Transcript VI. CHROMOSOMAL BASIS OF INHERITANCE, cont

UNIT VI - MENDELIAN GENETICS
Baby Campbell – Ch 9
Big Campbell – Ch 14, 15
Hillis – Chp 8
I. MENDEL
• Mendel’s Experiments
o Worked with ______________
pea plants
o Eliminated
________________________
and
self-pollination
controlled ____________________
cross-pollination
o P Generation - True-breeding pea
plants with one trait X true-breeding
pea plants with another trait
o Produced hybrids also known as F1
 ___________________________
TT x tt
 F1 Phenotype = 4 tall : 0 short
 F1 Genotype = 0 TT : 4 Tt : 0 tt
o F1 X F1 → F2
Tt x Tt
 ___________________________
 F2 Phenotype Ratio = 3 tall : 1 short
 F2 Genotype Ratio = 1 TT : 2 Tt : 1 tt
I. MENDEL, cont
• Mendel’s Principles
1)
2)
3)
4)
5)
Alternative versions of genes known
alleles
as ______________
account for
variations in inherited characters.
2
Organisms inherit _____
alleles for
each trait
If alleles at a locus differ; that is; if
heterozygous
the genotype is _______________,
the allele that shows is known as the
dominant
________________
allele.
Law of Segregation – two alleles for
a heritable character segregate
during meiosis
Law of Independent Assortment –
each pair of alleles segregates
independently of each other pair of
alleles during meiosis
Independent Assortment or Dependent Assortment???
P generation = YYRR x yyrr
Gametes = YR x yr
• F1 Generation = YyRr
• Predict F2 offspring…
If YyRr x YyRr….
• If dependent
assortment… then
gametes = YR yr
• If Independent Assortment…
Gametes are ____________
II. ANALYZING PROBABILITY OF TRAIT INHERITANCE
• Test Cross
o Organisms with dominant phenotype (PP or Pp) crossed with
______________________(pp)__
homozygous recessive
to determine genotype
• Punnett Square – tools used to predict the possible outcomes of meiosis
and fertilization.
• Multiplication Rule
o States the probability of 2 or more independent events occurring
together can be calculated by multiplying individual probabilities
o For example,
 Determine the probability of a homozygous recessive short plant
produced from F1 X F1




Cross = Tt x Tt
Probability of egg carrying t = ½
Probability of sperm carrying t = ½
Probability of tt offspring = ¼
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
 Tt x Tt
 Chance of egg carrying T = ½
 Chance of sperm carrying t = ½
 Chance of sperm carrying T = ½
 Chance of egg carrying t = ½
 Probability of Tt offspring = ¼ + ¼ = ½
II. ANALYZING PROBABILITIES, cont
• Crosses Involving Multiple Characters
o Determine the genotype ratios of the offspring for the
cross BbDD X BBDd
II. ANALYZING PROBABILITIES, cont
• Crosses Involving Multiple Characters
o Determine the genotype ratios of the offspring for the
cross YyRr X YyRr
II. ANALYZING PROBABILITIES, cont
• Crosses Involving Multiple Characters
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
Probability Practice Makes Perfect! 
In pea plants,
 long stems are dominant to short stems
 purple flowers are dominant to white, and
 round seeds are dominant to wrinkled.
 A plant that is heterozygous for all three loci self-pollinates and 2048 progeny are
examined. How many of the resulting plants would you expect to be long-stemmed
with purple flowers, producing wrinkled seeds?
III. VARIATIONS IN INHERITANCE
• Co-Dominance
o Both alleles affect phenotype in separate & distinguishable
ways
o Often designated with 2 different “big letters”
For example, __In chickens, BB = black WW = white
III. VARIATIONS IN INHERITANCE
• 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’
For example, in snapdragons, RR = RED R’R’ = WHITE
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
•
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?
Phenotype
A
B
AB
O
Genotype
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
• 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
• Pleiotropy – gene can have multiple phenotypic
effects. EX: the gene for flower color also affects
the gene for seed coat color.
III. VARIATIONS IN INHERITANCE, cont
• Environmental Impact on Phenotypes
• Phenotype of a trait depends on environment as
well as genotype.
• EX: size and shape of leaves on a tree also depends
on sun, wind, water, etc. in addition to alleles.
• EX: sun on skin, exercise on build, nutrition on
height, etc.
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
• Red-green colorblindness is caused by a sex-linked recessive allele. A colorblind man marries a woman with normal vision whose father was
colorblind. What is the probability she will have a colorblind daughter?
IV. SEX-LINKED INHERITANCE, cont
• The gene for amber body color in Drosophila is sex-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
Recessive Trait
Dominant Trait
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
965 GWgw
944 gwgw
206 Gwgw
Morgan realized variation in probabilities due to
185 gWgw
VI. CHROMOSOMAL BASIS OF INHERITANCE, cont
• Linkage Maps
o In crossing over, the further apart two genes are, the higher
the probability that a crossover will occur between them
and therefore, the higher the recombination frequency.
VI. CHROMOSOMAL BASIS OF INHERITANCE, cont
• Linkage Maps, cont
o Recombination Frequency =
# recombinants__ X 100
total # offspring
o One map unit = 1% recombination frequency
• 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 (vg) wings and body color (b) have a 17% crossover
rate.
 The gene for eye color (cn) and body color (b) have a 9% crossover rate.
 The gene for eye color (cn) and vestigial wings (vg) have a 9 ½%
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
Affects 1st 22 pair of chromosomes
• Sex-linked Disorder
o Affects last pair of chromosomes (X)
• Grouped according to path of inheritance
o Recessive
o Co-Dominant
o Dominant
VIII. INHERITED GENETIC DISORDERS
 Albinism
VIII. INHERITED GENETIC DISORDERS, cont
 Cystic Fibrosis
VIII. INHERITED GENETIC DISORDERS, cont
 PKU
VIII. INHERITED GENETIC DISORDERS, cont
 Tay-Sachs
VIII. INHERITED GENETIC DISORDERS, cont
o Autosomal Co-Dominant Disorders
 Sickle Cell Anemia
VIII. INHERITED GENETIC DISORDERS, cont
o Autosomal Dominant Disorders
 Huntington’s Disease
VIII. INHERITED GENETIC DISORDERS, cont
• Autosomal Dominant Disorders
 Marfan Syndrome
VIII. INHERITED GENETIC DISORDERS, cont
• Autosomal Dominant Disorders
 Achondroplasia
VIII. INHERITED GENETIC DISORDERS, cont
• Hypercholesteremia
VIII. INHERITED GENETIC DISORDERS, cont
• Sex-Linked Disorders
o Vast majority are recessive
o Affect mostly males
o Hemophilia
VIII. INHERITED GENETIC DISORDERS, cont
o Colorblindness
o 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 __karyotype_______
IX. CHROMOSOMAL MUTATIONS, cont
• Chromosomal Number Disorders
o Examples
 Down Syndrome
X. CHROMOSOMAL DISORDERS, cont
Turner Syndrome
X. CHROMOSOMAL DISORDERS, cont
• Klinefelter Syndrome
X. CHROMOSOMAL DISORDERS, cont
• Cri du Chat – Caused by deletion in chromosome 5
• Chronic Myelogenous Leukemia – Caused by reciprocal
translocation. Large piece of chromosome 22 exchanges
places with tip of chromosome 9. Resulting chromosome 22
easily recognizable; known as Philadelphia chromosome.
XI. TESTING FOR INHERITED GENETIC DISORDERS
• Identification of Carriers/Genetic Counseling
o Tests are 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
XI. 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
o Karyotype
XI. GENETIC TESTING, cont
• Newborn Screening
o PKU
• PGD
o Pre-Implantation Genetic Diagnosis