Transcript Introduction to Genetics

Introduction to Genetics
Chapter 11
What is inheritance?
• Genetics – the scientific study
of heredity.
• Gregor Mendel – an Austrian
monk who observed patterns
of inheritance in pea plants.
• Why peas? Breeding could be
easily controlled (pollen and
eggs), they grew fast, and had
variable genetic traits
(characteristics).
Peas had varying
traits:
Round, wrinkled,
green, yellow, tall,
short, axial flowers,
terminal flowers, etc…
Mendel’s observations
of peas:
• Some plants were true-breeding –
they always produced the same
offspring. Always tall, green…
• Pea plants could be selectively bred by
brushing on or inhibiting pollen.
• Certain traits would “disappear” for a
generation, then return again.
• Repeating patterns emerged.
Mendel’s observations
• P generation –
parent
generation.
• F1 generation –
first filial,
offspring of P
generation.
• Hybrid –
offspring
produced by
parents of
different traits.
Patterns emerging…
• When Mendel bred the purple
and the white, all the F1
generation were purple. The
white disappeared.
• White flower trait is recessive.
• All offspring were purple.
Purple flower trait was
dominant.
• Different versions of a gene are
called alleles.
Would the recessive trait return?
• Mendel bred the F1 generation to
produce an F2 generation: flower color,
pod color, shape…The recessive returned.
What
ratio
was
observed?
Mendel’s conclusions:
• Each trait has two alleles (versions).
Plant height: Tall (T) or Short (t)..
• These alleles segregate, or separate
when gametes (egg and sperm) are
formed. TT makes T gametes, Tt
makes T gametes and t gametes..
• The allele from a sperm and the
allele from the egg is the organism’s
genotype. TT, Tt, or tt..
• What the organism looks like is its
phenotype. Tall plant, short plant..
Genotype, phenotype, homozygous
(same) and heterozygous (different)
traits
Probability and Punnett
Squares:
• When gametes are formed, there is
a 50/50 chance that it will get one
version of a trait. Like a coin flip!
• The different possible offspring,
and ratios of genotypes produced in
a cross can be shown using a
Punnett square.
• Did the 3:1 ration for Mendel’s F1
cross make sense?
A Monohybrid
(one trait) cross:
Dihybrid (two traits)
cross, Trihybrid…
• In reality, all traits are passed on to
the gametes.
• Most traits go into the gametes
independent of each other.
• Example: A homozygous tall,
yellow plant (TTYY) produces only
the following gamete: TY.
• Another example: a heterozygous
tall, yellow plant TtYy can produce
the following gametes: TY, Ty, tY,
ty.
Try a Dihybrid cross:
Other modes of inheritance….
• Incomplete
dominance – the
dominant gene
does not
completely show
• The heterozygous
is in-between
• Japanese four-oclocks,
snapdragons
Other modes…
• Codominance – both alleles show up in the
phenotype.
• Examples: coat color in cows, speckled hens,
sickle-cell trait, blood type…
• Multiple alleles – more than two alleles for a
trait.
Meiosis – Gamete
production!
• In order to make an individual with
the full component of chromosomes
(diploid), gametes must be created
(haploid).
• Gametes are sex cells with half the
amount of chromosomes.
• We have 23 homologous (same)
pairs of chromosomes, total = 46.
• Gametes (egg or sperm) have 23.
Meiosis…
• Meiosis consists of 2 divisions of a
diploid or 2N cell, to create 4
haploid cells (1N each).
• Before the first division, the
chromosomes replicate to form the
tetrad (X X figure). The
homologous pairs and their copies
go through crossover.
• Crossover produces new
combinations of alleles, It makes
you unique!
Crossover…
• Crossover happens in prophase I, and
sections of one homologue are traded with
sections of another.
• Alleles are shuffled from one to another.
This is a random event, with an infinite
number of combinations!
Crossover…
• If genes are close together on an
arm of a chromosome, they are
more likely to transfer together.
• Because of this, they are said to
exhibit linkage.
• This helps scientists map the genes
on a chromosome!
Gene maps
Meiosis continued…
• Meiosis I – chromosomes
duplicate, crossover occurs, IPMAT,
results in 2 diploid cells.
• Meiosis II – PMAT, results in
haploid cells.
• Note that these 4 gametes are each
genetically unique!
• Non-disjunction - if the
chromosomes fail to separate
properly, disorders occur.
Meiosis