Transcript UNIT V – MENDELIAN GENETICS

UNIT VI - MENDELIAN GENETICS
Baby Campbell – Ch 9
Big Campbell – Ch 14, 15
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
II. ANALYZING PROBABILITY OF TRAIT INHERITANCE
• Test Cross
o Organisms with dominant phenotype crossed with
_____________________________
homozygous recessive
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 = 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?
II. ANALYZING PROBABILITIES, cont
• Pedigree Analysis
Recessive Trait
Dominant Trait…
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
• Multiple Alleles
o Many genes have more than 2
alleles
o Example, ABO blood groups in
humans
o Three alleles



•
A woman with O blood has a child with Type A
blood. The man she claims is the father has
AB 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
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
• 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. GENE MUTATIONS
o Change in the nucleotide sequence
o May be spontaneous mistakes that
occur during replication, repair, or
recombination
o May be caused by mutagens; for
example, x-rays, UV light, carcinogens
o If changes involve long stretches of
DNA, known as chromosomal
mutations
o Point mutations – change in a gene
involving a single nucleotide pair; 2
types
 Substitution
 Frameshift – due to addition or
deletion of nucleotide pairs
V. GENE MUTATIONS, cont
• Classification of Gene Mutations
o Traits may be described as dominant, recessive, etc . based on the
effect of the abnormal allele on the organism’s phenotype
o Instruction encoded by genes carried out through protein synthesis
o Vast majority of proteins are enzymes
o Abnormal allele → Defective enzyme
 If the enzyme produced by the normal allele is present in sufficient
quantities to catalyze necessary reactions,
No noticeable effect on phenotype
Defective allele is classified as recessive
 If the lack of normal enzyme production by defective allele cannot
be overcome by normal allele,
Organism’s phenotype is affected
Defective allele is classified as dominant
VI. INHERITED GENETIC DISORDERS
• Due to gene mutations
• Classified as autosomal or sex-linked, depending on
chromosome location of affected gene
• Autosomal Disorders – Grouped according to path of inheritance
o Autosomal Recessive Disorders
VI. INHERITED GENETIC DISORDERS
 Albinism
VI. INHERITED GENETIC DISORDERS, cont
 Cystic Fibrosis
VI. INHERITED GENETIC DISORDERS, cont
 PKU
VI. INHERITED GENETIC DISORDERS, cont
 Tay-Sachs
VI. INHERITED GENETIC DISORDERS, cont
o Autosomal Co-Dominant Disorders
 Sickle Cell Anemia
VI. INHERITED GENETIC DISORDERS, cont
o Autosomal Dominant Disorders
 Huntington’s Disease
VI. INHERITED GENETIC DISORDERS, cont
• Autosomal Dominant Disorders
 Marfan Syndrome
VI. INHERITED GENETIC DISORDERS, cont
• Autosomal Dominant Disorders
 Achondroplasia
VI. INHERITED GENETIC DISORDERS, cont
• Hypercholesteremia
VI. INHERITED GENETIC DISORDERS, cont
• Sex-Linked Disorders
o All
o Affect
o Hemophilia
VI. INHERITED GENETIC DISORDERS, cont
o Colorblindness
o Duchenne Muscular Dystrophy
VII. 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
• PGD – Preimplantation Genetic Diagnosis
VII. GENETIC TESTING, cont
• Fetal Testing
o Amniocentesis
 Performed between 14th-16th weeks of pregnancy
 Cells collected, tested
VII. GENETIC TESTING, cont
o 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
• Newborn Screening
o PKU
VIII. 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
VIII. 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
VIII. CHROMOSOMAL BASIS OF INHERITANCE,
cont
965 GWgw
944 gwgw
206 Gwgw
Morgan realized variation in probabilities due to
185 gWgw
VIII. 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.
VIII. CHROMOSOMAL BASIS OF INHERITANCE, cont
o Recombination Frequency = # recombinants__ X 100
total # offspring
o One map unit = 1% recombination frequency
……………………………………………………………………………………..
965 GWgw
944 gwgw
206 Gwgw
185 gWgw
VIII. 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.
IX. CHROMOSOMAL DISORDERS
• Alterations in Chromosome Number
o Most commonly due to nondisjunction
o
Results in aneuploid gametes
IX. CHROMOSOMAL DISORDERS, cont
IX. CHROMOSOMAL DISORDERS, cont
o Detected with karyotype
o Examples
 Down Syndrome
IX. CHROMOSOMAL DISORDERS, cont
Turner Syndrome
IX. CHROMOSOMAL DISORDERS, cont
• Klinefelter Syndrome
IX. CHROMOSOMAL DISORDERS, cont
IX. 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.