Transcript mendelian inheritance - msc-biology-2008

MENDELIAN
INHERITANCE
Gregor Mendel proposed that heredity is
passed on by what we now call genes.
GENOTYPES &
PHENOTYPES
• The genotype of an organism can be
defined as the genes that it possesses,
and is usually expressed as a combination
of alleles.
• The phenotype of the organism is the
physical features, proteins produced that
is determined by both the genotype and
the environment.
DOMINANCE &
RECESSIVENESS
• The characteristic (or trait) itself is
said to be dominant, when it is
expresses when there is at least one
copy of the allele present.
• The characteristic (or trait) is said
to be recessive when two copies must
be present for it to be expressed.
GENOTYPES
• In the trait of tongue rolling the dominant
characteristic is represented by R and the
recessive by r.
• The genotypes of a roller could be
- Rr
- RR
* The genotype of a non-roller would be
- rr
C0- & INCOMPLETE
DOMINANCE
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Examples of co-dominance include:
The ABO blood group in humans.
The appearance of roan cattle.
Examples of incomplete dominance
include:
- Pink Snapdragons.
IMPORTANT
TERMINOLOGY
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HOMOZYGOUS
HETEROZYGOUS
CO-DOMINANT
INCOMPLETE DOMINANT
HYBRID
PURE BREEDING
MENDELIAN
INHERITANCE
• The example of tongue rolling where R
represents the ability to roll and r
represents the inability to roll.
• Such a problem may be worded:
- What are the F1 & F2 generation
phenotypes of a monohybrid cross between
homozygous dominant & homozygous
recessive individuals?
TAKE A LOOK…..
Parents
RR +
rr
Gametes
all R +
all r
Genotype F1
all Rr
Phenotype F1
all tongue rollers
Therefore F2 parents Rr + Rr
Gametes (1/2 R + ½ r) + (1/2 R + ½ r)
MENDELIAN
INHERITANCE…..
• Genotype ratio 1:2:1
• Phenotype ratio 3:1
• The genotype ratio is the result of
genes separating when gametes form
& combing at fertilisation by chance.
• This is the Principle of Segregation,
or, Mendel’s First Law.
MENDEL’S SECOND
LAW
• When two different characteristics are
considered, such as colour & texture of
fruit, this is termed a dihybrid cross and
leads to an explanation of Mendel’s 2nd law.
• In this cross let G represent green fruit
and g represent yellow and let H represent
hairy fruit and h represent smooth.
• Cross a homozygous dominant plant
with a homozygous recessive plant
Parents
GGHH
+ gghh
Gametes all GH
+ all gh
F1 offspring
all GgHh
Parents for F2 GgHh + GgHh
Gametes 1/4GH+1/4Gh+1/4gH+1/4gh
PUNNETT SQUARE
MENDEL’S SECOND
LAW
• This is explained by the fact that the two
characteristics are independent of each
other.
• That is, they do not necessarily move to
the same gamete in the same combinations
every time meiosis occurs.
• Mendel’s second law is the Principle of
Independent Assortment.