Transcript 29 inheritance

Genetics: Inheritance
- Simple Mendelian genetics
Punnett squares
- Dihybrid crosses
gene linkage
- Other patterns of inheritance
Refer to chapter 11 in text
A sample of simple human traits
widow’s peak
have
____
don’t
____
dimples
____
____
unattached ear lobes
____
____
straight thumbs
(vs. hitchhiker’s thumb)____
____
tongue rolling
____
____
polydactyl
____
____
PTC tasting
____
____
↓Gregor Mendel in the garden.
How he assured the parentage
of his pea plants→
P (parental) generation
- true breeding
genotype = homozygous
- Includes two phenotypes
- Each can only make one
gamete type (P, p)
F1 (first filial) generation
- phenotypically uniform
showing dominant trait
(recessive trait is masked)
- genotype = heterozygous
- Gametes could be p or P
from any plant represented.
F2 (second filial) generation
- phenotypes show a 3:1 ratio
dominant : recessive
- 1:2:1 genotypic ratio
Punnett squares
You have a purple-flowered and a white-flowered pea plant:
What are their genotypes?
How do you test that?
p
p
P
P
P p
P p
p
Pp
p
P p
P
p
Pp
pp
Pp
pp
phenotypes?
phenotypes?
If you have a dominant-phenotype expressed
and cross with the recessive type,
it is called a test cross.
Dihybrid cross
…looking at two traits at once.
Assuming the traits sort
independently
(they are unlinked),
and they are autosomal
(not sex-linked, i.e. on a
sex chromosome)…
Any pairing of genes in
is equally likely.
(This is a Mendelian cross,
starting with true-breeding P)
What is the resultant
phenotypic ratio?
9:3:3:1
What is the phenotype ratio for each trait,
separately?
If genes are in a linkage group
(near each other on a chromosome)
they won’t sort as expected.
Purple flower (P)
dominant over red (p):
Long pollen (L)
dominant over short (l):
Resulting F2 from self-cross →
PL
pL
pl
PL
PPLL PPLl PpLL PpLl
Pl
PPLl
pL
PpLL PpLl ppLL ppLl
pl
PpLl
(Could you set up the 4x4 Punnett Square?)
example lifted from Allott
Pl
PPll
Ppll
PpLl
ppLl
Ppll
ppll
Expected phenotypic ratio (9:3:3:1)
in 6952 plants=
3910.5 : 1303.5 : 1303.5 : 434.5
observed
4831 : 390 : 393 : 1338
NB - Genotypes for linked autosomes
PL or pL
have a special way of being written:
pl
Pl
(What determines which it is?.... “=“ can be seen as chromosomes.
Any that are not like one of the parental types are recombinant.
How does this occur, if they are “linked”?
How often does recombination occur?
Hey… is this like that gene mapping thing?
Other patterns of inheritance
Mendel worked with dominant/recessive traits…
What if the heterozygous phenotype is
intermediate to the homozygous
phenotypes (a black chicken crossed
with a white makes speckled?)
Codominance
the heterozygote shares
the phenotypes;
both forms are seen together.
What if a red flower crossed with a white one
produces pink?
Incomplete dominance
(There is some argument that incomplete
dominance is just codominance on a local level…)
What if there are more than two choices for a given trait, like rabbit fur?
Multiple Alleles: more than two versions, or two alleles, for a given gene.
What if the trait shows a wide
range of phenotypes, like human
height or skin color?
Polygenic Inheritance
More than one gene
controls that trait
What if a trait shows a gender bias?
Queen Victoria was a carrier
← for hemophilia (XhXH)
Sex-linked - the allele is carried on the X (usually)or Y chromosome.
This also demonstrates the use of a pedigree to track a condition:
Note symbols (□, o, half and totally filled, Mader p. 198 has a good walk through).
Hemizygous: X?Y
What is the chance of Prince William having a child with hemophilia?
Human X chromosome
color blindness
is also X-linked
(Xb vs. XB)
Human Y chromosome
“Real human Y
chromosome”
http://www.life.uiuc.edu/bio100/lectures/s07lects/12s07-chromo.html
pleiotropy: some genes have more than one affect.
Sickle-cell is cited in text as an example:
The point mutation impacting Hb has many ramifications.
epistasis: one gene can specifically impact the expression of another.
Albinism is an example of this:
The genes for various colors might be there,
but homozygous recessive for a color-deposition gene (cc)
at a different locus, prevents the colors from being seen.
the ABO blood group is an
example of both multiple
alleles and codominance:
The alleles code for blood
proteins A, or B, or neither
(denoted IA, IB, and i, for
immunoglobulin A, etc.).
A and B are codominant,
both are dominant over O.
If you have protein A (only),
you make anti-B antigens,
and vice-versa…
Hence the concept of
universal donor (O) or
recipient (AB).
Rh factor is at another locus.
Other considerations:
Internal environment:
A gene might not be activated
until puberty,
may only be expressed in
combination with sex-inked
genes (bird plumage),
or when other compensating genes
shut down with age (gray hair).
External environment:
A gene might only be expressed
at a given temperature,
or might be activated by chemicals
or viruses.
Multifactorial:
Genes can be polygenic and
subject to environmental influence
Probabilities:
- If a man has type A blood, and his wife type B,
what types could their kids have?
What if his father was type O?
What if her parents were both AB?
- If your grandfather has Huntington’s disease (dominant),
what is the likelihood you will have it?
- If your sister is colorblind,
what is the likelihood your daughter also will be?
What is meant by linked genes?
How would you test for it?
How might environment affect gene expression?
Discuss two distinct aspects.
What is a test cross?
What are you testing?
Give the names and examples
of 3 non-traditional inheritance patterns.
One plant is homozygous for yellow seeds, which are wrinkled;
Another is heterozygous for both color and seed shape.
Determine the resulting phenotypic ratio.
Determine your parents’ blood types:
What were the possible types in their kids?
What was the probability of you getting yours?
genotype
dihybrid cross
polygenic inheritance
P
autosomal
sex linked
homozygous
sex chromosome
pedigree
phenotype
9:3:3:1
hemizygous
F1
3 to 1
carrier
dominant
1 to 2 to 1
pleiotropy
recessive
linkage group
epistasis
heterozygous
Punnett Square
ABO blood group
F2
recombinant
internal environment
test cross
codominance
external environment
unlinked
incomplete dominance
multifactorial
multiple alleles