Mendel & Genetics

Chapter 14. Mendel & Genetics
Gregor Mendel
Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas
used experimental method
used quantitative analysis
collected data & counted them
excellent example of scientific method
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 collected data for 7 pea traits
Looking closer at Mendel’s work
true-breeding
purple-flower peas
true-breeding
white-flower peas
X
P
What did Mendel’s findings mean?
Traits come in alternative versions
purple vs. white flower color
alleles
different alleles vary in the sequence of nucleotides at the specific locus of a gene
purple-flower allele & white-flower allele are 2 DNA variations at flower-color locus
different versions of gene on homologous chromosomes
Traits are inherited as discrete units
For each characteristic, an organism inherits 2 alleles, 1 from each parent
diploid organism
inherits 2 sets of chromosomes, 1 from each parent
homologous chromosomes
like having 2 editions of encyclopedia
Encyclopedia Britannica
Encyclopedia Americana
What did Mendel’s findings mean?
Some traits mask others
purple & white flower colors are separate traits that do not blend
purple x white ≠ light purple
purple masked white
dominant allele
fully expressed
recessive allele
no noticeable effect
the gene makes a non-functional protein
Genotype vs. phenotype
difference between how an organism “looks” & its genetics
phenotype
description of an organism’s trait
genotype
description of an organism’s genetic makeup
Explain Mendel’s results using
…dominant & recessive
…phenotype & gentotype
Making crosses
using representative letters
flower color alleles  P or p
true-breeding purple-flower peas  PP
true-breeding white-flower peas  pp
Pp
Looking closer at Mendel’s work
true-breeding
purple-flower peas
true-breeding
white-flower peas
X
100%
100%
purple-flower peas
25%
white-flower peas
75%
purple-flower peas
3:1
PP
pp
Pp
Pp
Pp
Pp
?
?
?
?
phenotype
100%
100%
purple-flower peas
F1
generation
(hybrids)
25%
white-flower peas
F2
generation
75%
purple-flower peas
3:1
P
Punnett squares
Pp x Pp
PP
Pp
pp
Pp
3:1
1:2:1
%
genotype
%
phenotype
Genotypes
Homozygous = same alleles = PP, pp
Heterozygous = different alleles = Pp
homozygousdominant
homozygousrecessive
Phenotype vs. genotype
2 organisms can have the same phenotype but have different genotypes
Dominant phenotypes
It is not possible to determine the genotype of an organism with a dominant phenotype by looking at it.
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
pp
is itPP or Pp?
PP
pp
Test cross
p
p
P
P
Pp
Pp
Pp
Pp
p
p
P
p
Pp
pp
pp
Pp
Pp
pp
100%
50%:50%1:1
Mendel’s laws of heredity (#1)
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
Law of Segregation
What meiotic event creates the law of segregation?
Monohybrid cross
Some of Mendel’s experiments followed the inheritance of single characters
flower color
seed 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 cross
true-breeding
yellow, round peas
true-breeding
green, wrinkled peas
x
YYRR
yyrr
P
YyRr
Y = yellow
R = round
y = green
r = wrinkled
What’s going on here?
How are the alleles on different chromosomes handed out?
together or separately?
Dihybrid cross
YyRr
YyRr
YR
Yr
yR
yr
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
x
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?
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
monohybrid cross
single trait
each allele segregates into separate gametes
established by Meiosis 1
Law of independent assortment
dihybrid (or more) cross
2 or more traits
each pair of alleles for genes on separate chromosomes segregates into gametes independently
established by Meiosis 1
Mendel chose peas wisely
Pea plants are good for genetic research
available in many varieties with distinct heritable features with different variations
flower color, seed color, seed shape, etc.
Mendel had strict control over which plants mated with which
each pea plant has male & female structures
pea plants can self-fertilize
Mendel could also cross-pollinate plants: moving pollen from one plant to another
Mendel chose peas luckily
Pea plants are good for genetic research
relatively simple genetically
most characters are controlled by a single gene
each gene has only 2 alleles, one of which is completely dominant over the other
Any Questions??