Patterns of Inheritance
Chapter 12
You are not responsible for
the section on epistasis
Read on your own the
sections on
-- Environmental influences
-- Pleiotrophy
Gregor Mendel, 1862
Mendelian Inheritance
Convent Garden, 1920s
1
What was Mendel’s contribution to Biology?
Mendel uncovered ‘rules’
of heredity
Augustinian Monk (Czech
republic) 1856-1865
Gregor Mendel, 1862
Why pea plants?
Variation in traits
Can control pollination
Peas normally selfpollinating
Mendelian Inheritance
Convent Garden, 1920s
2
What were some typical
results of Mendel’s
experiments?
Worked with “true breeding” varieties
P
P
F1
cross-pollinated true breeding:
P, F1 & F2 generations
F1 x F1
F2
some traits “skip” a generation
Why?
3 : 1 ratio
Mendelian Inheritance
3
Why do traits sometimes ‘skip’ a
generation?
P
P
F1
Mendel deduced:
True breeding: hold information for only a single trait
F1: possess information for both traits
‘Dominant’ trait is one that appears
‘Recessive’ trait is suppressed
Mendel saw many such relationships
Mendelian Inheritance
4
How can an organism possess
information for two different traits?
1) organisms possess 2 ‘genes’ for a trait
2) specific information is called an ‘allele’
Mendel’s “Principle of Segregation”
Each trait is inherited as a pair of alleles, which separate
in the gametes and recombine upon fertilization
Round ‘R’
Wrinkled ‘r’
What are three possible combinations of alleles?
2 dominants = ‘homozygous dominant’
1 dom & 1 rec = “heterozygous’
2 recessives = ‘homozygous recessive’
What is Phenotype vs Genotype?
-- genetic information
vs its physical expression
Genotype
RR
Rr
rr
A dominant allele
Is designated with
uppercase Letter.
A recessive allele
with the corresponding
lower case letter
Phenotype
round
round
wrinkled
Question
Mendelian Inheritance
5
How can a Punnett square be used to predict the outcomes of
crosses?
r
r
RR
P
X rr
=
F1
all Rr
True breeding traits must have a
homozygous genotype
R
Rr
Rr
R
Rr
Rr
R
F1 X
Rr
F1
X
=
F2
r
R
RR
Rr
Round
Round
r
Rr
rr
Round
wrinkled
Rr
Only the homozygous recessive
genotype will yield the recessive
phenotype
Genotypes of gametes
are placed on borders
Ratio of genotypes= 1:2:1
Ratio of phenotypes= 3:1
Crosses that examine 1 gene at a time are called “monohybrid”
Mendelian Inheritance
6
What are the characteristics of Autosomal
Recessive traits and disorders?
Carried on non-sex chromosomes
Phenotype can skip generations
People can be a carrier
And many disorders, e.g.
Cystic fibrosis & Tay Sachs
Question
hexadactyly
freckles
Mendelian Inheritance
7
What are the characteristics of Autosomal
Dominant traits and disorders?
Also on non-sex chromosomes
Phenotype does not skip generations
50 or 100% of children will get trait
And Various disorders, e.g.
Huntingtons
Question
Cleft chin
Mendelian Inheritance
8
How does meiosis explain Mendel’s Principle of Segregation?
Key questions:
1) Where do a pair of
alleles exist in the cell?
2) When does separation
of alleles occur?
3) When does recombination
occur?
Probability yields 3:1 ratio in phenotypes
Mendelian Inheritance
9
What is Mendel’s ‘Principle of Independent Assortment’?
Alleles for one trait sort independently
of the alleles for a different trait
--illustrated by a Dihybrid Cross
Gamete genotypes
Gamete genotypes
Question 1
Question 2
Mendelian Inheritance
10
How can the outcomes of
‘multiple-hybrid’crosses be
predicted mathematically?
How does meiosis explain
Mendel’s Principle of
Independent Assortment?
Ratios of ‘multiple-hybrid’ crosses are the
product of the ratios of monohybrid crosses
RrYy x RrYy
¾ Round : ¼ wrinkled
Yy x Yy = ¾ Yellow : ¼ green
Rr x Rr =
(¾R_ : ¼rr)
x (¾Y_ : ¼yy)___
9/16
R_Y_
Rnd Yel
:
3/16
:
R_yy
Rnd Grn
3/16
rrY_
Wrk Yel
:
1/16
rryy
Wrk Grn
In a cross of RrYy x RrYY what is the
expected frequency of “rrYy”?
¼x½=⅛
Question
Mendelian Inheritance
11
Why do inheritance patterns sometimes not
follow normal Mendelian ratios?
How is Incomplete Dominance
different from ‘normal’ dominance?
HS
-- heterozygote has intermediate phenotype
HC
HS
HC
HS HS
HS HC
Straight
Wavy
HS HC
HC HC
Wavy
Curly
e.g. Human hair form
-- two alleles: HS – straight
and HC – curly
In a mating of heterozygotes, what is expected frequency
of hair styles among children?
Other examples:
Pea flower color (see book)
Chicken “Naked neck” allele
homozygote NA NA -- normal neck feathers
homozygote Na Na -- lack neck feathers
heterozygote NA Na -- reduced # of feathers
Mendelian Inheritance
12
What is Codominance?
Tabby
-- heterozygote has distinctive phenotype
-- combination of alleles yields new trait
Tabby gene (T) affects patterning of cat fur
TSTS – striping of colored and white hairs = ‘Tabby’
TATA – no striping (hairs are mixed) = ‘Agouti’
TSTA – ‘chinchilla-like’ hair: color-tipped hairs
Agouti
chinchilla
Question
Mendelian Inheritance
13
How can “multiple alleles” for a gene influence inheritance?
-- When more than 2 alleles for a trait exist in the population
Inheritance of blood type
-- involves codominance and multiple alleles
Antigens: present on the cells
Blood
Group
Antigen
on cells
A
A
B
B
O
neither
AB
A&B
Alleles code for antigens
-- 3 alleles in species
-- each person can possess only 2
Allele & antigen
Expression
Associated
Blood group
Possible
genotype
IA yields A antigen
Codominant
A
IA IA or IA i
IB yields B antigen
Codominant
B
IB IB or IB i
i
Recessive
O
ii
yields no antigen
[Given tables such as these, you should be able to fill in missing values]
Mendelian Inheritance
14
Inheritance of blood type, con’t.
What is the theoretical frequency of genotypes and
phenotypes among children of heterozygous-A and a
heterozygous-B parents?
IB
If a child is heterozygous for B-type blood, what are the
possible blood types of the parents?
i
IA
i
IA IB
IB i
AB
B
IA i
ii
A
O
What causes blood type incompatibility?
-- role of antibodies
Blood
Group
Antigen
on cells
Antibodies
in serum
A
A
anti-B
B
B
anti-A
O
U-donor
Neither
anti-a & anti-b
AB
U-recipient
A&B
neither
Blood type and Paternity
If a child has type-O blood, could a man
With type-B blood be the father? Explain.
Question 1
Question 2
Mendelian Inheritance
15
ONON
What is a Lethal Allele?
OOON
-- homozygous recessive is fatal
Overo gene affects hair color pattern in horses
Two alleles: ON = normal OO = white
OOON X OOON yields ratio of 2 patterned : 1 solid
Why?
OOOO : lethal aganglionic colon
-- absence of nerves in colon
‘Creeper’ gene in birds; CA = ‘Creeper’ allele
causes deformed wings and legs;
heterozygotes (CNCA) display creeper trait
OOOO
This is impossible: Why?
CNCA X CACA
Mendelian Inheritance
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Sickle-cell anemia: incomplete dominance and lethal allele
(This topic is in Chapter 13)
Cause
SC anemia
Genetics
HbA: normal allele HbS: abnormal
Effects
-- ‘SC-anemia’: early death
-- ‘SC-trait’: moderate symptoms
Consider cross of heterozygotes
HbA
Why does a lethal allele persist?
HbS
HbA
HbS
HbA HbA
HbA HbS
Normal
SC trait
HbA HbS
HbS HbS
SC trait
SC anemia
Question 1
Question 2
Mendelian Inheritance
17
What are the characteristics of Polygenic Inheritance?
-- when more than 1 gene contributes to a phenotype
e.g., human height, intelligence, eye color, skin color, etc
Skin color
Melanin production
-- dominant
Why variation?
No longer reflect long-term
phylogenic relationships
May be 9 genes involved
-- 3 shown
Note: this is not albinism
Question
Mendelian Inheritance
18