Download Principles of Heredity

Principles
of
Heredity
Chapter 10
Gregor Mendel
• Generally considered
the ‘Father of Modern
Genetics’
• Worked with pea
plants, keeping careful
records of his
experiments
• Used statistical
analysis to establish
several important
genetic principles
Mendelian Genetics
• Things he knew
before he started:
– How to control pea
plant reproduction
– That he had truebreeding plant strains
– That hybrids between
these strains did NOT
breed true
Mendelian Genetics…
• Things he didn’t
know:
– What DNA or
chromosomes were
– Anything about mitosis
or meiosis
– That traits in hybrids
did not always ‘blend’
as was the idea of the
time
Mendelian Genetics…
• What he figured out:
– Blending of two distinct traits in the parents did not
always occur in the hybrid offspring
– Each kind of inherited feature in an organism is
controlled by 2 factors that behave like distinct
particles
– That some of these factors can mask others (that is,
some are dominant while others are recessive)
– The Principle of Segregation and The Principle of
Independent Assortment
The Principle of Segregation
• Alleles – alternate gene forms – are
located on corresponding loci on
homologous chromosomes
• During gamete formation in meiosis,
homologous chromosomes separate
(when?)
• During sexual reproduction, offspring
receive one of these homologous
chromosomes from each parent
Terms to review
•
•
•
•
•
•
•
Dominant / Recessive
Gene / Allele
Monohybrid cross / Dihybrid cross
Homozygous / Heterozygous
Phenotype / Genotype
Punnett square / Probability
Test cross
Monohybrid cross practice
1.) A TT (tall) pea plant is crossed with a tt
(short) pea plant
Monohybrid cross practice…
2.) A Tt pea plant is crossed with a Tt pea
plant.
Monohybrid cross practice…
3.) A heterozygous round seeded pea plant
(Rr) is crossed with a homozygous round
seeded pea plant (RR).
Monohybrid cross practice…
4.) A homozygous round seeded pea plant is
crossed with a homozygous wrinkled pea
seeded plant.
Monohybrid cross practice…
5.) In pea plants purple flowers are dominant
to white flowers. Cross two white flowered
plants.
Monohybrid cross practice…
6.) A white flowered pea plant is crossed
with a pea plant that is heterozygous for
the trait.
Monohybrid cross practice…
7.) Two pea plants, both heterozygous for
the gene that controls flower color, are
crossed.
Monohybrid cross practice…
8.) In guinea pigs, short hair is dominant
over long hair. Show the cross for a pure
breeding short haired guinea pig and a
long haired guinea pig.
Monohybrid cross practice…
9.) Show the cross for two heterozygous
guinea pigs.
What percentage of the offspring will have
short hair? ________
What percentage of the offspring will have
long hair? _______
Monohybrid cross practice…
10.) Two short haired guinea pigs are mated
several times. Out of 100 offspring, 25 of
them have long hair. What are the
probable genotypes of the parents? Show
the cross to prove it!
Dihybrid cross practice
IN PEAS:
R = round
r = wrinkled
T = tall
t = short
Y = yellow peas
y = green peas
P = purple flowers
p = white flowers
1.) Homozygous tall, round parent X pure
short, wrinkled parent
Dihybrid cross practice…
IN PEAS:
R = round
r = wrinkled
T = tall
t = short
Y = yellow peas
y = green peas
P = purple flowers
p = white flowers
2.) Heterozygous for both height and flower
color parent X short, white flowers parent
Dihybrid cross practice…
IN PEAS:
R = round
r = wrinkled
T = tall
t = short
Y = yellow peas
y = green peas
P = purple flowers
p = white flowers
3.) Green peas, short plant X Heterozygous
for yellow peas, homozygous for tall
parent
Dihybrid cross practice…
IN PEAS:
R = round
r = wrinkled
T = tall
t = short
Y = yellow peas
y = green peas
P = purple flowers
p = white flowers
4.) Heterozygous round, green peas X
wrinkled peas, Heterozygous yellow peas
Dihybrid cross practice…
IN PEAS:
R = round
r = wrinkled
T = tall
t = short
Y = yellow peas
y = green peas
P = purple flowers
p = white flowers
5.) Both parents heterozygous for height and
flower color
The Principle of Independent
Assortment
• Another of Mendel’s ideas
• Explains the results of these types of
dihybrid crosses
• Each different trait is inherited
independently from the other
• Now we know that this is due to meiosis –
homologous chromosomes separate
independently (again, when?)
Probability Rules
• The product rule:
– Predicts the combined probability of 2
independent events
– If two or more events are independent of each
other, the probability of both occurring is the
product of their individual probabilities
– Example: coin toss – heads two times in a row
• ½ X ½ = ¼ or one chance in 4
– Also: Bb X Bb parents – producing a bb child
• ½ b X ½ b = ¼ or one chance in 4
Probability Rules…
• The sum rule
– Predicts the combined probabilities of
mutually exclusive events
– If there is more than one way to get a result,
we combine the probabilities by summing
– Example: Bb X Bb parents chance of Bb child
•
•
•
•
2 possibilities: B egg + b sperm; b egg + B sperm
B egg (½) X b sperm (½) = ¼
And b egg (½) X B sperm (½) = ¼
Then … ¼ + ¼ = ½
Probability Rules…
• Most important –
– Chance has no memory!
– If events are truly independent, past events
have no influence on the probability of future
events…
– Even though we don’t like this idea – ‘my luck
is bound to change…’
Things Mendel didn’t know
• Linked genes - inherited together because they
are located on the same chromosome
– Linked genes do not undergo segregation or
independent assortment
– The rates of crossing over can be used to determine
the relative positions of genes on a chromosome
– Higher crossing over rates indicate greater separation
of genes on a chromosome
– Each %age of crossing over rate = one map unit
Things Mendel didn’t know…
• Sex chromosomes:
– Female = XX
- Male = XY
– Male produces the sex determining gamete
– In humans, Y chromosome has the SRY gene
(sex reversal gene on the Y) – this acts as a
genetic switch to cause testes to develop
– Developing testes produce testosterone which
determines other sexual characteristics
– Everyone has at least one X – female is the
‘default’ sex… need a Y to develop as a male
Things Mendel didn’t know…
• Sex-linked genes:
– Located on the X chromosome only
– Include genes for color perception and blood
clotting – things all humans need
– Females get two copies – can be either
homozygous or heterozygous
– Males only get one copy – they are
hemizygous
– Defects in these traits arise more in males
than in females
Things Mendel didn’t know…
• Dosage compensation:
– Makes equivalent the female’s 2 ‘doses’ of the
genes on the X chromosome to the male’s 1
– One X chromosome in each female cell is
inactivated – called a Barr body
– Individuals with heterozygous X-linked genes
will often have a variegated phenotype as
random X chromosomes are inactivated in the
body
Calico cats – an example of X
chromosome inactivation in action
Things Mendel didn’t know…
• Incomplete
dominance
– The phenotype of the
heterozygous
individual is a blending
of the two genes
Things Mendel didn’t know…
• Codominance:
– The phenotype of the heterozygous individual
expresses both genes, but without blending
ABO Blood
Types:
A and B alleles
are codominant
to each other
Both are
dominant to type
O allele
Things Mendel didn’t know…
• Multiple alleles:
– Three or more alleles exist in the population,
even though each individual only has two
– Examples:
• ABO blood types in humans
• Coat color in rabbits
Things Mendel didn’t know…
• Pleiotrophy
– One gene with many effects
– Often found in genetic diseases
– Example:
• Cystic fibrosis in humans
• Homozygous individuals produce abnormally thick
mucus in many body systems
Things Mendel didn’t know…
• Epistasis
– The presence of one
allele can prevent or
mask the expression of
a gene at another loci
– Example:
• Coat color in Labrador
retrievers
• Pigment gene is either B
(black) or b (brown)
• Recessive ee blocks the
expression of either
Black
BBEE
BbEE
BBEe
BbEe
Brown Yellow
bbEE
bbEe
BBee
Bbee
bbee
Things Mendel didn’t know…
• Polygenic inheritance
– Mulitple independent pairs of genes have
similar and additive effects on the phenotype
– The phenotypes in a population will generally
show a normal distribution curve
– Examples:
• Human skin and eye color
Things Mendel didn’t know…
• Environmental interaction
– Genetically identical individuals show different
phenotypes based on environmental factors
– Example:
• Human height and intelligence
– Nature versus nurture questions
• Problems with experimental methods to answer
these questions in humans….