Transcript Genetics notes 12 13

Mendelian
Genetics
Genetics
Study of heredity, or the
passing on of
characteristics from
parent to offspring.
*Reproduction Review*
Asexual
One parent
Sexual
Two parents
= genetically identical
= genetic variations
Results in a true
breed.
Results in a hybrid.
Other concepts for review
• Meiosis divides the genes found
on homologous chromosomes
from parent cells into separate
gametes.
• 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?
• 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…
But Mendel controlled
the pollination of two
true breed pea plants
(see picture at right).
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?*
1. There is some hidden factor that controls
inheritance. It’s called a gene. Remember
genes are segments of a chromosome and we
have pairs of homologous chromosomes: one
maternal and one paternal.
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.
*Mendel also realized*
3. Alleles may be dominant or recessive. The
dominant version of the trait will mask the
expression of the recessive version.
Alleles are carried on
opposite homologous
chromosomes (shown
at right).
**We’ll label the
dominant allele with a
capital letter P and
the recessive allele
with a lower case p.
*Mendel also realized*
4. Organisms inherit two alleles for each trait.
One from each parent (i.e. homologous pair).
There are 3 possible allelic combinations that can be
inherited. These are known as genotypes. Genotypes are
either homozygous (both alleles are identical; AA or aa) or
it is heterozygous (alleles are different; Aa). Genotypes are
expressed as phenotypes. These are the observable
characteristics.
See the table of genotypes and corresponding phenotypes
on the next slide.
Genotype
Phenotype
(allele type or “ingredients”)
(expression or “cake”)
TT
Tall
(homozygous=same)
(Dominant)
Tt
Tall
(heterozygous=diff.)
(Dominant)
tt
Short
(homozygous)
(Recessive)
*Mendel also realized*
5. Gametes play a role in genetics. The two
alleles of a person’s genotype segregate into
separate gametes during meiosis so each sex
cell only carries one allele for each trait. This
is known as the Law of Segregation.
If each organism has two alleles for each trait, there are four
possible genetic recombinations that can result from the
fertilization of the two parents’ gametes. Thus, genetic
outcomes of offspring can be predicted mathematically.
Watch the segregation of A, a, B, b genes by clicking here
*Predicting genetic outcomes*
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 a 50%
probability of inheriting from
either gamete).
*Predicting genetic outcomes*
1. One parent’s genotype is
segregated on top of the
square: one allele over
each column
2. The other parent’s
genotype is segregated on
the side of the square: one
allele beside each row
*Predicting genetic outcomes*
3. Then each allele from the
top is distributed down
into each box beneath it.
In the example at the
right B is distributed
down each column.
4. And each allele from the
side is distributed into
each box to the right. In
the example at right b is
distributed across.
*Predicting genetic outcomes*
Probability of inheriting traits:
• Of the four gametes produced by
meiosis (example shown at right),
two contain one of the homologous
chromosome pair and two contain
the other chromosome of the
homologous pair
• Thus, if the parent was heterozygous
for gene “A” (Aa) there’s a 50% (2
out of 4) chance that a gamete with
A may be fertilized and a 50%
chance that a gamete with a may be
fertilized. Likewise for the B gene.
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 green hybrids (Gg)
*Predicted Outcomes*
What probable phenotypes
are produced?
= Ratio of green to yellow
What probable genotypes
are produced?
= Ratio of GG:Gg:gg
How to solve a genetics problem
1. Write out the genotype key!!!
GG – green, Gg – green, gg – yellow
2.
3.
4.
5.
6.
Write out parent genotypes of test cross
Draw Punnett square
Segregate each parent’s alleles
Complete the square
Compare genotype & phenotype ratios
Try this one…
The ability to roll the tongue is dominant
over the inability to do so in humans.
If two heterozygous tongue-rollers have
children, what genotypes could their
children have?
Hint: T=tongue-rolling and t=non-tongue-rolling
Solution #1
• TT, Tt, tt (see Punnett Square)
T
t
T
TT
Tt
t
Tt
tt
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
Solution #2
• 3 out of 4 chance or 75% (see
Punnett Square)
F
f
F
FF
Ff
f
Ff
ff
Solution #3
• No – only ss genotypes are recessive
(long fingers) and no offspring have
this genotype
S
S
S
SS
SS
s
Ss
Ss
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.
Solution #4
• One parent must be bb because she is
blue-eyed
• The other parent
B
B
must be BB if
Bb
Bb
none of their kids b
are blue-eyed
b
Bb
Bb
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
PROBLEMO
Solution #5
• Blue-eyed man = bb
• One blue-eyed child = bb B
• Then, brown-eyed woman b
must be Bb to have a blueeyed child
• The father’s parents,
B
who are brown-eyed,
b
must both be Bb
b
b
bb
B
b
bb
* Finally Mendel also observed *
Of an organism’s many different traits, many genetic
combinations were observed. For instance, some pea plants
had white flowers, round seeds, yellow pods while others had
white flowers, wrinkled seeds, yellow pods. Others included:
white, round, green; white, wrinkled, green; purple, round,
yellow; purple, wrinkled, green…
This showed that versions of different traits
did not influence one another, which can only
be explained the Law of Independent
Assortment: alleles for different traits assort
separately and independently of one another.
Mendel In Summary
• 1st Law of Dominance: form of a trait masks
the expression of the other form
• 2nd Law of Segregation: alleles segregate
during gamete formation (meiosis)
• 3rd Law of Independent Assortment: alleles
for different traits do not influence the
inheritance of one another.
Lesson Summary
One father of genetics
Two alleles for every trait
Three laws of inheritance
Four possible genetic outcomes (Monohybrid
Punnett Square)