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 ______________
o Eliminated
________________________ and
controlled ____________________
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
 ___________________________
 F1 Phenotype =
 F1 Genotype =
o F1 X F1 → F2
 ___________________________
 F2 Phenotype Ratio =
 F2 Genotype Ratio =
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 Segregation
5)
Law of Independent Assortment
2)
3)
II. ANALYZING PROBABILITY OF TRAIT INHERITANCE
• 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 egg carrying t =
 Probability of sperm carrying t =
 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 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
II. ANALYZING PROBABILITIES, cont
• Pedigree Analysis
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’
• 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



•
Phenotype
Genotype
A
B
AB
O
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?
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
• Pleiotropy
• 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
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
o 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
Albinism
Cystic Fibrosis
PKU
VI. INHERITED GENETIC DISORDERS, cont
o Autosomal Recessive Disorders, cont
 Tay-Sachs
o Autosomal Co-Dominant Disorders
 Sickle Cell Anemia
VI. INHERITED GENETIC DISORDERS, cont
o Autosomal Dominant Disorders
 Huntington’s Disease
 Marfan Syndrome
 Achondroplasia
 Hypercholesteremia
VI. INHERITED GENETIC DISORDERS, cont
• Sex-Linked Disorders
o All
o Affect
o Examples
 Hemophilia
 Colorblindness
 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
• Fetal Testing
• Newborn Screeing
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
• Linked Genes, cont
o Instead Morgan counted
965 GWgw
944 gwgw
206 Gwgw
o 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.
o Recombination Frequency =
# recombinants__ X 100
total # offspring
o One map unit = 1% recombination frequency
IX. CHROMOSOMAL DISORDERS
• Alterations in Chromosome Number
o
o
o
o
Most commonly due to nondisjunction
Results in aneuploid gametes
Detected with karyotype
Examples
 Down Syndrome
 Turner Syndrome
 Klinefelter Syndrome
IX. CHROMOSOMAL DISORDERS, cont
IX. CHROMOSOMAL DISORDERS, cont
• Alterations in Chromosome Structure
o Often due to mistakes made during
• 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.