Download Genetics: Mendel and Beyond

2 Mendel’s experiments
(2015)
Genetics
Genetics is a biological discipline that studies:
• the transmission of traits from one generation
to the next
• gene distribution, variation and change in
populations
• evolution of genes
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
Character: observable physical feature
(e.g., flower colour).
Trait: form of a character (e.g., purple
flowers or white flowers).
A heritable trait is passed from parent to
offspring.
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
Mendel’s experiments
Genetics: Mendel and Beyond
Gregor Mendel and his garden
Mendel carried out the first study
on heredity in the second half of
the 19th century using garden
pea plants (Pisum sativum).
Plant
reproduction
6
In Mendel's time, inheritance
was thought to be the result of
a blending of two parents' characters
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
Mendel looked for well-defined, truebreeding traits (the observed trait is the
only one present for many generations).
True-breeding strains (pure line) were
isolated by inbreeding and selection.
He concentrated on seven characters,
each showing two alternate traits.
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
Controlling pollination and plant
generations:
P
• pollen from one parent was transferred
to the carpel of the other parent
parental generation = P;
• resulting offspring = first filial
generation or F1;
• if F1 plants self pollinate, produce
second filial generation or F2.
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
Mendel’s first experiment:
1.crossed plants differing in just one trait (P);
2.F1 generation are monohybrids;
3.the monohybrids were then allowed to self
pollinate to form the F2 generation - a
monohybrid cross;
4.Mendel repeated this for all seven traits.
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
One trait of each pair disappeared in the F1
generation and reappeared in the F2 —
these traits are recessive.
The trait that appears in the F1 is the
dominant trait.
P
F1
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
The ratio of dominant to recessive in the
F2 was about 3:1.
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
Mendel concluded that each plant possesses two
factors controlling a given characteristic; these
could be identical (as in true-breeding plants) or
different (as in hybrid plant) and in this case one
dominates the other; these factors separate and go
to different gametes when a parent reproduces
These “factors” are now called genes. The different
forms of a single gene are called alleles.
Try yourself
P
P
p
P
p
P
p
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Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
Mendel’s first and second law
The Law of Dominance: in a cross of parents that
are pure for contrasting traits, only one form of the
trait will appear in the next generation.
The Law of Segregation: each characteristic is
controlled by two factors, which separate and go to
different gametes when an organism reproduces.
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
Mendel’s second experiment
Mendel wondered whether different characteristics
are inherited together. For example, are purple
flowers and tall stems always inherited together?
Or do these two characteristics show up in different
combinations in offspring?
To answer these questions, Mendel next
investigated two characteristics at a time.
Genetics: Mendel and Beyond
Independent assortment
- What are the mendelian laws of inheritance?
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
Mendel’s third law
The Law of Independent Assortment
Factors controlling different characteristics
are inherited independently of each other
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
How modern genetics explain Mendel’s law of
segregation?
Today, we known that characteristics of organisms are
controlled by genes (Mendel’s “factors”) located on
chromosomes.
A gene is a sequence on a DNA molecule that resides
at a particular site on a chromosome—the locus—and
encodes a particular character.
Genes are expressed as proteins with particular functions.
Different alleles of a gene separate during meiosis.
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
Meiosis accounts for the segregation of alleles (part 1)
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
Meiosis accounts for the segregation of alleles (part 2)
Genetics: Mendel and Beyond
Meiosis accounts for independent
assortment of alleles
- What are the mendelian laws of inheritance?
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
When gametes unite during fertilization, the resulting
zygote inherits two alleles for each gene. One allele
comes from each parent. The alleles an individual
inherits make up the individual’s genotype, i.e. The
genetic makeup ov an organism.
The expression of an organism’s genotype produces its
phenotype, i.e.the the physical appearance of an
organism (e.g., spherical seeds).
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
An individual that inherits two alleles of the
same type is said to be homozygous for
the allele (e.g., AA or aa).
An organism with two different alleles is
called heterozygous (e.g., Aa).
homozygous
heterozygous
homozygous
Genetics: Mendel and Beyond
- What are the mendelian laws of inheritance?
Problem samples
1.
Classify the following as heterozygous or homozygous: RR, Rr, yy, YyRR
2.
What is the phenotype of the following: Yy, Ry, yy, YyRr
3.
What genotypes could have a purple flowered plant? PP, pp, Pp, Yp
4.
Which cross would produce both green and yellow seed?
YY x yy,
Yy x Yy,
Yy x yy,
yy x yy