Transcript Modified Mendelian Ratios I

Announcements
1. Exam 1 next week, 9/19, 9/20 in testing center. Covers chapters 1
through 4, with emphasis on material: from lectures through 9/13,
from “Monk in the garden”, and from lab. Part multiple-choice, part
short answer - emphasis on problem-solving. No time limit, but must
finish that day so choose a 2-3 hr. time block. Closed book; bring
calculator, #2 pencils and BLUE BOOK.
**Review sessions next week in lecture and in lab. Bring your
questions!
2. “Problem set 2” answers due Friday, 9/13 at start of class.
Also
practice Ch.4 problems this week (but do not turn in): 1, 7, 16, 27,
31.
3. re. printing power point slide files: when in computer room of
Brooks, please choose “handout” when asked print what and
print 6 slides/page. Do not print 1 slide per page.
Problem Set 2: due Friday 9/13 in class - show all your work.
1. The LM and LN alleles at the MN blood group locus exhibit codominance.
Give the expected genotypes and phenotypes (with their ratios) of
progeny from the following crosses:
a) LMLM x LMLN
b) LMLN x LNLN
2. A woman of blood group AB marries a man of blood group A whose father
was group O. What is the probability that:
a) their 2 children will both be group A?
b) one child will be group B and the other child group O?
3. In snapdragons, red flower color (R1) is incompletely dominant to white
(R2); the R1/R2 heterozygotes are pink. A red-flowered snapdragon is
crossed to a white colored one. Determine the ratios of the flower colors
in the progeny from a cross of an F1 with the red parent.
2 points each question part for 10 points total
Review of last lecture
I.
Chi-square revisited: small deviation from
expected yields small X2 value; this correlates
with high probability that deviation is due to
chance and you should NOT reject your
hypothesis
II. Pedigree analysis- recessive vs. dominant traits
- solving pedigree problems
Solving Pedigree Problems
• Inspect the pedigree:
– If trait is dominant, it will not skip generations nor be
passed on to offspring unless parents have it.
– If trait is recessive, it will skip generations and will
exist in carriers.
• Form a hypothesis, e.g. autosomal recessive.
• Deduce the genotypes.
• Check that genotypes are consistent with phenotypes.
• Revise hypothesis if necessary, e.g. autosomal
dominant.
Pedigree Example 2: p. 71, #26
Outline of Lecture 7
In all crosses discussed so far, one of two traits for a character has
been dominant to the other. ie. according to Mendel’s second
postulate of dominance/ recessiveness. Does the expression of all
genes occur in this way? ex. Are there only two colors of hair for
humans with one clearly dominant to the other? NO
I. Alleles alter phenotypes in different ways; a variety of symbols are used for
alleles
II. Incomplete dominance - where neither allele is dominant
III. Codominance - both alleles in a heterozygote are expressed
IV. Multiple alleles of a gene are studied in a population
V. Lethal alleles - recessive or dominant
VI. Modification of the 9:3:3:1 ratio
I. Alleles - alternate forms of the
same gene
• Wild-type allele - allele (form of gene) most frequently
found in nature (“normal”); specifies normal
phenotype and is usually dominant.
• Mutant allele specifies an altered phenotype.
• Mutation creates new alleles.
Gene Symbol Conventions
• ebony body color mutation in Drosophila: e
• Normal (wildtype) color is gray: e+
– e+/e+ or +/+ is homozygous wildtype
– e/e is homozygous ebony
– e+/e or +/e is heterozygous
• Other systems are also used, but symbol usually
reflects the function of the gene, e.g. cdc, leu-,
BRCA1
II. Incomplete Dominance
• Neither of two alleles is
dominant, e.g. snapdragon
flower color:
– R1 is red
– R2 is white
• Heterozygotes give an
intermediate (blended)
phenotype:
– P1 cross gives pink
flowers in F1
– F1 cross gives 1:2:1
red:pink:white in F2
III. Codominance
• When two alleles of a gene specify two distinct, detectable
gene products
• MN blood group in humans: LM, LN alleles
• MN locus codes for surface glycoprotein on red blood
cells; can detect immunochemically.
• LM LM gives M phenotype
• LM LN gives MN phenotype
• LN LN gives N phenotype
• LM LN X LM LN produces 1/4 LM LM, 1/2 LM LN, 1/4 LN LN
IV. Multiple Alleles: ABO Blood Groups
• When 3 or more alleles present (allelic series); can
only be studied in populations.
• A and B alleles code for glycoproteins on red blood
cells which can be detected immunochemically:
– mix blood sample with type A or type B antibodies
– look for clumping of RBC’s
• O allele carries neither antigen
ABO Blood Groups
A - A antigen only
B - B antigen only
AB - Both A and B antigens
O - Neither antigen
ABO Genotypes and Phenotypes
Genotype
Antigen
Phenotype
IAIA
A O
I I
B B
I I
B O
I I
IAIB
A
A
B
B
A, B
A
AB
IOIO
Neither
O
B
ABO, continued
• IA and IB are codominant
• Both IA and IB are dominant to IO
• All possible matings shown in Table 4.1
• Applications:
– testing compatibility of blood transfusions
– disproving parentage of a child
– forensic science
Biochemical Basis of ABO
• A and B antigens are on
carbohydrate groups
bound to fatty acids
(glycolipid) on RBC
membrane
• The A and B alleles code
for enzymes that
differentially process the
carbohydrate during
synthesis of the glycolipid:
– A enzyme adds Nacetylgalactosamine
– B enzyme adds
galactose
Complexity with ABO blood groups:
The Bombay Phenotype
Biochemical Basis of Bombay Phenotype
• h mutation prevents
addition of fucose to form
H substance.
• A and B enzymes no
longer recognize
structure, don’t add A and
B antigens.
• So individual is
phenotypically O but can
be genotypically A_, B_ or
AB.
• H/h acts upstream of A
and B in the pathway.
Bombay Phenotype: hh masks the
expression of ABO (Epistasis)
HhIAIO HhIAIB
hhIBIO
The Secretor Locus also affects the
expression of the ABO blood type
About 80% of human population have the A and B antigens
present in various body secretions - not only in blood.
Genetics - dominant allele, Se (Se/Se or Se/se)
In what societal application would the secretor locus
have significance?
Forensic science - ABO blood typing can be performed on
tissue samples other than blood.
Multiple alleles - a second example
White locus in Drosophila - over 100 alleles may occupy this
locus. This results in an allelic series of eye colors ranging
from pure white, to light buff to yellowish pink to deep ruby.
(Table 4.3 in text)
V. Lethal Alleles
• Recessive lethal if heterozygote
is viable, homozygous mutants
die; common, e.g.
– Yellow (AY) in mice
– Manx (M) in cats
– Curly wing (Cy) in
Drosophila
• Dominant lethal if either
heterozygotes or homozygous
mutants die; rare, e.g.
Huntington disease
Mouse coat colors
Agouti x agouti
Yellow x yellow
Agouti x yellow
All agouti
2/3 yellow, 1/3 agouti
1/2 yellow, 1/2 agouti
Effect of Dominant Yellow, Recessive
Lethal in F2
A
AY
Y
A
AA
agouti
AA
Yellow
AY
AAY
Yellow
AYAY
Yellow, lethal
Results in 2:1 monohybrid ratio
AY is dominant to A
AAY is yellow, but
AYAY is lethal
VI. Modified Dihybrid Cross: First
Consider Each Trait on its Own
Ex. 2 humans heterozygous for albinism and are blood type AB
Modified Dihybrid Cross: Next
Consider Both Traits Together