendel’s Law of Segregation
Parent 1
P
a
r
e
n
t
2
pp
P
P
p
Pp
Pp
p
Pp
Pp
(PP)
Gametes made; each has a
50% chance of getting an
allele from either homolog.
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Mendel’s Independent Assortment
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With these rules in mind, we can cross
individuals that have two different traits.
Dihybrid (vs. monohybrid) cross.
We can cross peas that have green pods (G)
which are inflated (I) with peas that have
yellow pods (g) which are constricted (i).
GG II X gg ii = Gg Ii
parents
F1
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The only “trick” to a dihybrid cross is setting
up the Punnett square.
GgIi →
GI, Gi, gI, gi
Like making a snack – take one of each
Chips: Fritos or Doritos
Fruit: apple or a peach
candy bar: snickers or butterfinger
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GI
Gi
gI
gi
GI
GGII GGIi
GgII
GgIi
Gi
GGIi GGii
GgIi
Ggii
gI
GgII
GgIi
ggII
ggIi
gi
GgIi
Ggii
ggIi
ggii
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Genotypes :
1 GGII : 2 GgII : 2 GGIi : 4 GgIi
1
GGii : 2 Ggii 1 ggII : 2 ggIi
1 ggii
Phenotypes:
9 Green inflated
3 Green constricted
3 Yellow inflated
1 Yellow constricted
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Sex-Linked traits
Some genes carried on the X chromosome
are missing from the Y chromosome.
 These traits show up in different ratios in
males and females and are called sexlinked traits
 Males are said to be hemizygous for these
traits since they can only have one gene
and a recessive gene will always be
expressed.
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Color blindness is carried on the X
chromosome (X’) Normal color vision (X)
X
X
XX
Y
XY
X’
XX’
X’Y
The females all have
normal color vision, but
half the males are color
blind.
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X
X’
X’
XX’
X’X’
Y
XY
X’Y
Here half the females
and half the males are
color blind.
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What happens when two traits are
located on the same chromosome?
They tend to be passed on together – this is
called genetic linkage.
Can these two traits be inherited separately?
The likelihood that two genes on the same
chromosome will be inherited separately
depends on the distance between them.
They can be separated during “crossing
over” that occurs during Prophase I of
meiosis.
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