Transcript Document

Introduction to Genetics
Maternal
Paternal
B
b
B
BB
Bb
b
Bb
bb
Patterns of Inheritance
twins
sisters
brothers
Father and son
Family
Mom and offspring
Diploid – 2
complete sets
of chromosomes
and 2 complete
sets of genes
Haploid – 1 set
of chromosomes
and 1 set of genes
Homologous chromosomes: Same genes, but different
versions, in the same location, each coming from one parent
Crossing Over: Results in the exchange of alleles between
chromosomes resulting in a new combination of alleles
Some genes appear to be inherited together, or
“linked.” If two genes are found on the same
chromosome, does it mean they are linked
forever?
Study the diagram, which shows four genes
labeled A–E and a–e, and then answer the
questions on the next slide.
1. In how many places can crossing over
result in genes A and b being on the same
chromosome?
2. In how many places can crossing over
result in genes A and c being on the same
chromosome? Genes A and e?
3. How does the distance between two genes
on a chromosome affect the chances that
crossing over will recombine those genes?
1. One (between A and B)
2. Two (between A and B and A and C);
Four (between A and B, A and C, A and
D, and A and E)
3. The farther apart the genes are, the
more likely they are to be recombined
through crossing over.
Prophase II
Meiosis I results in
two haploid (N)
daughter cells, each
with half the number
of chromosomes as
the original.
Metaphase II
Anaphase II
Telophase II
The chromosomes
line up in a similar
way to the
metaphase stage of
mitosis.
The sister
chromatids separate
and move toward
opposite ends of the
cell.
Meiosis II results in
four haploid (N)
daughter cells.
* Austrian monk
* Teacher of high school natural
science love of evolution,
nature, meteorology
* “for the fun of it”: crossed peas
and mice- saw inheritance
patterns
* pea plants- a formal test
1. The research
pea plants- why?
- structure (male and female parts
on same plant)
- distinctive traits
- rapid reproduction
- ability to control pollination and
fertilzation
Used “true-breeding”
plants
-if self-pollination
occurs, offspring
produced that are
identical to parent
**Cross-pollination
allowed for him to
prevent self-pollination
and study the results
1. Mendel studied the inheritance
of one trait (for example plant's height, color
of flowers or color and shape of seeds).
2. Mendel first cross pollinated tall pea
plants (identified asTT, height of plants in
this variety were about six feet tall) with
each other.
3. Mendel then cross pollinated short pea
plants (identified as tt, height of plants in this
variety were about one foot tall) with each
other.
X
In every generation of this plant only short
plants were produced. He concluded that
the pea plant must contain some factor for
height (in that variety - for shortness).
4. The next step of Mendel's experiment
was to crossed tall pea plants (TT) with
short pea plants (tt). The resulting plants
were labeled Tt and only tall plants were
produced.
F1 Generation – had
the recessive alleles
disappeared?
F1 generation
crossed by self
pollination to create
F2 generation
Tt
X
Tt
F1
Segregation
t
T
TT
Tt
t
T
Tt
tt
F2
Alleles segregate from each other so that each gamete carries
only a single copy of each gene. Alleles pair up again when
gametes fuse during fertilization
Mendel’s next question: Does the segregation of one pair of
alleles affect the segregation of another pair of alleles?
EX: Does the gene that determines whether a seed is round or
wrinkled in shape have anything to do with the gene for seed
color?
Conclusion: genes that segregate
don’t influence each others
inheritance. This is why we have
so many genetic differences in
plants and animals.
1. Incomplete Dominance – some alleles are neither dominant
or recessive
2. Codominance: both alleles contribute to the phenotype
3. Multiple Alleles: Genes that have more than two alleles.
EX: Blood type exists as four possible phenotypes: A, B, AB, &
O.
There are 3 alleles for the gene that determines blood type.
(Remember: You have just 2 of the 3 in your genotype --- 1
from mom & 1 from dad).
CODES FOR
The alleles are as follows:
ALLELE
IA
IB
i
Type "A"
Blood
Type "B"
Blood
Type "O"
Blood
GENOTYPES
I AI A
IAi
RESULTING
PHENOTYPES
Type A
Type A
I BI B
IBi
Type B
Type B
IAIB
Type AB
ii
Type O
1.A woman with Type O blood and a
man who is Type AB have are
expecting a child. What are the
possible blood types of the kid?
Step #1, figure out the genotypes of ma & pa
using the given info.
"Woman with Type O" must be ii, because that is
the one & only genotype for Type O.
"Man who is AB" must be IAIB, again because it is
the one & only genotype for AB blood.
So our cross is: ii x IAIB. The proper p-square
would look like this:
As you can see, our
results are as follows:
50% of kids will be
heterozygous with
blood Type A
50% will be
heterozygous with
blood Type B
Single gene, four different alleles
C = full color dominant
Cch=chinchilla; dominant to ch & c
Ch=Himalayan; dominant to C
c=albino; recessive
 What allele combinations can a chinchilla rabbit have?
 What are the possible offspring that would result when
a chinchilla rabbit mates with a himalayan rabbit?
Exceptions to Mendel’s Principles
Sex-linked Inheritance: The X and Y chromosomes in
mammals do not have exactly the same genes.
 In mammals, the Y-chromosome is small and does not
carry many genes.
 Sex-linked genes linked
to X chromosome
X-linked Inheritance
 Color blindness is one
example of X-linked
inheritance.
Exceptions to Mendel’s Principles
Polygenic Inheritance: A characteristic controlled by
more than one gene (many genes shaping one
phenotype)
 Human Examples:
 Skin Color
 Eye Color
 Height
Polygenic Inheritance: Human
Skin Color
Dominant alleles
O
aabbcc
 1-2
3
=
very light skin
AABbcc =
medium
AaBbCc
AaBBcc, etc.
AABBCC =
very dark skin
AaBbCc
x
AaBbCc
