Download Lecture#12 Page 1 BIOLOGY 207 - Dr.McDermid Lecture#12 Alleles

BIOLOGY 207 - Dr.McDermid
Lecture#12
Alleles, Dominance, and Segregation
Readings: Griffiths et al, 7th Edition: Ch. 2 pp 28 – 34; Ch. 4 pp 106, 109-111
Problems: Griffiths et al, 7th Edition: Tier 1: Ch. 2. #2,7,9,12; Ch. 4 #2,4,7,9
Tier 2: Ch. 2. #3,10,11,13; Ch. 4 # 1,5,6
Concepts:
How do genes behave in diploids?
1. From the wide variety of mutational possibilities for most genes, we can usually distinguish
only functional and non-functional alleles.
2. The functional allele is usually dominant to the non-functional allele in individuals with both
alleles (heterozygote).
3. Offspring from heterozygous parents may have a recessive phenotype, because of the
segregation of alleles in meiosis.
Genes and Alleles in Diploids
Allelic forms:
"normal" or wild type sequence or allele
variant allele
mutant allele
Diploids have two alleles for each gene locus
Having two alleles sometimes makes it difficult to ascertain the genotype from the phenotype
of a diploid
Genotype
Phenotype
Heterozygotes and homozygotes
If we start with two different forms of a gene: two alleles
One allele is functional Other allele is non-functional Lecture#12
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Possible combinations of alleles:
1) AA ---> homozygote
2) aa ---> homozygote
3) Aa - alleles are of different types --> heterozygote
Result: ---> phenotype is ????????
Dominance and recessive
Note: Because the A allele determines the phenotype in the diploid heterozygote it is
considered dominant to the a allele.
Because the a allele is masked by the A allele, the a allele is considered recessive to the A
allele.
For the simple situation where
A is dominant to a or a is recessive to A
A/A -->
A/a -->
a/a -->
Note:
1). The terms Dominant and recessive are always used in relation to another alleles
2). Dominance and recessive are not an innate property of an allele but a relative one.
Notation - Frequently capital letters are used to denote dominant alleles while the recessive
alleles are given the lower case equivalent
eg. Round (R) and wrinkled (r)
The Dominance/recessive relationship is not the only possibility:
Incomplete dominance
Intermediate (or blended) phenotype in the heterozygote
Example in "Four-o'clock" plants
Strains have flowers with either red or white petals:
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C Red / CRed
C White / CWhite
C Red / CWhite
Co-dominance
In certain circumstances both alleles can be seen in the phenotype.
Example: Human Blood Groups A, B, and O
Example: Human disease Sickle Cell Anemia
H bS and HbA
The Father of Genetics - Mendel:
The concepts of alleles, dominance, recessiveness, pairs of genes, homozygotes,
heterozygotes all come to us from the work of Gregor Mendel and his peas.
Gregor Mendel's peas
Gregor Mendel who did his experiments in the 1860's with the pea plant (Pisum sativum)
Advantages of peas:
1) Variety
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2) Self pollinate and cross pollinate
He had true-breeding strains
Some of the work Mendel involved the inheritance of the Round and wrinkled character of the
pea
The crosses that led Mendel to his understanding:
Parents:
Round (RR)
cross (x) to
wrinkled (rr)
Gametes:
R
P1
r
Progeny:
Round (Rr)
F1
Heterozygote-Rr self-pollinate
Round (Rr)
cross (x) to
Round (Rr)
P2
ovum
R
pollen
r
R
r
Resulting Progeny observed by Mendel:
Genotype
number
ratio
Phenotype
Ratio
RR
Rr
rr
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Mendel proposed the following to account for the observations he made:
1) Hereditary determinants are particulate in nature
2) Each individual has two alleles for each character studied
3) The alleles segregate (separate) equally into the gametes
4) Each gamete (pollen and ovum) carries only one allele of the pair
5) One gamete (1N) from each parent fuse to form the zygote (2N) of progeny. The gametes
combine at random.
Mendel's 1st Law
Law of Segregation of allele pairs
The two members of a gene pair segregate from each other into the gametes
Another example:
If Mendel had looked at Four-o’clock plants, crossing a true-breeding red with a true-breeding
white, CRed/CRed X C White/Cwhite,
The F1 would all be
The C Red/CWhite X C Red/CWhite would give:
Genotype
number
ratio
Phenotype
Ratio
C Red/CRed
C Red/CWhite
C White/CWhite
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Lecture notes: Copyright © 2002 Heather McDermid and the Department of Biological Sciences, University of Alberta
Images are Copyright©2000 by W.H. Freeman & Co. in Griffiths et al, Introduction to Genetic Analysis
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