Transcript UNIT VI Notesx

UNIT VI - MENDELIAN GENETICS
Hillis – Ch 8
Dual Campbell – 11,12
Baby Campbell – Ch 9
Big Campbell – Ch 14, 15
I. MENDEL
• Mendel’s Experiments
I. MENDEL, cont
• Mendel’s Principles
1)
4)
Alternative versions of genes
known as ______________
account for variations in
inherited characters.
Organisms inherit _____ alleles
for each trait
If alleles at a locus differ; that is;
if the genotype is
_______________, the allele that
shows is known as the
________________ allele.
Law of Independent Assortment
5)
Law of Segregation
2)
3)
II. ANALYZING PROBABILITIES
• Test Cross
o Organisms with dominant phenotype crossed with
_____________________________ to determine genotype
• Punnett Square
• Multiplication Rule
o States the probability of 2 or more independent events occurring
together can calculated by multiplying individual probabilities
o For example,
 Determine the probability of a homozygous recessive short plant
produced from F1 X F1
 Cross =
 Probability of tt offspring =
II. ANALYZING PROBABILITIES, cont
• Addition Rule
o States that the probability of 2 or more mutually exclusive events
occurring can be calculated by adding together their individual
probabilities
o For example,
 Determine the probability of a heterozygous plant produced from
F1 X F1

 Chance of egg carrying T =
 Chance of sperm carrying t =
 Chance of sperm carrying T =
 Chance of egg carrying t =
 Probability of Tt offspring =
II. ANALYZING PROBABILITIES, cont
• Crosses Involving Multiple Characters
o Determine the genotype ratios of the offspring for the cross BbCCDD X
BBccDd
II. ANALYZING PROBABILITIES, cont
o Determine the phenotype ratios of the offspring for the cross YyRrSS X
yyRrss
II. ANALYZING PROBABILITIES, cont
o In the cross, PpYyRr X Ppyyrr, what is the probability of offspring that are
purple, green, & round?
 P= purple, p = white
 Y = yellow, y = green
 R = round, r = wrinkled
III. VARIATIONS IN INHERITANCE
• Co-Dominance
o Both alleles affect phenotype in separate & distinguishable ways
o Often designated with 2 different “big letters”
• Incomplete Dominance
o Neither allele is dominant; heterozygotes show a blend of two homozygous
phenotypes
o One allele designated with “big letter’, the other with “big letter prime”;
for example T T’
III. VARIATIONS IN INHERITANCE, cont
• Epistasis
o Gene at one locus alters
phenotypic expression of a
gene at a second locus
o For example,
A dominant allele, P causes the production of purple pigment; pp
individuals are white. A dominant allele C is also required for color
production; cc individuals are white. What proportion of offspring will be
purple from a ppCc x PpCc cross?
III. VARIATIONS IN INHERITANCE, cont
• Multiple Alleles
o Many genes have more than 2 alleles
o Example, ABO blood groups in humans
o Three alleles



• Rh Factor
Phenotype
Genotype
A
B
AB
O
• A woman with O positive blood has a child with Type A negative blood. The
man she claims is the father has AB positive blood. Is it possible that he is the
father of this child?
III. VARIATIONS IN INHERITANCE, cont
• Polygenic Inheritance
o For example, AABBCC = very dark skin; aabbcc = very light skin.
o Intensity based on units; in other words, AaBbCc and AABbcc
individuals would have the same pigmentation
III. VARIATIONS IN INHERITANCE, cont
• Pleiotropy
III. VARIATIONS IN INHERITANCE, cont
• Environmental Impact on Phenotypes
IV. SEX-LINKED INHERITANCE
o First recognized by Thomas Hunt Morgan
 Drosophila melanogaster
Fruit flies
Excellent organism for genetic studies
 Prolific breeding habits
 Simple genetic make-up; 4 pairs of chromosomes → 3 pairs of
autosomes, 1 pair of sex chromosomes
 Crossed true-breeding wild-type females with true-breeding mutant
males
 Mutant trait showed up in ½ male F2 offspring ; was not seen in F2
females
o Determined mutant allele was on X-chromosome; thus inherited
differently in males versus females
 In females,
 In males,
IV. SEX-LINKED INHERITANCE, cont
• The gene for amber body color in Drosophila is X-linked recessive. The
dominant allele produces wild type body color. The gene for black eyes is
autosomal recessive; the wild type red eyes are dominant. If males with
amber bodies, heterozygous for eye color are crossed with females
heterozygous for eye color and body color, calculate the expected
phenotype ratios in the offspring.
IV. SEX-LINKED INHERITANCE, cont
• X Inactivation in Females
o During embryonic development, one X chromosome in female cells is
inactivated due to addition of methyl group to its DNA
o Dosage compensation
o Inactive X chromosome condenses; known as Barr body
IV. SEX-LINKED IN INHERITANCE, cont
o Occurs randomly
Females will have some cells where “Dad’s copy” of X is
inactivated, some where “Mom’s copy” is inactive
Therefore, females are a mosaic of cells
Preserved in mitosis
In ovaries, Barr body chromosome is reactivated for meiosis
and oogenesis
IV. SEX-LINKED INHERITANCE, cont
• X Inactivation
 Calico coloration in female cats
V. PEDIGREE ANALYSIS
V. PEDIGREE ANALYSIS, cont
VI. CHROMOSOMAL BASIS OF INHERITANCE
• Chromosomal Theory of Inheritance
o States genes occupy specific loci
on chromosomes
o During meiosis, chromosomes
undergo segregation &
independent assortment
• Linked Genes
o During Thomas Morgan’s work
with Drosophila, he recognized
 Two genes located on same
chromosome were linked;
that is, inherited together
 However, offspring
phenotypes showed this
wasn’t always true
VI. CHROMOSOMAL BASIS OF INHERITANCE, cont
• Linked Genes, cont
o In fruit flies, normal wild-type phenotype is gray body, normal wings –
both genes are located on same chromosome
 G = wild-type (gray) body; g = black body
 W = wild-type wings; w = mutant wings
o True-breeding wild type flies X true-breeding mutants
o F1 showed all
o F1 X test cross
 Counted 2300 offspring
 Should have counted
VI. CHROMOSOMAL BASIS OF INHERITANCE, cont
• Linked Genes, cont
o Instead Morgan counted
965 GWgw
944 gwgw
206 Gwgw
o Morgan realized variation in probabilities due to
• Linkage Maps
o In crossing over, the further apart two genes are …
185 gWgw
VI. CHROMOSOMAL BASIS OF INHERITANCE, cont
• Linkage Maps, cont
o Recombination Frequency =
# recombinants_
total # offspring
o One map unit = 1% recombination
• Morgan’s Data …
o F1 X test cross
 GW/gw X gw/gw
 Counted 2300 offspring
 Should have counted
 965 GWgw
944 gwgw
206 Gwgw
185 gWgw
VI. CHROMOSOMAL BASIS OF INHERITANCE, cont
• In a genetics experiment with fruit flies, P generation true-breeding flies
are crossed. The female parent is brown and wingless while the male
parent is black with normal wings. All of the F1 flies are brown with
normal wings.
• In the next cross, F1 females are test-crossed with black, wingless males.
The following offspring are counted:
 85 brown winged flies
 728 black winged flies
 712 brown wingless flies
 75 black wingless flies
• How many map units apart are the two genes?
VI. CHROMOSOMAL BASIS OF INHERITANCE, cont
• The genes for vestigial wings, black body color, and cinnabar eyes are linked genes.
• In controlled crosses …
 The gene for vestigial wings (vg) and body color (b) have a 17% crossover rate.
 The gene for eye color (cn) and vestigial wings have a 9% crossover rate.
 The gene for eye color and body color have an 8% crossover rate.
• Draw the chromosome.
VII. MUTATIONS
• Change in the nucleotide sequence of DNA
• May be spontaneous mistakes that occur
during replication, repair, or recombination
• May be caused by mutagens; for example, xrays, UV light, carcinogens
• Classification
 Gene
 Chromosomal
VIII. INHERITED GENETIC DISORDERS
• Most often due to gene mutations
• Classified according to type of chromosome on which affected gene is
located
• Autosomal Disorders
o
o
• Sex-linked Disorder
o
o
o
• Grouped according to path of inheritance
o
o
o
VIII. INHERITED GENETIC DISORDERS
o Autosomal Recessive Disorders
 Albinism
 Cystic Fibrosis
 PKU
 Tay Sachs
VIII. INHERITED GENETIC DISORDERS, cont
o Autosomal Co-Dominant Disorders
 Sickle Cell Anemia
o Autosomal Dominant Disorders
 Huntington’s Disease
 Marfan Syndrome
 Hypercholesteremia
VIII. INHERITED GENETIC DISORDERS, cont
• Sex-Linked Disorders
o Vast majority are
o Examples
 Hemophilia
 Colorblindness
 Duchenne Muscular Dystrophy
IX. CHROMOSOMAL MUTATIONS
• Due to abnormal chromosome structure or abnormal number of
chromosomes
• Abnormal Chromosome Structure
o Often due to mistakes in
IX. CHROMOSOMAL MUTATIONS, cont
• Abnormal Number of Chromosomes
o Most commonly due to nondisjunction
o Results in aneuploid gametes
o Detected with ________________________
IX. CHROMOSOMAL MUTATIONS, cont
• Chromosomal Number Disorders
o Examples
 Down Syndrome
IX. CHROMOSOMAL MUTATIONS, cont
 Turner Syndrome
IX. CHROMOSOMAL MUTATIONS, cont
 Klinefelter Syndrome
X. GENETIC TESTING
• Identification of Carriers/Genetic Counseling
o Gene testing is available for Tay-Sachs, sickle cell, cystic fibrosis,
Huntington’s, PKU, & many others
• Fetal Testing
o Gene Testing
Amniocentesis
 Performed between 14th-16th weeks of pregnancy
 Cells collected, tested
X. GENETIC TESTING, cont
 Chorionic Villus Sampling (CVS)
 Narrow tube inserted
through mother’s vagina,
cervix
 Small tissue sample
suctioned from placenta
(organ that transmits
nutrients, removes wastes
from fetus)
 Testing may be done earlier
in pregnancy but not
suitable for all types of
testing
• Karyotype
X. GENETIC TESTING, cont
• Newborn Screening
 PKU
• PGD
 Pre-Implantation Genetic Diagnosis