Transcript Inheritance

Inheritance
Chapter 29
Gregor
Mendal
1822 - 1884
“Father of Genetics”
What Mendal did
 He
bred peas in the monastery garden at Brno,
Czech Republic (then part of the
AustroHungarian Empire).
 Observed occasional variations in the
appearance of these plants.
 Selectively bred plants to consistently produce
“characteristics” that were unusual.
 Saw a pattern in the way that the unusual
characteristics showed up.
 Was the first to propose that these
characteristics were passed from one
generation to another by the gametes.
The Abby where Mendal worked
What Mendal did not do
 He
didn’t use the word “gene” to refer to
subject of his work.
 He didn’t see chromosomes.
 He never used a Punnett square.
 He never achieved fame in his lifetime for
his work.
Charles Darwin
1809 - 1882
•Proposed the “Theory of Evolution”.
• Actually, talked about “descent with
modification from a common
ancestor”. He didn’t use the word
“evolution” very often.
• Voyage of the Beagle 1831 – 1836.
• Presented paper with Alfred
Russell Wallace in 1858.
• Published first edition of “Origin of
Species” in 1859.
Some Vocabulary
– study of inheritance.
 Autosomes – the 22 pairs of
chromosomes that do not determine
genetic sex.
 Sex chromosomes – the 23rd pair, the X
and the Y.
 Karyotype – the diploid chromosomes
displayed in their condensed form and
paired as homologs
 Genetics
A typical karyotype
More Vocabulary
 Alleles
- a matched pair of two genes,
coding for the same or alternative forms of
a particular trait. Found at the same
location (locus) on homologous
chromosomes.
 Homozygous – having the same alleles for
a trait
 Heterozygous – having different alleles for
the same trait.
More words
– an allele that expresses itself and
masks its partner. Example: brown hair is
dominant over blond.
 Recessive – the reverse of the above. The
allele that is masked
 Allele pairs are expressed as a pair of letters
representing the trait. Example: Mendal’s peas
came in tall and short. Tall is the dominant allele
for height in peas. Therefore it is written as a
capital “T”.
 A heterozyote for height would be Tt, with the
lowercase t representing the recessive.
 Dominant
Genotype vs. Phenotype
– the actual alleles an organism has
is it’s genotype. In our heterozygote pea plant
that would be Tt.
 Phenotype – that which is expressed. Our pea
plant maybe genotypically heterozygotic but
phenotypically it is tall.
 Genotype
 Homozygote
dominant = TT phenotype = tall
 Homozygote recessive = tt phenotype = short
 Heterozygote
= Tt phenotype = tall
Sources of
variation:
segregation
&
independent
assortment
Assortment leads to many
possibilities as far as
gamete formation goes.
For any genome it can be
calculated as 2n, where
n=the number of
chromosome pairs.
So for a human with 23
chromosome pairs, the possible
23
combinations of gametes = 2
or
8,388,608!
(and that’s with out recombination)
Mendal’s Laws
 Mendal
discovered that if you bred plants
that had two alleles for each trait that you
would get the same ratios of phenotypes &
genotypes whenever you crossed
heterozygotes. It was like clockwork!
 This was because of independent
assortment and segregation, which
became known as “Mendal’s Laws”
It works like this…
Phenotypic ratio = 3:1
or
3 tall : 1 short
Genotypic ratio = 1:2:1
or
1 homozygote
dominant
2 heterozygotes
1 homozygote
recessive
Example: PKU
Violation of Mendal’s Laws
 Mendal’s

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


laws only hold if:
there is random fertilization
the alleles are located on separate
chromosomes
the alleles have a simple dominant/recessive
relationship
there are only two alleles for that trait
they are not lethal to the zygote
Recombination interferes with
Mendal’s laws
Types of inheritance
Aside from simple dominant/recessive
dominance – a dominant allele does
not completely mask the recessive (red flower +
white flower = pink flower).
 Codominance – both traits are expressed
together (red flower + white flower = stripes).
 Multiple alleles – More than one allele for a trait.
ABO blood group is an example.
 Polygene – several alleles interact to produce a
trait. Results are a continuous or quantitative
phenotype, as in skin color.
 Incomplete
Incomplete
dominance:
Sickle Cell
Codominance of multiple alleles
Polygenic inheritance
Sex-linked inheritance
 Males
only have one X chromosome.
Therefore, if a trait is found only on the X it will
be expressed in a male regardless of whether it
is dominant or recessive.
 X – inactivation occurs in females. Every
normal woman has two Xs but they only need
one. Therefore, one X chromosome turns off,
forming a Barr body.
 Because X – inactivation is random in most
cases, it leads to a fine mosaic of cells in
females.
22 autosomes & 1 set of sex chromosomes
Sex
determination
in humans
Colorblindness:
a sex-linked
trait
Environmental influences
– Developmental influences
impact genetic expression in ways that
appear to be genetic but are not
inheritable.
 Temperature, nutrition, non-genetic
pathologies can have impacts that are
expressed in ways that appear genetic.
 Phenocopy
Genetic defects
– a defective set of genes.
 Triploidy – an extra set of chromosomes
 Trisomy – an extra single chromosome
 Monosomy – a missing homolog
 Aneuploidy
of the 23rd chromosome –
XXX = “super female”
XXY = Klinefelter’s syndrome
 Trisomy of the 21st chromosome leads to
Down’s Syndrome.
 Trisomy
Down’s syndrome
Klinefelter’s a type trisomy
affecting the
sex
chromosomes
Turner’s Syndrome: monosomy of
the 23rd chromosome, X_
Monosomy of the 23rd chromosome
Name that condition!
A
Pedigree:
tracking
genetic
traits
A Peek
into the
Future:
Screening
for genetic
disorders
That’s all folks!