Transcript Genetics_Mendel and beyond

Patterns of inheritance:
Mendel and
beyond
Contrasting characters in peas
Experiment with garden peas I
In this case a truebreeding tall plant was
crossed with a truebreeding short plant.
All of the plants in the next
generation were tall.
These are the
results Mendel
obtained when
he crossed two
heterozygotes.
‘Paint-pot’ theory of inheritance
Mendel’s finding was contrary to the
prediction of the ‘Paint-pot’ theory of
inheritance
Mendel’s 1st lawlaw of segregation
Mendel’s law of segregation explanation of monohybrid ratio
Mendel described patterns of inheritance in the 1860s,
but it wasn’t until the early 1900s that inherited traits,
genes, were linked to cellular structures called
chromosomes. The number of chromosomes varies
among species, but all chromosomes contain genes
arranged linearly at specific locations, called loci.
This is how geneticists today represent the cross from
the previous screen. The original plants are the P
generation and their offspring are the F1 generation.
The T and t symbols represent dominant and
recessive alleles of a single gene.
we breed the offspring
to one another in the
cross Tt x Tt, and wish
to predict the results
we need to follow the
production of gametes
during meiosis.
These individuals
make gametes with
either T or t in equal
numbers.
The chance of gametes of different types encountering one
another is represented on a Punnett square.
The genotypic ratio of homozygous dominant:
heterozygous: homozygous recessive individuals is 1:2:1.
Many genetics problems will fall in the category of a
monohybrid cross. They all can be approached using
the model above
Testcross. a heterozygote
was crossed with a
homozygous recessive
individual.
A situation in which an
individual of dominant
phenotype, but of unknown
genotype, is crossed with
one or more recessive
individuals. This can
provide information on the
unknown genotype.
Dihybrid
cross
Dihybrid cross
Dihybrid
cross
The inheritance of
two contrasting
characters were
considered at the
same time
Law of independent assortment
Mendel’s
nd
2
law
Mendel’s
2nd law
explained by
observing
movement of
chromosomes at
meiosis
Cell division
The cell cycle
Interphase
Interphase
Mitosis – division of the nucleus
Division of
the nucleus
Prophase
metaphase
anaphase
telophase
Division of the cytoplasm
Produce identical daughter cells
Mitosis and cell cycle
Meiosis
• Reduction
division
• Generate
variability in
gametes
Meiosis-Interphase
Meiosis-prophase I
Meiosis-metaphase I
Meiosis-anaphase I
Meiosis-telophase I
Meiosis I -animated
Meiosis II
prophase II
metaphase II
anaphase II
telophase II
Meiosis II -animated
Meiosisoverall
Independent assortment
Crossing over
Anaphase
I and II
note new gene
combinations as a
result of crossing
over
Comparing
Mitosis
and
Meiosis
Comparing
Mitosis
and
Meiosis
Incomplete dominance
In some cases, alleles
may not interact in a
dominant/recessive
pattern. Heterozygotes
have an intermediate
phenotype or express both
alleles. The genotypic ratio
will match the phenotypic
ratio. Snapdragons
demonstrate incomplete
dominance.
Incomplete dominance II
Multiple allele
Linkage and crossing-over
Recombinant frequency
Skin colour_a pair of twin sisters
Continuous or discontinuous
Polygenic effects on genetic
variation
Polygenic inheritance_skin
colour
Skin colour
is a
continuous
variation
because…
How would U explain the very
different skin colour
of the twin sisters?
X-inactivation
X-inactivation