Download Lecture 13: May 24, 2004

Lecture 13: May 24, 2004
CH14: Mendel and the gene idea
*particulate inheritance – parents pass on discrete heritable units
*gene- unit of inheritance which occupies a specific chromosomal location (locus)
*allele- alternative forms of a single gene
Mendel’s hypothesis (to explain his results)
1. Alternative versions of genes account for variation in inherited
characters
2. For each character, an organism inherits two alleles, one from
each parent
3. If two alleles differ, one is dominant, the other recessive
4. The two alleles for each character segregate (separate) during
gamete production.
*genetic make-up – genotype
*appearance - phenotype
*Homozygous – 2 identical alleles for a character
*Heterozygous – 2 different alleles for a character
Sample problem
Albinism in humans is inherited as a simple recessive trait. Det ermine the genotypes
of the parents and offspring for the following families. When two alternative
genotypes are possible, list both. (A) Two non albino (normal) parents have five
children, four normal and one albino. (B) A normal male and an albino female have six
children, all normal.
1) establish gene symbols:
Move on to part (A): Parents are
both
phenotypically
normal,
genotypes could be EITHER AA
or Aa, an albino phenotype could
only result from an aa genotype.
*One a had to come from the
mother and one a had to come
from the father, so, the parents
must be genotypically Aa.
2) Establish: genotype
phenotype
Sample problem con’t:
(B) A normal male and an albino female have six children, all normal.
1) The female is phenotypically albino; genotype can only be aa
2) The male is phenotypically normal; genotype can be AA or Aa
3) Since all children are
4) BUT male COULD also be Aa !
normal one might assume
*IF the father was
the male to be AA
genotypically Aa, then
what is the likelihood
(chance or probability)
of this couple having 6
normal children?
Recall the product law!
Mendel’s Law of Independent Assortment
*What happened when he looked at two characters?
If they segregate together:
If they segregate independently:
Dihybrid cross- A genetic cross between two individuals involving two
characters
Punnett square and the law of
Example:
independent assortment:
P1
X
yellow, round
green, wrinkled
GGWW
ggww
F1
All
yellow, round
GgWw
Punnett square and the law of
independent assortment:
F1
F1
X
All
yellow, round
All
yellow, round
GgWw
GgWw
F2
9/16 yellow, round
9:3:3:1 Phenotypic ratio; Genotypic ratio as follows:
1/16 GGWW, 2/16 GGWw, 2/16 GgWW, 4/16 GgWw
3/16 yellow, wrinkled
1/16 GGww, 2/16 Ggww
3/16 green, round
1/16 ggWw, 2/16 ggWw
1/16 green, wrinkled
1/16 ggww
Mendelian inheritance is based on probability
Example- coin toss
*1/2 chance landing heads
*Each toss is an independent event
*Coin toss, just like the distribution
of alleles into gametes
*The rule of multiplication – determines
the chance that two or more independent
events will occur together
½ x ½ = ¼
The Rule of Addition
*The rule of addition
*The dominant allele could come from
the sperm and the recessive from
the ovum Probability = 1/4
*The dominant allele could come from
the ovum and the recessive from the
sperm Probability = 1/4
*The probability of a heterozygote =
¼ + ¼ = ½
Modifications of Mendelian Ratios
*Incomplete dominance
*Codominance
Example:
MN Blood group
*We can predict
genotypic and
phenotypic ratios
Modifications of Mendelian Ratios
*Multiple Alleles
Examples:
*100 alleles at a given locus in Drosophila
*ABO Blood group in humans
*Pleiotropy
Genotype
IAIA
IAIO
IBIB
IBIO
IAIB
IOIO
Antigen
A
A
B
B
A,B
Neither
Phenotype
A
A
B
B
AB
O
Modifications of Mendelian Ratios
*Epistasis
Example:
In Drosophila, gene:eyeless
*Black (B) is dominant to brown (b)
*Second gene responsible for allowing
pigment to be deposited in hair
C = presence, c = absence (colorless)
*Quantitative characters
*often due to polygenic inheritance
Example*skin color – controlled by 3 genes
*each gene with two alleles
light and dark
incomplete dominance
Phenotype depends on
environment and genes
CH15: The Chromosomal basis of inheritance
*We will cover pgs 269-270 (Mendel and
chromosomes) and pgs 276-279 (sex chromosomes)
Sex Chromosomes
*1909 Thomas Hunt Morgan
II
III
IV
XY
or
XX
Sex chromosomes
Autosomes
Example: In Drosophila and all mammals
sex chromosomes designated as X and Y
XX=female
XY=male
*Each chromosome has two regions:
*homologous region
*differential region
Sex-linkage
X-linkage
Transmission of sex-linked recessive traits:
Sex-linked disorders in humans
*Duchenne muscular dystrophy
*Hemophilia
The X Chromosome and Dosage Compensation
*Do females produce twice as much of each gene product for X-linked
genes?
Dosage compensation
*In mammals – X inactivation