Mendel, Genes, and Inheritance Chapter 12. Gregor Mendel Austrian Monk with a strong background in...
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Transcript of Mendel, Genes, and Inheritance Chapter 12. Gregor Mendel Austrian Monk with a strong background in...
Gregor Mendel Austrian Monk with a strong background in plant
breeding and mathematics Using pea plants, found indirect but observable
evidence of how parents transmit genes to offspring
Mendel’s Work Bred pea plants
cross-pollinated true breeding parents (P)raised seed & then
observed traits (F1) filial
allowed offspring to cross-pollinate & observed next generation (F2)
Mendel’s Experimentstrue-breeding
purple-flower peastrue-breeding
white-flower peasX
100%100%
purple-flower peasF1generation(hybrids)
25%white-flower peas
F2generation
75%purple-flower peas
3:1
P
self-pollinate
Genes Sequences of DNA that contain
information about specific traits, by coding for individual proteins
Passed from parents to offspring Each has a specific location (locus) on a
chromosome
Alleles Different versions of the same trait
Genes and inheritence
For each characteristic (gene), a diploid organism inherits 2 sets of alleles1 from each parent
Homologous chromosomesPairs of chromosomes that contain the same
traits (genes)For each trait there are two alleles (one on
each homologous chromosome)
Genetic Terms
purple-flower allele & white-flower allele are 2 DNA variations at flower-color locus
different versions of gene on homologous chromosomes
Mendel’s Findings: Dominance
Some alleles for a given traits mask others Dominant allele
Fully expressed in a hybridDesignated by a capital letter
e.g. P = Purple allele
Recessive allele no noticeable effect in a hybridDesignated by a lowercase letter
E.g. p = White allele
Genotype vs. phenotype Phenotype
Description of an organism’s trait“visible” characteristic
Genotype Description of an organism’s genetic makeup;
i.e. which alleles are present in the organismAlleles may be the same or they may be
different:
Homozygous = same alleles; PP, pp Heterozygous = different alleles; Pp
Looking closer at Mendel’s worktrue-breeding
purple-flower peastrue-breeding
white-flower peasX
100%purple-flower peas
25%white-flower peas
75%purple-flower peas
3:1
PP pp
Pp Pp Pp Ppself-pollinate
????
Phenotype:
100%
100%purple-flower peasF1
generation(hybrids)
F2generation
75%purple-flower peas
3:1
P
Punnett squares
Pp x Pp
P pmale / sperm
P
p
fem
ale
/ eg
gs
PP
Pp pp
Pp
PP
Pp
Pp
pp
75%
25%
3:1
25%
50%
25%
1:2:1
%genotype
%phenotype
Phenotype vs. genotype
2 organisms can have the same phenotype but have different genotypes
PP homozygous dominantpurple
Pp heterozygouspurple
Dominant phenotypes It is not possible to determine the
genotype of an organism with a dominant phenotype by looking at it.
So how can you figure it out?
PP?
Pp?
Test cross
Cross-breed the dominant phenotype — unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele
x
ppis itPP or Pp?
Mendel’s laws of heredity (#1)
PP
P
P
pp
p
p
Pp
P
p
Law of segregation when gametes are produced
during meiosis, homologous chromosomes separate from each other
each allele for a trait is packaged into a separate gamete
Monohybrid cross Some of Mendel’s experiments followed the
inheritance of single characters flower colorseed color monohybrid crosses
Dihybrid cross
Other of Mendel’s experiments followed the inheritance of 2 different characters seed color and
seed shape Dihybrid crosses
Dihybrid crosstrue-breeding
yellow, round peastrue-breeding
green, wrinkled peasx
YYRR yyrrP
YyRr
100%F1generation(hybrids)
yellow, round peas
self-pollinate
F2generation
9/16yellowround peas
9:3:3:13/16
greenround peas
3/16yellow
wrinkledpeas
1/16green
wrinkledpeas
Y = yellowR = round
y = greenr = wrinkled
Dihybrid Cross How are the alleles on different chromosomes
handed out? together or separately?
YyRr
YR yr
YyRr
Yr yRYR yr
Dihybrid crossYyRr YyRr
YR Yr yR yr
YR
Yr
yR
yr
YYRR YYRr YyRR YyRr
YYRr YYrr YyRr Yyrr
YyRR YyRr yyRR yyRr
YyRr Yyrr yyRr yyrr
x9/16
yellowround
3/16greenround
3/16yellow
wrinkled
1/16green
wrinkled
Mendel’s laws of heredity (#2)Law of independent assortment: each pair of alleles segregates into gametes
independently 4 classes of gametes are produced
in equal amounts YR, Yr, yR, yr
only true for genes on separate chromosomes
YyRr
Yr Yr yR yR YR YR yr yr
Law of Independent Assortment What meiotic
event creates the law of independent assortment?
Meiosis 1
The chromosomal
basis of Mendel’s laws…
Trace the genetic events through meiosis, gamete formation & fertilization to offspring
Review: Mendel’s laws of heredity Law of segregation
each allele segregates into separate gametes Law of independent assortment
Observable in dihybrid (or more) cross 2 or more traits
each pair of alleles for genes on separate chromosomes segregates into gametes independently
Each gamete carries one allele of each trait
Extending Mendelian genetics Mendel worked with a simple system
peas are genetically simplemost traits are controlled by a
single geneeach gene has only 2 alleles, 1 of which is
completely dominant to the other The relationship between
genotype & phenotype is rarely that simple
Incomplete dominance Heterozygotes show an intermediate
phenotypeRR = red flowersrr = white flowersRr = pink flowers
make 50% less color
Incomplete dominancetrue-breeding
red flowerstrue-breeding white flowers
X
100%
100% pink flowers
F1generation(hybrids)
25%white
F2generation
25%red 1:2:1
P
self-pollinate50%pink
Incomplete dominance
CRCW x CRCW
CR CW
male / sperm
CR
CW
fem
ale
/ eg
gs CRCR
CRCW CWCW
CRCW
25%
1:2:1
25%
50%
25%
1:2:1
%genotype
%phenotype
CRCR
CRCW
CRCW
CWCW
25%
50%
Codominance
BOTH alleles express in heterozygotes Example: In chickens, feather color trait
has two alleles:B= Black feathers
W = White feathers
What is phenotype of BB, WW?What is the phenotype of BW?
Multiple Alleles
More than two possible alleles for one trait Example: in ABO blood group, 3 possible
alleles: IA, IB, i
4 blood types: A, B, O, AB How?
Pleiotropy Most genes are pleiotropic
one gene affects more than one phenotypic character
wide-ranging effects due to a single gene: dwarfism (achondroplasia) gigantism (acromegaly)
Epistasis One gene masks another
coat color in mice = 2 genes
pigment (C) or no pigment (c)
more pigment (black=B) or less (brown=b)
cc = albino, no matter B allele
9:3:3:1 becomes 9:3:4
Epistasis in Labrador retrievers 2 genes: E & B
pigment (E) or no pigment (e) how dark pigment will be: black (B) to brown (b)
Polygenic inheritance (continuous variation)
Some phenotypes determined by additive effects of 2 or more genes on a single characterphenotypes on a continuumhuman traits
skin color height weight eye color intelligence behaviors
Environmental influence
The expression of some genes can be influenced by the environment
for example: coat color in Himalayan rabbits and Siamese catsan allele produces an enzyme that allows
pigment production only at temperatures below 30oC