–Heredity
Gregor Mendel – (1865) “Father of Genetics”
1st to demonstrate the principles of inheritance, not recognized until 1900
Inheritance – Transmission of genetic info from parents to offspring.
Locus - Location of a gene on a chromosome (It’s address)
Allele – One of several varieties of a specific gene ( Ex. eye color Bb, height Tt)
- humans have 23 pairs of chromosomes, so each gene has 2 alleles.
Dominant – Allele that is expressed if present. Indicated using a capital letter
Recessive – Allele that is expressed only in the absence of a dominant allele.
Indicated using a lower case letter.
Homozygous – Same alleles on homologous chromosomes.
Heterozygous – Different alleles on homologous chromosomes.
How to know what alleles an organism has?
1. Examine physical characteristics (phenotyope) Ex. tall or short
2. Do genetic cross to determine the types of genets present (genotype) Ex. TT, Tt,
or tt
*Alleles are inherited in the gametes, gametes are formed by meiosis
Mendel’s law of dominance: Dominant traits are expressed if present.
Mendel’s 1st law of genetic inheritance:
The law of segregation – Alleles of a gene randomly segregate (separate) during the
formation of gametes (Meiosis). Ex. a Female that is Tt will give either a T or a t to each
egg (Not a T to one and a t to another).
Ex.
Diploid
Tt
Meiosis
T
1/2
t
1/2
Possible Haploids
Mendel’s 2nd law of genetic inheritance:
The law of independent assortment – Homologous chromosomes don’t influence the
migration of other homologous chromosomes.
Ex. Chromosome 5 will not influence the assortment of Chromosome 3.
PROBABILITY - Mathematical expression of a degree of confidence that a
certain event will or will not be realized.
Punnett Squares
T
Monohybrid Cross:
T
Tt x Tt
Mother
TT
t
Father
Gametes: T or t for both
Mother
t
Tt
Tt
tt
Possible
Offspring
Father
p tt = ¼ = 25%
p TT = ¼ = 25%
p Tt = 2/4 = ½ = 50%
p T_ = ¾ = 75%
Genotypic ratio TT : Tt : tt = 1:2:1
Dihybrid Cross:
TtRr x TtRr
Mother
Father
Gametes: TR, Tr, tR, tr for both
TR
Tr
tR
tr
TR TTRR TTRr
Tr TTRr TTrr
TtRR TtRr
TtRr
Ttrr
tR TtRR
tr TtRr
TtRr
ttRR
ttRr
Ttrr
ttRr
ttrr
p TTRR = 1/16
p TTRr = 2/16 = 1/8
p TTrr = 1/16
p TtRR = 2/16 = 1/8
p TtRr = 4/16 = 1/4
p Ttrr = 2/16 = 1/8
p ttRR = 1/16
p ttRr = 2/16 = 1/8
p ttrr = 1/16
Rules of probability for solving
genetic problems
Multiplication rule (rule of “and”):
The probability that independent events will occur
simultaneously is the product of their individual
probabilities.
Ex. 1. Probability that 3 coins flipped at the
same time will
result in 3 heads? ½ x ½ x ½ = 1/8
2. In a Mendelian cross between pea plants that
are
heterozygous for flower color (Pp),
what is the
probability that the
offspring will be homozygous
recessive (pp)?
Mother
Father
Pp
Pp
P
p
1/2
1/2
Possible
Gametes
pp
P
p
1/2
1/2
1/2 x 1/2 = 1/4
Offspring
Probability that an egg from the Mother (Pp) will
receive a “p”
allele is p = 1/2.
Probability that a sperm from the Father (Pp)
will receive a “p”
allele is p = 1/2.
The overall probability that two recessive
alleles (pp) will
unite, one from the egg and
one from the sperm,
simultaneously, at
fertilization is obtained by
multiplying both individual probabilities:
pp = egg x sperm = 1/2 X 1/2 = 1/4.
Practice question: If AaBbCc is crossed with AaBbCc,
what are the chances that the gametes are abc?
Addition rule (rule of “or”):
The probability of an event that can occur in two
or more independent ways is the sum of the separate
probabilities of the different ways.
For Example:
In a Mendelian cross between pea plants that are
heterozygous for flower color (Pp), what is the
probability of
the offspring being a heterozygote
(Pp)?
Mother
Father
Pp
Pp
P
p
1/2
1/2
Possible
Gametes
Pp
Pp
Offspring
P
p
1/2
1/2
1/2 x 1/2 = 1/4
1/2 x 1/2 = 1/4
1/2
There are two ways in which a heterozygote may be
produced:
1. The dominant allele (P) may be in the egg
and the
recessive allele (p) in the
sperm
2. The dominant allele may be in the sperm and
the
recessive in the egg.
So, the probability that the offspring will be
heterozygous (Pp) is the sum of the probabilities of
those two possible ways:
1. Probability that the P will be in the egg
with the p in
the sperm is 1/2 X 1/2
= 1/4.
2. Probability that the P will be in the sperm
and the p in
the egg is 1/2 X 1/2 = 1/4.
So, the probability that a heterozygous offspring
will be produced is 1/4 + 1/4 = 1/2.
“Cheater” method for poly-hybrid crosses:
Do monohybrid crosses for all gene
possibilities ( AA, BB,
CC, …..) calculate
specific probability for each cross, and
multiply
all probabilities together.
For Example:
AaBbCc x AaBbCc
offspring?
A
probability of AABBCC
a
A AA Aa
a Aa
aa
B
b
Probability of AA is
1/4
B BB Bb
b Bb
bb
C
c
Probability of BB is
1/4
C CC Cc
c Cc
cc
Probability of CC is
1/4
So, the probability of having homozygous dominant
offspring for all three genes is obtained by
multiplying the probability for each gene.
Prob. Of AABBCC = AA x BB x CC = 1/4 x 1/4 x1/4
= 1/64
Non-Mendellian Genetics
Incomplete dominance:
- One allele is not completely dominant over
the other.
- The heterozygote has a phenotype that is
intermediate between the phenotype of the
two
homozygotes.
- Ex. Red flower (RR), White flower (rr), Pink
(Rr)
Codominance:
- Each allele is expressed in the phenotype
- Ex. Red Petals (RR), White petals (rr),
Red/White/Red/White petals (Rr)
Multiple Alleles:
- More than 2 alleles cause a certain trait
- Ex.Blood type A,B,&O alleles
Pleiotrophy:
- One gene with many phenotypic effects.
- Ex. The gene for growth influences height,
weight, density, etc.
Epistasis:
- Genes that influence the expression of other
non-allelic genes. (“genetic peer pressure”)
Polygenic:
- Many genes have an additive effect on the
phenotype expression.
- Ex. Skin color
Chromosome Theory of Inheritance
- Genes are located on chromosomes
- Chromosomes segregate & independently assort
Thomas Hunt Morgan
- 1st to associate a specific gene with a specific chromosome (early
1900’s)
- Experiment provided convincing evidence that genes are located
on chromosomes
- Fruit fly (Drosophila melanogaster) was the organism of choice.
Why were fruit-flies the organisms of choice?
- Easily cultured in the lab
- Prolific breeder
- Short generation time
- Wild-type is designed by a superscript ( Wildtype is dominant)
Wild Type – Normal or most frequently observed phenotype in a population.
Mutant – Phenotypes which vary from wildtype due to mutations.
Alterations of Chromosome Structure
Breakage - Chromosome breakage can alter chromsome structure.
Deletion – Lost segment of genetic material from a chromosome
(lack a centromere)
Duplication – Lost segment joins the homologous chromosome,
causing repetition.
Inversion – Lost segment reattaches to the original chromosome in
reverse order
Translocation – Lost segment joins to a non-homologous chrom.
Most common is reciprocal translocation.
Deletions & duplications are likely to occur during meiosis.
Homologous (non-sister) chromatids sometimes break & rejoin at
“incorrect” places, so that one gives more than it receives –
Nonreciprocal translocation results in 1 chrom. w/a duplication.
Errors and exceptions in chromosome inheritance
Meiotic errors & mutagens can cause major chromosomal change due to
altered chrom # or altered chrom structure.
Alterations in Chromosome Number:
Nondisjunction – Failure of chrom. to separate & move to opposite poles.
Results in loss or gain of chrom. 1 gamete receives 2 of the same type
of chrom & another gamete receives no copy.
Aneuploidy – Condition of having an abnormal # of certain chrom.
Result of a normal gamete uniting w/an abnormal one produced as a
result of nondisjunction. Aneuploid cell that has a chrom. in triplicate
– Trisomic – for that chrom. (Ex. Down’s Syndrome is trisomy of
#21) Aneuploid.
Polyploidy – Chromosome # that is more than 2 complete chromosome sets
Triploidy = 3n
Possible result of fertilized diploid egg produced by nondisjunction
Tetraploidy = 4n
May result if a diploid zygote undergoes mitosis w/out cytokinesis.
Subsequet normal mitosis would produce a 4n embryo.
Polyploidy common in plants & important in plant evolution
Extremely rare in animals
Polyploids are more nearly normal in appearance than aneuploids
Gene Mapping
One of Morgan’s students figured out a method of constructing a genetic
map (or an ordered list of genetic loci on a particular chromosome) after the
discovery of linked genes & recombination.
Recombination frequency calculated from experiments reflect the distances
between the genes on the chromosome.
Assuming that the chance of crossing over is approx. equal at all points on
the chromosome he predicted that the further apart the 2 genes are the higher
the probability that the crossover will occur & then result in a higher
frequency % of recombination.
With this in mind he began using recombination data from fruit fly crosses
to assign the relative positions of genes on the chromosomes ( MAP
GENES)
Maps give:
Names of genes, distance between genes, order of genes
1% recombination = 1 map unit = 1 cM (centimorgan)
Loci
b vg
cn b
cn vg
Recom. Freq.
18.5%
9%
9.5%
Approx map units
18.5
9
9.5
Chromosome map:
b
cn
9
vg
9.5
Sex Determination
In 1990, British research team identified a gene for the development of
testes. Named SRY (sex determining region of the Y), it codes for the
protein TDF (testes determining factor) that regulates many other genes
required for male development. If no SRY, female develops.
X-Inactivation
X-chromosome is larger than Y, therefore there are more X-linked traits than
Y-linked traits.
How does an organism compensate for the fact tht some individuals have a
double dose of sex-linked genes ( Ex. Human females) while others have
only one (Ex. Human males)?
X-Inactivation in female mammals – most diloid cells have only 1 fully
functional X chromosome. The other becomes almost completely inactivated
during embryonic development ( approx. 16 days after fertilization) due to
attachment of methyl groups (-CH3) to cystine ( one of DNA’s nitrogenous
bases. The inactive X in each cell condenses into a compact object (Barr
body). It lies along the inside of the nuclear envelope. Most of the genes
within the chromosome that forms the Barr body are not expressed, although
some remain active. They are reactivated in gonadal cells to undergo meiosis
to form gametes.
Mary Lion – British geneticist demonstrated that the selection of which of
the 2 X’s will form the Barr body is random and independent.
What determines which of the 2 X chromosomes will be methylated?
Recently discovered gene XIST (X inactive specific transcript) is active only
on the Barr body. The product of the XIST gene is an RNA molecule that
apparently attaches to the X chromosome, inactivating it.
Still don’t know: How XIST initiate inactivation? What determines which
chromosome in each cell will have active XIST gene and become Barr
body?