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Theoretical Genetics
GENETICS
Gregor Mendel – The Father of Genetics
 High School teacher and minister
 One of side jobs was as a gardener
 While gardening he completed
the work that changed biology
forever.
 Pea plants!!!
 LUCKY LUCKY LUCKY MAN!
MENDEL’S PEAS
During sexual reproduction, male and
female reproductive cells join in a
process called Fertilization
Pea plants in most gardens are selfpollinating which results in offspring
identical to itself.
Mendel knew this and used this
information to “mix it up” a bit
PASS THE PEAS PLEASE
Mendel looked at 7 traits that he observed
in the plants which included:
Height – Tall or short
Seed Color – Yellow or Green
Seed Shape – Round or wrinkled
Pod shape – Smooth or constricted
Seed Coat color – Gray or white
Pod Color – Green or yellow
Flower Position – Axial or Terminal
LETS TALK ABOUT SEX (PEA STYLE)
Mendel crossed the plants by removing the male
portions of the plants and controlling pollination
of the plants.
Mendel crossed plants with each of the seven
contrasting characteristics and studied their
offspring.
The offspring of crosses between parents of
different traits are called hybrids.
WHAT HE SAW!
After studying the offspring he came up with two
conclusions:
 Biological inheritance is determined by factors that are
passed from one generation to the next. Today scientists
call them genes.
 Principle of Dominance: some alleles are
dominant(CAPITAL LETTER) and others are
recessive(lower case letter).
Dominant alleles will always exhibit that form of the
trait. Recessive alleles are only seen when there is no
dominant allele present
Important terms
·Genotype = the alleles of an organism (e.g. one brown
eyed allele and one blue eyed allele, Tt, tt, TT)
·Phenotype = the physical characteristics of an organism
(e.g. brown eyes, tall, short, tall)
·Homozygous = having two identical alleles for the same
gene (TT, tt)
·Heterozygous = having two different alleles for the same
gene(Tt)
·Dominant
allele = an allele that has the same affect on
the phenotype whether it is present in the homozygous
or heterozygous state (e.g. brown eye allele, Tall gene in
pea plants Tt and Tt are both tall)
- Represented by a capital letter in the genotype
·Recessive allele = an allele that only has an affect on
the phenotype when present in the homozygous state
(e.g. blue eye allele)
- Represented by a lowercase letter in the
genotype
Carrier = an individual that has one copy of a
recessive allele that causes a genetic disease in
individuals that are homozygous for this allele (e.g.
carriers for sickle cell have a resistance to malaria
but can pass on the sickle cell disease to their
children)
·Test cross = testing a suspected heterozygote by
crossing with a known homozygous recessive
Punnett Squares
·Can be used to determine the offspring
of a monohybrid (one trait) cross
·Male gametes go on one side of the
square while female gametes go on the
other side
·Potential offspring are inside the square
·In the example R is the dominant allele
and codes for Red petals while r is the
recessive allele and codes for white
petals
What percent of offspring will be red?
What percent will be white?
·Some genes have multiple alleles
While multiple alleles may exist in the population,
an individual can still only have two
·Multiple alleles increase the variety of
phenotypes in the population
·Alleles may be dominant, recessive, or
codominant to each other
example, rabbit coat color has four alleles: C, cch, ch, and c
·These dominance of these alleles follows the order in which
they are listed
·Produces several possible phenotypes.
·For
ch
h
C>c >c >c
Full color
C_
Albino
cc
Chinchilla
cchcch, cchch, cchc
Himalayan
chch , chc
·Codominant alleles = pairs of alleles that both affect the
phenotype when present in a heterozygote
·E.g. A cow with the allele for white hair and the allele for
brown hair will be roan (white with brown spots)
Human blood type:
gene with multiple
alleles that are
codominant
·Three alleles and four
phenotypes
·The ‘I’ in the
genotype stands for
immunoglobin
·i represents the
recessive allele that
codes
for
type O blood
A
B
·I and I are
codominant to each
other
·The
allele
hierarchy
A
B
is I = I > i
Sex Chromosomes
·Chromosome pair number 23 are
referred to as the sex chromosomes
and determine gender
- the only pair that might not
match in size/shape
- X chromosome is longer
and contains many more
genes.
·X is always donated to the offspring by the
mother while the sperm may donate either an X or
aY
·An embryo with XX genotype will be female while
an embryo with XY genotype will be male
Some genes are present on
the X chromosome but absent
from the Y
- Y chromosomes have
fewer loci and therefore
fewer genes than X
·Sex Linked Gene: any gene
with its locus on the X or Y
chromosome
- often affect one gender
more than the other.
·
·Color blindness as an example of recessive sex linked gene
4 Sex-Linked Traits:
Normal Color Vision:
A: 29, B: 45, C: --, D: 26
Red-Green Color-Blind:
A: 70, B: --, C: 5, D: -Red Color-blind:
A: 70, B: --, C: 5, D: 6
Green Color-Blind:
A: 70, B: --, C: 5, D: 2
dominant allele is XB while the recessive allele is Xb
·
A female with the genotype Xb, Xb will be color blind
B
b
A female with the genotype X , X is a carrier(not color blind
herself but can pass the recessive gene to her children)
·Any male children who receive the Xb allele from their
mother will be color blind because there is no allele on the
Y chromosome to dominate over it (their genotype will be
b
X ,Y)
·The
·Hemophilia as another example of recessive sex linked gene
Research on your own:
- Characteristics of disease:
- Inheritance pattern:
Pedigrees
·Often geneticists will carry out planned
experiments in which breeding pairs are selected
and the offspring phenotypes counted.
·However this is not acceptable or possible when
working with humans.
·Instead geneticists have to collect information
about individuals and relatives within a family and
construct diagrams of inheritance (family trees)
called pedigrees.
·Pedigrees are often used to track the inheritance of
a disease in a family
·In pedigrees circles represent females while squares represent
males
·Shaded individuals are affected with the condition
·Individuals who are half shaded are carriers (some pedigrees don’t
do the half shading and you have to figure out whether or not they are
carriers by looking at the individuals parents and offspring)
·Horizontal lines connect two people who have mated
·Vertical lines connect parents with children
Trait: Colorblindness
ID the genotype of each family member.
Phenylketonuria (PKU) is a
metabolic disorder and a
recessive genetic condition.
·Which individuals can we
be sure about their
genotype?
·
Since it was not possible to identify the
condition of 12 and 13 suggest their
genotype and phenotype and how the
diagram may need modifying?