Principles of heredity,
Mendelian laws
Marie Černá
Lecture No 403-Heredity
1865 – Gregor Mendel
Mendel’s experiments
Pisum sativum
pod/flower location
7 characters described in work „Experiments with plant hybrids“.
Terms
•
•
•
•
•
character (gene) – a heritable feature (flower color)
trait (allele) – each variant for a character
(purple or white for flower color)
hybrid - the result of breeding (hybridization)
P generation (parental generation)
F1 generation (first filial generation)
F2 generation (second filial generation)
B1 backcross (F1 x parent)
Testcross (PP or Pp x pp)
monohybrid cross - only one character is studied
dihybrid cross - two characters are studied simultaneously
Relation of alleles in a heterozygote
dominance
– manifestation of both AA and Aa genotypes
dominant allele masks the expression of
recessive allele in heterozygote
recessivity – manifestation of aa genotype only
incomplete dominance
heterozygote’s phenotype is intermediate between
that of both homozygotes (dominant and recessive)
Dominant / recessive alleles
at the level of
a) all organism – complete dominance
b) biochemical – incomplete dominance
c) molecular – codominance
• Round seed shape
• Wrinkled seed shape
• Low osmotic pressure
• High osmotic pressure
(no water absorption
(water absorption that
into seeds)
is then lost)
• Enzyme for metabolism • Defective form of
sugar into starch
enzyme
• HOMOZYGOTE
– two identical alleles at a given locus on a pair of
homologous chromosomes => 1 type of gametes
• HETEROZYGOTE
– two different alleles at a given locus on a pair of
homologous chromosomes => 2 different gametes
A
A A
A
A
meiosa
A
A
A
a
a
meiosa
A
A
A
A
a
a
Monohybrid cross
Monohybrid = crosser that is different in 1 allelic pair (gene)
P
x
AA
aa
Aa
F1
Gametes
P(A)=P(a)=1/2
F2
the 1st filial generation
heterozygote
a
A
Genotypes
of gametes
parental generation
homozygotes (dominant and recessive)
A
a
A
AA
Aa
a
Aa
aa
the 2nd filial generation
genotypes of zygotes
the 2nd filial generation
genotype ratio: 1 : 2 : 1
AA : Aa : aa
phenotype ratio: 3 : 1
A- : aa
B1 backcross – verification of hybrid’s heterozygosity (F1 x parent)
Aa x AA
AA AA
Aa Aa
Aa x aa
Aa Aa
ratio 1
aa
aa
:
1
Testcross
(F1 x recessive homozygote)
Dihybrid cross
2 allelic pairs (genes) carried
by different pairs of homologous chromosomes
– segregate independently
two types of parental crosses
P: AABB x aabb
AAbb x aaBB
F1:
AaBb double heterozygote
genotypes of gametes: AB Ab aB ab
1 : 1 : 1 : 1
Punnett square:
F2
Genotypes
of gametes
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
aB
AaBB
AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb
aabb
F2
Genotypes:
AABB AABb AAbb AaBB AaBb Aabb aaBB aaBb aabb
1
: 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1
= (1 : 2 : 1)(1 : 2 : 1)
Phenotypes:
A-BA-bb
aaBaabb
9
:
3
:
3
:
1
= (3 : 1)(3 : 1)
Breeding news = homozygotic recombined forms =
new combinations of parental properties
For parental cross:
AABB x aabb → AAbb, aaBB
AAbb x aaBB → AABB, aabb
B1 backcross - testcross
AaBb x aabb
heterozygote x recessive homozygote
AB
ab
Ab
aB
ab
AaBb Aabb aaBb aabb
1
:
1
:
1
:
1
Trihybrid cross
3 allelic pairs (genes)
four types of parental crosses
P: 1) AABBCC x aabbcc 2) AABBcc x aabbCC
3) AAbbcc x aaBBCC 4) AAbbCC x aaBBcc
F1:
AaBbCc
8 types of gamets:
ABC ABc AbC Abc aBC aBc abC abc
1 : 1 : 1 : 1 : 1 : 1 : 1 : 1
F2:
64 of zygotic combinations
Calculations of ratios:
• by combinatory square (Punnett square)
• by combination of monohybrid ratios
(1AA:2Aa:1aa)(1BB:2Bb:1bb)(1CC:2Cc:1cc)
• by the calculation of probabilities
Polyhybrid cross
n – number of followed traits
• number of gametes 2n
• number of zygotes 4n
• genotype ratio: ( 1 : 2 : 1 )n
• phenotype ratio: ( 3 : 1 )n
MENDEL’S LAWS
1. Uniformity of F1 generation
identity of reciprocal crosses
During formation of gametes:
2. Principle of segregation
two alleles of one gene separate from each other into
two gametes
3. Principle of combination - independent assortment
alleles of two or more allelic pairs (genes) assort
independently of one another
there are as many types of gametes as possible random
combinations of paternal and maternal chromosomes
(genes on different chromosomes behave independently)
Differences from the Mendel’s laws
•
•
•
•
•
•
•
•
•
•
•
Lethality, decreased vitality
Decreased penetrance
Different expressivity
Sex linked heredity
Gene linkage
Gene interactions
Polygenic heredity
Mitochondrial heredity
Gene imprinting (transcription only of one allele)
Dynamic mutations (amplification of triplet repeats)
Structural balance chromosomal aberrations
Lethality, decreased vitality
all genotypes are not vital in the same way
- zygote dying or individuals do not live to fertility
- change of ratios
COMPLETE RECESSIVE LETHALITY
Lethal is a homozygote (dominant or recessive)
0 AA : 2 Aa : 1 aa or 1 AA : 2 Aa : 0 aa
Ex. In mice: A – yellow coat, a – black coat
AA = lethal, yellow mice only Aa
Allele A – 2 expressions (pleiotropy)
- dominant = yellow coat
- recessive = premature embryo dying
COMPLETE DOMINANT LETHALITY
Lethal is a dominant homozygote and a heterozygote
0 AA : 0 Aa : 1aa It forms by new mutation and is not transmitted.
COMPLETE RECESSIVE LETHALITY
gametes
zygotes
adult
individuals
Decreased vitality
- recessive
1 AA : 2 Aa : 1 aa
1 AA : 2 Aa : 1 aa
- dominant
1 AA : 2 Aa : 1 aa
1 AA : 2 Aa : 1 aa
1912 - Thomas Hunt Morgan
Mendel´s 3rd law only applies to allelic pairs (genes)
carrying on different pairs of homologous chromosomes
(independent assortment)
Genes on 1 chromosome – are not assorted
independently - are linked, transmitted together
into gametes
Genes on 1 chromosome = linkage group
Strength of linkage depends on their distance:
More are farther from each other – higher
probability of crossing over
Morgan’s laws
• Genes on chromosome are in linear order
• No of linkage groups is equal to No of
homologous chromosomes pairs
Gametes with assortment of linked alleles,
that is different from parental one, form only
in the case of recombination (crossing over)
Frequency of these recombinations sets
strength of linkage
1944 – Avery, MacLeod, McCarty
the demonstration of DNA as genetic material
Role of genetics in medicine
• Universal validity of Mendel´s laws
• 1902 Garrod: the first human disease
with Mendelian heredity - alkaptonuria
(disorder of metabolism of amino acids phenylalanine and tyrosine)
• Use in medical practice: genetic
counselling –calculation of probability in
monogenic and polygenic disorders
ABO blood groups
ABO blood groups
ABO blood groups
Frequencies (%) of blood group system in
the U.S.A. population
Blood Blood Antibody
group antigen in serum
White
Blacks
Asians
American
Indians
Caucasians
O
H
anti-A,B
45
49
40
79
A
A
anti-B
40
27
28
16
B
B
anti-A
11
20
27
4
AB
AB
4
4
5
1
-
Rh blood group
Rh blood group
Rh blood group
Blood group donors and recipients
Literature
Biology, eighth edition,
Neil A. Campbell, Jane B. Reece
Pearson International Edition, 2008
Unit three: Genetics
Chapter 14: Mendel and the Gene Idea
Chapter 15: The Chromosomal Basis of
Inheritance
pages 262 – 304