Download Theoretical Genetics

Theoretical Genetics
Beyond Mendel
Topics
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Non-Disjunction
Co-Dominance
Polygenic Traits
Pedigree Chart
Sex Linkage
Linkage Group
Non-Disjunction
• Abnormal segregation of homologues during
Anaphase I of meiosis
• Tetrads fail to separate
• Homologous chromosomes migrate into the
same cell
• Results in gametes with extra, or missing,
chromosomes.
• Ex. Trisomy 21 – Down Syndrome
Trisomy 21 - Characteristics
ncbi.nlm.nih.gov
www.cdc.gov/Features/d
sDownSyndrome/
Co-Dominance
• Also known as incomplete dominance
• Two dominant alleles can both be expressed
in an organism’s phenotype
• example
– Blood type: A and B are both dominant
oftentimes
more than one
gene
influences a
phenotype
Polygenic Traits
• A phenotype (physical trait) that is
determined by more than one gene, often
many
• Usually results in phenotypic values that
display an incremental range in a bell shaped
curve within a population
• Ex. Human height and weight
Bell Shaped Curve – Normal
Distribution
Polygenic Traits show Normal
Distribution
Pedigree Chart
Sex Linkage
• Traits, or genes, that are located on the X or Y
sex chromosome
• Recall: XX = female
XY = male
• Many sex linked diseases are recessive and
found on the X chromosome
• Females can be heterozygous carriers (or
afflicted), males are always afflicted.
• X HX h
XhY
Sample Question
What is a difference between autosomes and sex chromosomes?
A. Autosomes are not found in gametes but sex chromosomes
are.
B. Sex chromosomes are found in animal cells and autosomes
are found in plant cells.
C. Autosomes are diploid and sex chromosomes are haploid.
D. Sex chromosomes determine gender and autosomes do not.
Sample Question – Sex Linkage
A male and female with normal color vision each have a father
who is color blind. They are planning to have children. Predict,
showing your working, the possible phenotypes and
genotypes of male and female children.
Key
male
female
affected male
affected female
?
Linkage Group
• Mendel’s Law of Independent Assortment
states that alleles of different traits segregate
independent from one another.
• But, this is only true if the genes are located
on different chromosomes. Why?
Linkage Group
• A linkage group consists of genes for different
traits, found on the same chromosome.
• The alleles of the traits do not separate, they
must move into the same gamete together
during meiosis.
• The exception is if the alleles are separated by
crossing over during prophase I.
The closer two genes are located on a chromosome, the less
likely the alleles will separate during crossing over
Sample Question
In garden peas, the pairs of alleles coding for seed shape and
seed colour are unlinked. The allele for smooth seeds (S) is
dominant over the allele for wrinkled seeds (s). The allele
for yellow seeds (Y) is dominant over the allele for green
seeds (y).
If a plant of genotype Ssyy is crossed with a plant of
genotype ssYy, which offspring are recombinants?
A.
B.
C.
D.
SsYy and Ssyy
SsYy and ssYy
SsYy and ssyy
Ssyy and ssYy
Sample Question
A cross is performed between two organisms
with the genotypes AaBb and aabb.
What genotypes in the offspring are the result
of recombination?
A. Aabb, AaBb
B. AaBb, aabb
C. aabb, Aabb
D. Aabb, aaBb
Sample Question
A polygenic character is controlled by two genes
each with two alleles. How many different
possible genotypes are there for this
character?
AaBB
aaBB
A. 2
AABB
AABb
B. 4
AAbb
aabb
C. 9
Aabb
AaBb
D. 16
aaBb
Sample Question
In Zea mays, the allele for coloured seed (C) is dominant over the allele for
colourless seed (c). The allele for starchy endosperm (W) is dominant over
the allele for waxy endosperm (w). Pure breeding plants with coloured
seeds and starchy endosperm were crossed with pure breeding plants
with colourless seeds and waxy endosperm.
(a) State the genotype and the phenotype of the F individuals produced as a
result of this cross.
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genotype
...............................................................................................................
phenotype
................................................................................................................
(b) The F plants were crossed with plants that had the genotype c c w w.
Calculate the expected ratio of phenotypes in the F generation, assuming
that there is independent assortment. Use the space below to show your
working.
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