Transcript Inheritance

Inheritance
Chapter 29
Gregor
Mendel
1822 - 1884
“Father of Genetics”
What Mendel 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 Mendel worked
What Mendel 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
Mendel’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 Mendel’s Laws
 Mendal’s





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
Penetrance
 Percentage
of individuals with particular
genotype that shows “expected”
phenotype
Expressivity
 Extent to which particular allele is
expressed
Teratogens
 Factors
that result in abnormal
development
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)
Suppression
1
gene suppresses other
 Second gene has no effect on phenotype
Complementary Gene Action
 Dominant
alleles on 2 genes interact to
produce phenotype different from when 1
gene contains recessive alleles
Sources of Individual Variation
 During
meiosis, maternal and paternal
chromosomes are randomly distributed
 Each gamete has unique combination of
maternal and paternal chromosomes
Crossing Over and
Translocation
Figure 29–17
Genetic Recombination
 During
meiosis, various changes can
occur in chromosome structure, producing
gametes with chromosomes that differ
from those of each parent
 Greatly increases range of possible
variation among gametes
 Can complicate tracing of inheritance of
genetic disorders
Crossing Over
 Parts
of chromosomes become
rearranged during synapsis
 When tetrads form, adjacent chromatids
may overlap
Translocation
 Reshuffling
process
 Chromatids may break, overlapping
segments trade places
Genomic Imprinting
 During
recombination, portions of
chromosomes may break away and be
deleted
 Effects depend on whether abnormal
gamete is produced through oogenesis or
spermatogenesis
Chromosomal Abnormalities
 Damaged,
broken, missing, or extra
copies of chromosomes
 Few survive to full term
 Produce variety of serious clinical
conditions
Mutation
 Changes
in nucleotide sequence of allele
Spontaneous Mutations
 Result
of random errors in DNA replication
 Errors relatively common, but in most
cases error is detected and repaired by
enzymes in nucleus
 Errors that go undetected and unrepaired
have potential to change phenotype
 Can produce gametes that contain
abnormal alleles
A Map of Human Chromosomes
Human Genome Project
 Goal
is to transcribe entire human genome
 Has mapped more than 38,000 human
genes
Karyotyping
 Determination
of individual’s complete
chromosomal complement
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
Carriers
 Individuals
who are heterozygous for
abnormal allele but do not show effects of
mutation
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.
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 syndrome
Klinefelter’s a type trisomy
affecting the
sex
chromosomes
Turner Syndrome: monosomy of
the 23rd chromosome, X_
Monosomy of the 23rd chromosome
Name that condition!
A Peek
into the
Future:
Screening
for genetic
disorders
Thanks for a great term!