Transcript File

Start-up for Tuesday, November 13, 2012
1.
Complete the following crosses. Show your work
in Punnett Squares.
T = Talll
R = Round
t = Short
r = Wrinkled
A. Single Trait = If you cross a heterozygous tall
plant with a short plant, what percent of the
offspring do you expect to be tall?
B. Two traits = What is the phenotypic ratio if
you cross a pure tall round pea plant with a short
wrinkled pea plant?
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Genetics – Mendelian Exceptions
Multiple Alleles and
Sex-linked Genes
Chapter 14 Section 2
Objectives: Students will
A) Identify the genotypes of males compared to females
B) Define sex-linked traits
C) Complete punnett squares for sex-linked traits
D) Predict offspring outcomes of multiple allele crosses
E) Read and predict genetic outcomes from pedigrees
2
Exceptions to Mendel’s Laws
The elegance of Mendel’s experiments
was partly due to the complete
consistency between his observations
and hypotheses he developed.
However, after Mendel’s work was
rediscovered, it became clear that the
simple Mendelian model did not
adequately predict experimental
observations in all situations.
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Exceptions to Mendel’s Laws
Variations on Mendel
1.
2.
3.
4.
5.
Incomplete Dominance
Codominance – Smiley Face Lab
Multiple Alleles
Polygenic Inheritance
Sex-linked Traits
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A) Identify the genotypes of males compared
to females.
Human males are the
heterogametic sex with two
different sex chromosomes,
(XY).
Human females are the
homogametic sex (XX).
During fertilization, what is
the probability that a female
will result?
50%
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Obj. B) Define sex-linked traits
Chromosomes carry genes. Genes are located on
autosomes and sex chromosomes.
Genes on
Autosomes – body traits
Sex chromsomes- sex traits
However, certain genes present in the sex
chromosomes control the body traits.
The characters which are controlled by such genes
are called sex-linked traits and transmission of
such traits from one generation to next is called
sex-linked inheritance
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Obj. B) Define sex-linked traits
Discovery and types of Sex linked inheritance
Was first discovered by Thomas H. Morgan in
1910 on Drosophila melanogaster (fruit flies).
It can be classified into three types depending
upon the chromosomes (X or Y) having sex linked
genes.
1. X – Linked inheritance
2. Y - Linked inheritance
3. XY - Linked inheritance
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Obj. B) Define sex-linked traits
1. X- Linked inheritance
Certain sex-linked genes are located only on X
chromosomes and their alleles are absent in Y
chromosomes.
These genes are called X–linked genes;
Their mode of inheritance is called X-linked
inheritance
This pertains to the inheritance of traits which are
controlled by genes located in the non-homologous
part of X-chromosomes
Examples: Colorblindness, Hemophilia,
Duchenne Muscular Dystrophy
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Obj. C) Complete punnett squares for sexlinked traits
Color blindness –
the inability to see
red and green
XBXB
- Normal female
XBXb
-
Carrier female
XbXb
- Affected female
XBY
- Normal Male
XbY
- Affected male
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Sex-linked
Recessive Practice
Obj. C) Complete punnett
squares for sex-linked traits
Trait: Color blindness
Cross: Carrier female x Color Blind male
XBXb
x
XbY
Use genotypes on previous slide.
Xb
XB
XBXb
Y
XBY
Phenotypes:
25% Carrier Females
25% Color Blind Females
25% Normal Males
Xb
XbXb
XbY
25% Color Blind Males
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Sex-linked
Recessive Example
Obj. C) Complete punnett
squares for sex-linked traits
Trait: Color blindness
Cross: Carrier female x Normal male
XBXb
XB
XB
XBXB
x
Y
XBY
XBY
Phenotype:
25% Normal Females
25% Carrier Females
25% Normal Males
Xb
XBXb
XbY
25% Color Blind Males
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Obj. B) Define sex-linked traits
See page
X – Linked Recessive:
345 for a list
1. Affects males more than females
2. No female offspring of an affected male are
affected, the trait skips generations in
pedigrees. When is there an exception?
Exception to this pattern occurs in the rare
instance when the affected male mates with a
female carrier, producing an affected female
offspring.
X – Linked Dominant:
1. Affected male produce all affected female
offspring and no affected male offspring
2. Approximately half the offspring of affected
females are affected, regardless of their sex.
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Multiple Alleles
It is generally accepted that a gene has two alternative
forms called Alleles.
Usually, one of the two alleles of a gene is dominant
over the other, which is recessive e.g, Tall (TT)
But in many cases, several alleles of a single gene govern
the concerned trait and is known as Multiple alleles.
Both prokaryotes and Eukaryotes show multiple alleles.
Examples:
1. Human Blood Type
2. Fur color in rabbits and other mammals.
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Obj. D) Predict offspring outcomes of multiple allele crosses
Multiple Alleles – Blood type
Safe to
Safe to
Transfuse Transfuse
to
From
A, AB
A, O
B, AB
B, O
AB
A, B, AB,
O
A, B, AB,
O
O
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Obj. D) Predict offspring outcomes
of multiple allele crosses
Example: homozygous male Type B (IBIB)
x
heterozygous female Type A (IAi)
Genotype:
IB
IB
IA
IAIB
IAIB
i
IBi
IBi
50% = IAIB
50% = IBi
Phenotype:
50% = AB blood
type
50% = B blood
type
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Obj. D) Predict offspring outcomes
of multiple allele crosses
Practice: male Type O (ii)
x
female type AB (IAIB)
i
i
IA
IB
IAi
IBi
IAi
IBi Phenotype:
Genotype:
50% = IAi
50% = IBi
50% A blood type
50% B blood type
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Obj. E Read and predict genetic outcomes from pedigrees
Pedigree Symbols
Male =
Female=
Sexual Union=
Offspring=
Always read the
legend
Obj. E) Read and predict genetic outcomes from pedigrees
Shaded =
color blind
No shade =
normal vision
1.
2.
3.
A.
B.
C.
What does a line between a male and female
Sexual Union
indicate?
What do the circles and squares beneath this
mean? Children
What are the genotypes of the following individuals:
b
Individual #8, Generation III
XY B b
Individual #3, Generation II
X
X
B
b
Individual #2, Generation III X X
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Obj. C) Complete punnett squares for sex-linked traits
Hemophilia
Hemophilia is another sex-linked trait.
Hemophilia is characterized by delayed blood
clotting. People with hemophilia can easily bleed
to death from very minor wounds.
Otherwise called “Bleeders disease”.
Hemophilia is sex-linked recessive trait (hh) and
the genes are located on X chromosomes.
It is more common in men and rare in women.
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Understanding Check – Sex-linked
This disease appeared as a mutant in
Queen Victoria and from her it was
transmitted to her descendants.
“Royal disease” = Hemophilia, Bleeder’s
Disease
XH XH
XH Xh
XH Y
Xh Y
Xh Xh
- Normal female
- Carrier female
- Normal Male
- Affected male
- Affected female
Cross a carrier female with a normal male. Identify the
phenotypes.
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Understanding Check – Multiple Alleles
Cross a heterozygous type B blood type with a AB
blood type
_______ x _______
Phenotype Percentages:
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