Transcript AP BIOLOGY 11 - Gorsic's Gameplan

AP BIOLOGY 11
Genetics: Chapter 14 & 15
Pg. 247 - 286
Mendel’s Laws
• Gregor Mendel is
credited with
discovering the basic
principles of genetics
• These principles or
Laws have seen many
modifications but
basic genetics begins
here.
Mendelian genetics
• Character
(heritable feature, i.e., fur color)
• Trait
(variant for a character, i.e., brown)
• True-bred
(all offspring of same homozygous
variety)
• Hybridization
(crossing of 2 different true-breds)
• P generation
(true-breeding parents)
• F1 generation
(first filial generation - hybrid)
• F2 generation
(result of self-pollination from F1)
Genetic vocabulary…….
• Allele: one alternative form of a given
gene pair; more than two alleles can
exist for any specific gene, but only
two of them will be found within any
individual
• Allelic pair: a combination of two
alleles
• Punnett square: grid to predict the
results of a genetic cross between
individuals of known genotype
• Homozygous: pair of identical alleles
for a character
• Heterozygous: two different alleles
for a gene
• Phenotype: the appearance
(expression) of a trait in an individual
• Genotype: specific allelic
combination for a certain gene or set
of genes
• Testcross: breeding of a recessive
homozygote X dominate phenotype
(but unknown genotype)
Dominance and Recessive
• The ability of one allele
(the dominant) to either
express its phenotype at
the expense of an alternate
allele, or mask the effect
of the other (recessive)
member of the pair.
• Generally the dominant
allele will make a gene
product (e.g. protein) that
the recessive cannot
• Dominant is not a
reflection of the
abundance in a
population
Mendel’s Laws
Example: yellow peas dominant over green; round
pea is dominant over wrinkled
Note: The dominant recessive interaction can be
interpreted differently depending on your level of
study as - a whole organism, a bio-chemical level
or a molecular level E.g. Tay-Sachs disease pg. 256
Mendel’s Principles of Heredity
Law of Segregation
The alleles for each character segregate
(separate) during meiosis I.
(homologous pairs separate)
Only one of the allelic pair are in each
sex cell
Law of Independence
Assortment
During meiosis, the different
homologous chromosome assort
independently from other
homologous chromosomes
Important Genetic Terms
Monohybrid cross
Cross between parents that differ at a single allelic
pair (usually AA x aa)
Dihybrid Cross
Cross between parents that differ at two allelic
pairs (AaGg X AaGg)
Mendel’s Laws
Law of Independent Assortment
Chromosomal Basis of Mendel’s
Laws
Mendel’s Laws
• Using a Punnett Square, there are 16 posssible
offspring with a phenotype ratio of 9:3:3:1
• Segregation of gene pairs
• Independent assortment of unlinked genes,
• and the Fertilization of an egg by a sperm
Are All Random Events……
• Random events occur independently of each other
and are unaffected by previous events.
Probability
• Mendelian inheritance reflects the rules of
probability.
• If we know the genotypes of parents, we can
predict the genetics of the offspring by using
the Laws of Probability
Probability
• Probability events occur in a range from 0 to 1
• 1 means that an event is 100% certain to
occur.
• 0 means and event is 0% certain to occur.
• The probability of all possible outcomes in an
event must total 1 (100%)
• Ex. Coin toss, throw of a die
Probability
1. Rule of Multiplication
• The probability that independent events will occur
simultaneously (i.e. in a specific combination) is
equal to the product (x) of their individual
probabilities.
• Ex: If two parents are Pp for a trait, what’s the
probability of them having a homozygous
recessive offspring? (ovum and sperm must carry the p allele)
Probability
• Probability of a “p” from egg = 1/2
• Probability of a “p” from sperm = 1/2
• Overall probability of two “p’s” uniting at
fertilization:
• 1/2 x 1/2 = 1/4 (.25)
Problem: If two parents are YyRr, what is
probability of them conceiving a yyrr offspring?
Probability
2. Rule of Addition
• The probability of an event that can occur in
two or more independent ways is the sum (+) of
the the separate probabilities of the different
ways.
• Ex: if two parents are Pp, what is the probability
of them producing a heterozygous offspring?
Probability
• There are two independent ways to make a heterozygous offspring: the
dominant allele could come from the mother and the recessive from the
father, or vice versa
.
The probability of the dominant from mother and recessive from
father is: 1/2 x 1/2 = ¼
The probability of the dominant from father and recessive from
mother is: 1/2 x 1/2 = 1/4
Therefore, the overall probability is the sum of these two
probabilities: 1/4 + 1/4 = ½
Answer: The probability of them producing a
heterozygous offspring is a 1 in 2 chance or a
50% chance
Probability
Practice!
1. What is the probability of rolling two 6’s
from two dice at the same time?
2. What is the probability of getting a heads tails combination when tossing two coins
simultaneously?
3. Two parents are AaBbCc - what is their
probability for a aabbcc offspring?
Mendel’s Laws
Practice Problems:
1. Pigeons can be either plain or checkered color. A series of
crosses were made, and the following ratios were observed:
Cross 1: Plain x Plain = All Plain;
Cross 2: Checkered x Checkered = All Checkered;
Cross 3: Checkered x Plain = 1 Checkered:1 Plain;
Cross 4: Checkered x Checkered = 3 Checkered:1 Plain.
Which phenotype is dominant? Create your own gene symbol
and give the genotypes of the parents (as close as you can
determine) for each cross.
Mendel’s Laws
2. In humans, the allele which confers the ability to roll the
tongue is dominant over the allele which does not confer
the ability to roll the tongue. If a man who can roll his
tongue and whose mother could not mates with a woman
who cannot roll her tongue, what proportion of the children
would be expected to be able to roll their tongues if they
have a large number of children? What are the genotypes
which are possible among the children?
Mendel’s Laws
3. In humans, the gene for dimples is recessive to its allele
which produces no dimples. Also, the gene for 5 digits per
hand and foot is recessive to its allele for 6 digits per hand
and foot. Cross a man who does not have dimples, who has
6 digits per hand and foot, heterozygous for both these
traits with a women who has dimples and 5 digits per hand
and foot. What phenotypes might be present in their
children? What is the probability at conception for each
phenotype occurring?
Mendel’s Laws
4. A student did a series of genetic crosses on pea plants
examining tallness and seed color. What type of cross
would produce the following offspring genetic ratios?
a) 3:1
b) 1:1
c) 1:2:1
d) 9:3:3:1
e) 1:1:1:1
f) 2:1
Extending Mendel’s Genetics
• We have learned much since Mendel’s work
with garden peas. We now know that there
are many exceptions to “Mendel’s Laws”.
• Mendel was fortunate that he chose peas
and that the seven traits he studied all
happened to be on different chromosome
pairs.
Non-single gene genetics
• Incomplete dominance:
Both alleles are expressed resulting in a
blending of traits
Example: snapdragons can be red or white,
but in the heterozygous condition, they
are pink
• Codominance:
This occurs when both members of an
allelic pair are expressed - not blended.
Example: blood types - Type A and Type B
are co-dominant. The heterozygous
condition is Type AB
Note: Dominance ranges from complete
dominance, through to various degrees
of incomplete dominance, to
codominance
Extending Mendel’s Genetics
Multiple Alleles
• Mendel only ever considered two possible
alleles for a trait, ex. Tall (T) or Short (t).
• In many traits there are more than two
possible alleles.
• For example, in human blood types there
are three possible allele: A, B, or O
Extending Mendel’s Genetics
Pleiotropy
• This is when one gene or allele affects more
than one trait.
• Ex: Yellow mice are heterozygous; gray
homozygous recessive. The homozygous
dominant doesn’t exist - it’s lethal. The gene
for color also affects viability
Ex: sickle-cell anemia.
•Incomplete dominance –organism
•Codominance at the molecular level
•Heterozygous form is beneficial to
ward of malaria pg. 262
More Than One Gene Involved
Epistasis: a gene at one locus
(chromosomal location) affects
the phenotypic expression of a
gene at a second locus. The
interaction between two or
more genes to control a single
phenotype Ex: mice coat color
Ex: Chicken combs are
controlled by two genes
producing four possible
phenotypes (dihybrid cross
Rose
Single
Pea
Walnut
Extending Mendel’s Genetics
•Polygenic Inheritance:
An additive effect of two
or more genes on a
single phenotypic
character Ex: Skin color
and height are controlled
by a series of genes
which effects multiple
depending on each genes
dominance
9 B_D_ (black)
3 B_dd (dilute black)
3 bbD_ (brown)
1 bbdd (dilute brown)
Extending Mendel’s Genetics
Sex Determination
• Unlike most traits, sex
is determined by a pair
of chromosomes
• Females have two “X”
chromosomes while
males have an”X” and
a “Y”.
Extending Mendel’s Genetics
Sex Linkage (pg. 277-279)
•
•
Fathers= pass X-linked alleles to
all daughters only (but not to sons)
Mothers= pass X-linked alleles to
both sons & daughters
•
X-inactivation: 2nd X chromosome
in females condenses into a Barr
body
•
Sex-Linked Disorders: Colorblindness; Duchenne muscular
dystropy (MD); hemophilia
Males have only one X
chromosome so express sex linked
genes even if recessive; females
show normal inheritance (have two
alleles for the trait).
•
The white eyed fruit fly is a
male that inherited a single
sex linked mutant gene for
eye color. The red eye male is
normal.
More practice problems:
1. In fruit flies, eye color is sex-linked. In females, the
allele for red eyes is dominant over the allele for
white eyes. List phenotypes for the offspring of each
of the following crosses and the expected ratio of
each phenotype.
a. A male with red eyes crossed with a female with white
eyes
b. A heterozygous female crossed with a male with red eyes
c. A heterozygous female crossed with a male with white eyes
d. A homozygous red-eyed female crossed with a male with
white eyes
More practice problems:
2. A couple have a son with sex-linked muscular dystrophy.
a. From which parent did the son receive gene(s) for muscular
dystrophy?
b. If their next child is a girl, what is the probability she will inherit
sex-linked muscular dystrophy?
c. If their next child is a boy, what is the probability he will inherit
sex-linked muscular dystrophy?
d. What is the probability of them having three sons in a row all
with muscular dystrophy?
e. This couple is expecting yet another child (after the three in d),
what is the probability of this child having muscular dystrophy?
Chromosomal errors
•
Nondisjunction:
The pair of homologous
chromosomes do not separate
properly during meiosis I or sister
chromatids fail to separate during
meiosis II
•
Aneuploidy: chromosome number
is abnormal
•
Monosomy~ missing
chromosome
• Trisomy~ extra
chromosome (Down
syndrome)
• Polyploidy~ extra sets
of chromosomes
Extending Mendel’s Genetics
Sex Nondisjunction
• Occurs when the egg
or sperm have an
abnormal number of X
or Y chromosomes baby gets abnormal
number
• Ex. Kleinfelters XXY
& Turners, _X
Pedigrees
• The generation time for humans is very long
( 20 - 30 years).
• Further, we produce relatively few offspring
• As a result, Mendelian inheritance is
difficult to study in us.
• One useful method is the development and
analysis of human family histories in
diagrams called Pedigrees.
Pedigrees
What can you decipher
From the pedigree?
When boy III-1 (outlined in blue) died suddenly at a football game at
the age of 19, his mother II-2, brother and sisters, friends and doctors
were confused. An autopsy showed that the young athelete had died
from familial hypertrophic cardiomyopathy (HCM), an inherited disease
of the heart muscle. condition.