Download Chapter 3 – Basic Principles of Heredity

Chapter 3 – Basic Principles
of Heredity
Johann Gregor Mendel (1822 –
1884)
• Pisum sativum
• Rapid growth; lots of
offspring
• Self fertilize with a
single plant; cross
fertilize between two
plants
Pisum sativum
• 7 characteristics
– Each had only 2 forms
– True-breeding varieties
• When allowed to self-fertilize, all offspring had same parental
trait
Modern Genetic Terminology
• Gene – inherited factor that codes for a
specific characteristic
• Locus – physical location of a gene on a
chromosome
• Allele – alternate forms of a gene
– what specifically the gene codes for (black
hair, blond hair)
Modern Genetic Terminology
• Genotype – set al individual’s alleles; its
genetic makeup
– Homozygous – 2 of the same allele for a gene
– Heterozygous – 2 different alleles for a gene
• Phenotype – outward expression of a
gene
– An allele may be present but not expressed in
the phenotype
Monohybrid cross
• Cross between plants
that differ in a single
characteristic
• P (paternal)
generation
– True-breeding for trait
Monohybrid cross
• F1 (filial) generation
– All have trait of one
parent
– Reciprocal cross – sex
of parent with trait
made no difference
Monohybrid cross
• F2 generation
– Phenotype ratio 3:1
• 3 = trait in F1
• 1 = trait not seen in
F1; seen in P
generation
– “lost” phenotype
reappeared
Monohybrid cross conclusions
• Each plant has two “factors” (modern terms genes)
• In heterozygotes, one allele will be expressed;
other will be masked, but can be passed on and
expressed in offspring
– Dominant allele – expressed
• Capital letter
– Recessive allele – masked
• Lowercase letter
• 2 alleles separate with equal probability
Principle of Segregation
• Each diploid organism has 2 alleles for
each gene
• Alleles segregate from each other
randomly in gamete formation
Punnett square
• Illustrates possible
gametes formed and
possible fertilization
combinations
Probability
• Likelihood of the occurrence of a particular
event expressed as a fraction or a decimal
• Multiplication rule “AND”
– Probability of two or more independent
events occurring together
Multiplication rule
• Probability of rolling a
four – 1/6
• Probability of rolling a
four AND then a 3
– 1/6 x 1/6 = 1/36
Multiplication rule
• Cross between two heterozygous purple
flowered plants (Pp x Pp)
• ? Probability of having a purple offspring,
AND then a white
• ? Probability of having two white offspring
Addition rule
• “either/or”
• Probability of having 2
or more mutually
exclusive events
occur together
• Probability of rolling a
three OR a four
– 1/6 + 1/6 = 2/6 (1/3)
Albinism – autosomal recessive
disorder
• 2 carriers mate (Aa x Aa)
• ? Probability of having three children with
albinism
– ¼ x ¼ x ¼ = 1/64
• ? Probability of having 2 “normal” and 1 albino
(order not important)
–
–
–
–
1st affected = ¼ x ¾ x ¾ = 9/64
2nd affected = ¾ x ¼ x ¾ = 9/64
3rd affected = ¾ x ¾ x ¼ = 9/64
Add all possible combinations = 27/64
Binomial expansion
• a = probability of albinism (1/4)
• b = probability of “normal” pigmentation
(3/4)
• 5 children (a + b)5
• a5 + 5a4b + 10a3b2 + 10a2b3 + 5ab4 + b5
Binomial expansion
• a5 + 5a4b + 10a3b2 + 10a2b3 + 5ab4 + b5
• Term number = n +1
• First term has an, second term an-1b, etc
– a always loses 1; b gains 1
• For coefficient (number in the front)
– 1st term is always 1
– 2nd term – same power as binomial (5)
– 3rd term – multiply preceding coefficient (5 from 2nd
term) by exponent of a in the 2nd term (4), then divide
by term # (2) = (5x4)/2 = 10
• Coefficient for 3rd term is 10
Binomial expansion
• ? probability of 2 carriers of albinism
having 5 albino children and 1 “normally”
pigmented child
• (a + b)6
Test Cross
• Determination of genotype of a dominant
phenotype individual
• Cross with homozygous recessive
individual
– If any offspring demonstrate recessive
phenotype, unknown must be heterozygous
Types of genetic crosses
• Reciprocal
– Sex of parents with a specific trait is switched
• Test
– Cross of unknown dominant with recessive
• Back
– Cross of individual with a parent
Dihybrid cross
• 2 different traits are
examined at the same
time
• P generation – true
breeding for both
traits
Dihybrid cross
• F1 exhibits both
dominant forms of the
traits
• Heterozygous for both
– can form 4 different
types of gametes
Dihybrid cross
•
•
•
•
F2 generation
9:3:3:1 ratio
9 – both dominant traits
3 – dominant for color;
recessive for shape
• 3 – recessive for color;
dominant for shape
• 1 – both recessive traits
• Principle of Independent
Assortment
– Alleles at different loci
segregate independent from
one another
Branch diagram
• Uses probability rules
• 1st column lists
proportions of
phenotypes of 1st trait
• 2nd column lists
proportions of
phenotypes of 2nd trait,
etc
• Faster than a Punnett
square when dealing with
multiple loci
– Specifically when you need
one particular phenotype
Ratios
• Punnett squares and Branch diagrams
deal with probability
• Observed ratio is rarely EXACTLY the
expected ratio
• Goodness of fit Chi-Square test
– Indicates probability that deviation between
observed and expected ratio is due to chance
alone
Chi-Square example
Chi-Square example cont
• Number is squared,
so it’s always a
positive number
• X2 = 2.0
• Need Table
• Degrees of freedom =
n – 1, where n =
possible phenotypes
Chi-Square example cont
• Df = 1
• X2 = 2
• .1< p < .5
• 10% < p < 50% that
variability is due to
chance – hypothesis is
accepted
• Cut off is usually p =
0.05 (5% variation due
to chance)
Cat example – Chi-square
• Assuming black is dominant to gray, a
cross between Bb x Bb yields an expected
ratio is 3:1
• Offspring = 30 black cats and 20 gray
• Accept or reject hypothesis?
Cat example calculations