Download Genetics Part 1

Mendelian
Genetics
Genetics
Study of heredity, or the
passing on of
characteristics from
parent to offspring.
Important concepts for review
• Meiosis divides the genes found
on homologous chromosomes
from parent cells into separate
gametes.
Important concepts for review
• Fertilization combines genes
found in opposite sex cells in
order to form new offspring with
half of each parent’s traits.
How was genetics
discovered?
• Genetics was
discovered by
Gregor
Mendel. So he
is also known
as the Father of
genetics
Some Terms
• True-breeding –
produce offspring
identical to themselves
if they self pollinate
• Hybrids – offspring
of crosses between
parents with different
traits
• P generation – the
parents of a cross
• F1 generation – the
offspring of the P
generation
• F2 generation –the
offspring of the F1
generation
How was genetics discovered?
• Gregor Mendel observed traits or
characteristics of the garden pea.
• Some are short-stemmed, some seeds
are round, some are yellow...
• He observed that different pea traits
are produced by different varieties of
parent plants.
How do traits get inherited?
Pea plants self-pollinate
Resulting in true-breeds – pea plants
with only one form of a
characteristic: only the short allele,
or only round seeds, only the yellow
pea pod color…
Mendel controlled the
pollination of two true
breed pea plants.
Mendel’s Experiments
What would result if a true breed yellow pea plant was
crossed with a true breed green pea plant?
What happened to the yellow?
Did yellow disappear or is green simply dominant?
Mendel then crossed
the F1 generation that resulted from the previous cross…
Yellow did not disappear; green is dominant
*What did he realize?*
Key terms highlighted
1. There is some hidden factor that controls
inheritance. It’s called a gene.
2. Genes can be of alternate forms called
alleles.
E.g. one form of the gene that determines pod color was
green; the other was yellow.
*What did he realize?*
Key terms highlighted
3. Alleles may be dominant or recessive. The
dominant trait is expressed or shown while
the recessive is hidden.
Alleles are carried on
opposite homologous
chromosomes.
We’ll label the
dominant allele with a
capital letter P and
the recessive allele
with a lower case p.
*What did he realize?*
Key terms highlighted
4. Organisms inherit two alleles for each
trait. One from each parent.
Allele combinations are either homozygous (both alleles
are identical; AA or aa) or heterozygous (alleles are
different; Aa).
5. Gametes play a role in genetics. Sex cells
only carry one allele each because alleles
segregate into separate gametes during
meiosis. This is known as the Law of
Segregation.
Watch the segregation of A, a, B, b genes by clicking here
Through Mendel's observations we are
now able to predict the probability of the
genes a offspring may have. Base upon
the parents traits.
*Predicting genetic outcomes*
Monohybrid cross
A Punnett Square is used to
predict the probability of genetic
outcomes. Here’s how…
• The square has 2 columns
and 2 rows
• Each row and column
represents one of the 2
possible alleles carried by the
sex cells of each parent (i.e.
accounting for the 50%
probability of inheriting from
either gamete).
*Predicting genetic outcomes*
1. The dad’s genotype is
segregated on top of the
square
2. The mom’s genotype is
segregated on the side of
the square
*Predicting genetic outcomes*
3. The 4 boxes in the square
are filled in with the gene
type from dad’s gametes
(B)
4. The 4 boxes in the square
are filled in with the gene
type from mom’s gametes
(b)
Terms
• Dominant – the trait that is expressed (T)
• Recessive – the trait that is hidden (t)
• Homozygous – has two of the same allele
(TT or tt)
• Heterozygous – has one of each allele (Tt)
What’s being expressed
SIMPLE DOMANCE
• TT- dominate trait
• Tt- Dominate trait
• tt- recessive trait
*Predicting genetic outcomes*
Accounting for probability in the
monohybrid cross (Punnett
square):
• 50% chance of inheriting 1
of 2 gamete types from father
(columns)
• 50% chance of inheriting 1
of 2 gamete types from
mother (rows)
• Thus, there’s a 25% chance
that sperm 1 fertilizes egg 1
What did
Mendel’s cross
look like?
True-breeding yellow pod
plants have 2 recessive
alleles (gg) for pod color.
True-breeding
green pod
plants have 2
dominant
alleles (GG) for
pod color.
The resulting plants are hybrids (Gg).
*Allele Expression*
Genotype
Phenotype
(allele type or “ingredients”)
(expression or “cake”)
TT
Tall
(homozygous=same)
(Dominant)
Tt
Tall
(heterozygous=diff.)
(Dominant)
tt
Short
(homozygous)
(Recessive)
How to write ratios
• Phenotype –
How many look
like what
• Genotype- How
many have what
alleles
Try this one…
The ability to roll the tongue is dominant
over the inability to do so in humans.
(A) If two heterozygous tonguerollers have children, what genotypes
could their children have?
Hint: T=tongue-rolling and t=non-tongue-rolling
A little different….
• A man and a woman are heterozygous
for freckles. Freckles (F) are dominant
over no freckles (f). What are the
chances that their children will have
freckles?
• A woman is homozygous dominant for
short fingers (SS). She marries a man
who is heterozygous for short fingers
(Ss). Will any of their children have long
fingers (ss)? yes / no
Requires some deep thought…
Start by writing what you know!
An allele for brown eyes B is dominant
over that for blue eyes b. A couple of
whom one is brown-eyed and the other
blue-eyed have eight children, all brown
eyed. What would be the genetic make
up of each parent in this regard? For
each parent state whether they are
homozygous or heterozygous.
Here’s a tricky one…
• A blue-eyed man, both of whose
parents were brown-eyed, marries a
brown-eyed woman. They have one
child who is blue-eyed. What are the
genotypes of all the individuals
mentioned?
• FYI: BROWN IS DOMINANT OVER
BLUE, USE B’S JUST LIKE LAST
PROBLEM
*Reproduction*
Asexual
One parent
Sexual
2 parents
= one form of the gene
= variations in gene
Self-pollination
results in a true
breed.
Cross-pollination,
conjugation or
fertilization results
in a hybrid.
Mendel In Summary
• 1st Law of Dominance: A dominate trait
masks or hide the expression of the other
trait
• 2nd Law of Segregation: alleles segregate
during gamete formation (meiosis)
• Offspring inherit 2 alleles for each trait
• Allele combinations are either homozygous
or heterozygous, which influences
expression